Results from a 2015 and 2016 Survey to Determine the Distribution and Frequency of Herbicide- Resistant Horseweed (Conyza canadensis) in Missouri

Eric Oseland, Mandy Bish, Kevin Bradley, University of Missouri, Columbia, Mo.

Horseweed (Conyza canadensis) has been classified as one of the ten most troublesome and common weeds in the U.S according to a recent Weed Science Society of America survey. Quantitative data regarding the distribution and frequency of herbicide resistance in horseweed in Missouri is lacking. Horseweed seed from 112 separate horseweed populations was collected from infested fields throughout Missouri just prior to soybean harvest in 2015 and 2016. In greenhouse experiments, seed were planted in 10-cm pots and maintained in the greenhouse until they reached a 10-cm rosette for characterization of glyphosate, glufosinate, 2,4-D, dicamba, and cloransulam resistance in a greenhouse experiment. Each population was sprayed with twice the recommended field use rate of glyphosate, glufosinate, 2,4-D, dicamba, and cloransulam. Horseweed populations were classified as resistant if visual injury assessed at 28 days after application (DAA) was less than 60%. Glyphosate resistance was confirmed in all 112 populations and these populations were distributed across 42 counties in Missouri. Cloransulam resistance was confirmed in 99 out of 112 populations and these populations were distributed across 40 counties. Thirteen populations survived the application of glufosinate, which were spread across 12 counties. Two populations survived the application of 2,4-D while all populations were found to be susceptible to dicamba. The results of this survey will provide Missouri producers with valuable data regarding the distribution of herbicide-resistant horseweed populations in the state. 

Cross-resistance patterns to ALS-inhibitors in beggarticks (Bidens spp.) in Brazil

Rafael R Mendes*^1/, Rubem S Oliveira Jr. ^1/, Jamil Constantin^1/, Vanessa V Silva^1/, Fellipe G Machado^1/, Liriana B Cantagalli^1/

^1/Maringá State University – Maringá, Paraná - Brazil. *Present author.

Resistance to ALS-inhibitors is complex and diversified because of the number of species selected, to the several target site mutations and to the level of resistance that is found in field. Beggarticks (Bidens spp.) has been recognized as ALS-resistant since the 90’s in Brazil, however we have observed different cross-resistant patterns in populations from soybean, corn and cotton fields. Our objective was to characterize the cross-resistance patterns in populations of beggarticks resistant to ALS-inhibitors. Thirty-seven populations were screened with a discriminatory-dose assay for imazethapyr (imidazolinones), chlorimuron (sulfonylureas) and diclosulam (triazolopyrimidines). Three resistant populations (R1, R2 and R3) were further evaluated in dose-response trails for all herbicides. Resistance was found in 81% of the populations for imazethapyr, 62% for chlorimuron and 51% for diclosulam. The R1 population was resistant to imazethapyr (13.2-fold), chlorimuron (3.9-fold) and diclosulam (7-fold). The R2 population showed broad cross resistance to imazethapyr (>30-fold), chlorimuron (15.7-fold) and diclosulam (>271-fold). Finally, the R3 population was resistant to imazethapyr (11.4-fold), but presented very low resistance to chlorimuron (1.19-fold) and diclosulam (2-fold). There are at least three cross-resistance patterns in beggarticks resistant to ALS-inhibitors in Brazil and cross-resistance patterns observed in R1, R2 and R3 can also be found in other populations dispersed throughout different sampled areas.

E-mail senior-author: rafaromero.mendes@gmail.com / rafamendes.consultoria@gmail.com

Cotton (Gossypium hirsutum L.) Response to Combinations of Mepiquat Chloride, Glyphosate, and Dicamba. T. Buck¹, A. York², D. Stephenson³, B. Woolam³, M. Askew², S. Rustom¹, ¹LSU Ag Center, Baton Rouge, LA, ²North Carolina State University , Raleigh, NC, ³LSU AgCenter, Alexandria, LA,

Abstract

Research was conducted at Alexandria, LA and at individual locations in Clayton and Rocky Mount, NC in 2017 to evaluate the effect of mepiquat chloride, glyphosate, and dicamba application alone or in combination on injury of XtendFlex cotton.  Experiments were arranged in a randomized complete with three to four replications. Dicamba, glyphosate, and mepiquat chloride rates were applied alone or in combination at 92, 208, and 8 g ae or ai ha^-1, respectively, at early bloom followed by two wk after early bloom.  Visual cotton injury was evaluated 7 d after each application timing.  Cotton height and total node count were recorded at each application timing and just prior to harvest. Cotton yield and total boll count were also collected, cotton yield was converted to lint yield.  Cotton heights, total node number, total boll number, and yield were converted to percent of the nontreated prior to analysis.

Averaged across application timings, cotton injury was 0 to 8% following all treatments, regardless of their combination.  Dicamba mixed with glyphosate and the three-way mixture of dicamba, glyphosate, and mepiquat chloride were the only treatments resulting in 8% injury.  Injury was greatest following the early bloom application at 8% following the three-way mixture, but decreased to 5% following the second application.  Cotton height was reduced as much as 19%, but only occurred when mepiquat chloride was present in the treatment.  Despite the reduction in height, total nodes were not affected and ranged from 22 to 25, regardless of treatment.  Similarly, the amount of total bolls were not different between treatments with a range of 87 to 100 harvestable bolls per plant.  Cotton yield was not affected by any of the combinations of treatments, with yields following all treatments greater than 100% of the nontreated.

These results indicate that it is safe to combine mixtures of dicamba, glyphosate, and mepiquat chloride to cotton at early bloom in Louisiana and North Carolina.  While cotton showed small levels of injury in response to some mixtures, yield and fruiting were not affected.  By applying herbicide and growth regulator in one application, growers can cut cost by avoiding making multiple trips across a field.

Effects of Simultaneous Fertilizer and Preemergence Herbicide Applications on Nutrient Uptake and Leaching on Tifway 419 Bermudagrass (Cynodon dactylon).

L. O. R. Maia, T. W. Shaddox, R. Leon, and J. K. Kruse

Abstract

Certain classes of pre-emergence herbicides influence the root development of non-target turfgrasses. Nutrients applied according the best management practices (BMPs) on turfgrass exhibiting root inhibition promoted by pre-emergence herbicides may be prone to leaching to groundwater. The objective of this study is to determine the effects of simultaneous fertilizer and pre-emergence herbicide applications on nitrogen (N) and phosphorus (P) uptake and N and orthophosphate (ortho-P) leaching from Tifway 419 bermudagrass [Cynodon dactylon (L.) Pers. × C. transvaalensis Burtt-Davy]. A field experiment was arranged in a randomized complete block design with four replications. Treatments were applied every four months and included a negative control (no-turf), a control (no herbicide), oxadiazon (224 kg ha^-1), prodiamine (1.17 L ha^-1), and indaziflam (0.33 L ha^-1). Sprigs were planted on February 23^rd on a mixture of 90% sand and 10% peat and first data was collected June 1, 2017. Fertilizer [15-02-12 (N-P-K)] was applied at 49 kg ha^-1 60 d^-1 to all plots except the negative control. Leachate was collected as needed and analyzed for NO₃-N, NH₄-N, and ortho-P. Tissue N content was determined once a month. Roots were collected 4 and 8 weeks after treatment (0-5 cm and 5-15 cm) and analyzed for length, length density, and surface area. No differences were observed on root parameters at both depths. Leached N peaked for each herbicide treatment one week after application (WAA) and decline to concentrations equivalent to the negative control three WAA. Ortho-P leaching was similar among treatments on each collection date. Ortho-P leaching from fallow plots were similar to fertilized turfgrass.  Due to the early stage of the study, we failed to reject the null hypothesis that pre-emergent herbicides do not increase N leaching, ortho-P leaching, or decrease N uptake. Turfgrass may provide an ecological benefit by reducing the amount of N and P that leaches naturally.

______________________________________________________________________________

L. O. R. Maia, M.S. student, Fort Lauderdale Res. and Educ. Center, Univ. of Florida, Fort Lauderdale, FL 33314-7799.

T. W. Shaddox, Assistant Professor, Fort Lauderdale Res. and Educ. Center, Univ. of Florida, Fort Lauderdale, FL 33314-7799. 

Palmer amaranth (Amaranthus palmeri S. Wats) has become the most impactful weed in the history of Georgia agriculture. Glufosinate, dicamba, and 2,4-D systems are available to assist cotton growers in managing this pest. Although dicamba and 2,4-D systems are effective and important, it is the glufosinate system that is the most critical to Georgia as it offers a more sustainable system without off-target movement concerns. However, research is needed to improve the effectiveness of the system. Therefore, an experiment was conducted at three locations to determine if the interval between POST 1 and POST 2 applications influenced weed control. The randomized split plot design included the whole plot being herbicide system (glufosinate 656 g ai/ha alone or mixed with glyphosate 1,261 g ae/ha) and the split-plot being time interval (1, 3, 5, 7, 10, or 14 d) between sequential application of the same treatment when weeds initially reached 16 cm. A no sequential POST 2 treatment and a non-treated control were included for comparisons. A POST 3 application of the same herbicide treatment followed the POST 2 application 14 days later. All herbicide treatments were made at 140 L/ha using flat fan nozzles and no adjuvants were included. Crop injury evaluations, height, and yield data were collected. Palmer amaranth and large crabgrass (Digitaria sanguinalis (L.) Scop.) control and biomass data were collected as well as the number of Palmer amaranth plants present at harvest. Glyphosate resistant Palmer amaranth control at harvest was greater than 98% for both herbicide systems with intervals of 1, 3, 5, or 7 d between applications; plant populations were less than 1560 plants/ha with a biomass less than 13,260 g/ha. When no POST 2 was applied, or there was an interval of 10 or 14 d between sequential applications, control ranged from 64 to 86% with 15,600 to 234,780 plants/ha having a biomass ranging from 54,600 to 503,880 g/ha. Intervals of 1, 3, 5, or 7 d between glufosinate applications or 1, 3, 5, 7, or 10 d between glufosinate plus glyphosate applications noted similar large crabgrass control (93-99%) with a biomass less than 49,140 g/ha. Extending the interval between applications with either system increased the level of crabgrass biomass at harvest by at least 8 times. Neither cotton injury nor heights of XtendFlex cultivar DP 1646 B2XF were influenced by treatments. Yield followed trends primarily noted with crabgrass control. Yields (2930 to 3120 kg/ha) were similar with 1, 3, 5, and 7 d intervals between glufosinate applications; yield decreased 16 to 40% by extending intervals to 10 or 14 d or when no POST 2 was applied. Yield from all sequential glufosinate plus glyphosate applications were similar and at least 35% greater than when no POST 2 application was made. Results suggest glyphosate-resistant Palmer amaranth and large crabgrass control can be improved by making sequential applications of glufosinate or glufosinate plus glyphosate with a 1 to 10 d interval between applications.

The increasing occurrence of herbicide-resistant weeds coupled with the recent registration of dicamba-resistant soybean will lead to the increased use of dicamba in Michigan. Several broadleaf crops are extremely sensitive to dicamba, raising concerns about the implications of tank-contamination. Dry edible beans are one of these sensitive crops that are of economic importance to Michigan farmers. In 2017, a field trial was conducted in Richville, MI to investigate the effects of dicamba and dicamba + glyphosate tank-contaminates with commonly used postemergence herbicide treatments in dry edible beans. The objective of this research was to gain a better understanding of how dry edible beans respond to sub-lethal rates of dicamba when applied in combination with other herbicides labeled for use in dry edible beans.  ‘Zenith’ black beans were exposed to the contaminated treatments at the V2 and V8 (preflower) stages of growth. These timings were chosen based on typical herbicide application periods in dry edible bean production. Dicamba and glyphosate was applied at 1% of the labeled field rate, assuming use rate of 0.56 kg ae ha^-1 of dicamba and 1.27 kg ae ha^-1 of glyphosate. Labeled dry bean treatments included: 1) imazamox (35 g ha^-1) + bentazon (280 g ha^-1) + crop oil concentrate + ammonium sulfate and 2) fomesafen (280 g ha^-1) + crop oil concentrate. Dry bean injury was evaluated 7, 14, 21, and 28 d after treatment (DAT). Dry beans were harvested, yield was adjusted to 18% moisture, and seed weight was determined. Maturity was recorded weekly prior to harvest. Dry bean injury was similar for both application timings. Contaminates of dicamba + glyphosate in the imazamox + bentazon treatment caused 26% injury, 14 DAT. Contamination with dicamba + glyphosate prolonged dry bean injury symptoms compared with dicamba alone. Maturity was delayed by contaminated treatments at both application timings. At the V2 timing, contaminated treatments delayed maturity to 50% by 8-26 d compared with the control. The addition of glyphosate to dicamba caused the greatest delays. At this timing, seed weight was not reduced. At the V8 timing, contaminated treatments delayed maturity, 55-64 d. Seed weight was also lower (9-15%). We are currently examining what effects these treatments have on seed and germination and quality and will be repeating this research in 2018.  

PPO converts protoporphyrinogen IX (protogen IX) to protoporphyrin IX (proto IX). Two nuclear encoded genes produce isoforms of this enzyme, PPO1 in the chloroplast and PPO2 in the mitochondria. To date, 13 weed species have confirmed resistance to PPO-inhibitors. From these weed species, the target-site resistance has only been identified in dicot species, i.e., Amaranthus spp. and Ambrosia artemisiifolia. The first case was reported in lactofen resistant biotypes of A. tuberculatus, in which three nucleotides were deleted resulting in a deletion at the 210 position (ΔGly210) in the PPO2 gene. In PPO-resistant populations of A. artemisiifolia, a Arg-98-Leu mutation was identified in the PPO2 gene. The most recent case was two new mutations at the Arg-98 position in A. palmeri, Arg-98-Gly and Arg-98-Met. The objective of this study was to characterize a Lolium rigidum population collected in an olive tree field from Jaén (Spain), resistant to the PPO herbicide oxyfluorfen. Dose response studies confirmed it as PPO-resistant (R) when compared to an oxyfluorfen-sensible (S) population. Relative growth reduction (GR) and lethal (LD) doses were calculated at both 50% and 90% levels. The R-population exhibited a resistance index (RI) ranging from 20.1 (GR₅₀) to 70.2 (GR₉₀). The complete PPO-2 gene was sequence and analyzed for both R- and S-population of L. rigidum. In the R-population, an amino acid substitution in the PPO-2 gene and the first time in a monocot weed species was found. The substitution was Arg-132-His, a region that is highly conserved in the PPO2 gene, in which the analogous region in A. palmeri and A. artemisiifolia is Arg-98. This current study represents the first report of a target site exchange in PPO2 for monocot species.

Keywords: Lolium rigidum; PPO2 gene; resistance; target-site resistance; Arg-132-His.

The release of dicamba tolerant soybeans by Monsanto will aid growers in weed control. However, several challenges are also forthcoming, one being sprayer hygiene. Glyphosate is very water soluble, allowing it to be easily removed from spray tanks through three rinses with water alone. However, synthetic auxin herbicides are not as water soluble and therefore can be difficult to completely remove from sprayer components. Additionally, many crop species are highly sensitive to synthetic auxins at very low concentrations. The objective of this study was to determine which commercial tank cleaners are most effective in the removal of auxin herbicides from spray tanks using a standard washout procedure. Field experiments were conducted in 2016 and 2017 in Brooksville and Starkville, MS. Eight different cleaners were evaluated, along with a no cleanout treatment and a treatment consisting of only 3 rinses with water. In 2016 a large scale sprayer was used while in 2017 a small scale sprayer was designed to replicate the cleanout procedures used on commercial sprayers. During both years the cleanout procedure was identical. The system was first contaminated with dicamba (Clarity in 2016, Xtendimax in 2017) and rhodamine WT dye. The sprayer then underwent a 3 rinse cleanout, utilizing one of the tank cleaners during the second rinse step. During each rinse, the solution was recirculated through the system for 15 minutes and samples were collected for both field and lab analysis. Once the sprayer was cleaned using the triple rinse procedure it was filled with a 867 g ae ha^-1 rate of glyphosate (Roundup Powermax), and another sample was collected. All samples were sprayed over actively growing soybeans at the R1 growth stage. Visual ratings for phytotoxicity were taken 7, 14, 21, and 28 DAT and plant heights were taken 14, 21, and 28 DAT. Samples collected during each rinse were analyzed using HPLC to determine auxin herbicide concentrations as a means to evaluate cleaner efficacy. Plants were harvested at end of the growing season for yield. Data revealed at 28 DAT, cleanouts utilizing Incide-Out resulted in 31% injury, higher than all other cleaners. Plant injury decreased significantly from rinse 1, 2, 3, and the glyphosate solution resulting in 57, 36, 17, and 9% injury averaged across cleaners and years, respectively. Plant height reductions did not differ from the untreated check following the second rinse. Data also revealed that a yield reduction occurred at the first and second rinses; however, the third rinse and glyphosate solution did not cause yield reductions compared to the untreated check. HPLC analysis revealed that following the first rinse, no differences were detected between any of the eight cleaners tested.

In order to avoid the herbicide-resistance fate of previous herbicides, best management practices such as mode of action diversification must be adopted to properly steward new auxin herbicide technologies such as in the dicamba-resistant Xtend Weed Control System. Chloroacetamides provide good control of small-seeded broadleaf weeds and can be applied flexibly within cotton and soybean production systems utilizing this technology. Experiments were conducted to determine the optimal application timing (PRE, E/LPOST, PRE+E/LPOST) of s-metolachlor or acetochlor in Xtend soybean and cotton production systems at four locations across Mississippi and Nebraska. Control, density, and biomass of kochia and pigweed species were recorded at various timings along with crop yield and injury information. Head-to-head comparisons of s-metolachlor vs acetochlor were not significant for responses relative to any parameters. However, inclusion of chloroacetamides as PRE applications alone led to a 500-900 kg ha-1 reduction in yield and a twofold average increase in weed densities at harvest. Use of a chloroacetamide as a PRE application only resulted in a 25-50% approximate reduction in visible weed control. Significant crop injury was noted from some POST and split POST applications of chloroacetamides but was limited to a low magnitude ranging from 2-5 percent. Results of these studies suggest that a chloroacetamide should be applied PRE and either early or late POST to maximize weed control and crop yield. Future research will characterize the roles of other residual herbicide families in auxin-resistant production systems.

Field and greenhouse experiments were conducted in 2017 in Starkville, MS to investigate the effect of carrier volume and weed size on glufosinate efficacy when applied from two different drift-reduction nozzles. In the greenhouse study, TTI and TDXL nozzles were used to apply 0.66 kg ai ha^-1 of glufosinate at 94 or 140 L ha^-1 to 7.6 and 15.2 cm waterhemp plants. Likewise in the field study, TTI and TDXL nozzles at 94 and 140 L ha^-1, respectively, were used to apply 0.66 or 0.74 kg ai ha^-1 of glufosinate to 7.6 and 15.2 cm waterhemp in soybean. In the greenhouse study, common waterhemp control was consistently ~10% higher across three evaluation dates when plants were treated at 7.6 cm compared to 15.2 cm, regardless of nozzle or carrier volume. Likewise, fresh biomass of waterhemp plants harvested 21 DAT were over 6-fold higher in plants treated at 15.2 cm compared to 7.6 cm. In the field, waterhemp control was reduced by treatment of 15.2 cm waterhemp when TDXL nozzles at 140 L ha^-1 were used to apply 0.74 kg ai ha-1 or TTI nozzles at 94 L ha-1 were used to apply 0.66 kg ai ha^-1. Applications to 7.6 and 15.2 cm waterhemp in the field corresponded to the V4 and R2 soybean growth stages. Soybean injury to plots treated at the 7.6 cm timing was 19% which translated into a yield loss of 10%. Conversely, the 15.2 cm weed size treatment caused only 3% soybean injury and no yield loss. The soybean injury observed is likely due to higher glufosinate rates used. Therefore, these results suggest that timely glufosinate applications to small waterhemp at typical use rates are key to avoiding the need for higher rates and the possibility of soybean injury and yield loss. However, if higher rates are needed, soybean tolerance is much higher when applied at later growth stages. 

Modern sugarcane production can result in sugarcane residues exceeding 10 mt ha^-1 following harvest. These high residue levels can impact herbicide performance by physically intercepting the herbicide and potentially retaining it even with subsequent rainfall. An herbicide’s chemical properties can be used to predict how it will sorb to residue. The objective of this research was to evaluate indaziflam, imazapic, and amicarbazone interception, adsorption, and desorption by sugarcane residue. A typical batch-equilibrium study was conducted to determine adsorption and desorption across a range of herbicide concentrations. Sugarcane residue (0.27 gm) was combined with a range of herbicide concentrations (0.125, 0.5, and 1 ppm) plus 0.24 KBq indaziflam, 0.26 KBq imazapic, or 0.20 KBq amicarbazone. Indaziflam, imazapic, and amicarbazone adsorption were evaluated 24, 48 and 120 hours respectively, after contact with sugarcane residue. Herbicide and sugarcane residue were allowed to equilibrate for various time peroids, then centrifuged, and 1ml of supernatant was transferred to a scintillation vial followed by the addition of 10 mL of scintillation cocktail. Herbicide concentrations in the supernatant were determined by liquid scintillation spectroscopy (LSS) and by difference used to determine adsorption. For desorption, the herbicide solution was removed from test tubes and replaced with 8 ml of 0.02 M CaCl₂ solution once or multiple times depending on the herbicide. Indaziflam adsorption was greater than 80% at all concentrations, while imazapic adsorption was less than 7 % at all concentrations. Amicarbazone adsorption was less than 20% across all concentrations. Indaziflam desorption was 30%, 28.5% and 27.5% at 0.125, 0.5 and 1 ppm respectively, after 5 days. The maximum desorption for amicarbazone was observed at 1 ppm with 11%. The desorption for imazapic was not determined due to low adsorption . In a second experiment herbicide interception by sugarcane residue and rainoff using simulated rainfall was determined at several rainfall amounts (3, 6, 12 and 24 mm). Dried sugarcane residues were spread evenly on top of a stainless steel screen, and then the screen was placed on top of a Pyrex® pan. Pans were used to capture herbicide solutions during simulated rainfall events. Residue amounts were 5 mt ha^-1  and 10 mt ha^-1.  For indaziflam, allowing the herbicide to interact with sugarcane residue for seven days and using a 24 mm rainfall event resulted in removal of only 25% of the adsorbed herbicide. Herbicide characteristics, such as water solubility and Kow, could be used to determine how difficult it will be to remove herbicides from crop residues.

Glyphosate-resistant Palmer amaranth (Amaranthus palmeri) is the most problematic weed in row-cropping systems. In years past, growers have relied heavily on group 14 (PPO inhibitors) and group 15 (long-chain fatty acid inhibitors) for control GR Palmer amaranth populations. As a result, PPO-resistant Palmer amaranth biotypes were discovered in 2015 in Tennessee populations. However, data is lacking on the residual efficacy of group 14 and 15 herbicides on PPO-resistant Palmer amaranth. Considering these herbicides are utilized on nearly every acre to control Palmer amaranth, studies were conducted to evaluate the efficacy if group 14 and 15 residual herbicides on PPO-resistant Palmer amaranth.

Two studies were conducted at both Ripley, Tennessee (PPO-R) and Dyersburg, Tennessee (PPO-S) in 2017. In study one (group 14 herbicides), treatments included a nontreated, sulfentrazone, saflufenacil, flumioxazin, sulfentrazone + metribuzin at 0.5X, 1X, 1.5X, and 2X field-use rate for a silt loam soil. In study two (group 15 herbicides), treatments included a nontreated, pyroxasulfone, S-metolachlor, dimethenamid-P, and pyroxasulfone + fluthiacet-methyl at 0.5X, 1X, 1.5X, and 2X field-use rate. Plots were 1.5 m wide and 9.1 m in length. Treatments were replicated three times and arranged within a randomized complete block design. Visual control was assessed 21, 28, 35 and 42 days after treatment and expressed as ED₇₅ (effective dose to control 75% of Palmer amaranth) and as a percentage compared to the nontreated check. Data were subject to an analysis of variance using the PROC Glimmix in SAS 9.4. Using a 3-parameter sigmoidal equation, trends from results were constructed in Sigma Plot 14.

In study one, ED₇₅ values of sulfentrazone was less at PPO-S site, 109 g ai ha^-1, compared to PPO-R site. ED₇₅ values of flumioxazin at PPO-R site (121 g ai ha^-1) were 10 times greater than the PPO-S site (12 g ai ha^-1)  35 DAT. In study two, ED₇₅ values of S-metolachlor were less at PPO-S site (178 g ai ha^-1) compared to PPO-R site (634 g ai ha^-1), 28 DAT. Percent control of Palmer amaranth with pyroxasulfone at 90 g ai ha^-1 was greater at PPO-S site (92%) compared to PPO-R site (79%) 35 DAT.

ED₇₅ values suggest that group 14 herbicides applied were less effective on Palmer amaranth at this PPO-resistant site. In fields that are infested with PPO-resistant Palmer amaranth, premixes of residual herbicides that contain effective modes of action should be utilized. Growers that rely on group 15 herbicides to control PPO-resistant Palmer amaranth should be cognizant of the inconsistencies in control shown in these data.

The recent commercialization of Roundup Ready^® Xtend soybean will allow growers to use dicamba (low-volatile formulations) to control glyphosate-resistant weeds, including kochia. Although dicamba-resistant kochia has been reported in 5 states in the western US, it is still not widespread. Proactive measures must be taken to prevent development of dicamba and especially glyphosate and dicamba multiple-resistant kochia in Roundup Ready^® Xtend soybean system. Field experiments were conducted in 2017 at the Montana State University Southern Agricultural Research Center, Huntley, MT to develop effective herbicide programs to control glyphosate- and dicamba-resistant kochia in Roundup Ready^® Xtend soybean. Nine different herbicide combinations were evaluated, which included sulfentrazone + glyphosate or pyroxasulfone + glyphosate alone or with dicamba (Engenia^™) PRE, or PRE followed by (fb) dicamba + glyphosate with or without pyroxasulfone POST. Treatments were arranged in a randomized complete block design, with four replications. Plots were infested with an equal proportion of glyphosate- and dicamba-resistant kochia at the time of soybean planting. Data on crop injury, kochia control, density, biomass, and seed production were recorded. Preemergence (PRE) application of sulfentrazone or pyroxasulfone alone or with dicamba did not influence soybean emergence. Pyroxasulfone alone or with dicamba PRE did not cause any significant injury to soybean. However, plots treated with sulfentrazone PRE had 10 to 15% visual injury to soybean until 3 weeks after PRE (WAPRE). Plots treated with sulfentrazone PRE fb pyroxasulfone + dicamba + glyphosate POST showed 14 to 20% visual injury to soybean at 2 weeks after POST (WAPOST). Nevertheless, crop injury by 6 WAPOST was reduced to ≤ 8% in those plots. A single application of sulfentrazone PRE provided complete, season-long control of glyphosate- and dicamba-resistant kochia. Addition of dicamba with pyroxasulfone PRE program improved kochia control to 89 to 91% compared with 53 to 69% control with pyroxasulfone alone PRE at 3 to 9 WAPRE. Pyroxasulfone PRE fb pyroxasulfone + dicamba + glyphosate POST provided 94 and 100% kochia control at 2 and 4 WAPOST, respectively. Similarly, pyroxasulfone + dicamba PRE fb dicamba + glyphosate POST program provided 98 and 100% kochia control at 2 and 4 WAPOST. Kochia density in plots treated with pyroxasulfone + dicamba PRE was 94% less than pyroxasulfone PRE alone program at 9 WAPRE. Similarly, kochia dry biomass and seed production at harvest was reduced by 93 and 91%, respectively, in pyroxasulfone + dicamba PRE compared with pyroxasulfone PRE alone; a follow-up POST program was much needed with pyroxasulfone alone (in the absence of dicamba) PRE program to prevent kochia seed production and soybean yield reductions. Low to moderate levels of early-season soybean injury caused by sulfentrazone did not translate into yield loss. In conclusion, PRE soil-residual herbicides investigated in this study will serve as a foundation for dicamba- and glyphosate-resistant kochia management in Roundup Ready^® Xtend soybean.

The competitive ability, reproductive potential, and fecundity of Palmer amaranth have been studied extensively. Previous experiments have reported that a single female Palmer amaranth is capable of producing 200,000 to 600,000 seeds. Since hand-counting all seeds from harvested Palmer amaranth is prohibitively time consuming, subsamples are hand-counted, weighed, and extrapolated to calculate total seed number. However, the relative error of extrapolation associated with seed production estimates has yet to be explored. A Computerized Particle Analyzer II (CPA; W.S. Tyler Group), an image analysis instrument, was evaluated and used to determine the relative error of extrapolation from two subsampling methods, 100-count seed weights and 0.5 g seed counts. Forty-six hand-counted subsamples ranging from 500 to 5000 Palmer amaranth seeds determined that the relationship of hand-counts vs. CPA counts was described by the following linear equation: y = 3.5 + 0.987x; R² = 0.999. A slope of 1 would represent perfect agreement between counting methods, so CPA seed counts were considered to be highly accurate. With the accuracy of the CPA established, CPA seed counts of eight Palmer amaranth seed collections (25 g) were used as a standard to calculate the relative error of extrapolation for both subsampling methods. Relative seed count error was calculated by subtracting the extrapolated count from the CPA count then dividing the difference by CPA count. ANOVA determined that there was no significant difference in relative error between subsampling methods, and the relative errors were 1.6 and 0.8% for the 100-count and 0.5 g methods, respectively. Thus, either subsampling method can be used to estimate Palmer amaranth seed production accurately.

Intermittent and prolonged emergence period, rapid plant maturity, and prolific production of highly-dispersive seeds make S. oleraceus as difficult-to-control weed, worldwide. Evolutionary herbicidal resistance, genetic diversity, innate seed dormancy, and absence of natural predators contributed towards its successful distribution as aggressive weed across the globe. Over the last 10 years, shift towards conservation tillage systems raised the status of this weed from relative obscurity to a most troublesome and economically damaging invasive weeds, particularly in Australia. Numerous populations has been discovered resistant to group B and M, including chlorosulfuron, atrazine, and glyphosate in this country. However, application of post-emergent herbicides, such as carfentrazone, florasulam, bromoxynil octanate, and sulfentrazone, either singly or in mixture form, were found effective in suppressing S. oleraceus in small and non-stressed infestations. Double-knock tactic would be considered effective approach for preventing seed set to achieve high level weed control through preventing soil seedbank replenishment. However, integration of weed control approaches has been reported to be more reliable and efficient for the long-term control of S. oleraceus. This article was intended to highlight the current scenario and future prospects of this economically damaging invasive weed, particularly in Australia. Understanding related to the physiological aspects regulating the invasion biology of S. oleraceus will help in predicting its agro-ecological impacts and has pragmatic implications in designing management strategies. 

Sourgrass is a perennial rhizomatous grass native of the American tropics that grows in clumps. Sourgrass seeds have low dormancy potential and high dispersal ability. Due to the selection of glyphosate-resistant biotypes, it is considered the most troublesome weed in the Brazilian grain-producing areas and the problem has spread throughout other South American countries. To evaluate the effects of plant regulators on sourgrass, field and greenhouse studies were carried out with applications of ALS-inhibitors (sulfometuron-methyl), gibberellin inhibitors (mepiquat chloride and trinexapac-ethyl) and ethylene precursors (ethephon), either alone or combined with herbicides (glyphosate and clethodim) in plants at different stages of development (two-tillers, five-tillers, pre-flowering and flowering). Applications of sulfometuron with glyphosate in stages of two and five tillers provided control >80%. Trinexapac-ethyl also provided control of sourgrass, however the best results were found with its mixture with glyphosate when the application was carried out in at initial stages. Trinexapac-ethyl applied alone or with glyphosate (at pre-flowering stage) reduced seed germination in 87 and 67%, respectively, when compared to the non-applied treatment. The use of plant growth regulators provided effects in sourgrass growth, number of panicles per plant and seed germination. Therefore, plant growth regulators may have a potential role to improve control and suppress viable seed productions in glyphosate-resistant sourgrass management programs.

Eragrostis plana (tough lovegrass) is an invasive weedy grass in several native pasture areas of South America. This species was already reported in some localized areas in United States. It has low forage quality compared with Paspalum notatum, which is one of most important native forages distributed in livestock areas, specially in South Brazil. E. plana has traits of an invasive grass that confer competitive advantage under degraded areas, including rapid growth, potential allelopathic effect, persistent seed bank and ability to tolerate freezing and drought conditions. Considering the known competitive ability of this species as well as current and predicted global changes in precipitation patterns, we performed a greenhouse experiment to determine whether drought conditions would increase Eragrostis plana competitiveness with the forage grass, Paspalum notatum. In this study, the species were grown from emergence to grain filling stage using three plant proportions (100:0, 50:50 and 0:100 for P. notatum and E. plana, respectively) to simulate the competition under well-watered conditions, then at panicle emergence, plants were allowed to compete also under drought condition for 50 days. Relative water content (RWC), stomatal conductance (g_s), chlorophyll a fluorescence parameters, chlorophyll index, plant height, tiller and panicle number, and above-ground biomass were measured at the end of water-stress period. The analysis of variance revealed no significant interactions (p ≥ 0.05) among plant proportions and water stress factors. E. plana had a negative impact on P. notatum overall growth by reducing plant height, tiller and panicle number, above-ground biomass and chlorophyll a fluorescence in both water conditions. Likewise, the forage grass had reduced quantum yield of electron transport (Φ_Eo) where it could increase the dissipated energy flux (DI₀/RC) when competing with E. plana. In addition, the low competitive ability of P. notatum was confirmed by favoring the above-ground biomass production and increasing the panicle number in E. plana. The water stress had a significant impact on both species by reducing g_s, RWC, plant height and panicle number for E. plana and RWC and above-ground biomass in P. notatum. Additionally, water restriction promoted reduction of electron transport flux (ET₀/RC) and quantum yield of electron transport (Φ_Eo) on P. notatum. However chlorophyll a fluorescence in E. plana was not altered in drought stress condition underlining its tolerance to drought. Overall, our results suggest that E. plana could overcome the forage grass P. notatum either in well-watered or drought conditions, emphasizing the invasive potential of E. plana.

Palmer amaranth (Amaranthus palmeri) has extend its range into selected areas of South Dakota as a swine manure contaminant.  Palmer amaranth is problematic due to fast growth rate, poor control once it reaches a height of 4 cm, high seed production, and herbicide resistance to many different herbicide modes-of-action in Southern states.  The Corsica infestation, first noted in 2015, has spread in area over the past 3 years.  In 2017, emergence started in mid-June in soybean, but new seedlings were also observed in late July and by mid-August these late emerging plants were flowering.  In greenhouse studies, seeds taken in 2016 from the Corsica infestation were treated preemergence or seedlings at the 2 to 4- true leaf stage were treated postemergence with several different mode-of-action herbicides.  Neither pre-, post-, or a mixture of pre + post atrazine applications controlled the plants. Thifensulfuron at high rates and glyphosate were also ineffective. S-metolachor, applied preemergence, dicamba, and glufosinate, applied postemergence, provided the greatest and most consistent control.  

Herbicide options are limited for the control of wild carrot in field crops in Ontario. This study which comprised 25 field trials, 9 in corn (4 PRE and 5 POST herbicides trials), 8 in soybean (3 PP and 5 POST herbicides trials) and 8 in winter wheat (8 POST herbicides trials) was completed to determine the most efficacious herbicides for the control of wild carrot in these crops. There was no significant visible injury in corn, soybean and winter wheat with herbicides evaluated. Atrazine, dicamba, dicamba/atrazine, isoxaflutole + atrazine, mesotrione + atrazine, saflufenacil, and saflufenacil/dimethenamid-p applied PRE provided inadequate (39 to 75%) control of wild carrot in corn. Prosulfuron + dicamba applied POST at 10 + 140 g ai ha^-1 controlled wild carrot 93 to 97% with density and biomass similar to the weed-free control in corn. Among PP herbicides evaluated in soybean, only glyphosate (2700 g ai ha^-1) and glyphosate + imazethapyr (900 + 100 g ai ha^-1) controlled wild carrot >80%. None of the POST herbicides evaluated in soybean provided adequate control of wild carrot. In winter wheat, prosulfuron + bromoxynil (10 + 140 g ai ha^-1) controlled wild carrot 83 to 86% compared to other herbicides which controlled wild carrot 21 to 73%.

Corn, soybean and dry beans are of huge economic importance to agriculture in the USA and Canada. Crop losses due to weed interference have a dramatic economic impact on net returns for corn, soybean and dry bean producers. Earlier Weed Science Society of America (WSSA) Weed Loss Committee reports by Chandler et al. (1984) and Bridges (1992), provided a snapshot of crop yield losses throughout various regions of North American if weeds were left uncontrolled. This study is a report from the current WSSA Weed Loss Committee on potential crop yield losses due to weeds in corn, soybean and dry beans based on data collected from research trials in various regions of the USA and Canada. Averaged across 2007 to 2013, weed interference reduced corn yield an average of 50% in the USA and Canada which equates to a loss of 148 million tonnes valued at over $26.7 billion annually. Weed interference in soybean during the same period caused a 52% yield loss which equates to a loss of $16.2 billion in the USA and $1.0 billion in Canada annually if no weed management tactics were employed. Averaged across 2007 to 2017, weed interference in dry bean in the USA and Canada caused an average of 71% yield loss which equates to a loss of $622 million in the USA and $100 million in Canada if no weed control tactics are used. These data further emphasize the continued need for weed science research to develop integrated weed management strategies that are efficacious, environmentally sustainable, and economically feasible for corn, soybean, and dry bean producers in the USA and Canada.

Early season management of Palmer amaranth (Amaranthus palmeri S. Wats) is critical for cotton production in fields where this weed is present. Timely application of herbicides can be difficult to accomplish which can result in inadequate weed control and greater interference of this weed with cotton. Palmer amaranth plants that escape applications of herbicides early in the season can significantly reduce cotton yields. The objective of this study was to evaluate Palmer amaranth, annual grass control, cotton yield, and economic return associated with various timing sequences of postemergence (POST) herbicides including dicamba, glufosinate, and glyphosate.

Research was conducted in North Carolina from 2015-2017 across six environments near Clayton and Rocky Mount. No preemergence herbicides were applied at planting. Treatments consisted of herbicides applied 2, 3, 4, and 5 weeks after planting (WAP); 3, 4, and 5 WAP; 4 and 5 WAP; and 5 WAP only. Additional treatments included herbicides applied 2 WAP only, 2 and 3 WAP; and 2, 3, and 4 WAP.  Glufosinate was applied 2 and 3 WAP at 543 g ai ha^-1.  At 4 and 5 WAP, glyphosate plus dicamba (946 g ae ha^-1 + 560 g ae ha^-1) were applied.  A non-treated control was also included.  Cotton was planted in a conventional-tillage system.  Visible estimates of percent weed control were recorded 3, 4, 5, 6, 7, and 8 WAP using a scale of 0 to 100 where 0 = no control and 100 = complete control.  Fresh weight of weeds from 1 m² area of each plot was determined within 4 weeks prior to harvest.  Cotton was machined harvested with a spindle picker and seedcotton weight was recorded.  Estimated economic return was calculated based on the North Carolina Cooperative Extension Service budget for cotton with a total production cost, excluding seed, ginning, and herbicide costs, set at $1,268 ha^-1. Seed cost was set at $250 ha^-1, to reflect specific seeding rate and seed type used for the study. Ginning cost was based on seed cotton yield for each plot at a price of $0.24 kg^-1. Herbicide costs were based on local, chemical retailer prices in 2017. Economic net return was calculated as the difference between the product of yield (45% lint at $1.65 kg^-1 and 55% seed at $0.26 kg^-1) and total production cost.

Palmer amaranth was controlled at least 98% 8 WAP when three or more herbicide applications were made regardless of timing sequence. Control declined to 92, 82, 71, and 58% with the 4 and 5, 2 and 3, 5, and 2 WAP application timings, respectively. Annual grasses (primarily broadleaf signalgrass, Urochloa platyphylla (Munro ex C. Wright) R.D. Webster, and large crabgrass, Digitaria sanguinalis L.) were controlled greater than 99% when all four herbicide timings were used.  At least 94% control and no greater than 99% was observed with three herbicide applications or with the 4 and 5 WAP timing. Control declined to 87, 80, and 64% with the 2 and 3, 5, and 2 WAP application timings, respectively. Yield reductions of 30, 40, 61, and 83% were observed with the 4 and 5, 2 and 3, 5, and 2 WAP timing sequences, respectively. Similar to trends with Palmer amaranth control, when three or more herbicide applications were utilized, no difference in yield was observed. Trends for economic return were similar to those observed for cotton yield. Regardless of timing, there was no difference in economic return when at least three herbicide sprays were made. Although yields were similar, the 4 and 5 WAP application timing economic net returns were higher compared to the 2 and 3 WAP application timing. This can be attributed to the higher herbicide cost of the 2 and 3 WAP program.  This research demonstrates that even when adequate weed control is obtained with larger weeds, early season weed interference can still adversely affect cotton yield.  

Research was established in 2011 to determine the impact of continuous use of glyphosate plus dicamba on weed species richness and density in two cotton fields near Rocky Mount, North Carolina. Fields received two or three postemergence applications of glyphosate plus dicamba each year. Based on soil cores removed from each plot immediately after planting, after six years of repeated use of these herbicides, weed richness did not change for the following weeds: Palmer amaranth, carpetweed, common ragweed, common lambsquarters, eclipta, common purslane, corn spurry, prickly sida, entireleaf morningglory, tall morningglory, pitted morningglory, spurred anoda, yellow nutsedge, annual sedge, broadleaf signalgrass, large crabgrass, goosegrass, crowfootgrass, and bermudagrass.   However, density of all weed species remained the same or decreased over the six years, most likely because these herbicides controlled weeds that emerged almost completely and limited contributions of seed from escaped weeds to the soil seedbank. 

Palmer amaranth (Amaranthus palmeri (S.) Wats.) is a weed that growers across Virginia and the entire southeast United States struggle to control due to many populations being resistant to glyphosate and ALS inhibiting herbicides.  Two studies were initiated to evaluate preemergence herbicide options to control glyphosate and ALS resistant Palmer amaranth in South Hill, VA in 2016 and 2017, in glyphosate resistant soybean.  Treatments in study 1 included saflufenacil + pyroxasulfone (25 + 120, 21 + 100, and 19 + 90 g ai ha^-1), saflufenacil + dimethenamid + pyroxasulfone (25 + 219 + 120 g ai ha^-1), sulfentrazone + cloransulam-methyl (217 + 28 g ai ha^-1), chlorimuron-ethyl + flumioxazin + pyroxasulfone (21 + 77 + 98 g ai ha^-1), saflufenacil + dimethenamid + metribuzin (30 + 219 + 263 g ai ha^-1), saflufenacil + dimethenamid (30 + 481 g ai ha^-1), flumioxazin + metribuzin (107 + 263 g ai ha^-1), and chlorimuron-ethyl + flumioxazin + thifensulfuron (26 + 82 + 8 g ai ha^‑1).  Treatments in study 2 included sulfentrazone + S-metolachlor (172 + 1544 g ai ha^-1), sulfentrazone + S-metolachlor + metribuzin (172 + 1544 + 278 g ai ha^-1), sulfentrazone + metribuzin (176 + 265 g ai ha^-1), sulfentrazone + chlorimuron-ethyl (175 + 22 g ai ha^-1), sulfentrazone + chlorimuron-ethyl + metribuzin (175 + 22 + 278 g ai ha^-1), metribuzin (278 g ai ha^-1), chlorimuron-ethyl + metribuzin (30 + 180 g ai ha^-1), and chlorimuron-ethyl + flumioxazin + thifensulfuron (26 + 82 + 8 g ai ha^-1).  A nontreated check was included in each study.  Both studies were arranged in a randomized complete block design with 4 replications.  Plots measured 3 by 7.6 m.  Treatments were applied with a handheld spray boom calibrated to apply 140 L ha^-1 of spray solution.  Data collected included visible Palmer amaranth control and soybean injury on a biweekly basis on a 0 (no control or injury) to 100 (complete control or necrosis) scale.  Data were subjected to ANOVA and means separated using Fisher’s Protected LSD (α=0.05) in JMP Pro 13.  In study 1, 4 wk after treatment (WAT) all treatments containing flumioxazin resulted in 100% Palmer amaranth control.  All other treatments resulted in <83% Palmer amaranth control.  By 6 WAT the best performing treatment was chlorimuron-ethyl + flumioxazin + thifensulfuron with 83% Palmer amaranth control.  The other two flumioxazin containing treatments, chlorimuron-ethyl + flumioxazin + pyroxasulfone and flumioxazin + dimethenamid had Palmer amaranth control of 74 and 56%, respectively.  In study 2 all treatments performed well 2 WAT with >94% Palmer amaranth control.  However, by 4 WAT, Palmer amaranth control was poor (8 to 55%).  These studies indicate that PPO plus ALS based preemergence herbicide options can control glyphosate and ALS resistant Palmer amaranth.  PPO-based herbicides are also used heavily POST in soybean; it is critical to use as many modes of action as possible to steward this class of chemistry. Future research should investigate these herbicide options on PPO resistant populations of Palmer amaranth found in the mid-Atlantic region. 

Research was conducted at the LSU AgCenter Dean Lee Research and Extension Center near Alexandria, LA in 2017 to evaluate the effect of simulated isoxaflutole drift on non-HPPD-tolerant soybean.  Experimental design was a factorial arrangement of application timings and isoxaflutole rates in a randomized complete block design with four replications.  Application timings were unifoliate, 2-trifoliate, or 4-trifoliate soybean.  Isoxaflutole rates were1/16x, 1/8x, or 1x of 105 g ai ha^-1, which is the field use rate.  Visual estimations of percent overall injury, with injury divided into chlorosis, necrosis, and height reduction, were recorded 3, 7, 14, 28, and 42 d after treatment (DAT).  Soybean height was recorded 14, 28, and 42 DAT and just prior to harvest.  Soybean node number was also recorded just prior to harvest.  Yields were collected at harvest and adjusted to 15% moisture prior to analysis.  Soybean heights, node number, and yield were converted to percent of the nontreated prior to analysis. 

Averaged across evaluation date, overall injury (17 to 75%) increased with increasing isoxaflutole rate at all application timings.  Chlorosis (7 to 22%) and necrosis (1 to 8%) were observed following all application timings and isoxaflutole rates, except necrosis was not present following isoxaflutole at 1/16x when applied to unifoliate soybean.  Regardless of application timing or isoxaflutole rate, overall injury (30 to 39%) 3 and 7 DAT was observed predominately as chlorosis (20 to 33%) and necrosis (3 to 9%) with injury seen as height reduction (43 to 49%) 28 and 42 DAT.  Nontreated soybean heights 14, 28, and 42 DAT and just prior to harvest were 360, 710, 1040, and 1170 mm, respectively.  Differences in height as percent of the nontreated slightly differed 14 and 28 DAT among application timings, but height just prior to harvest was decreased as isoxaflutole application was delayed with height following the unifoliate, 2-trifoliate, and 4-trifoliate application at 93, 86, and 77% of the nontreated, respectively.  Regardless of application timing, soybean height as a percent of the nontreated was decreased with increasing rate 28 and 42 DAT.  Soybean node number was 92 to 97% of the nontreated for all application timings.  However, only isoxaflutole at 1x reduced soybean node number to 88% of the nontreated compared to 99 and 96% for the 1/16 and 1/8x rates, respectively.  Nontreated soybean yield was 3870 kg ha^-1.  At all application timings, soybean yield was 90 to 98% of the nontreated following isoxaflutole at 1/16 and 1/8x rates with the 1x rate (43 to 62% of the nontreated) reducing yield greater than both simulated off-target rates.  The 1x isoxaflutole rate reduced soybean yield more following the 4-trifoliate application timing compared to the 2-trifoliate application.  Preliminary data indicates that injury following isoxaflutole application is observed initially as chlorosis with some necrosis, then injury observed as height reduction over time.  Soybean height compared to the nontreated was reduced more as off-target movement was delayed from unifoliate to 4-trifoliate soybean possibly because unifoliate soybean are able to compensate from early-season injury.  Study will be repeated in 2018.

Forage radish has the ability to alleviate soil compaction and scavenge nutrients from deeper soil levels, which can become available to the surrounding crop after the forage radish taproot decomposes. Incorporating forage radish into winter wheat production has the ability to offer these benefits but must not interfere with wheat yield to be a viable option for growers.

Two studies were conducted in Blacksburg, Virginia in 2015 and 2016 to explore the potential for intercropping forage radish into winter wheat. Both experiments were designed as a 5 by 2 factorial with factors of forage radish seeding rate (0, 2.24, 4.48, 6,72, and 8.96 kg ha^-1) and nitrogen fertilization (0 or 56 kg N ha^-1) in a randomized split block design with 4 replications. In the first experiment, conducted in 2015 and 2016, forage radish and wheat, at a rate of 134 kg ha^-1, were planted at the same time in mid-October. The second experiment was only conducted in 2016. The difference between these experiments was forage radish planting date. Rather than planting forage radish and wheat at the same time, in the second experiment forage radish was planted on August 31^st and wheat planting followed on October 18^th. For both experiments in mid-March, fertilization was applied in appropriate plots and a broadcast application of thifensulfuron-methyl + tribenuron-methyl at 17.51 g ai ha^-1 and 8.76 g ai ha^-1 was applied to the whole experiment. Data collected were: stand counts taken 6 weeks after wheat planting; soil tests at initiation of the experiment, directly prior to fertilization in March, and 6 weeks after fertilization; and wheat yield. Soil tests were analyzed using flow injection analysis to determine plant available nitrogen. Data were analyzed using JMP Pro 12. Stand counts of wheat were analyzed using a simple linear regression. Wheat yield and soil analyses were subjected to ANCOVA and effects were considered significant when p < 0.05.

Stand counts of wheat decreased as forage radish seeding rate increased in both experiments. In the experiment in which forage radish was planted earlier than winter wheat, an increase in seeding rate of 1 kg ha^-1 led to a decrease of 33,254 wheat plants ha^-1. In the experiment where forage radish and wheat were planted concurrently, an increase in 1 kg ha^-1 of forage radish, led to a decrease of 28,820 wheat plants ha^-1. While there was a decrease in wheat stand caused by forage radish planting, there was no difference detected in final wheat yield due to forage radish planting. The soil analysis indicated no differences in plant available nitrogen across forage radish seeding rate but increases in plant available nitrogen were detected in treatments that received fertilization in mid-March. These results indicate that forage radish does not negatively impact wheat yield and that it does not have an effect on soil plant available N at the timings that were investigated in this study. Future research is needed to check for competitive effects of forage radish in other environments and test for other soil benefits provided by forage radish.

Tolpyralate (development code: SL-573) is a newly registered HPPD inhibiting active ingredient. EPA approved the registration of tolpyralate for POST weed control in all types of field corn, sweet corn, and popcorn in July of 2017. Since 2010, over 100 field corn varieties have been evaluated in more than 20 US States for tolerance to tolpyralate. In the corn studies, tolpyralate was applied alone at rates ranging from 30 to 80 g ai ha^-1 and in a tank mixture with atrazine at rates ranging from 560 to 1120 g ai ha^-1. In addition, crop tolerance was evaluated when various spray adjuvants including MSO (0.5% v/v), COC (1.0% v/v), NIS (0.25% v/v), and UAN (2.5% v/v) were included in the spray solution when tolpyralate alone and tolpyralate plus atrazine was applied. The field corn varieties that were tested demonstrated excellent tolerance to tolpyralate alone and tolpyralate plus atrazine. In adjuvant studies, the addition of MSO, COC, NIS, and/or UAN to the spray solution did not affect the crop tolerance. In cases where bleaching symptoms were observed following tolpyralate applied alone or when tolpyralate was applied with atrazine, the average bleaching symptomology was less than 1 % (n=210 for tolpyralate solo application and n=186 for tolpyralate plus atrazine application). When tolpyralate was applied at 60 and 80 g ai ha^-1 (2x the registered low and high rate, respectively) up to 12 % bleaching was observed 7 days after application; however, the crop quickly recovered and the symptoms had no effect on the yield. When sweet corn and popcorn varieties were tested, results similar to field corn studies were observed. The results of experiments suggest that corn has excellent tolerance to the registered rates of tolpyralate applied POST across all US corn production regions.

Tolpyralate (development code: SL-573) is a newly registered HPPD inhibiting active ingredient. EPA approved the registration of tolpyralate for POST weed control in all types of field corn, sweet corn, and popcorn in July of 2017. The registered rate range for tolpyralate is 29.3 to 39.5 g ai ha^-1. A key component in the development of a new herbicide active ingredient is the evaluation of rotational crop flexibility following applications. In order to characterize the rotational crop flexibility following POST applications of tolpyralate in corn, a series of experiments were conducted from 2009 through 2014 in 18 US states. Tolpyralate was applied in the spring at rates ranging from 20 to 100 g ai ha^-1. Following the spring applications, rotational crops were planted in the fall of the same year or in the spring of the next year.  Rotational crops included alfalfa, spring barley, dry beans, cabbage, spring canola, cotton, peas, peanuts, potatoes, grain sorghum, soybeans, squash, sugarbeet, sunflower, tomato, spring wheat, winter wheat, and various grasses grown for seed. No injury was observed in all spring planted crops, except alfalfa (≤ 4%), peanut (≤ 3%), and squash (≤ 5%), following treatments of tolpyralate (20 to 100 g ai ha^-1) applied the previous year. When alfalfa, winter wheat, and grasses grown for seed were planted in the fall after spring treatments of tolpyralate, no injury was observed in treatments that received 20 to 40 g ai ha^-1 and injury was observed when tolpyralate was applied at rates greater than 40 g ai ha^-1, however, injury did not exceed 8%. In replant crop studies, tolpyralate was applied in the spring two weeks prior to planting the replant crops. The replant crops that were evaluated included dry beans, corn, grain sorghum, soybean, sunflower, and spring wheat. No injury was observed in corn when tolpyralate was applied preplant at 20 to 80 g ai ha^-1. Injury to sorghum and spring wheat was less than 5% and inconclusive levels of injury to dry bean, soybean, and sunflower was observed following preplant applications of tolpyralate at 20 to 80 g ai ha^-1. The results of the studies suggest that tolpyralate will provide growers with acceptable rotational and replant flexibility in US corn production regions.

In 2015, Palmer amaranth (Amaranthus palmeri) with resistance to glyphosate and protoporphyrinogen oxidase (PPO) inhibitors was confirmed throughout West Tennessee. Historically, growers have relied on PPO herbicides for control of Palmer amaranth preemergence (PRE) and postemergence (POST). However, with auxin, glufosinate and 4-hydroxyphenlpyruvate dioxygenase inhibitor (HPPD) tolerant crops on the horizon growers have POST options to control multiple-resistant Palmer amaranth. Preliminary research at PPO-resistant sites in Tennessee have revealed a higher tolerance to the aforementioned herbicide modes of action (MOA). Therefore, the objective of this study was to compare the efficacy of POST herbicides in PPO-resistant and susceptible Palmer amaranth populations.

Studies were conducted on-farm near Ripley, TN (PPO-resistant) and at the Agricenter International in Memphis, TN (PPO-susceptible) in the 2017 growing season to evaluate available options for control of Palmer amaranth. Plots were 1.5 m wide and 7 m in length. Treatments were applied when Palmer amaranth were 6.5 cm in height. There were 20 treatments total consisting of herbicides used alone, as tank-mixtures or in a two-pass system with the following herbicides: atrazine at 2.24 kg ai ha^-1 plus COC at 1% v/v, mesotrione at 70 g ai ha^-1 plus NIS 0.25% v/v, glufosinate at 660 g ai ha^-1, fomesafen at 270 g ai ha^-1 plus MSO at 1% v/v, dicamba at 560 g ae ha^-1, 2,4-D at 1.05 kg ae ha ^-1 and 2,4-D at 1.05 kg ae ha^-1 + glyphosate at 1.12 kg ae ha^-1. Treatments were arranged within a randomized complete block design and replicated four times at each location. Palmer amaranth control was visually assessed 7, 14, 21 and 28 DAI (days after initial application). All data were subjected to ANOVA and estimates of the least square means were used for mean separation with α = 0.05.

In Memphis, regardless of herbicides applied alone, in tank-mixtures or in a two-pass system, Palmer amaranth control was ≥ 99% 14 DAI. However, mesotrione applied alone provided 88% control 14 DAI this location. Contrarily, in Ripley mesotrione and fomesafen applied alone provided 38 and 34% control, respectively, of Palmer amaranth. Whereas, atrazine, glufosinate, dicamba and 2,4-D + glyphosate provided greater than 90% control when applied alone. Treatments that included 2,4-D + glyphosate, 2,4-D or dicamba plus/ or fb glufosinate provided greater than 92% control. These data provide that PPO-resistant Palmer amaranth are effectively controlled by herbicides that are the framework of LibertyLink, Enlist and Xtend systems. However, like fomesafen, less activity was observed with mesotrione at the PPO-resistant location. Further research is needed to evaluate HPPD herbicides on the PPO-resistant population near Ripley, TN for potential resistance.

Burndown programs for cotton in the southeast frequently include 2,4-D and dicamba for control of glyphosate and ALS inhibitor resistant weeds. Sensitive cotton varieties planted into soil treated with 2,4-D and dicamba in burndowns can result in crop stunting and stand loss if the plant back interval is too short. With the introduction of 2,4-D and dicamba tolerant cotton varieties, producers can apply 2,4-D or dicamba in burndowns very close to planting or use them with preemergence (PRE) treatment. However, with this program, if the cotton stand fails and a susceptible shorter season cotton has to be replanted, injury may be seen if intervals between applications and replanting are short and this injury may further delay maturity when the grow season is already short. Therefore, the objective of this study was to evaluate cotton injury and yield responses as resulted from 2,4-D and dicamba residues in the soil. In 2016, field studies were conducted in Macon and Baldwin counties in Alabama and Santa Rosa county in Florida.  In 2017, cotton at Henry, Macon, and Baldwin counties were also evaluated. Treatments of 2,4-D included 532 and 1063 g ai h^-1 applied 3 weeks before planting and 53, 160, 266, 532, 1063 g ai h^-1 applied at planting as PRE. Treatments of dicamba included 560 and 1120 g ai h^-1 applied 3 weeks before planting and 56, 168, 280, 560, 1120 g ai h^-1 applied at planting as PRE. Stand counts and plant heights were collected 3 weeks and 7 weeks after planting, with a final yield collected to assess herbicide residual injury on the cotton. 2,4-D and dicamba treatments applied 3 weeks before planting did not have any adverse effects on cotton establishment or yield. The only significant height reduction observed over all locations for both years was in the 2016 Macon county study for dicamba 560 g ai h^-1, with a stunting of 27% at 21 days after planting (DAP) and 38% at 51 DAP, respectively.  In 2016, 560 g ai h^-1 of dicamba showed significantly reduced stands by 36 % at 21-24 DAP which did not recover at 50-51 DAP over all locations. In 2017, stands were reduced by 2,4-D at 266 g ai h^-1 by 26% and by 168 g ai h^-1 dicamba by 17% at 20-23 DAP respectively but recovered by 47-48 DAP. Dicamba at 280, 560 and 1120 g ai h^-1 produced significantly reduced stands by 17-25% also 532 and 1063 g ai h^-1 of 2,4-D had stand reductions of 29-36% at 20-23 DAP over all locations in 2017. None of these stand losses recovered by 47-48 DAP but they did not cause any significant yield loss at harvest. Our data suggests cotton stunting and stand reduction may occur if susceptible cotton varieties are planted too close to a burndown application with 2, 4-D and dicamba, but final yields may not be affected after a full growing season. 

The widespread occurrence of glyphosate-resistant (GR) kochia has dramatically increased the utility of synthetic auxins (dicamba and fluroxypyr) in the US Great Plains. This frequent reliance on dicamba applications for controlling GR kochia may also escalate the evolution of dicamba-resistant (DR) kochia. The main objective of this study was to investigate the variation in response to dicamba applied POST among kochia accessions collected from western Kansas. Seeds of four putative kochia accessions (KS-104, KS-107, KS-110, and KS-113) were collected from research plots in a fallow field near Hays, KS. The sampled field was under continuous fallow–wheat–sorghum rotation, with frequent use of dicamba in glyphosate-based burndown treatments. In contrast, seeds of a susceptible kochia accession (SUS) were collected from a pasture land, with no history of dicamba use. Kochia seedlings from each accession were grown in a greenhouse at KSU Ag Research Center, near Hays, KS. Single-dose (560 g ae ha^-1 of dicamba) experiments were conducted by using about 100 seedlings from each accession. Three accessions, viz. KS-110, KS-113, and SUS were further selected for dose-response experiments to characterize the variation in fresh and dry weight response to dicamba. Dose-response experiments were performed in randomized complete block design, with 12 replications and repeated twice. Dicamba doses ranging from 0, 280, 560, 1120, 1680, 2240, and 2800 g ae ha^-1 were tested. Visible injury assessments were made and individual plants were harvested to determine the fresh and dry weights at 28 d after treatment (DAT). Results from single-dose study revealed that kochia accessions KS-104, KS-107, KS-110, and KS-113 had about 97, 98, 100, and 88% survivors at 28 DAT. Results from dose-response study indicated that 5.5- and 3.1-fold higher dicamba dose was required to obtain 50% fresh weight reduction (I₅₀) of KS-110 and KS-113 accessions, respectively, relative to the SUS accession. Furthermore, about 1,334 and 837 g ha⁻¹ of dicamba was needed to achieve a 50% dry weight reduction (GR₅₀) in KS-110 and KS-113 accessions, respectively. Based on dry weight response, the KS-110 and KS-113 accessions exhibited an 8.2- and 5.1-fold resistance levels to dicamba, respectively. Differential fresh and dry weight response exhibited by kochia accessions from a single field and the fact that higher dicamba doses (>560 g ha⁻¹) were needed to achieve I₅₀ and GR₅₀ values demonstrate that repeated and frequent use of dicamba for kochia control may escalate the evolution of DR kochia. With recent commercialization of newly-developed DR crops, growers should adopt dicamba use stewardship programs, and utilize multiple effective modes of action herbicides and other ecological-based approaches to prevent evolution of DR kochia on their production fields. 

With the availability of the Roundup Ready Xtend and LibertyLink soybean systems, growers face a decision as to what trait, and thereby what herbicide, they can utilize to control glyphosate-resistant Palmer amaranth in the Midsouth. Prior research has documented glufosinate efficacy is influenced by humidity, temperature, and time of the day the application occurs. In contrast, dicamba is a systemic herbicide that appears to be less affected by those application parameters that reduce glufosinate efficacy. An experiment was conducted at the University of Arkansas Altheimer Lab to investigate the impact of light intensity on efficacy of dicamba (Engenia herbicide) and glufosinate (Liberty herbicide) on Palmer amaranth. Palmer amaranth plants were sprayed with dicamba using TeeJet TTI nozzles (UC droplets), glufosinate using TTI nozzles, or glufosinate using TeeJet XR nozzles (fine droplets). Six rates for each herbicide were used to generate a dose response curve with rates ranging from 1/16X to 2X of 595 g ai ha^-1 and 560 g ae ha^-1 of glufosinate and dicamba, respectively. After establishment in a greenhouse, plants were transferred to a growth chamber producing either low or high light intensity (220 or 1050 µmol m^-2 s^-1) for 48 hours before, and after, herbicide treatment. Percent mortality and biomass data were collected 3 weeks after application and fit to a nonlinear model. Using the nonlinear model, a lethal dose for 50% of the population (LD₅₀) and dose needed to reduce biomass by 50% (ED₅₀) could be calculated for each treatment. Dicamba LD₅₀ and ED₅₀ values were not impacted by light treatment, whereas glufosinate had greater LD₅₀ and ED₅₀ values when applied under high light conditions with both nozzles. Additionally, glufosinate was more efficacious when applied using XR nozzles compared to TTI nozzles under high light conditions (LD₅₀= 0.25X and 0.33X, respectively). These results suggest that glufosinate may perform better with some cloud cover than in full sunlight, although solar light intensity is often much greater than that simulated in this experiment. 

Downy brome (Bromus tectorum L.) is one of the most troublesome winter annual grass weeds in wheat production across the U.S. Great Plains. In summer of 2016 and 2017, two downy brome populations (R1 and R2) with putative resistance to acetolactate synthase (ALS) inhibitors were collected from two separate wheat fields: one in Carter County and the other one in Toole County, MT. The main objectives of this research were to: (1) confirm and characterize the resistance levels in R1 and R2 downy brome populations to commonly used ALS-inhibiting herbicides relative to a susceptible (S) population, (2) investigate the target site-based mechanism(s) of resistance; and (3) determine the effectiveness of pyroxasulfone alone or in combination with other herbicides for downy brome control in winter wheat. Seeds of S downy brome population were collected from a wheat field near Huntley, MT. Whole plant dose-response experiments indicated that the R1 population was highly resistant (110.1-fold) to imazamox, and low to moderately cross-resistant to pyroxsulam (4.6-fold) and propoxycarbazone (13.9-fold), respectively, but susceptible to mesosulfuron. However, the R2 population was cross-resistant to all four ALS-inhibiting herbicides, i.e., imazamox, propoxycarbazone, pyroxsulam, and mesosulfuron. The nucleotide and amino acid sequence analyses showed that Ser₆₅₃Asn and Pro₁₉₇His substitutions in the ALS genes conferred cross-resistance to ALS-inhibiting herbicides in R1 and R2 downy brome populations, respectively. This is the first molecular confirmation of target site (ALS gene) mutation at Pro₁₉₇His in this weed species. In separate field experiments, pyroxasulfone (89 or 178 g ai ha^-1) applied preemergence (PRE) alone in the fall provided up to 84% control of downy brome. Furthermore, pyroxasulfone (89 g ai ha^-1) applied PRE in the fall followed by imazamox (44 g ai ha^-1) applied POST in the spring provided excellent (99%) end-season control of downy brome in Clearfield^™ winter wheat. In conclusion, these results confirm the first occurrence of downy brome populations with cross-resistance to ALS-inhibiting herbicides in wheat production. Pyroxasulfone applied PRE can be effectively utilized as an alternative site-of-action herbicide for downy brome management in winter wheat. 

Multiple herbicide-resistant (HR) Palmer amaranth (Amaranthus palmeri) is an increasing management concern for growers across the United States. After the initial confirmation of Palmer amaranth populations with glyphosate resistance in 2011, random field surveys were initiated in fall 2014 to determine the distribution, frequency, and levels of resistance to most commonly used herbicides in Kansas cropping systems. Over last three years, approximately 180 Palmer amaranth populations were collected from different counties in the northwestern and southcentral Kansas. Fully-matured seeds of Palmer amaranth plants (40 to 50 per field) were randomly sampled from chemical fallow, wheat, corn, sorghum, soybean fields along with field edges/fence lines, and roadsides. Seeds of each population were sown into 10 by 10-cm plastic pots filled with commercial potting mix under greenhouse conditions at the KSU Agricultural Research Center near Hays, KS. Discriminate dose experiments were conducted by treating 10 to 12 plants from each population with glyphosate (1,103 g ha^-1), 2,4-D LV6 (882 g ha^-1), and chlorsulfuron (26 g ha^-1), when those plants were 8- to 10-cm tall. The frequency of resistant or tolerant plants in each population was determined, and treated plants were harvested to determine the fresh and dry weights at 21 d after treatment (DAT). Four confirmed HR Palmer amaranth populations (BT12, SH1, KW2, and PR8) along with a known susceptible (SUS) population were further characterized for levels of resistance to selected herbicides in dose-response experiments. Data on percent visible injury were recorded at 7, 14, and 21 DAT, and shoot dry weights were determined at 21 DAT. Screening of 47 populations indicated that 19, 42, and 19 populations had resistant or tolerant individuals to glyphosate, chlorsulfuron, and 2,4-D herbicides, respectively. Based on percent injury and shoot dry weight response (LD₅₀ and GR₅₀ values), the PR8 and BT12 populations had 7- to 13-fold levels of resistance to glyphosate, and up to 20-fold levels of resistance to chlorsulfuron. Furthermore, the BT12, KW2, and PR8 populations exhibited 2- to 5-fold level of resistance to mesotrione based on percent injury response (LD₅₀). These results confirm the evolution of multiple HR Palmer amaranth population (resistant to glyphosate, chlorsulfuron, and mesotrione) in Kansas. Dose-response studies on these populations for atrazine, 2,4-D, and dicamba are still under progress. Future studies will also investigate the underlying mechanism(s) of multiple herbicide resistance in these populations. Growers should adopt multiple control tactics, including chemical and non-chemical (tillage, crop rotation, cover crops) to manage the multiple HR Palmer amaranth.

Environmental factors, such as antecedent soil moisture, may impact dicamba dissipation in plants exposed to drift. These effects are poorly understood, however, and literature reports vary regarding levels of dicamba symptomology in plants exposed to drought conditions. The objective of this study was to identify metabolic transformations underlying observed differences in dicamba symptomology between well-watered and water-stressed soybean. These transformations were measured as a suite of dicamba metabolites over time, including 5-OH dicamba, dichlorosalicylic acid (DCSA), dichlorogentisic acid (DCGA), and the glucosides 5-OH glucoside, DCSA-glucoside, and DCGA-glucoside. Preliminary LC-MS/MS analysis of dicamba and DSCA concentration in soybean tissue showed no detectable DSCA after 63 days, suggesting metabolites other than DSCA are produced. Plants commonly add a glucose to herbicides as a metabolism pathway. However, analytical standards of glucoside metabolites are not commercially available, and a proof of concept method for indirect estimation of glucoside concentrations was tested. Samples were analyzed for DCSA using LC-MS/MS before and after acid hydrolysis, which removes the glucose from DCSA-glucoside and converts it to DCSA. The difference in DCSA concentration in a sample before and after hydrolysis is therefore an indirect estimate of DCSA-glucoside metabolite concentration. Understanding the impact of environmental factors on dicamba dissipation in plant tissues could help agencies target persistent analytes for drift-complaint cases and provide additional information for approximating initial concentrations, estimating drift timing, and predicting damages.

The increase in confirmed cases of herbicide-resistant (HR) and multiple herbicide-resistant (MHR) kochia [Kochia scoparia (L.) Schard] over recent years has become a very serious concern for sustainable crop production in the western US. We hypothesize that the use of competitive crops in crop rotations could be a viable and ecologically-sound weed management strategy to mitigate the effects of herbicide-resistant weed populations. A four-year study was conducted in Huntley, MT; Powell, WY; Lingle, WY; and Scottsbluff, NE to determine how to best utilize the canopy effect of competitive crops in a crop rotation program directed to reduce seed bank of an herbicide-resistant weed population, apart from utilizing tillage and herbicide diversity. In fall of 2014, kochia seeds with a known ALS R:S ratio (5%) were uniformly broadcasted in the field to establish an experimental weed seed bank. A split-split plot in a randomized complete block design with four replications was used, with tillage (conventional till or minimum till) as the whole plot factor, crop rotation diversity (corn/corn/corn/corn, corn/sugar beet/corn/sugar beet, corn/dry bean/sugar beet/corn, or corn/dry bean/wheat/sugar beet) as the split-plot factor, and herbicide use pattern (complete reliance on ALS inhibitors, mixture of ALS and non-ALS inhibitors, or annual rotation to ALS inhibitors) as the slit-split plot factor. Experimental plots were 4 m wide by 15 m long, with a total of 96 plots. Data on kochia seedling density, percent control, seeds plant^-1, days to flower initiation, days to flowering, days to seed set, and residual soil seed bank plot^-1 were collected in each growing season (2014 through 2017). The first three years (2014, 2015, and 2016) of the study showed that the effect of crop rotation on kochia seed bank reduction was significant. Averaged across tillage and herbicide use pattern treatments, corn and barley were the most competitive crops in the 3-year rotation for reducing late-season kochia density and seed production, followed by dry bean, while sugar beet was the least competitive crop. The sugar beet-corn-sugar beet rotation was the least effective, with the highest late-season kochia density (35 plants m^-2) and seed production (58,113 seeds m^-2). Competitive crops, viz., barley or corn grown in two out of three years in the rotation reduced both the kochia density m^-2 and seed production m^-2 by 95% (1.6 plants m^-2 and 26,570 seeds m^-2, respectively). More so, addition of late-planted (June) dry bean in the first two of the three-year rotation combined with a highly competitive crop either in the first or third year, reduced both the kochia density and seed production by 98% (<1 kochia density m^-2 and 9,800 seeds m^-2, respectively). Besides the crop canopy effect, the reduced end-season seed bank inputs in barley plots in the rotation was also attributed to the early harvest of barley when kochia seeds were not fully matured (non-viable), compared to corn, sugar beet, or dry bean. Canopy effects of competitive crops such as barley and/or corn alone or in conjunction with late-planted crops such as dry bean would be an effective non-herbicidal strategy to reduce kochia soil seed bank returns and delay herbicide resistance evolution. 

Evaluation of Herbicide Programs in Oklahoma Soybean

T.A. Baughman, R.W. Peterson, D. Teeter

Abstract 

Weed resistance has continued to increase in severity on many Oklahoma soybean acres.  Resistant weed species include horseweed (Erigeron canadensis), Palmer amaranth (Amaranthus palmeri), tall waterhemp (Amaranthus tuberculatus), and giant ragweed (Ambrosia trifida).  The most abundant and problematic being Palmer amaranth.  Recently with the development new herbicide technologies like Roundup Ready Xtend and Balance GT soybean herbicide systems growers may have potential tools to deal with these resistant weeds.  Studies were conducted during the 2017 growing season to evaluate weed efficacy with both systems.  Weed control trials where established at the Oklahoma State University Mingo Valley Research Station near Bixby, OK. The soil type is a Radley silt loam with 0.6% OM and 6.4 pH.  Soybean were planted on May 30, 2017 in 76-cm rows.  The first trial was planted to soybean cultivar “FG72” that was tolerant to isoxaflutole (Balance Bean) and glyphosate. Isoxaflutole was applied at 0.11 kg ai ha^-1 in combination with various PRE herbicides.  These include s-metolachlor (1.07 kg ai ha^-1), sulfentrazone (0.17 kg ai ha^-1), metribuzin (0.28 kg ai ha^-1), flumioxazin (0.07 kg ai ha^-1), acetochlor (1.26 kg ai ha^-1), or pyroxasulfone (0.09 kg ai ha^-1).  All PRE herbicides were followed with a POST application of glyphosate at 1.54 kg ai ha^-1.  Three additional studies were planted to Roundup Ready 2 Xtend soybean cultivar “AG47X6”.   The first study evaluated various PRE residual herbicides including: s-metolachlor (1.38 kg ai ha^-1) + sulfentrazone (0.15 kg ai ha^-1), s-metolachlor (1.42 kg ai ha^-1), flumioxazin (0.07 kg ai ha^-1), metribuzin (0.42 kg ai ha^-1), acetochlor (1.26 kg ai ha^-1), acetochlor (1.19 kg ai ha^-1) + fomesafen (0.26 kg ai ha^-1), pyroxasulfone (0.09 kg ai ha^-1), or pyroxasulfone (0.09 kg ai ha^-1) + saflufenacil (0.02 kg ai ha^-1) + imazethapyr (0.5 kg ai ha^-1).  All PRE herbicides were followed with a POST application of dicamba (0.56 kg ai ha^-1) + glyphosate (1.54 kg ai ha^-1).  The second study evaluated residual herbicides applied either PRE or POST at their labeled rates including s-metolachlor (1.42 kg ai ha^-1), acetochlor (1.26 kg ai ha^-1), acetochlor (1.19 kg ai ha^-1) + fomesafen (0.26 kg ai ha^-1), or pyroxasulfone (0.09 kg ai ha^-1).  All POST herbicide combinations included dicamba (0.56 kg ai ha^-1) + glyphosate (1.54 kg ai ha^-1).  The final study evaluated PRE residuals either alone or in combination with metribuzin (0.28 kg ai ha^-1).  These included s-metolachlor (1.42 kg ai ha^-1), flumioxazin (0.07 kg ai ha^-1), acetochlor (1.26 kg ai ha^-1), or pyroxasulfone (0.09 kg ai ha^-1).  All PRE herbicide programs were followed by dicamba (0.56 kg ai ha^-1) + glyphosate (1.54 kg ai ha^-1).  All POST herbicides treatments were applied at the V2-V3 growth stage. Palmer amaranth and ivyleaf morningglory (Ipomoea hederacea) were visually evaluated season long for control with late season evaluations presented.  Plots were harvested with a small plot combine to determine yield.  Late season Palmer amaranth control was 99% when isoxaflutole was applied in combination with either flumioxazin or acetochlor PRE and followed by glyphosate POST.  Variation in control with sulfentrazone may have been due to potential PPO-resistant Palmer amaranth at this location.  Ivyleaf morningglory control was at least 98% except when isoxaflutole was applied in combination with sulfentrazone or pyroxasulfone.  Soybean yields were similar across all treatments.  Palmer amaranth control was 100% and ivyleaf morningglory control was at least 98% regardless of the PRE herbicide program when followed by a POST application of dicamba + glyphosate.  Palmer amaranth control and ivyleaf morningglory control was at least 97% regardless if the residual herbicide was applied PRE and followed by a by a POST application of dicamba + glyphosate or if the residual herbicide was applied in combination with the POST herbicide.  While PRE + POST applications were just as effective as PRE followed by POST combinations the ability for a grower to makes these POST combinations in a timely fashion across all his acres should be considered.  Late season Palmer amaranth and ivyleaf morningglory control was 100% whether a PRE herbicides was applied alone or in combination with metribuzin.  Soybean yields exceeded 3000 kg ha^-1 with all Roundup Ready Xtend programs.  A residual herbicide is an important component of the Roundup Ready Xtend soybean system but the Xtend system allows more flexibility in the choice of that residual.  This research indicates that both the Balance GT and Roundup Ready Xtend programs can be effective tools for weed management in Oklahoma soybean. 

Along with the commercial introduction of dicamba-resistant soybean cultivars, there have been numerous reports of soybean fields without the dicamba-resistance trait showing synthetic auxin-herbicide symptoms. Many studies have been conducted to quantify the relationship between dicamba dose and soybean response. Egan et al. (2014) previously conducted a meta-analysis of published research in order to better quantify the response of soybean and cotton to 2,4-D and dicamba. Since that time, at least four additional studies have been published reporting the effects of dicamba on soybean yield. These new studies provide additional exposure timings and dicamba doses compared to the studies analyzed by Egan et al. (2014). Therefore, the purpose of this analysis was to update the meta-analysis by Egan et al. (2014), and to add potentially useful information regarding the relationship of visible injury symptoms to soybean yield loss based on the new data that has been published in the last several years. Data from 11 studies were included in this meta-analysis, four of which were not included by Egan et al. (2012). Means from each dose response series for each study were extracted from the published papers, and converted to percentage of control (zero-dose) values. Response variables included soybean injury 14 days after dicamba exposure, and soybean yield at maturity. For combined analysis, the growth stages were grouped into the following categories: early vegetative (V1 to V3), late vegetative pre-bloom (V4 to V7), flowering (R1 to R2), and pod fill (R3 to R4). The dicamba dose required to cause 2.5% soybean yield loss and the amount of observed in-season injury associated with a 2.5% yield loss were estimated for each growth stage from each study. Weighted means were then calculated, using the inverse of the standard error estimate from each growth stage in each study as the weighting factor.

No evidence of hormesis (increased soybean yield caused by low levels of dicamba exposure) was observed in this analysis. The weighted mean dicamba dose causing 2.5% soybean yield loss (YL_2.5) ranged from 0.13 to 16 g ha^-1 depending on soybean growth stage at exposure. Soybean was 18 to 126 times more susceptible to dicamba if exposed at the flowering stage (R1 to R2) compared to vegetative growth stages. The 95% confidence interval around the YL_2.5 estimates contained zero for all soybean growth stages; therefore, based on this analysis, no 'safe dose' of dicamba can be determined for any growth stage. When soybean was exposed at the flowering stage (R1 to R2), 9% observable injury 14 days after exposure (95% confidence interval = 4% to 14%) was associated with 2.5% soybean yield loss at the end of the season. Based on these results, if 9% dicamba injury (± 5%) is observed in the field after soybean plants begin flowering then measurable soybean yield loss is likely.

Evaluation of Inzen Technology as a Weed Management Tool in Southern Great Plains Grain Sorghum

R.W. Peterson, T.A. Baughman, P.A. Dotray, W.J. Grichar, J.W. Keeling, D. Teeter

Abstract 

Weed management has always been problematic for grain sorghum producers in the Southern Great Plains.  This is due to many factors including the limited number of herbicides available, potential carry over to rotational crops, and potential drift to other crops. The Inzen Sorghum Trait is a new technology development that is tolerant to nicosulfuron herbicide.  This technology will hopefully assist producers in managing weed issues in grain sorghum.  Trials were established in Oklahoma and Texas to evaluate this new technology as a weed management tool.  A sorghum hybrid containing the Inzen Sorghum Trait was planted at each location.  Phytotoxicity and efficacy was evaluated throughout the growing season.  Late season weed efficacy evaluations are presented.  Some visual grain sorghum injury was evaluated in Oklahoma early season but was transient in nature and was not visible later in the season.  No grain sorghum injury was observed in Texas.  Palmer amaranth (Amaranthus palmeri) and large crabgrass (Digiteria sanguinalis) control was at least 98% with all treatments at Bixby, OK.  The only treatment that controlled large crabgrass control at Chickasha, OK more than 65% was rimsulfuron + thifensulfuron + s-metolachlor PRE followed by nicosulfuron + atrazine POST.  This variability in control between the two locations was likely due to drier conditions during the growing season at Chickasha.  Palmer amaranth control was 100% with all treatments at Chickasha.  Texas millet (Urochloa platyphylla) control was greater than 60% when rimsulfuron + thifensulfuron was applied with atrazine or s-metolachlor PRE and followed by nicosulfuron + atrazine POST at Fort Cobb.  The only treatment that controlled Palmer amaranth at least 85% was s-metolachlor + atrazine PRE alone or followed by nicosulfuron + atrazine POST.  Ivyleaf morninglory (Ipomoea hederacea) control was greater than 85% with all treatment combinations at Fort Cobb.  Barnyardgrass (Echinochloa crus-galli) and Palmer amaranth control was at least 90% with all treatments except rimsulfuron + thifensulfuron + s-metolachlor PRE followed by nicosulfuron POST and for s-metolachlor + atrazine PRE alone for barnyardgrass control.  This research indicates that when used as part of an overall weed management program the Inzen Sorghum Trait can help producers with many of their weed problems in grain sorghum.  

A field study in a fallow setting was conducted in 2017 in Pennsylvania (Landisville, Lancaster Co.) as a follow-up to a similar 2016 study to evaluate glyphosate-resistant horseweed/marestail (Conyza canadensis) control with POST herbicides. Studies were arranged in a randomized complete block design with three replications. Herbicides were applied with a small-plot, CO2-backpack sprayer system that delivered 15 GPA thru TeeJet AIXR110015 nozzles on May 18 and marestail ranged from 4 to 12 inches tall (8 inch average height). Treatments included: glyphosate (1.13 lb ae/A), cloransulam (0.0315 lb ai), chlorimuron (0.0105 lb), metribuzin (0.28 lb), saflufenacil (0.0223 lb), flumioxazin (0.08 lb), sulfentrazone (0.25 lb), glufosinate (0.66 lb), 2,4-D ester (0.5 lb ae), dicamba + diflufenzopyr premix (0.175 lb ae), halauxifen (0.0045 lb), pyrasulfotole + bromoxynil premix (0.241 lb), glyphosate plus fluthiacet + fomesafen premix (1.13 + 0.17 lb A), glyphosate + mesotrione + atrazine (1.13 + 0.0.94 + 0.5 lb A), bentazon + atrazine (1 + 0.5 lb), and paraquat ± metribuzin ± 2,4-D ester (0.75 ± 0.28 ± 0.5 lb). All of the spray mixtures included liquid AMS at 2.5% v/v; all contained MSO at 1% v/v except glufosinate, and pyrasulfotole + bromoxynil premix. In addition, a broadcast application of clethodim was applied to control weedy grasses. Visual weed control ratings were taken periodically throughout the growing season.

Results from mid-season ratings (approx. 5 WAA) revealed that metribuzin, flumioxazin, sulfentrazone, and the bentazon + atrazine mix ranged from 17 to 30% marestail control. Glyphosate and glyphosate plus fluthiacet + fomesafen premix provided about 60% control. Chlorimuron, saflufenacil, 2,4-D ester, paraquat alone, and paraquat + metribuzin mix were 80 to 88% effective on marestail. Cloransulam, glufosinate, pyrasulfotole + bromoxynil premix, dicamba + diflufenzopyr premix, halauxifen, glyphosate + mesotrione + atrazine mixture, paraquat + metribuzin + 2,4-D ester mixture provided 90 to 94% marestail control.

Comparing marestail control data (%) from repeated treatments across both years (2016 and 2017 data, respectively): glyphosate (66 and 62); cloransulam (81 and 94); chlorimuron (71 and 84); saflufenacil (87 and 81); glufosinate (95 and 93); 2,4-D ester (83 and 82); dicamba + diflufenzopyr premix (87 and 93); and pyrasulfotole + bromoxynil premix (96 and 90).

In summary, although the average marestail height was greater than what is suggested for good control it was realistic to when most are sprayed. We assume better and more consistent control might be observed by some (possibly all of the Group 4 herbicides, atrazine, cloransulam, chlorimuron, and paraquat) but not all of the treatments if applied to smaller weeds. Furthermore, it was assumed that saflufenacil would provide more effective control with MSO (as recommended on the product label) compared to COC (used in 2016 study), but that was not necessarily the case in these studies. Aside from saflufenacil, none of the other Group 14 herbicides provided acceptable control of marestail. Certain herbicide combinations did provide better control compared to single active ingredients. In particular, glyphosate + mesotrione + atrazine mixture and paraquat + metribuzin + 2,4-D ester mixture provided better marestail control in combination than their individual components. However, some combinations (i.e., glyphosate plus fluthiacet + fomesafen premix and bentazon + atrazine) still did not provide effective marestail control. Some of the treatments (e.g., glufosinate) may have been less active if applied earlier in the year due to cooler temperatures. Higher spray volumes and different nozzles could potentially influence control as well. We plan to repeat and expand this study to assess some of these factors and other herbicides in the future.

The use of dicamba-tolerant cotton (Gossypium hirsutum) will increase the number of preplant and postemergence options to control glyphosate-resistant Palmer amaranth (Amaranthus palmeri), but there is serious concern about the risk of off target dicamba movement to non-target crops.  A field study was conducted at the Texas Tech New Deal Research Farm equipped with subsurface drip irrigation in 2016 and 2017 to evaluate cotton response to dicamba when applied at four crop growth stages (first square + 2 weeks, first flower, first flower + 2 weeks, and cut-out).  Dicamba (Clarity 4L) at 0.56 (1X), 0.056 (1/10X), 0.0112 (1/50X), 0.0056 (1/100X), and 0.00112 (1/500X) kg ae/ha was applied at 15 GPA using TTI11004 nozzles.  Plots, four rows spaced 102-cm apart by 9.1 meters, were replicated three times.  Visible injury was recorded throughout the growing season.  Cotton was box mapped prior to harvest to determine boll distribution.  Plots were machine harvested to determine lint yield and samples were sent to the Texas Tech Fiber and Biopolymer Institute for fiber analysis.  Visual cotton injury at each application timing was rate dependent and injury was noted from all dicamba rates 1/100X and greater.  Injury symptomology following each dicamba rate varied across application timings and in general was greater from applications made at first square + 2 weeks.  Visual injury ranged from 3 to 77% following first square + 2 week applications, whereas injury ranged from 0 to 47% following applications made at first flower + 2 weeks.  Lint yield was reduced following dicamba at 0.56 kg ae/ha at all application timings and following dicamba at 0.056 kg ae/ha applied at first square + 2 weeks.  Lint yield from the non-treated control was 1583 kg/ha.  The greatest change in boll distribution followed the 1/50X, 1/10X, and X dicamba rates applied at first square + 2 weeks.   

Recently, soybean with tolerance to 2,4-D was commercialized by Dow AgroSciences. With the prevalence of glyphosate-resistant Palmer amaranth in South Carolina soybean production, additional options are needed for effective and economical management of glyphosate-resistant Palmer amaranth.  Field experiments were conducted in 2017 at Edisto Research and Education Center located near Blackville, SC to evaluate preemergence (PRE) and postemergence (POST) herbicide systems for weed control in 2,4-D tolerant soybean. Soybean was planted on May 22, 2017 on 96 cm row spacing.  Flumioxazin plus chloransulam at 0.11 and 0.04 kg/ha was applied shortly after planting.  Postemergence treatments glyphosate at 0.84 kg/ha, 2,4-D choline at 1.12 kg/ha, s-metolachlor at 1.22 kg/ha, glufosinate at 0.59 kg/ha, fomesafen at 0.27 kg/ha, and acetochlor at 1.26 kg/ha were applied on June 19, 2017 and July 7, 2017. Percent weed control and soybean injury were collected 2 weeks after PRE (2WAP), at POST1, at POST2, and 2 weeks after POST2 (2WAP2).  Soybean plots were harvested for yield on October 27, 2017.  Experimental design was a randomized complete block design with three replications.  Percent soybean injury, soybean yield, and percent weed control were analyzed using ANOVA and means separated at the P = 0.05 level.  The PRE treatments were activated using supplemental irrigation shortly after planting.  At 2 WAP, Palmer amaranth, pitted morningglory, and goosegrass control was 100% across all treatments while pitted morningglory and goosegrass control ranged from 90 to 100%.  At the POST2 timing, Palmer amaranth, pitted morningglory, and goosegrass control was excellent.  At 2WAP2, pitted morningglory control ranged from 98 to 100% control.  Overall, no soybean injury was observed at any of the treatment evaluation timings.  Soybean yields across all treatments were not significantly different and ranged from 2354 to 3564 kg/ha.  These results showed that a foundation preemergence program followed by glyphosate, glufosinate, and 2,4-D choline systems was the most effective treatment in Palmer amaranth and pitted morningglory when applied at the correct weed size (< 10 cm in height).

Sugar beets account for fifty-five percent of the sugar produced in the United States with profits totaling nearly $3 billion in 2015/2016. A significant challenge for weed management in sugar beets is controlling herbicide-resistance weeds, as no herbicides with new mode of action have been developed in over twenty years, leaving us to rely on currently available herbicides. Herbicide safeners that induce herbicide tolerance in monocots can increase the range of herbicides used in crop species. However, safener usage has not been explored extensively in dicots. Pre-emergence application (PRE) of herbicides can control broadleaf weeds in sugar beets, but sugar beets can be damaged. We therefore propose sugar beets may benefit from safener seed treatment and experience less or no damage with herbicide PRE. Seeds from six sugar beet varieties (A-F; Betaseed) were treated with the safener fluxofenim (Syngenta; 0.25 g/kg seeds). Treatments for each variety were carried out as follows: untreated control, safener only, herbicide only, and herbicide + safener. Pots were arranged in a randomized complete block design; all aboveground tissue of sugar beets was harvested and dried for total biomass at 21 days after herbicide treatment. This experiment has been repeated twice with three replications in each run. Greenhouse results indicated safener fluxofenim decreases damage in a genotype-specific manner from three herbicides: pendimethalin, S-metolachlor, and ethofumesate. Pendamethalin + fluxofenim showed an increase in dry biomass for varieties A (+ 49%) and C (+23%) compared with untreated control. Additionally, varieties A and B showed induced herbicide tolerance for S-metolachlor + fluxofenim with dry biomass being the same as untreated control and significantly higher than herbicide only treated plants. Varieties E and F showed significantly improved dry biomass with S-metolachlor + fluxofenim compared to herbicide only. Finally, ethofumesate caused little damage to the all sugar beet varieties, however, variety E showed increased dry biomass when safener was seed-applied. Interestingly, safener only treatment significantly improved plant growth for three varieties, A, B and C compared with untreated control. A standard least-squares model (JMP) examining genotype, treatment and replicate as model effects indicates there is an environmental effect on our results. Future directions for this work is to examine these results in a large-scale field experiment.

The presence of other species shortly after corn (Zea mays) emergence has been shown to increase the variability of growth properties among corn plants. Our objective was to compare ear size as it was affected by the presence or absence of a winter annual weed adjacent to an emerging corn plant and nitrogen rate.  Corn was planted on April 13, 2017 at 88,000 seeds ha^-1 in 76 cm rows.  Plots were 3 x 46 m, with 3 replications arranged in a randomized block design.  Shortly after emergence 10 plants in each plot that were adjacent (within 30 cm) of a weed were marked with a wooden stake, and 10 other randomly selected corn plants that were not near a weed (greater than 80 cm distance) were also marked.  The weed most frequently present was common chickweed (Stellaria media).  Herbicides were applied May 12 and May 26 to control emerged weeds.  Nitrogen (urea ammonium nitrate solution [32-0-0]) was applied May 26 at 0, 84, 140, 196, and 252 kg N ha^-1 on a 1.5 m spacing.  After corn had reached R6 and dried below 20% moisture ears were harvested from the marked plants and from one plant on each side.  Ears were weighed, and kernel counts were estimated by counting rows and number of ears per row.  Yield of each plot was measured using a plot combine to harvest the middle two rows.  

The response of Palmer amaranth (Amaranthus palmeri S. Wats.) to mixtures of glufosinate with either 2,4-D or dicamba were measured in fallow fields at University of Arizona experiment stations in hot, arid environmental conditions. In both 2016 and 2017, a range of plant sizes where sprayed. In 2016 at the Red Rock Agricultural Center (RRAC), about 30% of the plants had 2.5 leaves, 63% had six leaves and were about 10 cm tall and 10 cm in diameter, and 7% were larger plants with 12 leaves that were about 23 cm tall.  In 2017, a similar range of plant sizes were sprayed but two plants per plot were flagged and measured just prior to spraying; flagged plants averaged 7.6 ± 3.6 cm tall (mean ± std. dev.) with about 11.6 ± 3.3 leaves (mean ± std. dev.) at RRAC and 18.2 ± 2.8 cm tall with 13.9 ± 1.2 leaves at the Maricopa Agricultural Center (MAC). When flagging and measuring plants, adjacent plants, if any, were removed so that the foliage of other plants did not block spray contact with the leaves of flagged plants. The herbicides were applied on July 14, 2016 (RRAC) and on June 28 (MAC) and June 30, 2017 (RRAC) using a CO2 pressurized backpack sprayer 412 kPa (45 PSI) and a 4-nozzle boom (TeeJet AI110015) that sprayed a 203 cm (80”) swath at a carrier volume of 148 L/ha (15.8 gal/A). Herbicide efficacy based on chlorosis, stunting and necrosis was visually rated in both 2016 and 2017 on a whole plot basis. Additionally in 2017, the response of the flagged plants was visually rated and plant heights were measured. The data were transformed as needed and subjected to analysis of variance and mean separation. In the RRAC 2017 experiment with flagged plants at 21 DAT, there were no differences between treatments except the glufosinate (0.79 lb ai/A) plus glyphosate (1.13 lb ai/A) treatment at 73% control compared to other treatments, including glyphosate (1.13 lb ai/A) at 100% control, glufosinate (0.79 lb ai/A) at 92% control and 2,4-D-choline (0.95 lb ae/A), dicamba (0.5 lb ae/A), and tank mixes of 2,4-D or dicamba with glufosinate all with efficacies between 97 and 99% control. Similarly, in the MAC 2017 experiment with flagged plants, there were no differences in efficacy between the above treatments with values ranging between 90 and 100% control at 21 DAT. The density of plants at the RRAC was lower than at MAC and the RRAC plants were more robust compared taller, more etiolated plants at MAC. This might explain the differential response to glufosinate between the RRAC and MAC experiments with individual plants. The RRAC data indicated that there is antagonism between glyphosate and glufosinate under some conditions. But for the range of flagged plant sizes sprayed, there were no differences between the auxin herbicides alone versus the auxin herbicide mixed with glufosinate (i.e., no antagonism). The whole plot efficacy data from RRAC in 2016 and 2017 experiments and the MAC 2017 experiment, also found evidence of antagonism between glyphosate and glufosinate in mixtures but not between the auxin herbicides and glufosinate in mixtures. Whole plot treatment efficacies were much lower than the individual plant efficacies. This was likely due to the presence of some larger plants in the plots and the importance of other factors such as plant density and interference with spray coverage between adjacent plants.

Quelex, a premixture of florasulam + halauxifen, is a new herbicide labeled for postemergence control of broadleaf weeds in wheat. In Oklahoma, florasulam + halauxifen may improve the management of horseweed (Erigeron canadensis L.) in winter wheat. To evaluate the efficacy of florasulam + halauxifen on horseweed, a trial was conducted in Altus, Perkins, and Ponca City, Oklahoma in the spring of 2017. Visual weed control was evaluated every two weeks throughout the growing season. In Altus, 2 inch horseweed rosettes were controlled at least 84% at the end of the season with the exception of chlorsulfuron + metsulfuron + MCPA and metsulfuron + 2,4-D. At Perkins 8 weeks after treatment, 4 inch rosettes were controlled at least 86% with the exception of low and high rates of bicylclopyrone + bromoxynil and 2,4-D alone. At Ponca City 8 weeks after treatment, the only treatments that achieved above 90% control of 6 inch horseweed rosettes were those that included florasulam + halauxifen alone or in tank mixture. Overall, several successful treatments were identified when applications were made at the 2 to 4 inch rosette stage. However, once horseweed rosettes were approximately 6 inches in diameter, treatments containing florasulam + halauxifen provided the greatest control.

Alfalfa is an important crop for the low desert region of California. Saflufenacil is used for dormant alfalfa; however, there is lack of information on crop safety for non-dormant alfalfa grown in the low desert region. A field study was conducted to evaluate injury from saflufenacil applied during summer-slump on established non-dormant alfalfa. Treatments consisted of saflufenacil alone at 50 or 100 g ai ha^-1; and saflufenacil at 100 g ai ha^-1 tank-mixed with imazamox, bromoxynil, and metribuzin at 53, 420, 1120 g ai ha^-1, respectively. All the treatments consisted methylated seed oil and ammonium sulfate at 1 and 2% v/v, respectively. At 2 days after treatment (DAT), alfalfa injury was least (21%) from saflufenacil at lower rate and greatest (48%) from saflufenacil plus bromoxynil treatment. The injury progressed for 2 wk after treatment (WAT) and with injury was >36% for all the treatments except saflufenacil at a lower rate. However, crop recovery was observed at 3 WAT and minimal injury (<3%) was observed at 4 WAT. Alfalfa height was comparable across the herbicide treatments and the non-treated check. At 4 WAT, alfalfa height from each treatment was 0.4 m or greater. The injury observed during early weeks did not translate in to yield reduction, and there was no difference in crop yield. Alfalfa yield was 1053 kg ha^-1 or greater when harvested at 30 DAT. The result from this study illustrated that the saflufenacil and its tank-mix with some POST herbicides caused injury to non-dormant alfalfa during early weeks, but crop recovered later in the season without reduction on height and yield.

Palmer amaranth (Amaranthus palmeri S. Watson) has become one of the most difficult to control weeds in South Carolina soybean production. Palmer amaranth exhibits prolific growth rates allowing it to compete with crops for light, nutrients and water. The ability to germinate throughout the growing season makes chemical or mechanical control difficult. These attributes have made Palmer amaranth an economic nuisance to soybean producers. Previous research on above-ground effects of weed competition is well documented; however, few studies have been conducted on below ground weed competition on crops. In 2016, field experiments were conducted at Edisto Research and Education Center located near Blackville, SC to evaluate how the presence of Palmer amaranth affects the soil moisture availability in soybean. The experimental design was a 3 x 2 x 2 factorial arranged in a split plot. The treatment factors consisted were: neighbor (Palmer amaranth, soybean, no neighbor), divider (with and without) and irrigation (with and without). Plastic sheet dividers were placed in the soil profile to a depth of 60 cm between crop and neighbor to eliminate the effects of root competition. Soil moisture content (volumetric water content) was measured using a Decagon GS1 sensor placed 30 cm deep within the root zone of the soybean crop. In the non-irrigated plots, a significant difference was observed with divider (p = 0.012; α = 0.05). The volumetric water content was greater in treatments which had a divider (0.1094 ± 0.0047 m³/m³) than in treatments without a divider (0.0897 ± 0.0047 m³/m³). Palmer amaranth as a neighbor in non-irrigated plots lowered soil moisture content more than soybean (0.0973 ± 0.0058 m³/m³ and 0.1079 ± 0.0058 m³/m³, respectively). A significant difference was observed between non-irrigated treatments with and without a divider and AMAPA as a neighbor (p = 0.018, α = 0.05). These results indicated that by eliminating Palmer amaranth root competition more soil moisture was then available for soybean. However, in irrigated system, there was not significant differences found among neighbor factors (i.e., lack of crop and weed competition) which was  attributed to the surplus soil moisture in the profile.

There is a need for new, effective, and less intensive weed management solutions to improve the yield and profitability of organic vegetables. Organic vegetable growers cannot use synthetic herbicides to manage weeds. Instead, they typically have to turn to a suite of management options that includes mulching, cultivation, hand weeding, rotation, and cover cropping. These labour- and knowledge-intensive strategies can be demanding on the producer, and can contribute to the higher cost of organic produce. With this in mind, there is an opportunity to develop new, naturally-derived weed control products that could be used as herbicides in organic production. Such products are additionally attractive as they are biodegradable and safer for the environment and people. Low-risk, organically-permitted, naturally plant-derived weed control products were evaluated for their effectiveness in controlling weeds in the organic production of vegetable crops. The products tested included (i) Manuka oil, an extract derived from the Oceania-native manuka tree, (ii) Weed Zap, containing clove and cinnamon oils, (iii) Suppress, containing caprylic and capric acid and (iv) Pine oil (100%). These novel organic herbicides were tested against another organically permitted herbicide already in use like Horticultural Vinegar (acetic acid). The organic herbicides were tested in field trials at the University of Guelph’s Simcoe Research Station with tomato, sweet corn and peppers. The organic herbicides were applied after weeds emerged, 3 weeks after the crop was planted and again after another 4 weeks. Emerging weeds were monitored, identified and counted, crop injury was noted and percent weed control and final crop yield determined. The best overall weed control came from applying a mixture of Manuka oil with either Weed Zap or vinegar. Use of these mixtures improved weed control by 25-30% beyond that seen when any of the products were used alone. Manuka oil is a unique organically-permitted herbicide, active in the soil and so can continue to impact newly emerging weeds. Furthermore, Manuka oil can move and be active throughout plant tissues, rather than affecting only the tissues that it contacts directly. Both of these aspects result in enhanced weed control activity over many other organically-permitted weed control products. Manuka oil, especially when applied with other organically-permitted substances with weed control capabilities, provides effective weed control in organic vegetable production. 

Infestation of strawberry (Fragaria × ananassa) fields in Florida by the plant-parasitic, sting nematode (Belonolaimus longicaudatus) can cave profoundly adverse effects on growth and yield of the crop. The sting nematode has a wide host range and thus many weeds that occur in strawberry fields during the summer off-season can serve as alternative hosts. Organic strawberry growers cannot use the soil fumigants that are routinely employed in conventional strawberry production for weed, soilborne pest, and disease management. Therefore, identifying effective integrated, nonchemical approaches to strawberry crop protection will be critical to foster the expansion of organic strawberry production in Florida. A study was conducted in summer 2017 with the objective of comparing 4-component cover crop mixtures with monocultures of the component species in order to assess whether mixtures can provide equivalent or superior biomass production and weed suppression. The study was conducted in Citra, Florida utilizing a randomized complete block design with 4 replications. Monocultures of Crotalaria ochroleuca, Aeschynomene americana, C. juncea cv. AU Golden, and Indigofera hirsuta were compared with 4 mixtures that contained all 4 species in differing proportions by seed weight: Mix 1 (1:1:1:1), Mix 2 (1:2:1:2), Mix 3 (2:1:1:1), and Mix 4 (2:2:1:3). Initial cover crop establishment was slow, so that at 4 weeks after planting (WAP), cover crop biomass ranged from a low of 49 kg/ha with A. americana to 401 kg/ha with Mix 4. By 8 WAP, cover crop biomass was 2208 kg/ha with C. juncea, which was higher than the biomass obtained with A. americana and C. ochroleuca, but was not significantly different from I. hirsuta and the 4 mixes. Weed pressure was very high (1347 plants/m^-2) in the weedy control and cover crops were not effective in suppressing total weed density. However, total weed biomass was suppressed to lower than the weedy control by C. juncea, I. hirsuta, Mix 2, Mix 3, and Mix 4. Only Mix 4 and C. juncea resulted in significant decreases in photosynthetically active radiation penetrating the canopy by 4 WAP, which indicated more rapid canopy closure than the other treatments. The results indicate that a mix (2:2:1:3) seeded at 53.8 kg/ha can provide cover crop biomass production and weed biomass suppression equivalent to the best performing monoculture - C. juncea at seeded at 44.8 kg/ha.

The investigation of potential herbicides for weed control in sweetpotato is critical due to the limited number of registered herbicides and the development of populations of herbicide resistant weeds. Therefore, field studies were conducted at the Horticultural Crops Research Station, Clinton, NC and the Pontotoc Ridge-Flatwoods Branch Experiment Station, Pontotoc, MS to determine the effect of oryzalin rate and application timing on sweetpotato tolerance. Oryzalin at 0.6, 1.1, 2.2, 3.4, and 4.5 kg ai ha^-1 was applied 0 to 2 or 14 d after sweetpotato transplanting (DAT). At Pontotoc 4 wk after application (WAA), sweetpotato injury (leaf distortion) was < 6% when oryzalin was applied 0 to 2 DAT regardless of application rate. However, when oryzalin was applied at 14 DAT greater leaf distortion was reported from 3.4 and 4.5 kg ai ha^-1 (12 to 13%) than 0.6, 1.1, and 2.2 kg ai ha^-1 (4 to 6%). At Pontotoc 8 WAA, sweetpotato injury (stunting) was < 4% regardless of oryzalin application rate and timing. At Clinton 6 WAA, sweetpotato injury (leaf distortion) was < 1% and 4 to 8% when oryzalin was applied 0 to 2 and 14 DAT, respectively. Sweetpotato stunting at 6 WAA was greater with oryzalin applied at 14 DAT (13%) than 0 to 2 DAT (6%). Oryzalin rate and timing did not affect yield of no.1, jumbo, and marketable sweetpotato. Based on these results, oryzalin herbicide has potential for registration in sweetpotato. 

Control of broadleaf weeds in caladium is difficult due in part to a lack of selective postemergence (POST) herbicides. Cultivation is not an option due to the dense canopy and potential for tuber injury.  As a result, growers currently rely on preemergence (PRE) herbicide and hand-weeding. The objective of this research was to evaluate the control of four common broadleaf weeds of field grown caladium with POST applications of halosulfuron, thifensulfuron-methyl, and trifloxysulfuron, and determine the tolerance of caladium cultivars ‘Florida Fantasy’ and ‘Florida cardinal’ to POST applications of halosulfuron. At 4 weeks after treatment (WAT), thifensufluron-methyl at 28 g ai ha^-1 and trifloxysulfuron at 84 g ai ha^-1 provided approximately 90 and 70% common purslane control, respectively, while 210 g ai ha^-1 halosulfuron provided 55% suppression. Trifloxysulfuron controlled >90% spotted spurge at 42 g ai ha^-1, whereas the highest rate of halosulfuron and thifensulfuron-methyl only achieved 60% suppression. The evaluated sulfonylurea (SU) herbicides were less efficacious on hairy indigo and sharppod morningglory as control never exceeded 65 and 50%, respectively. The evaluated halosulfuron rates ranging from 26 to 420 g ai ha^-1 did not significantly reduce caladium tuber weight from the nontreated control. Averaged over halosulfuron rates, ‘Florida Fantasy’ damage was 5 and 6% at 2 and 4 WAT, respectively, while ‘Florida Cardinal’ damage was 11%. We conclude that none of the herbicide treatments effectively controlled all species evaluated. Sequential treatments, higher rates, or tank-mixtures may be necessary to adequately control these species. We also conclude that halosulfuron is safe on caladium cultivars ‘Florida Fantasy’ and ‘Florida Cardinal’. Further research is needed to evaluate caladium tolerance to other SU herbicides. 

Florida cabbage production occurs September through March, allowing for a fallow period during the summer months. Many farmers use sorghum Sudangrass as a covercrop or previous research has shown two applications of glyphosate in a weed fallow program can reduce nutsedge by as much as 64%. The research objective was the weed population reduction and species shift in cabbage after multiple years of glyphosate tolerant crops and alternative cover crops during the fallow period. The fallow treatments were corn with and without glyphosate, soybean with and without glyphosate, sorghum Sudangrass with and without s-metolachlor and atrazine, sunnhemp with and without pendimethalin, and weedy fallow with and without glyphosate. The subplot in each cover crop treatment is s-metolachlor after transplanting. The s-metolachlor application in cabbage reduced weed populations in all fallow treatments. The use of herbicides in the corn, soybean, and sorghum Sudangrass reduced the number of weeds in the cabbage. No differences in weed counts between sunnhemp with pendimethalin and sunnhemp alone. After the second year of fallow crops, crops have reduced the number of eclipta and increased the number of yellow nutsedge, smooth pigweed, and lesser swinecress. The corn and soybean provides excellent weed control and additional income for the additional inputs during the fallow period. The sunnhemp provides nematode supression and nitrogen to the system, however, additional research is needed to find a more suitable method for planting and weed control.

Resistance to glufosinate has recently been confirmed in an Italian ryegrass (Lolium multiflorum) population in a pear orchard in northwest California. During a survey conducted in 2015 to evaluate the distribution of glyphosate resistance in Italian ryegrass from a diversity of crops in the Central Valley, a few populations were identified with multiple resistance to glyphosate and glufosinate at the labeled field rates (867 g ae h^-1 and 984 g ai h^-1, respectively). Further greenhouse studies revealed that two Italian ryegrass populations (Res1 and Res2) were resistant to both herbicides at two times the labeled field rates. The objectives of this study were to determine the level of resistance to glufosinate in the resistant populations and to determine the possible mechanism(s) of resistance to both herbicides. Greenhouse studies were also conducted to evaluate alternative herbicide options. Seeds of the resistant populations were collected from an infested grain field (Res1) and a vineyard (Res2) in San Joaquin County, and seeds of a known susceptible population (Sus) were used from our previous study. All herbicide treatments were applied to 3- to 4-leaf stage plants using an automated sprayer calibrated to deliver 20 GPA of each herbicide. Based on the lethal dose needed to kill 50% of plants (LD₅₀), Res1 and Res2 populations were 4.5 and 2.5-fold less sensitive to glufosinate, respectively, than the Sus population. Whole-plant bioassays revealed that both resistant populations were effectively controlled with the labeled field rates of paraquat and acetyl-coenzyme A carboxylase (ACCase)-inhibiting herbicides (e.g. sethoxydim, clethodim, fluazifop, and cyhalofop) from two different chemical families. Complete control (100%) of the resistant plants was attained with four times the labeled rate of glyphosate and glufosinate. Based on the in vitro glutamine synthetase leaf disk assay, accumulation of ammonia caused by inhibition of photorespiration by glufosinate in Sus plants was significantly greater than in resistant plants 24 h after light pretreatments. The glufosinate doses (µM) resulting in 50% ammonia accumulation were 1.8 and 1.7-fold greater in Res1 and Res2, respectively, than in the Sus leaf discs. This result indicates that glufosinate reaches the target enzyme of both Res and Sus plants but at significantly different levels. Sanger sequencing of the entire plastidic glutamine synthase (GS2) gene revealed a previously reported missense mutation resulting in an Asp₁₇₁-to-Asn substitution in 21% and 28% of Res1 and Res2 individuals, respectively. Based on RT-PCR analysis, GS2 gene expression levels were similar in Res and Sus plants. Alignment of EPSPS gene sequences around codon site 106 revealed a missense mutation (Pro₁₀₆-to-Thr) in Res plants that has previously been shown to confer resistance to glyphosate in weeds. This research indicates that glufosinate resistance in these populations may be due to both target site and non-target site mechanisms, and that resistance to glyphosate is at least partially due to an amino acid substitution in EPSPS enzyme of Res plants. Studies on glufosinate uptake, translocation and metabolism remain to be conducted.

Tall fescue is susceptible to injury from many ALS-inhibitors used for broadleaf weed control in turfgrass. Florasulam is an ALS-inhibitor that selectively controls broadleaf weeds in tall fescue, but the mechanisms for selectivity are not well understood. The objectives of this research were to evaluate the physiological basis of tall fescue tolerance to florasulam. In greenhouse experiments, florasulam rates required to injure tall fescue 20% (I₂₀) and white clover 80% (I₈₀) measured 320 and 65 g ai ha^-1, respectively. The I₂₀ and I₈₀ values of another ALS-inhibitor, flucarbazone, on these species measured 33 and 275 g ai ha^-1, respectively.  In laboratory experiments, the time required to reach 50% foliar uptake for ¹⁴C-florasulam and ¹⁴C-flucarbazone measured 23 and 62 h for white clover, respectively, and >72 h for both herbicides in tall fescue. The half-life of florasulam and flucarbazone in tall fescue were 15 and 40 h, respectively, while the half-life in white clover was >72 h for both herbicides. The concentrations of florasulam and flucarbazone required to inhibit ALS-enzymes 50% in excised leaves of tall fescue measured >1000 and 32 mM, respectively. The selectivity of florasulam for white clover control in tall fescue is associated with differential levels of absorption and metabolism between species.  Tall fescue has faster metabolism and less ALS-enzyme inhibition from florasulam as compared to a more injurious ALS-inhibitor, flucarbazone, which contributes to the differential tolerance levels between these herbicides. 

Experiments were conducted to evaluate the safety of late winter / early spring and summer applications of FeHEDTA over a diversity of woody and herbaceous ornamental plants in containers.  Fiesta (FeHEDTA) was applied three times at approximately 14-day intervals at 25, 50 or 100 oz/1000 ft².  The dormant-season test was initiated on 11 March 2016 and repeated on 10 February 2017.  Treatments to actively growing plants were initiated on 9 June 2016 and repeated 2 June 2017.  Percent crop injury was visually evaluated about 3, 7, and 14 days after each application and 26 days after the last application  Coniferous plants, juniper, arborvitae, Chamaecyparis exhibited little or no injury from dormant season applications with the exception of Cephalotaxus which had about 33% mortality from the highest dose but no injury from the 2 lower doses.  When applied in the summer to actively growing plants junipers were uninjured but other species exhibited some tip necrosis, severity increasing with dose.  Broadleaf evergreen shrubs differed in their tolerance to dormant-season applications but were injured by summer applications, with the exception of Ilex crenata that was not injured by summer applications. Dormant herbaceous perennials such as hosta were uninjured but actively growing plants were severely injured by FeHEDTA.  Container grown ornamental grasses were exhibited very little foliar discoloration from FeHEDTA. 

The main trouble weeds in golf turf of China  

Xue Guang^1, 2*, Du Jinrong², Zhang Zhaosong³, Li Chunyan² and Shen Zhenggao⁴

¹Plant Protection Institute, Jiangsu Academy of Agricultural Science, Nanjing, China

²East China Weed Technology Institute, Nanjing, China

³Nanjing Green Touch Golf Course Architecture Co., Nanjing, China

 ⁴Jiangsu iGreen Golf Course Maintenance Itc. Suzhou, China

Abstract: This study was conducted in more than 100 golf courses of China from 1998 to 2017. There are three turfgrass zones including warm turf zone, transition turf zone Ⅱ and cool turf zone Ⅲ. The investigation and observation showed that there were 40 kinds of trouble weed in total more than 400 kinds of weed in golf turf of China. The trouble weeds were divided three groups including main trouble weed (group 1), potential trouble weed (group 2) and special trouble weed (group 3) which transformed from other turf into golf turf (also can be called “mixed turf”). There were 20 kinds of main trouble weed in group 1, 12 kinds of potential trouble weed in group 2 and 8 kinds of mixed turf weed in group3. The weed in group 1 was Cynodon dactylon, Digitaria ischaemum, Poa annua, Eleusine indica, Panicum repens, Brachiaria villosa, Dactyloctenium aegyptium, Digitaria sanguinalis, Imperata cylindrica var. major, Eragrostis pilosa; Cyperus rotundus, Kyllinga brevifolia var. leiolepis; Alysicarpus vaginalis, Centella asiatica, Desmodium triflorum, Euphorbia humifusa, Hedyotis corymbosa, Hydrocotyle sibthorpioides, Taraxacum mongolicum and Polygonum aviculare. The weeds in group 2 was Paspalum distichum, Paspalum conjugatum, Paspalum dilastatum, Calamagrostis pseuophragmites, Phragmites communis, Hydrocotyle sibthorpioides var. batrachium. Conyza canadensis, Commelina communis, Euphorbia hirta, Mimosa pudica, Viola japonica and Oxalis corniculata. The weed in group 3 was Cynodon dactylon× C.transvaavlensis, Axonpus compressus, Eremochloa ophiuroides, Paspalum vaginatum, Zoysia japonica, Agrostis stolonifera, Lolium perenne, Festuca arundinacea and Poa pratensis. One of them was in both of group 1 and 3 acting as “wild bermudagrass (weed)” and “Burmudagrass (mixed turf)” in different situation respectively. Bermudagrass damaged seashore paspalum or Zoysia or kenturckey blugrass or tall fescue which were typical examples. Another example was bentgrass invaded into bermudagrass or kenturcky bluegrass. The main trouble weed in group 1 and 3 was always keeping with turf each other company and liked as “turf mate”. The potential trouble weeds were getting more and more. They threated pressure to golf turf. The distribution of main trouble weeds in golf turf of China and the growth habit of stems of each main trouble weed were introduced. The study showed that weed competition ability was more differences on different growth habit of stems of weed in golf turf where the water and the nutrients were abundant.

Weed control is one of the largest challenges that producers of container-grown ornamental plants have to overcome due to the lack of postemergence herbicide options, heavy reliance on hand weeding, and high crop species diversity. Most of the prior research on weed control within ornamental production has focused on various chemical and/or non-chemical approaches to weed management and the crop response to these approaches. The impact that fertilization methods have on weed control and herbicide performance is an area that has been subject to less investigation. The primary objective of this experiment was to provide a better understanding of how common fertilization methods (incorporation, subdress, and topdress) that are currently being impact weed growth and the performance of preemergence herbicides. Nursery containers (0.9 L) were filled with a pinebark:peat substrate and fertilized with a controlled-release fertilizer at two different rates (4.2 and 9 g representing low and high manufacturer recommendations, respectively) using one of three placements including topdressing (placing on media surface), subdressing (placing in a layer 3.8 cm below media surface), or incorporating. After filling, nursery containers were treated with either dimethenamid-P (Tower®), flumioxazin (SureGuard®) or prodiamine (Barricade®) at the highest recommended label rate. Seeds of spurge (Chamaesyce maculata), eclipta (Eclipta prostrata), and crabgrass (Digitaria sanguinalis) were then surface sown to pots treated with dimethenamid-P, flumioxazin, and prodiamine, respectively. A control was established for each fertilizer rate/placement and weed species that was not treated with herbicide and used for comparison. Weed counts were collected bi-weekly and shoot fresh weights were determined at trial conclusion. Data were converted to percent control using the formula [(weight nontreated –weight of treated)/weight nontreated] × 100. For herbicide treated pots, fertilizer rate was not a significant factor in any case. Fertilizer placement had no significant effect on percent control in crabgrass or eclipta as prodiamine provided > 97% of crabgrass and flumioxazin provided 89 to 97% of eclipta regardless of how fertilizer was applied. In spurge, the highest level of control was obtained when the fertilizer was subdressed (94%) followed by topdressing (90%) and incorporation (83%). Results suggest fertilizer placement could potentially affect herbicide efficacy in nursery conditions. However, adequate weed control would be likely achieved when herbicides are applied at recommended label rates and chosen properly for target weed species. 

Management of Dalmatian and yellow toadflax throughout the Intermountain West is challenged by the increasing occurrence of vigorous self-sustaining hybrid populations originating from gene flow between these invasive species. We used MaxEnt niche modeling to identify environmentally suitable areas for Dalmatian, yellow and hybrid  toadflax in Montana, Wyoming, and Colorado. One model identified areas environmentally suitable for co-occurrence of the parent species; a second model predicted areas at risk of hybrid invasion based on known hybrid occurrence and associated environmental conditions. Combining output from both modeling approaches identified hot spots of potential hybrid invasion in western Montana, northwestern, northeastern, and southeastern Wyoming, and the Western Slope and Front Range of Colorado. Our model output also predicted establishment of hybrid populations in parts of the three-state study area not currently invaded by the parent species. Model-based maps of potential toadflax distributions provided to weed managers will enable more effective direction of personnel and resources for proactively locating and controlling these populations.

Sericea lespedeza (Lespedeza cuneata) has been used in mine reclamation for many years in the past.  It is no longer recommended as it can be aggressive in growth and dominate an area interfering with establishment of other species and regeneration of trees and shrubs.  This trial was established to test the use and timing of some herbicides to control lespedeza to facilitate establishment of native grasses and forbs.

The trial was established at the Wendell H. Ford Regional Training Center, Greenville Kentucky on an area with a mix of lespedeza and Indian grass with a 6 x 2 factorial set of treatments and 4 replications arranged in a randomized complete block design.  The six herbicide treatments also had either dormant seeding done or not.  There were two timings for each of the three herbicides.  All the herbicides included primarily foliar active products as well as aminopyralid as a foliar and soil residual product.  The herbicides were Opensight (aminopyralid + metsulfuron), PastureGard HL (triclopyr + fluroxypyr) + Milestone VM (aminopyralid), and Garlon 4 Ultra (triclopyr) + Milestone VM (aminopyralid).  Plots were 3 m by 9 m with running unsprayed checks (3 m) between each of the plots.  All applications were at 187 L/ha and included a non-ionic surfactant (Activator 90) at 0.25% v/v. 

The first applications were in the early fall on Sept. 26, 2013 for all three herbicides, with the lespedeza at 90 cm height, marestail at 125 cm, common ragweed at 115 cm, annual marsh elder at 105 cm and Indian grass at 175 cm.  The second application was on Oct. 21, 2013 for the late Opensight application.  There was no distinct visual difference between the previously sprayed plots and unsprayed strips.  Dormant seeded plots were sown March 18, 2014 by mixing the seed mix with vermiculite to increase the volume and then broadcasting over the plot areas.  The last herbicide application was in the spring on June 8, 2014 for PastureGard + Milestone and Garlon + Milestone with the lespedeza at 60 to 90 cm height and the common ragweed at 30 cm.

Lespedeza control (%) was assessed at the time of the last application on June 6, 2014 for the first set of treatments (253 Days after Treatment) (DAT) and 228 DAT for the late Opensight application.  Lespedeza control (%) was assessed on Oct. 23, 2014 as well as % cover of lespedeza, grasses, other broadleaves, and bareground.  This was 392, 367, and 139 DAT after the first, second, and third application dates, respectively.  Percent cover of lespedeza, grasses, other broadleaves, and bareground was assessed on Oct. 7, 2015.  This was 741, 716, and 488 DAT after the first, second, and third application dates, respectively.  Data were analyzed using ARM software and treatment means were compared using Fisher’s LSD at p = 0.05. 

The spring after fall applications of Opensight had good control (94%) of lespedeza with less control when applied later in the season (83 to 89%). The fall applications of PastureGard + Milestone and Garlon + Milestone were less effective.  Application in late summer on smaller plants may have been more effective. 

A year after our fall application (392 DAT) the first Opensight treatment still had good control (73 to 81%) while the late fall application (367 DAT) was less (32 to 43%).  The spring applications of PastureGard + Milestone and Garlon + Milestone had good control of lespedeza (94 to 98%) 139 DAT.   These treatments along with the early Opensight treatment had 3 to 27% lespedeza cover and 78 to 52% grasses as vegetative cover.  The grasses were predominantly previously established Indian grass.  There was a mix of other broadleaf species but most of the cover was from common ragweed.  No plants from the dormant seeding were observed at any of the assessments.  Perhaps we would have had better results if had sown the seed mix on the snow so it had good moisture availability early in the season. 

By fall in 2015 (741 DAT) lespedeza was dominant in many plots.  We still had good control with the early Opensight (55 to 65% cover) and spring applications (14 to 31% cover).  These treatments had 31 to 84% grasses as cover.  There were not many other broadleaf species at this end of season rating.  We had a wet July with 4.6 inches more precipitation than the long term average which may have resulted in good growth of the lespedeza and Indian grass plants.

An early fall application of Opensight was effective for lespedeza control while a very late application was not.  Fall applications of PastureGard + Milestone and Garlon + Milestone were not very effective but spring applications were, when the plants were smaller and actively growing.  Controlling lespedeza resulted in more growth of already established grasses like Indian grass.  Herbicides can be effective management tools in promoting desirable prairie species.

PastureGard HL and Opensight can both provide excellent control of lespedeza when timed correctly.  The land manager has to be aware of this as well as the residual activity of the product applied.  Given the lack of soil residual activity from PastureGard HL, a quicker plantback time can be achieved but it’s more likely other broadleafs can reinvade.  Opensight on the other hand provides excellent residual control of other broadleafs but requires a delay in overseeding.  It’s recommended that PastureGard be used June through early September and Opensight be used in September and early October.

Management of weedy grasses in grass forages has relatively few selected and effective herbicide options available.  For example, imazapic has been used for selective control of many annual and perennial weedy grasses including crabgrass, vaseygrass, and dallisgrass in bermudagrass.  However, a postemergence application of imazapic can stunt and injure bermudagrass resulting in lower forage quality and productivity.  Pastora (a premix of metsulfuron and nicosulfuron) was recently commercialized for control of annual and perennial grass weeds in bermudagrass hayfield and pastures.  Annual sandburs are one of the most common and troublesome weed in bermudagrass production.  For effective control of annual sandburs, they need to be treated with Pastora at the 2-3 leaf stage. For larger sandburs (>3 leaf), adding a low-rate of glyphosate with Pastora provides increased control than Pastora alone.  However, crop response to glyphosate in this tank mix combination has not been characterized. The objective of this experiment was to quantify coastal bermudagrass crop response to different rates and combinations of glyphosate at 0.14, 0.28, 0.42, 0.56, and 0.7 kg/ha alone and in combination with metsulfuron and nicosulfuron at 0.7 kg/ha.  A standard rate of Impose at 0.42 kg/ha was included as a comparison. Field experiments were conducted at Edisto Research and Education Center (EREC) located near Blackville, SC. An established coastal bermudagrass ‘TIF 85’ field was clipped approximately 4 weeks prior to initiation of this study. Experimental design was a randomized complete block design with six replications. Postemergence herbicide treatments were applied in water on July 24, 2017 (mid-summer) and study was repeated on another site on September 20, 2017 (late summer). Percent bermudagrass injury was evaluated at 2 and 4 weeks after treatment (WAT). Bermudagrass regrowth biomass was collected using a 0.14 m2 quadrat at 4 WAT (2 subsamples per plot). Percent bermudagrass injury and regrowth biomass were analyzed using ANOVA and means separated at the P = 0.05 level. Bermudagrass injury increased significantly as the rate of glyphosate alone or with metsulfuron and nicosulfuron increased from 0.14 kg/ha to 0.7 kg/ha. A similar pattern was observed in the tank mixture of metsulfuron and nicosulfuron plus glyphosate. The untreated bermudagrass yield was significantly higher in both studies at 8393 and 4552 kg/ha compared to the herbicide treated plots. Bermudagrass yield was higher in the treatment where the glyphosate alone rate was 0.14 kg/ha; however, metsulfuron and nicosulfuron plus glyphosate (regardless of rate) tank mixture yields were more variable (with overall trend of decreasing yield with higher rates of glyphosate). Overall, Bermudagrass productivity was impacted by use of low rate of glyphosate; however, presence of grassy weeds, including sandburs have greater impact on marketability of forage Bermudagrass.

Invasive species can negatively affect the function of natural, agricultural, and built ecosystems, resulting in ecosystem damage, economic losses and expenditures for invasive species eradication. Currently, the Animal and Plant Health Inspection Service uses a linear regression Weed Risk Assessment (WRA) model to determine risk of a plant proposed for import becoming weedy. The assessment includes physiology and life history variable of the proposed plant or its near relatives as well as stakeholder inputs. In this study, we compared the logistic regression model to the tree-based random forests model. The random forests model was a better predicted of major and minor invasive species compared to the logistic regression model. Further, we identified the key variables that were the major predictors for invasiveness risk. Therefore, random forest model has the potential to be used a first analysis to assess the necessity of preforming a full WRA, and may reduce the assessment time for the proposed import of a new species by one-third.

The Weed Science Society of America (WSSA) has co-sponsored a number of educational tours for EPA staff since 2009. The tours have provided a firsthand learning experience on a wide range of weed management issues, including herbicide resistance, aquatic use permits, pollinator protection, and application technologies in crop and non-crop areas that impact herbicide registrations and use guidelines.  A hallmark of these tours has been the opportunity for direct dialogue between EPA personnel and growers, applicators, crop consultants, land and water managers, food processors, equipment manufacturers, and university research and extension.  Previous tours have included stops in FL, NM, MO, IL, AR, MD, DE, and IA.  In August 2017, a 3-day tour in western Kansas was organized by Phillip Stahlman, Kansas State University and Michael Barrett, WSSA-EPA Liaison.  The arid High Plains region of the U.S. poses a unique set of challenges for weed management.  Fourteen EPA staff from the Office of Pesticide Programs participated in the tour, which was hosted by Kansas State University with support from WSSA and several commodity organizations.  The goals of the tour were to: 1) help EPA staff better understand dryland cropping systems and the difficulties of managing herbicide resistant weeds in rainfall-limited environments; 2) provide EPA staff an opportunity to visit with local farmers, crop advisors, and applicators about the regulatory process and the practicality of different application requirements; and 3) allow farmers and crop advisors to provide feedback on the tools they need to successfully manage herbicide resistant weeds.  Some of the key points raised by farmers and applicators included: 1) the most problematic weeds in the High Plains regions include Palmer amaranth, kochia, horseweed, and tumble windmillgrass; 2) herbicide resistant weeds are threatening the continued use of no-till cropping systems, which are critical for soil and water conservation, soil structure, soil health, crop yields, yield stability, and profitability; 3) continued availability of atrazine, dicamba, 2,4-D, and paraquat are important to help manage weeds in dryland cropping systems; 4) barriers to develop and register new herbicide sites of action need to be minimized; 5) avoid application requirements that are impractical and consider differences between geographies and different production systems; and 6) solicit input from practitioners regarding critical registration and application requirement decisions.

Temperature inversions have often been implicated in facilitating drift of herbicides to non-target sites.  Several herbicide labels now prohibit application during a temperature inversion, but many applicators are not familiar with when and how frequently temperature inversions occur. Inversions are anomalies in the lowest layer of the atmosphere, when temperatures increase with height, usually correlated to the loss of longwave radiation from the Earth’s surface. Unfortunately, it is often very difficult to discern the presence of an inversion in the field. For this reason, the Kansas Mesonet underwent an upgrade that included adding a second 30 foot (10 meters) high temperature/humidity sensor to tower stations. This upgrade coincides with the already existing 6 foot (2 meters) temperature humidity sensor. With two temperature measurements at different heights, the Mesonet is able to provide a small vertical profile of the lower atmosphere. This lowest layer provides insight into the vertical mixing from inversion development and their subsequent influence on smoke dispersal, spraying results, and temperature forecasting. Utilizing the 30 and 6 foot temperature measurements, the Mesonet is able to provide regional guidance on inversion development, strength, and climatology at respective weather stations on the network. This data is provided free of charge on the Kansas Mesonet webpage (mesonet.k-state.edu/agriculture/inversion) and is updated every five minutes. Through this tool, users can determine the presence of inversions and monitor their trends at each respective station. Historical inversion and wind data may also provide valuable insight regarding the average number of hours suitable for spraying during critical periods and with diagnosing possible drift problems.

Glyphosate penetration through plant cuticle is concentration-dependent. Therefore, adjuvants that increase glyphosate adherence by lowering the spraying solution surface tension do not increase but rather decrease herbicide penetration. This is because high-contact-angle spraying solution droplets render small, thick, concentrated stains, while low-contact-angle droplets render spread-out, thin, diluted stains. The effect of a new pine-tree-based adjuvant (SprayFix) on both tensioactive and glyphosate penetration parameters was studied under laboratory-controlled conditions using wild oat (Avena fatua L.) as the experimental model. The parameters studied were spraying-solution contact angle (adaxial and abaxial), percentage of foliar coverage (adaxial and abaxial), total spraying-solution adherence, and glyphosate penetration. Glyphosate concentration in all the experiments was equivalent to a field rate of 720 g a.i. ha^-1 applied at a spraying volume of 300 L ha^-1. The SprayFix concentration was 0.25% (v/v). Glyphosate formulations containing SprayFix enhanced all of the parameters studied, compared to those of the technical glyphosate solutions used as the controls. Thus, the contact angle was reduced from 122.24° (control, adaxial) to 51.63° (SprayFix, adaxial), while the percentage of foliar coverage increased from 6.38% (control, adaxial) to 14.54% (SprayFix, adaxial). The adaxial and abaxial results were quite similar. This enhancement in both adherence and droplet spreading resulted in greater total retention values, which increased from 0.464 to 1.139 mL of spraying solution g^-1 DW. However, this increase in droplet spreading due to the presence of SprayFix did not lead to a reduction in glyphosate penetration, as the percentage of penetrated herbicide was similar between the non-amended control (16.60%, 48 HAT) and the SprayFix formulation (20.23%, HAT). Unlike other adjuvants, the new pine-tree-based adjuvant SprayFix managed to increase surface tension-related traits without decreasing glyphosate penetration.

New dicamba products have led to increasing complexity of labeling which makes selecting among the various technologies a greater challenge for growers. Discerning which tank-mix partners to use within these labels have created a flurry of questions from growers and applicators which need answering. When tank-mixing with herbicides that require the use of a drift reduction adjuvant (DRA), questions about which DRA is the best choice for growers and applicators have arisen. DRA's vary in their composition, which will invariably impact the tank mix physical and chemical properties. A study was conducted at Mississippi State University to determine the effect that tank-mix DRAs labeled with Xtendimax and Engenia have on the tank-mix pH, viscosity and droplet evaporation. Results indicate a difference with respect to DRA type, where viscosities and pH's were affected to varying degrees with each product. Results from this study will aid in making recommendations for growers and applicators.  

FV074 and FV075 are two new adjuvants derived from natural, renewable resources. FV074 is a hydrolyzed protein with a molecular weight < 10,000 Da derived from the animal tissue hydrolysate (≥ 35% w/w), while FV075 is a phospholipid plus sphingolipid extract from animal tissue. The effects of these two adjuvants on both the penetration and tensioactive parameters of a tallow amine-free commercial formulation of glyphosate were studied under laboratory-controlled conditions. Rigid ryegrass (Lolium rigidum Gaud.) was used as the experimental model, and a commercial lecithin-based adjuvant (LI700), as a benchmark. The parameters studied were spraying-solution contact angle (adaxial and abaxial), percentage of foliar coverage (adaxial and abaxial), total spraying-solution adherence, and glyphosate penetration. Both adjuvants presented similar results compared to those of the non-amended commercial formulation in terms of contact angle values and percentage of foliar coverage, with the benchmark showing improved results. However, the total spraying solution adherence was greater for FV074 (0.513 mL gr DW^-1) and similar for FV075 (0.483 mL gr DW^-1) compared to that of both the benchmark (0.430 mL gr DW^-1) and the commercial formulation alone (0.477 mL gr DW^-1). The percentage of penetrated herbicide was similar in both the commercial glyphosate (30.54%, 48 HAT) and the benchmark formulation (26.81%, 48 HAT), while FV074 increased herbicide penetration up to 68.92% (48 HAT), and FV074, up to 44.09% (48 HAT). Therefore, these two new experimental adjuvants doubled glyphosate uptake compared to the controls without negatively affecting herbicide adherence.

Ammonium sulfate (AMS) is recommended as a tank-mix adjuvant with glyphosate. It increases glyphosate uptake into the weed and reduces glyphosate complexing with divalent and trivalent cations (e.g., Ca²⁺ or Fe³⁺) in the spray solution or on the surface of the plant leaf. Because of the inconvenience associated with mixing granular AMS suppliers have developed liquid AMS adjuvants. The cost of these adjuvants is much greater per pound of AMS added to the spray solution than granular AMS, leading some to recommend increasing the rate of glyphosate in lieu of adding a liquid AMS adjuvant to the spray solution. The objective of this study was to quantify the benefit of additional glyphosate equal to the cost of granular or liquid AMS products.  Glyphosate was applied at a base rate of 0.42 or 0.84 kg ae ha^-1.  Granular AMS was applied at 1% and 2% w/v, and a liquid AMS formulation (420 g AMS l^-1) was applied at 0.5% and 1.0% v/v.  Glyphosate equal to the monetary value of the 1% and 2% granular AMS, and the 1% liquid AMS was added to the base rates, resulting in glyphosate rates of 0.48, 0.54, and 0.69, or 0.93, 0.98 and 1.12 kg ha^-1. Multiple species bioassays (velvetleaf, buckwheat, sunflower, waterhemp or amaranth, and corn or giant foxtail) were grown on Western Illinois University's Kerr Agronomy Farm and in the greenhouse. In the field study, plants were treated when they were approxiately 30 cm in height, and in the greenhouse plants were approximately 12 cm tall.  Treatment solutions were applied at 93 l ha^-1. Visual evaluations on a scale of 0 (no effect) to 100 (plant death) were made 7-14 days after treatment.  

Waterhemp (Amaranthus tuberculatus) is one of the most important weeds in the Corn Belt of North America. More than 55 cases of herbicide resistance are known for waterhemp (Canada 3; USA 52). Only two cases of waterhemp resistance to auxinic herbicides are recorded in www.weedsceince.org: Nebraska (2009), with multiple resistance to three sites of action (including auxins); and Illinois (2016) with multiple resistance to five sites of action (including auxins). Tested auxins were 2,4-D in IL and NE, and aminopyralid in NE. Dicamba was not tested.

Transgenic crop technologies for resistance to auxinic herbicides (2,4-D or dicamba) were developed recently and are expected to be deployed broadly and intensively across the Corn Belt. Evolution of resistance to these herbicides is expected, but at what rates and from what starting points? To help answer these questions we determined background levels of tolerance of waterhemp populations originating from seed accessions collected prior to the widespread adoption of Xtend, Enlist, and similar technologies in the Corn Belt.  

Waterhemp seeds were collected from 25 fields from Indiana, Iowa, Kentucky, Michigan, Minnesota, Missouri, Nebraska, North Dakota, Ohio, Wisconsin, and Texas. The seeds were sown in hydrated peat pots and incubated at 20º/30ºC (12/12h). Once seedlings emerged, pots were placed in a greenhouse and thinned to one plant per pot. When plants 3-15 cm tall, heights were recorded and groups of ten plants were treated with one of seven rates of 2,4-D or dicamba in a cabinet sprayer. Rates were 0X, 0.1X, 0.2X, 0.5X, 1X, 2X, and 5X. The 1X rates were equivalent to 500 g/ha of 2,4-D and 560 g/ha of dicamba.

At 7 DAT a visual injury score (0-10), height, and aboveground fresh weight was recorded for each seedling. Variables were analyzed by ANOVA and regression using Infostat, Statistix, and R programs. Dose response curves were generated and LD50s calculated. No population appeared tolerant to either herbicide. However, at lower doses, 2,4-D was typically more damaging to waterhemp than dicamba. These results suggest that prior to widespread adoption of Enlist and Xtend technologies, no geographic trends in baseline tolerances were apparent. Thus, waterhemp populations from across the Corn Belt appear generally and equally sensitive to 2,4-D and dicamba at a range of application rates.

Goosegrass (Eleusine indica) is a common and troublesome weed in agronomic crops and turfgrass throughout the world.  It is an annual species that can perenniate in tropical environments and is considered one of the top ten weeds worldwide.  Little is understood regarding the phenotypic diversity of goosegrass.  A garden plot experiment was conducted in 2017 to evaluate 11 goosegrass ecotypes collected from managed turfgrass, agronomic crop, and non-crop areas.  Ecotype names were abbreviated and hyphenated with either NC (non-crop), GC (golf course), GCPG (golf course putting green), or Crop (agronomic row-crop) to distinguish the ecotypes from the type of area from which they were collected.  Ecotypes were originally collected between 2012 and 2014 with seed increases occurring in years following.  Various morphological measurements were taken including but not limited to length of stem, spike, and leaf parts, and seedhead, tiller, and spikelet numbers.  Data were analyzed using principle components analysis using XLSTAT.  Two distinct groups could be separated based on principle components analysis.  The first group was comprised of ecotypes collected from crop-types areas – TN River, Wire Road, and PBU.  A second group was comprised of ecotypes collected from turfgrass/golf course areas – Clanton, NC-WT, RB, CCV, Craft, and Woodward.  These ecotypes were collected from areas such as golf course fairways and rough that received mowing 1-3 times per week.  Aug-NC is a population collected from a gravel parking lot in Augusta, GA, (thus labeled NC for non-crop).  Aug-NC was intermediate between turfgrass/golf course types and crop types.  Texas-GCPG was completely separate from all other types.  Texas-GCPG is a dwarf mutant goosegrass ecotypes collected from a golf course putting green.  Texas-GCPG produced twice as many seedheads and tillers per plant as all other plant and did not exceed 25 cm length from base to spike.  Overall goosegrass collected from golf courses had shorter stem lengths, shorter leaf lengths, shorter length between leaves, more prostrate growth habit, and more tillers per plant compared to ecotypes collected from crop areas.  It was hypothesized that goosegrass ecotypes collected from golf courses and undergo regular turfgrass management practices such as low, frequent mowing are plants that simply acclimate to such management practices and with the removal of such management practices these ecotypes would return to a “normal” ecotype present in non-mowed crop collected ecotypes.  From this research we conclude that ecotypes collected from managed turfgrass environments possess a distinct separate phenotype that likely has been selected for through these intensive management practices.  Ecotypes do not revert back to the normal wild-type phenotype as possessed by ecotypes collected from crop environments.  Rather, the ecotypes maintain a different phenotype best suited for environment from which they were selected. 

The introduced meadow knapweed (Centaurea x moncktonii), a hybrid of black (C. nigra) and brown (C. jacea) knapweeds, appears to be common and expanding in New York agricultural lands, including pastures, meadows and waste areas. The biology and ecology of the hybrid is mostly unstudied, such as its germination characteristics which may influence the seasonal timing of seedling emergence. We evaluated the effect of temperature, light, seed stratification, scarification, and population on percentage germination in four experiments over two years. Temperature and light were factors in all experiments. Mature capitula were collected from four populations in August. Seeds were separated from dried capitula with a rubbing board (scarified) or gently removed by hand (non-scarified) and stored dry at 4ºC for 4-28 weeks prior to an experiment. Lots of 50 seeds were placed in sealed petri dishes with moist blotting paper and germinated immediately (non-stratified) or, for one experiment, held for 4 weeks at 4ºC before the test (cold-wet stratified). Dishes were uncovered (light) or covered with aluminum foil or placed in black bags (dark). Temperature treatments were 15:5ºC (day:night), 20:10ºC, 25:15ºC, or 30:20ºC using a 14:10 hr (day:night) photoperiod. Groups of 5 dishes per treatment were checked for germination over 4 weeks; dark-treated seeds were checked for germination in a dark room with a dim green light. Percentage germination was mostly similar among meadow knapweed populations for different treatments of temperature, light and stratification. Scarification did not increase percentage germination. Cold-wet stratification appeared to increase germination rates by 4-57% relative to non-stratified seeds, particularly at lower temperatures in either light treatment, but it was not necessary for germination to occur. Long durations (e.g., 28 weeks) of cold-dry storage also appeared to promote germination. Light significantly increased germination rates in the three lower temperature treatments by 15-86% compared to dark-treated seeds. Germination was most rapid at warmer temperatures (25:15 and 30:20ºC) and was 87-97% with light. Seeds in the dark treatment at the lowest temperature (15:5ºC) generally had the lowest germination (3-21%). Warm temperatures, light, and stratification stimulate germination in meadow knapweed.

A study was conducted to assess how crop phases affect weed diversity over a full rotation period in a bahiagrass–bahiagrass–peanut–cotton rotation. The results demonstrated recurrent fluctuation in weed seed banks with bahiagrass crop phases promoting increases and peanut and cotton phases decreases in weed density and diversity. Peanut and cotton yields were not negatively impacted by the transient increases in weed populations during the bahiagrass phases. Incorporation of grazing in the system had limited effects on the general weed seed bank. However, grazing benefited overall weed management by reducing Palmer amaranth (Amaranthus palmeri) seed banks, which is among the most troublesome weed species in the Southeastern USA. The results of the present study support the idea that proper crop phase selection and sequence in crop-livestock systems allow for both temporary increases in weed diversity as well as maintaining yield and weed management in cash crop phases.

Junglerice (Echinochloa colona) is one of the most important summer crop weed in Argentina and cause large yield losses. Due to wide emergency period, which can occur from September to January, foliar-applied herbicides would be an economically and environmentally unsustainable practice. To rationalize the use of herbicides, an experiment was conducted in Pergamino (34° S, 60° W, Argentina) to evaluate the timing application of s-metolaclor plus byciclopyrone around the seedling emergence. Herbicides were applied at -130 GDD, 0 GDD, 130 GDD and 260 GDD respect to the weed emergence moment under field conditions. The seedlings reduction respect to control was greater than 90% in all treatments, but when applied at 130 GDD, the seedling emergency-free period was 100 days, while the two previous applications achieved a weed-free period of 61-64 days. The last application of herbicide (although it significantly reduced the number of seedlings), did not control those individuals present due to a more advanced development.

Weeds were adapted to different environments, causing possible changes in the vegetative and reproductive structures of the plant. An experiment was conducted in field conditions with three densities of junglerice (0.5, 2 and 250 pl.m-2) growing with natural weed infestation of Digitaria sanguinalis, Chenopodium album and Bidens subalternans). Junglerice plants that grew in isolated plant conditions (0.5 pl.m^-2) showed a prostrate growth habit, an average diameter between 173-220 cm, a height of 50 cm, 196 g.plant^-1 of aboveground dry matter (ADM) and 25-50000 seeds.plant^-1. On the other hand, in the treatment of 2 pl.m^-2 (low density) a semi-prostrate growth habit was observed, an average diameter between 113-142 cm, a height less than 50 cm, 25.7 g.plant^-1 of ADM and 2300-7800 seeds.plant^-1. Finally, in high density an erect growth habit was observed, with an average diameter of 15 cm, a height of around 86 cm, with an ADM less than 7 g.plant^-1 and a production of 650 seeds.plant^-1. The results obtained reveal the important phenotypic plasticity of the weed.

Camellia oleifera Abel is a major kind of woody oil plant in the southern part of China. At present, only single herbicide formulation such as glyphosate or glufosinate ammonium is applied for weeds control in C. oleifera forest, whereas most regularly adopted are still weeds pulling and soil digging especially when the forest is during its young stage under three years, which are time-consuming and uneconomic. In this paper, the weeds species and occurring regularities of C. oleifera forest in Hunan were surveyed, and ten tests of herbicides mixed in barrels by using glyphosate, glufosinate ammonium, fomesafen, or quizalofop-p-ethyl were carried out in  C. oleifera forest with an area of 10 m² each. The results indicated that there were 80 weeds species belonging to 31 families in C. oleifera forest in Hunan, China. Among them, there were 13, 9, 6, and 5 species in the Asteraceae, Gramineae, Rosaceae, and Leguminosae families, respectively. The main weeds communities in C. oleifera forest belonged to the Gramineae family. The optimal herbicide formulation mixed in barrels for weed control in C. oleifera forest was the mixture of glyphosate and quizalofop-p-ethyl, and no lateral buds of weeds were observed at 30 day after spraying the mixture. These tests could provide an efficient, economical, and practical way to control the weeds in C. oleifera forest. *Corresponding author, Y. Hu, e-mail: huyhongwangyi@163.com

Cover crops offer many benefits including weed suppression.  Growers seeking to optimize cover crop management for weed management may face trade-offs as greater residue produced to suppress weeds may also interfere with crop establishment and growth.  Conversely, growers who manage cover crops optimally for cash crop planting (that is, less cover crop residue to manage) may reduce their weed management potential.  There is interest in the Mid-South region and beyond in utilizing cover crops as forage or for grazing.  Removing aboveground cover crop biomass for these purposes may provide additional revenue, but then little weed-suppressing residue remains in the field.  

A field experiment was used to study how cover crop utilization affects weed management potential in strip-tilled tobacco.  Main plot treatments were combinations of two cover crop mixtures (wheat plus crimson clover and rye plus crimson clover) and planting date (early tobacco variety, with earlier cover crop planting date as well, and late planted tobacco with later cover crop planting).  Five cover crop utilization treatments were examined as subplot treatments—cover crop termination time (early and late relative to tobacco planting; residue left in situ), residue cut for forage and removed from the field, rotational grazing, and mob grazing.  Sulfentrazone plus carfentrazone-ethyl was applied to all plots PRE, with hand-weeding and a POST application of sethoxydim as well.  Weeds were counted in all plots prior to hand-weeding; residue remaining on the soil surface was also sampled.  Weed density and biomass at tobacco harvest was also determined.  Because the field was strip-tilled, all weed measurements were conducted both in and between the tobacco rows. 

Between row, there were more weeds following the wheat mixture than the rye mixture, likely due to increased biomass production in the rye mixture across all utilization treatments and planting dates.  The early planting date generally had more weeds than the later planting date; this occurred in all utilization treatments except the forage removal and the late termination time.  Killing the cover crop later rather than earlier, but leaving all residue in the field, resulted in lower weed density for the early planted crops, but not the later planted ones.  Removing residue by cutting or grazing also resulted in changes in weed density.  For both early and late planted crops, weed density was similar following the rotational and mob grazing.  Cutting for forage, however, reduced weed density in the early planted crops but not in the late.  In the crop rows, the forage removal resulted in the lowest weed density, while late cover crop termination resulted in the highest weed density.  This, combined with personal observations, indicates that increased biomass remaining on the surface following late termination may have interfered with the strip-tillage operation, perhaps resulting in reduced early-season tobacco growth.  Despite differences in mid-season weed density, weed biomass at harvest, following a hand-removal pass and an application of graminicide, was not affected by the cover crop utilization treatment.  Weed biomass following the rye mixture was greater in the early planted crops than following the late planted crops.  These results demonstrate how cover crop utilization can influence their weed management potential.

Burial depth and burial duration are two of the most important factors that determine Palmer amaranth and waterhemp seed viability, hence seedbank persistence. Knowing how these parameters interact efficient long-term management programs can be implemented. This is of major importance due to great seed production potential and consequently the high seedbank replenishment potential of both species. Seedlots of Palmer amaranth and waterhemp were collected from five different locations across the United States and were placed at seven sites with different soil and climatic conditions. Seeds exposed to two burial treatments (i.e. remained on the soil surface or were buried to a 15-cm depth) for three years. Each year seeds were retrieved and seed viability was assessed by exposing them to tetrazolium test. Higher seed damage was found for seeds exposed at Illinois (51.3% and 51.8%) conditions followed by Tennessee (40.5% and 45.1%) and Missouri (39.2% and 42% for Palmer amaranth and waterhemp, respectively) most probably due to higher volumetric water content at these locations. Burial depth affected significantly (α=0.001) the percentage seed damage of both species. Rates of seed demise were inversely proportional to burial depth whereas the percentage of viable seeds recovered after 36 months on the soil surface ranged from 4.1 to 4.3% compared to 5 to 5.3% of these at the 15-cm depth for Palmer amaranth and waterhemp, respectively. Viability of intact Palmer amaranth and waterhemp seeds remained on the soil surface was lower for the entire duration of the study compared to that recorded for the buried seeds.

Sagittaria pygmaea is a perennial weed in continuous cropping rice field of central Hunan province in China, where has the farmland with porous soil, slightly acidic pH, high organic content and long-term wetness. This weed has character of strong shade tolerance, and the plants of the weed still develop themselves in the environment that the plants of rice grow up to form the shelter. They  would disarrange ecological environment of rices in the field, and they would provide the breeding ground for pests or microorganism surviving. It is resulted that the rice has production decline and quality depression.

    With field investigation, plants of S. pygmaea have rhizomes, and the top of rhizomes and roots expands to corms. Stems and roots underground have a lot of fibrous roots. Their Leaves grow at the base of plant, and they show the shape of linear lanceolate. They have uni-sexual flowers with stalks upright and petals in whorls. Their achene fruits show the shape of broad ovate, and have wings with irregular hackle. The optimum temperature of seed germination ranges from 19 to 30 centigrade. About 15 days after sprouting, the top of rhizome come to being corms.  Formation and growth of the rhizomes range from June to August, or to the latter part of October partly. And the growth period of rhizomes is up to 50 days or more per year. The plants of S. Pygmaea have their generation in both sexual reproduction way and vegetative reproduction way. The achene with wings can spread with the flow of water for long distance transmission. Their corms distribute underground with a period of dormancy. They have good tolerance to drought, light and cold. And the corms can survive for five years or more in the farmland. Generally, the plants of S. pygmaea have great adaptability to the environment. In transplanting rice fields, the S. Pygmaea plants burgeon from 7 days to 10 days after transplanting rice seedlings, and their culminating period of germination is during 15 days -25 days after transplanting. In throwing rice fields, the S. Pygmaea plants burgeon from 5 to 7 days after throwing rice seedlings, and their culminating period of germination is during 10 days -20 days after throwing.

In our research, it shows that some agricultural measures can be used to control the S. Pygmaea plants for destroying their growth environment. Such as altering dry land crops and rices, altering dry and wet in the fields, and ploughing and basking soil. And the weed seeds in the fields can be reduced by refloatation before seedlings sowing or transplanting or throwing. However, those agricultural control measures are time-consuming and laborious. The results show that some chemical measures can achieve better control effect. After drying up the water in the fields, the S. Pygmaea plants can be controlled with spraying  the pesticides of cinosulfuron, bispyribac-sodium or penoxsulam at the early growth stage of the weed in a sunny day.

Common ragweed (Ambrosia artemisiifolia L.) is frequently observed in Québec carrot fields. Carrot growers essentially rely on linuron, a photosystem II inhibitor, to control this broadleaf weed. A linuron resistant biotype had been identified but its prevalence was unknown and the genetic basis of resistance was not established. Consequently, a survey was conducted and plants suspected to be resistant were collected in 2012 and 2013. Progeny from these plants were sprayed with a diagnostic rate of linuron and the psbA gene was partially sequenced to test for the presence of target site. Common ragweed was the most reported species (95% of accounts) and 94% of populations were diagnosed as resistant. A new target site mutation (Val₂₁₉Ile) was found in 37.5% of resistant populations tested. No mutations in the psbA gene, known to confer resistance to linuron, were found in the other resistant populations. Except for two populations, target site resistant plants were located in the muck soil production area while those diagnosed as non-target site resistant were found in sandy fields located in a different area. To our knowledge, this is the first report of a Val₂₁₉Ile mutation in the psbA gene of common ragweed and of evolved non-target site resistance to linuron.

Bird rape mustard (Brassica rapa) is a weed reported in Canada in the province of Quebec since 1908. It is a close relative of the most widely grown species of canola, B. napus. Almost all of the canola grown in Quebec is transgenic (glyphosate or glufosinate resistant). Crop-weed hybrids have been observed in 2001 in locations where the crop is grown and the weed was present. In 2005, the introgression of the transgene that confers glyphosate resistance was detected in a B. rapa plant located in one of the hybrid swarms identified in 2001 (near St-Henri, QC). In 2015, unidentified mustard plants growing in a glyphosate resistant corn field located less than 130 km southeast of St-Henri (near Victoriaville), survived a glyphosate application. The field history was recorded and seeds from this population were grown in the greenhouse. Seedlings were sprayed with 900 and 1800 g ae ha^-1 of glyphosate. Plants were also grown to maturity to identify the mustard at the species level using morphological characteristics and generate leaf and seed material. Leaf samples were tested for the presence of the transgene and species-specific markers while seeds were tested for fatty acid and glucosinolate content. The mustard plants survived the application of glyphosate, the transgenic construct was present, the seeds had 15% erucic acid and 110 micromoles g^-1 of glucosinolates and plants were identified as Brassica rapa using both mophological traits and genetic markers. In 2017, leaf and seed material from unidentified mustard plants growing in two other fields sprayed with glyphosate (RR corn and soybean) located in the same area (near Victoriaville) were also diagnosed as Brassica rapa weeds with the transgenic construct. 

Hydrothermal time (HTT) models are widely used to predict the germination and emergence responses of seeds to temperature and water. Although these models have shown to be very useful for describing germination at sub-optimal temperatures, they have limited adequacy to account for thermoinhibition of germination often observed at supra-optimal temperatures. Here, we introduce a new HTT model that not only provides new insights into the potential cause for the thermoinhibition phenomenon but also gives adequate fits to germination data over a wide range of temperatures encompassing both sub- and supra-optimal ranges. We also provide explicit mathematical solutions for estimating optimal temperature(s) and other cardinal temperatures within a HTT modelling framework. Using the germination data from two winter annual weeds species (Hordeum spontaneum and Phalaris minor), our model also shows that the optimal temperature is not a fixed parameter but rather changes with water availability and varies among seed sub-populations. That is, the optimal temperature decreased proportionally with decreasing water potential and became cooler for higher (slower) germination fractions than for the lower (faster) ones. Base water potential (the water potential at or below which germination stops) also increased linearly with temperature across both sub- and supra-optimal ranges. The new modelling approach shed light on the adaptive strategies evolved by plant species to optimize their germination timing under the various temperature and moisture environments.

Weeds are one of the most significant, and controllable, threats to crop production in North America. Crop losses in yield and quality due to weed interference, as well as costs of controlling weeds, have a significant economic impact on crop production. Previous WSSA Weed Loss Committee reports, as chaired by Chandler (1984) and Bridges (1992), provided snapshots of the comparative losses due to weeds across geographic regions and crops within these regions. This report summarizes grain sorghum [Sorghum bicolor (L.)] yield losses due to weeds. Yield loss estimates were determined from comparative observations of grain sorghum yields between the weedy control and plots with greater than 95% weed control in studies conducted from 2007 to 2015. Data were gathered from weed control reports and from researchers in Texas, Arkansas, Kansas, Nebraska, and South Dakota. Across the nine years, at least 10 individual comparisons for each state were documented, were averaged within a year, and averaged over the nine years. These percent yield loss values were used to determine total grain sorghum yield loss in bu/ac based on average grain sorghum yields for each state as well as current commodity prices for a given year as summarized by USDA-NASS. Annual yield losses were 60.3% in Texas, 39.5% in Arkansas, 32.8% in Kansas, 56.2% in Nebraska, and 50.1% in South Dakota. Averaged across 2007 to 2015, weed interference in grain sorghum caused 47.8% yield loss. For example, in 2015 sorghum for grain in the US was harvested from 7.85 million acres with an average yield of 76.0 bu/ac for a total production of 597 million bushels. Using an average grain sorghum price across 2007 to 2015 of US $7.83/cwt, farm gate value would be reduced by US $1,254 million annually if no weed management tactics were employed.

Weedy red rice is one of the most important weeds of cultivated rice. It not only contributes to significant yield reduction in rice production but most importantly, decreases the quality of rice. In this study, the morphological and physiological traits of weedy red rice were investigated to understand the nature of weedy red rice existing in California. A total of 103 weedy red rice collected in 2006 and 23 weed red rice collected in 2016 were grown in the greenhouse along with twenty-three cultivated California rice varieties. For morphological traits, we examined the leaf sheath, ligule and auricle colors, the pubescence of leaves, the presence of awn, plant height, grain type, hull and pericarp colors. For physiological traits, we evaluated the leaf color intensity or chlorophyll content, heading date, seed shattering and seed dormancy. In a separate study, DNA profiling indicated the presence of five genetically distinct ecotypes of weedy red in California. Analysis of variance indicated significant differences among weedy red rice ecotypes and cultivated rice varieties in plant height, leaf color intensity, days to heading, seed shattering and seed dormancy. In general, all weedy red rice ecotypes have pubescent leaves, lighter green leaves, red pericarp and are taller than cultivated rice varieties. Moderate to high level of seed shattering and dormancy were retained in all ecotypes while days-to-heading of weedy rice ecotypes ranged from earlier, at the same time, and later than cultivated rice varieties. Multivariate analysis of morphological and physiological traits indicated that CA weedy red rice ecotypes were phenotypically distinct from CA rice varieties. The results of these characterization will aid in early detection of weedy red rice in the field.

Weeds are a primary cause of yield loss in American crops. Recent research in weed emergence makes it possible to use weather data to predict what percentage of problem species have emerged so far, and improved weather modeling allow farm-scale weather data. Such models have been created for the Midwestern US, northern Italy, and other regions, but one for the Northeast has not yet been developed. In 2016 and 2017, we collected weed seedling emergence data at two sites in upstate New York to inform such a model, and used data from Myers et al. (2004) to develop a preliminary weed emergence model.  The first site is an experimental field crop farm on Honeyoe/Lima silt loams; the second is an experimental organic vegetable farm on Howard/Phelps gravelly loams. Drought and heavy moisture impacted the two sites differently, and weed emergence timing was affected. Primary weeds in the Honeyoe silt loam were Setaria spp. and Chenopodium album in 2016 and Plantago major and Veronica peregrina in 2017; at the Howard site Galinsoga quadriradiata and Capsella bursa-pastoris dominated in 2016 and Amaranthus powellii and Chenopodium album in 2017. We propose a weed emergence model to help farmers manage their problem weeds, improving, reducing costs, and minimizing environmental impact.  The first draft model incorporates temperature at 5 cm soil depth to predict seedling emergence.  We hope to continue the project by collecting till and no-till seedling emergence across the Northeast over several years to capture more of the range of possible interactions between temperature, precipitation and soil type.  We hope to develop a model that will help farmers and extension educators predict when problem weeds are emerging and better target control efforts.

One of the key features of the life cycles of obligate parasites of the Orobanchaceae is the ability of their seeds to germinate specifically in response to a host-derived germination stimulant. The best-characterized germination stimulants of Orobanchaceae seeds are strigolactones (SLs), and recent work implicates that SLs are perceived by members of the KARRIKIN-INSENSITIVE2 (KAI2) gene family. This work suggests that within parasitic Orobanchaceae the KAI2 genes have undergone duplication and specialization, and these divergent KAI2 (KAI2d) genes are thought to be responsible for detecting SL germination stimulants. We are using genetic and genomic approaches to determine whether and how KAI2 genes control the germination specificity in Orobanche species. To this end, we are characterizing two closely related species, Orobanche cumana and Orobanche cernua, which differ in host preference and germination stimulant specificity. Whereas O. cernua parasitizes tomato and responds to the SL orobanchol, O. cumana parasitizes sunflower and germinates in response to dehydrocostus lactone (DCL). We have sequenced all KAI2 genes from each species and found that they differ in quantity and sequence. Orobanche cernua contains four KAI2d genes (OceKAI2d1-4), while O. cumana contains six genes (OrcuKAI2d1-6). Crosses of O. cumana and O. cernua produced hybrids that segregate for stimulant specificity, creating a tractable genetic system. DNA from 106 F₃ hybrids was used in a sequence capture method to identify all the KAI2 genes in each hybrid. Results indicate that most of the KAI2d genes are linked, and does not identify a single gene as being responsible for conferring specificity to orobanchol or DCL. We are currently using a cross-species complementation assay, whereby Orobanche KAI2d genes were inserted into an Arabidopsis kai2 mutant background, to test the ability of specific KAI2d genes to recover the mutant phenotype when exposed to strigolactones (including orobanchol, 5-deoxystrigol and GR24) and DCL. To date, we have identified one O. cernua KAI2d gene, OrceKAI2d2, as being responsive to strigolactones. Interestingly, the homolog of this gene in O. cumana, OrcuKAI2d5, is also responsive to strigolactones. Taken together, we hypothesize that gene expression and non-KAI2 genes may be involved in regulating stimulant specificity. To look beyond KAI2d for genes functioning in stimulant specificity, we are conducting a genotyping-by-sequencing analysis on the F₃ hybrids to identify markers that can be associated with the stimulant specificity phenotype.

Parasitic plants have evolved to live as plant pathogens, yet relatively little is known about how they interact with the host plant immune pathways as compared to other classes of pathogens (e.g., bacteria, fungi, etc.), which have received substantially more attention. We set out to leverage the tools and paradigms of molecular plant pathology to investigate how parasitic plants overcome host plant immune responses. One major effort was to screen 47 Arabidopsis mutants defective in known aspects of immune responses against other classes of plant pathogens for altered resistance to virulent P. aegyptiaca. Perturbations in jasmonic acid (JA) and salicylic acid (SA) signaling result in lower susceptibility to P. aegyptiaca infection. Mutant plants with knocked out Della genes, which are thought to control the balance of JA and SA, were also less susceptible to P. aegyptiaca parasitization. In a separate experiment, in which host plants were exposed to SA with or without parasitism, the presence of P. aegyptiaca was associated with suppressed expression of PR1. Taken together, these results suggest that the parasite actively relies on manipulation of SA and JA pathways for parasitization. The second aspect of this project was to consider the possibility of immunity-suppressing effector proteins, which most classes of plant pathogens secrete in order to overcome host immune responses. We cloned over 30 candidate secreted proteins from the P. aegyptiaca transcriptome, and screened them for the ability to suppress plant immune responses when transiently expressed in N. benthamiana leaves. None of the candidate secreted proteins were able to suppress cell death, but five cloned proteins suppressed a pattern-triggered immunity response. These proteins are top candidates for being considered effector proteins in P. aegyptiaca. The two most intriguing effector candidates are a 217 amino acid protein that contains three LRR domains, and a 523 amino acid putative cannabidiol acid synthetase that is also able to improve the growth of a non-pathogenic bacterial strain. We are working to confirm secretion of these proteins from P. aegyptiaca into host roots and their immunity suppressing function to validate them as true effector proteins. Our goal is to understand parasite manipulation of host defense processes in order to make host plants more resistant to parasitization by P. aegyptiaca and related parasitic weeds.  

Field surveys were conducted across the Blacklands region of Texas during 2016-2017 to identify and characterize different species of ryegrass (Lolium spp.) infesting winter wheat production fields in the region. A total of 55 populations were grown in the greenhouse for morphological characterization and evaluating the level of resistance to one ALS-inhibitor herbicide [mesosulfuron-methyl (Osprey^®)], two ACCase-inhibitors [diclofop-methyl (Hoelon^®) and pinoxaden (Axial XL^®)], and glyphosate. Evaluations were conducted based on the recommended field rates (1X) of 14.5, 375, 59, 870 g ai/ae ha^-1 for mesosulfuron-methyl, diclofop-methyl, pinoxaden, and glyphosate respectively. Initial screening was conducted with a 2X rate of each herbicide, followed by dose-response assays with eight rates (0.5, 1, 2, 4, 8, 16, 32, and 64X) for the resistant population and six rates (0.0625, 0.125, 0.25, 0.5, 1, and 2X) for the susceptible population. The experiment was conducted in a completely randomized design with 3 replications (5 plants per replication), with two experimental runs. Species characterization has indicated that 90% of the surveyed populations were a complex of Italian ryegrass inter-mixed with perennial types (considered as L. perenne ssp. multiflorum), 5% were rigid ryegrass, (L. rigidum Gaudin), and the rest 5% were poison ryegrass (L. temulentum L.). Initial herbicide screening showed that 98, 96 and 31% of the populations had survivors to diclofop-methyl, mesosulfuron-methyl and pinoxaden, respectively. Only 2 populations were found resistant to glyphosate. Results of subsequent dose-response assays revealed high survival rate of the resistant populations for up to 64X (the maximum dose used) for mesosulfuron-methyl and diclofop, and up to 4X for pinoxaden. Compared to the susceptible standard, 40-, 200- and 23- fold resistance was observed in the resistant population for mesosulfuron-methyl, diclofop and pinoxaden, respectively. Multiple resistance in ryegrass to ALS- and ACCase-inhibitor herbicides severely limits the herbicide options available for effective ryegrass control and is a serious threat to wheat production in the study region. Future research will focus on understanding the physiological and genetic mechanisms conferring herbicide resistance in these populations. 

Callisia repens (creeping inchplant) is a plant of the Commelinacea family, common in low lands of Costa Rica where bananas and other perennial crops are planted. Its growth habit and asexual reproduction make it suitable as a native living mulch for weed management in tropical perennial plantations. The objective of this study was to evaluate the potential of C. repens as a living mulch for perennial plantations. Four experiments were carried out in a greenhouse at the University of Costa Rica, during 2017. Cuttings of C. repens were planted in 3 liter pots and placed in tunnels simulating 0%, 30%, 50%, and 90% shade, mimicking the growth of a perennial plantation over time. Fresh weight of roots and aerial parts was determined at three different growth stages. The second experiment evaluated percent cover of the soil by C. repens in the above mentioned shade tunnels over a period of 80 days after planting, using the software Image J. To study the susceptibility of C. repens to the nematode Radopholus similis, a common pest in banana plantations, two populations of the nematode were inoculated into pots at two different growth stages of the plant, and the reproductive factor for each population was estimated 50 days after inoculation. Finally, the tolerance of C. repens to several postemergence herbicides was also evaluated. Root growth of C. repens was greatest at 0% shade and decreased as shade increased. There were no differences in aerial growth at 0%, 30%, and 50% shade, but it was lowest at 90% shade. C. repens percent cover of the soil was similar in all the shade conditions. Initial R. similis population decreased more than 90%, but the reproductive factor was 0.089 and 0.074 for each of the populations inoculated. Callisia repens was tolerant only to graminicide herbicides. These results show that C. repens has the potential to grow and effectively cover the soil in multiple shaded and non-shaded environments, but its susceptibility to the nematode R. similis needs further exploration.

A substantial damage is imposed on food and fodder plants by mass incident of the parasitic weed plants, Orobanche spp. Although a considerable number of herbicides have been tested as means for controlling broomrape, none of them had an effective control measure. In addition, high cost and toxicity of chemical herbicides also limit their application. Indigenous, weed-specific fungal pathogens can be used as safe and effective bioherbicides. As an alternative or adjunct to conventional weed control technology of chemical and mechanical controls, the bioherbicides offer excellent means of ecologically sound weed management. There is now a unique opportunity to develop fungal pathogens of Orobanche crenata as bioherbicides in legumes. One strain of Fusarium oxysporum in addition to one strain of F. semitectum have been shown to be effective bioherbicide candidates for O. crenata. Results from laboratory and greenhouse trials have confirmed the high feasibility of using these fungal strains to control broomrape of faba bean (Vicia faba). In an attempt to develop these bioherbicides, various formulations of the fungus and its impact on broomrape control were determined. Pesta-pelletized mycoherbicides were mass produced in a production line consisted of a mixer (for fungus-infested dough preparation), an extruder (to form the dough bearing the biocontrol fungus into granular form), and a static dryer (to dehydrate the granular bioherbiicides into the final product). Pesta-pelletized mycoherbicides were evaluated against O. crenata emergence under open field conditions. The number of emerged Orobanche shoots, Orobanche shoots height, and Orobanche shoots dry weight were significantly decreased with increasing the pesta dose of the two Fusarium isolates as compared with the control treatment. There was a positive response of growth parameters (shoot height, dry matter content and leaf area index) of host plants (faba bean) to treatments with the Pesta-pelletized mycoherbicides in comparison with the control treatment. This positive effect of Pesta on growth parameters of the faba bean significantly increased with increasing the Pesta doses.

Digitaria ischaemum (Schreb) Schreb ex Muhl Schreb. and Poa annua L. are competitive early successional species usually considered weeds in agricultural and turfgrass systems. Bacteria and fungi associated with these weeds may contribute to their competitiveness. D. ischaemum and P. annua are annuals that reproduce exclusively through seed. These seeds may be a mechanism to vector important microbes. We hypothesized that bacteria associated with D. ischaemum and P. annua seeds would affect seedling growth and antagonize competitor forbs such as Taraxacum officinale, Trifolium repens. Bacteria and fungi associated with seeds of D. ischaemum and P. annua were isolated for study in axenic culture. Twenty-four bacterial strains and two fungal species were isolated. Twenty-four bacterial strains were inoculated onto T. officinale seeds. Ten strains were antagonistic to T. officinale seedling growth and four of those were antagonistic enough to cause significant seedling mortality. All four bacterial strains that increased T. officinale mortality were isolated from D. ischaemum seed while none of the 14 isolates from P. annua seed increased mortality. Two of the four bacterial isolates (characterized as Pantoea spp.) were evaluated further on D. ischaemum, T. repens (a competitor forb) and P. annua (a competitor grass) alone and in combination with a Curvularia sp. fungus also isolated from D. ischaemum seed. These bacteria caused >65% T. repens seedling mortality but did not affect P. annua seedling mortality. Effects on D. ischaemum seedling mortality were inconsistent. Whether alone or in combination with bacteria, Curvularia was highly pathogenic to D. ischaemum and T. repens but not P. annua. This Curvularia sp. was growth promotional in P. annua, increasing the gravitropic response of P. annua roots. These experiments demonstrate that bacteria associated with D. ischaemum seeds may be antagonistic to competitor forbs. The weedy character of D. ischaemum could at least in part stem from possession of bacteria that are antagonistic to competitor species. Future research should further explore the effect of these bacteria on D. ischaemum competitiveness and elucidate the mechanism by which D. ischaemum bacteria cause mortality in competitor species.

Hemp sesbania (Sesbania exaltata) (> 30 cm tall) sprayed with hot water (45-95° C), followed by spray applications of fungal spores of Colletotrichum truncatum (1.0 x 10⁷ spores/ml) suspended either in: 0.2% Silwet L-77 surfactant (SW); unrefined corn oil/distilled water (1:1, V:V; UCO); or 0.2% SW in UCO were controlled by 80-95%, 12 DAT under greenhouse conditions.  These treatments also similar caused dry weight reductions of this weed.  Plants treated with hot water sans fungus were wilted at temperatures ≥45° C, but these treatments caused no mortality or reductions of dry weight accumulation.  Dew treatments (25° C, 12 h), combined with the treatment protocols above, had little or no effects on weed mortality or dry weight reduction.  Under field conditions, >80% control of hemp sesbania was achieved following a pre-treatment with hot water (65° C), 12-15 DAT, with concomitant dry weight reductions.  Plants in the field tests, treated with the fungus sans the hot water treatment were visually unaffected, with no plant growth reductions or plant biomass reductions recorded 15 days after treatment.  These results suggest that use hot water may be an important tool for improving the infectivity and bioherbicidal potential of some plant pathogens.

Since the beginning of the 1990s the use of herbicides has been the  
most widely used tool for Chilean orchard weed control. In general,  
herbicides have been applied continuously year after year in most  
cases without rotating between herbicides with different mechanisms of  
action (MOA). These practices have selected herbicide-resistant  
biotypes, leading in some cases to the evolution of multiple  
resistance to herbicides with different MOA such as Epilobium  
ciliatum. This paper study PS I resistance and the evolution of  
multiple resistance in E. ciliatum biotypes harvested in Chile.
Resistance studies were performed with E. ciliatum seeds directly  
harvested in a field treated with diquat / paraquat (R1), which were  
selected recurrently with pyraflufen-ethyl, 2,4-D and glufosinate for  
three generations (progeny R2 , generation F3), and seeds of plants  
never treated with synthetic herbicides (S). Studies on whole plants  
show that the S biotype was perfectly controlled at the field dose of  
all herbicides tested. The R1 biotype exhibited resistance to diquat,  
paraquat and flazasulfuron and natural tolerance to glyphosate, while  
the progeny R2 biotype also exhibited multiple resistance to  
glufosinate, 2,4-D and pyraflufen-ethyl.  In all the R1 and progeny R2  
cases the LD50 values were higher than the field doses. Physiological  
and biochemical studies concluded that resistance to diquat was due to  
a lower translocation of the herbicide in R1 compared to the S  
biotype. The resistance to flazasulfuron was  confirmed by the high  
values of I50 in the biotypes R1 and R2 with respect to biotype S of  
E. ciliatum. The similar accumulation of shikimate (µg of shikimic  
acid g-1 fresh weight) in glyphosate treated plants (1000 &#956;M) in  
the three S, R1 and progeny R2 biotypes explained the existence of a  
natural tolerance to glyphosate in this species. Multiple resistance  
to glufosinate, in the progeny biotype R2 was determined by a higher  
resistance of the GS activity. While the multiple resistance to 2,4-D  
and  pyraflufen-ethyl was due to a greater accumulation of ethylene  
and proto IX in the biotype S of E. ciliatum compared to progeny R2,  
respectively.
Taking into account all these previous results, we can conclude that  
E. ciliatum  harvested in an olive grove located in the IX region of  
Chile (Lolol) is resistant to diquat / paraquat and flazasulfuron  
inhibiting herbicides and natural tolerance to glyphosate and after  
selection with three recurrent applications with pyraflufen-ethyl,  
2,4-D and glufosinate the progeny biotype R2 showed multiple  
resistance to these herbicides. This is the first case of E. ciliatum  
in the world with multiple resistant to these herbicides.

Locoweeds (Astragalus and Oxytropis spp.) contain swainsonine (SWA), a toxic alkaloid that causes severe economic losses to livestock producers in the western U.S. Fungal endophytes (Alternaria spp. section Undifilum) are responsible for SWA synthesis in locoweeds. To investigate endophyte effects on the host plant, a common garden study initiated in 2011 near Bozeman MT measured overwinter survival, gas exchange, flower and seed production, and seed germination rates over a 5-year period in plants germinated from seeds with endophyte (E+) and without endophyte (E-) in the seed coat. The locoweed-fungal endophyte complex appeared to be physiologically asymptomatic in field-grown plants. A legacy study was therefore initiated to determine whether endophyte presence in the maternal parent produces epigenetic effects in progeny. Seeds were collected from 10 E+ and 10 E-, 1-year-old O. sericea plants in 2014 and five seedlings from each E+ and E- maternal parent were established in the common garden in Fall 2015. After two winters, there were no detectable differences in survival rates, gas exchange or fecundity between plants derived from E+ and E- maternal parents. Seedlings from two plants from each 5 E- and 5 E+ maternal families were germinated in the greenhouse and transplanted to the garden during spring 2017, but no difference in gas exchange between E+ and E- derived plants was detected after 2 months after transplant. Our results indicate that in contrast to other endophyte-plant complexes, the SWA-producing fungal endophyte in locoweeds has no physiological effect either directly on the host plant or indirectly via epigenetic embryo effects in host plant progeny.

Different amino acids substitutions in the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene have been suggested as contributing to glyphosate resistance. However, only those substitutions occurring in the conserved region of the EPSPS gene (⁹⁵LFLGNAGTAMRPL¹⁰⁷) could be responsible for this resistance. In this research, the molecular bases of glyphosate resistance in Amaranthus hybridus, collected in GR-maize fields from Cordoba, Argentina, were characterized. First, the dose-response assays confirmed the resistance of the R population, requiring between 84 and 101 times more glyphosate to reduce 50% of the dry matter (GR₅₀) or 50% of the survival plants (LD₅₀) compared to the S population. The latter showed GR₅₀ and LD₅₀ values less than 20 and 40 g acid equivalent of glyphosate ha^-1, respectively. There were no differences in the EPSPS gene copy number or expression between the R and S populations. However, a triple substitution of amino acids was found in the EPSPS gene of the R population from TAP (wild-type) to IVS (triple mutant). The nucleotide substitutions consisted of ATA¹⁰², GTC¹⁰³ and TCA¹⁰⁶ instead of ACA¹⁰², GCG¹⁰³ and CCA¹⁰⁶. The contribution to glyphosate resistance of mutations occurring at positions Thr¹⁰² and Pro¹⁰⁶ is well known, and has been described in other weed species. On the contrary, the contribution of the novel mutation at position Ala¹⁰³ to Val is unknown, but its occurrence in the conserved region of the EPSPS gene coding, suggests that it could also be contributing in the high level of glyphosate resistance observed in A. hybridus.

Ragweed parthenium (Parthenium hysterophorus L.) is a troublesome annual weed native of the Gulf of Mexico, limiting crop production. Since 2014, several P. hysterophorus populations have been identified as being resistant to glyphosate. Therefore studies of their resistant mechanisms need to be carried out. In this work, three glyphosate-resistant populations (GR1, GR2 and GR3), collected in citrus groves from Mexico, were used to study their resistance mechanisms comparing them to one susceptible population (GS). Dose-response and shikimic acid accumulation assays confirmed the glyphosate resistance of these three populations. Higher doses of up to 720 g ae ha⁻¹ (field dose) were needed to control 50% of the plants of these populations. The enzyme activity of 5-enolpyruvyl shikimate3-phosphate synthase (EPSPS) was not different in the GS, GR1, GR2 populations. The GR3 population exhibited 8.2 times greater EPSPS activity than the other ones. Additionally, this population showed a higher EPSPS basal activity and a substitution in the codon 106 from Proline to Serine in the EPSPS protein sequence. The EPSPS gene expression in the GR3 population was similar to the other populations. These results described, for the first time, the glyphosate resistance mechanisms developed by P. hysterophorus resistant populations of citrus groves found in Mexico.

Weed growth and management are highly affected by environmental conditions. Climate changes can bring great challenges to food production. Environmental conditions during and after herbicide application and the conditions in which the target plants grow, impact the efficacy of herbicides. Increase in temperature and atmospheric CO₂ concentration may affect herbicide absorption and translocation in plants. The objective of this study was to evaluate the effects of increased atmospheric CO₂ concentration and temperature on absorption, translocation and efficacy of cyhalofop-butyl in multiple-resistant (MR) and susceptible Echinochloa colona ecotypes. The MR ecotype had low-level resistance to cyhalofop. Maximum ¹⁴C-cyhalofop-butyl absorption occurred at 72 h after the herbicide treatment regardless of temperature or [CO₂]. Neither temperature nor [CO₂] affected herbicide absorption and translocation in the plants. Almost all absorbed cyhalofop was retained in the treated leaf (>98%). Visible phytotoxicity was higher among susceptible plants than resistant ones, regardless of [CO₂] concentration and temperature. Plants grown under elevated [CO₂] and high temperature were taller than those in ambient conditions. Visible phytotoxicity was lesser on MR plants grown under elevated [CO₂] and high temperature. Elevated [CO₂] and high temperature do not affect the absorption and translocation of cyhalofop-butyl. However, these conditions can increase the resistance level of Echinochloa colona ecotypes already showing low-level resistance to certain herbicides. Further, elevated [CO₂] and high temperature increase the growth of Echinochloa, indicating increased weed competition even with C4 plants like Echinochloa, which are not expected to respond significantly to these conditions.

The application of chlorophyll fluorescence technique in plant physiology research including stress conditions such as nutrient deficiencies and shade has been proven to be an invaluable tool for the quantification of plant responses to abiotic stresses. The employment of chlorophyll fluorescence revealed significant differences in Palmer amaranth gender photochemistry due to nutrient and light-stress treatments. Electron transport rate (ETR) and chlorophyll content were significantly higher for females under low and medium (i.e. 150 and 450,µmol m^-2 sec^-1) light intensities whereas a reverse trend in favor of males was observed under high (1300 µmol m^-2 sec^-1) intensities particularly under K (for chlorophyll) and P (for ETR) deficiencies. ETR is a measure of a leaf’s photosynthetic capacity while photosynthesis occurs and is estimated as a ratio of variable fluorescence (FV): maximum fluorescence (FM), which evaluates the maximum quantum yield in PS II. Significant differences were found between the parameters of ETR but also the content of chlorophyll a, chlorophyll b, and total chlorophyll for the two Palmer amaranth genders. Measurements of the nutrient content from plant leaf samples revealed different responses between Palmer amaranth genders. The application of multivariate analysis showed opposite trends between various macro- and micro-nutrients within Palmer amaranth gender. Strong correlations between Ca with N and Mn, for example, were revealed in female plants whereas the opposite was found for male plants. Further research is required for better understanding the physiology of Palmer amaranth gender and how this can be used in Palmer amaranth management.

In Arkansas, the prevalence of glyphosate, ALS-, and PPO-inhibitor resistance in Palmer amaranth (Amaranthus palmeri) is widespread and it is expected soybean growers will become increasingly dependent on auxinic herbicides in auxinic-resistant crops for its control. In the recent growing season, there were reports of reduced dicamba activity on PPO-inhibitor resistant Palmer amaranth accessions and thus we initiated a study to determine if a correlation existed between PPO-inhibitor resistance and reduced dicamba sensitivity in Palmer amaranth accessions. In total, 130 accessions were collected from Arkansas and the progeny (50 seedlings run^-1 and 2 runs) at the 4-6 leaf stage were sprayed with fomesafen at 395 g ai ha^-1 or dicamba at 560 g ae ha^-1. Mortality data was collected at 14 and 21 days after applications for fomesafen and dicamba treated plants, respectively. Fomesafen surviving plants were genotyped using a TaqMan qPCR assay for the presence or absence of target-site mutations in the PPX2 gene. Only accessions positive for a target-site mechanism were considered a resistant accession. Correlation analysis between dicamba sensitivity and PPO-inhibitor resistance or susceptibility will be presented. 

Palmer amaranth (Amaranthus palmeri S. Wats.), ranked as the most troublesome weed in North America, has evolved resistance to several modes of action herbicides. Repeated use of any single herbicide can lead to evolution of resistance, and therefore, a better understanding of the process of resistance evolution is essential for the management of Palmer amaranth and other weed species. The objective of this study was to assess the potential for low-dose mesotrione (Callisto^®) to select for reduced susceptibility over multiple generations in two different Palmer amaranth populations (one collected in 2001 and the other in 2007). F0 plants (200 each) from the 2001 population were treated with mesotrione at 0.5 and 1x rates (x = 105 g ai ha^-1) and evaluated for percent survival three weeks after treatment. All plants were controlled at the 1x rate; however, 2.5% of the plants that survived the 0.5x treatment were allowed to grow and cross-pollinate to produce F1 seed that served for the next generation of low-dose mesotrione selection. The process was continued until the F3 generation. An increase in the survival percentage (31 and 10%) was observed when F3 plants (100 each) were treated with 1.5 and 2x rates of mesotrione, respectively. The results indicate that after three rounds of recurrent mesotrione selection, the F3 generation was less susceptible to mesotrione than the initial F0 population. Similarly, the 2007 population treated with 0.5x mesotrione rate showed an increase in survival percentage in the F1 (39%) when compared to F0 (7%) generation. Dose-response studies are currently being conducted to quantify the shift in sensitivity in the 2001 population. The results have significance in light of the soon to be released HPPD-resistant soybean and cotton varieties in the marketplace.

Acetolactate synthase (ALS)-inhibiting herbicides are frequently used to control ryegrass species in winter cereals worldwide. Recently, multiple resistance to herbicides glyphosate, paraquat and sethoxydim have been confirmed in two Italian ryegrass populations (MR1, MR2) in northern California. These populations are cross-resistant to ACCase-inhibitors from two chemical families. Resistance to paraquat in MR2 plants was negligible compared to MR1. These multiple-resistant populations were also resistant to imazamox (45 g ai h^-1) in a preliminary greenhouse study. The objectives of this research were to determine the resistance level to ALS-inhibiting herbicides (imazamox and mesosulfuron-methyl) in selected Italian ryegrass biotypes with different herbicide resistance patterns and to characterize the mechanism of resistance to these ALS-inhibiting herbicides. Alternative herbicide options to control the resistant biotypes were also evaluated. Seeds of MR1 biotypes were increased by crossing glyphosate/sethoxydim (MR1-A) and glyphosate/paraquat (MR1-P)-resistant plants in separate greenhouses. Seeds of the MR2 biotype were also increased by crossing glyphosate/sethoxydim plants. Seven doses of imazamox and mesosulfuron-methyl were applied individually to 25 plants of each biotype at the 3- to 4-leaf stage. Plant mortality was recorded 21 days after treatment (DAT), and data were subjected to probit analysis using PROC PROBIT in SAS to generate dose-response curves. In a second experiment, Italian ryegrass biotypes were treated with labeled field rates of 4 ALS-inhibiting herbicides from four different chemical families [sulfonylurea (e.g. rimsulfuron), imadazolinone (e.g. imazethapyr), pyrimidinyl benzoate (e.g. bispyribac-sodium) and triazolopyrimidine (e.g. penoxsulam)] and three non-ALS-inhibiting herbicides (e.g. glufosinate, propanil, and pinoxaden). These herbicides are recommended to control ryegrass species in different cropping systems. Aboveground biomass was harvested at 21 DAT, oven dried at 60 °C for 48h and weighed. Dry weight reductions were expressed as a percentage of the mean of the non-treated controls and data were subjected to ANOVA using PROC MIXED in SAS. Means were separated using Fisher’s LSD at α=0.05. To identify mutations previously shown to endow resistance to ALS-inhibitors in different weed species, Sanger sequencing was conducted. ALS enzyme assay was used to ascertain the resistance mechanism. MR1-A, MR1-P and MR2 biotypes were 38, 29, and 8-fold, and 37, 63 and 4-fold less sensitive to imazamox and mesosulfuron-methyl, respectively, compared to Sus. Only MR1-P and MR2 plants were cross-resistant to rimsulfuron whereas both MR1 biotypes were cross-resistant to imazethapyr. Bispyribac-sodium, penoxulam and propanil were not effective on any Italian ryegrass plants and pinoxaden [ACCase-inhibitor (Phenylpyrazoline 'DEN')] only controlled MR2 plants at the labeled field rate. However, all plants were effectively controlled (> 99%) with the labeled field rate of glufosinate. The ALS enzyme from MR1-A, MR-P and MR2 plants was 712, 1104, and 3-fold and 10, 18, and 5-fold less responsive to mesosulfuron-methyl and imazamox, respectively, than the Sus plants. Alignment of ALS gene sequences of resistant plants revealed a missense SNP resulting in a Trp₅₇₄-to-Leu substitution in MR1-A and MR1-P, but not in MR2 biotypes. This amino acid substitutions is known to endow a high level of resistance to ALS-inhibiting herbicides in weed species. The results suggest an altered target site as the mechanism of resistance in MR1 plants and non-target site based resistance to both herbicides in MR2 plants. Further studies on metabolism/absorption and translocation of ALS-inhibitors will elucidate the resistance mechanism in MR2 plants.  Meanwhile, prudent use of glufosinate is needed to decrease selection pressure for the evolution of resistance to glufosinate in these multiple resistant Italian ryegrass populations in northern California.

Metribuzin is an herbicide used extensively throughout the Brazil, and can be easily leached to groundwater sources. Bonechar (BC), as well as biochar have been shown to strongly sorb organic compounds and could be used as an adsorbent to increase herbicide sorption and decrease leaching, although its application form is still little known. The aim of this research was to assess the absorbent effect of a cow BC on the mobility of metribuzin in a sandy loam tropical soil under laboratory conditions. The mobility studies were performed in soil thin-layer chromatography (TLC) (9 cm wide, 15 cm long, and 2 cm thick). Deionized water was added to the soil, which was then spread in a homogeneous layer (0.1 cm) on the surface of the TLC plate. Four treatments were performed in duplicate: unamended soil (control), BC (5% w w^-1)-amended soil, soil + BC band (1 cm) in the middle (4.5 cm from the base of application), and soil + BC band in the top (9.0 cm from the base of application). ¹⁴C-metribuzin (1 µL, 0.53 kBq) was added to the soil base with 2 cm from the edge of the TLC plate. The plates were visualized by autoradiography with X-ray film. The ¹⁴C distribution of herbicide in the soil was then determined using a Cyclone Plus Phosphor Imager, which provides the mobility coefficient or retention factor (R_f) (end of the peak area). The R_f values were 0.88 (mobile), 0.29 (low mobile), 0.42 (moderate), and 0.88 (mobile) for unamended soil, BC-amended soil, soil + BC (middle), and soil + BC (top), respectively. We conclude that the cow BC is an excellent absorbent to reduce mobility of metribuzin in the soil through the high sorption of this herbicide, regardless of application form of the material. Thus, it is possible to minimize the pollution of soil-applied herbicide simultaneously as the BC, and then herbicide can remain in the soil for a long time.

Much research has been completed investigating the persistence of atrazine in various agricultural systems and edaphic conditions. Atrazine bioavailability, however, has not been characterized despite atrazine being the second most commonly applied agricultural pesticide as of 2012 according to the USEPA. It was hypothesized bioavailability would vary inversely with soil organic matter content. In order to assess the bioavailability of atrazine, a greenhouse study was conducted using a sensitive bioindicator species, Brassica napus L. (canola), to assess the availability of the xenobiotic compound. By comparing bioindicator growth in atrazine treated and non-treated pots planted with canola, the bioavailability of atrazine within an organic matter content level was evaluated. Atrazine (1.7 kg ai ha^-1) was broadcast sprayed to unique pots at trial initiation and soil sampled (0-7.62 cm depth) as well as seeded with canola 0, 14, and 28 days after treatment. Next, above-ground biomass was harvested three weeks after each respective seeded timing. It was found that despite longer persistence at higher soil organic matter contents, sorption by the organic matter reduced atrazine bioavailability. Half-life values ranged from 49.5 to 54.0 days across soil organic matter contents. Based on the bioindicator response, when planted 14 and 28 days after treatment, bioavailability decreased with increased organic matter content, and half-live values variation was small across soil organic matter content. This study suggests atrazine is most available when applied immediately at the time of seeding and in systems with lower organic matter content. High organic matter content is beneficial for crop production, but it may inhibit the effectiveness of particular herbicides. Furthermore, soil organic matter content increased atrazine sorption, making it less bioavailable and less of an environmental threat.

Soybean are extremely susceptible to damage by exposure to synthetic auxin herbicides, such as dicamba. While much research has focused on damage resulting from drift exposure, there are no reports documenting the risk posed to soybean exposed to dicamba dissolved in irrigation water, such as may occur in recycled tailwater. The hypothesis for this research was that susceptible soybean would experience increasing rates of injury and reduced height as dicamba concentrations increase in irrigation water.  A susceptible soybean variety was grown and exposed to dicamba dissolved in irrigation water using furrow irrigation. Treatments were applied to plants at V4 only, R2 only, and at V4+R2 growth stages of soybean.  Plant response was documented as percent injury and change in height compared to controls.  Controls were irrigated with water with no added dicamba.  Results indicate significantly reduced height and increased injury in response to dicamba exposure across all treatment rates and growth stages.  Exposure to dicamba at the V4+R2 growth stages produced significantly increased injury and reduced height compared to controls (p < 0.0001), as well as to the V4 only (p < 0.0364 for height and p < 0.0001 for injury) and R2 only (p < 0.0001) applications. Results indicate dicamba dissolved in irrigation water can result in damage to soybean. Therefore, care must be taken in managing tailwater recovery systems that recycle water between dicamba-resistant and dicamba-susceptible soybean fields.  

Recent advancements have made unmanned aerial vehicles (UAVs) a valuable asset for agricultural producers and land managers to employ in monitoring crop status and the detection of pest issues. Because UAVs offer a relatively cost effective and rapidly deployable platform with which to acquire color, multispectral or hyperspectral aerial imagery, questions have been generated regarding their utility for weed detection as part of a management program employing site specific weed management (SSWM). If UAVs and their resultant aerial imagery are ever to be adopted on a wide scale for SSWM, accurate detection and discrimination between various weed species as well as an understanding of the spectral behavior of weed species is of critical importance.

In 2017, two field studies were initiated to examine the spectral behavior of Palmer amaranth (Amaranthus palmeri), sicklepod (Senna obtusifolia) and common ragweed (Ambrosia artemisiifolia) at various sizes and to identify how accurately a supervised image classification algorithm could delineate the three species from one another. Aerial multispectral imagery consisting of five wavebands corresponding to green, blue, red, near infrared and red-edge light was collected and subject to subsequent image analysis. Supervised classification accuracy ranged from 20-100% depending on the weed species and size being observed. Additionally, the spectral behavior of the three weed species was highly dependent on an interaction present between the species and the size at which it was observed across every waveband; indicating that the spectral reflectance properties of these three species is highly dependent on size and would be expected to change throughout the season as growth occurs.

Harvest weed seed control (HWSC) is an emerging set of techniques that target weed seeds that have not dispersed by the time of crop harvest. While these seeds often pass through the harvester and get spread along with field residues, HWSC destroys them by burning, crushing, or other methods. HWSC could be feasible in many agronomic cropping systems, but it has rarely been studied in double crop systems. Field experiments were initiated in double crop soybean (Glycine max (L.) Merr.) and winter wheat (Triticum aestivum L.) fields infested with Italian ryegrass (Lolium perenne L. ssp. multiflorum (Lam.) Husnot). Two weed management programs featuring HWSC techniques, windrow burning and field residue removal, were compared to two weed managment programs without HWSC, no-tillage and conventional tillage programs. Italian ryegrass populations and wheat yield were measured in the years before and after HWSC use. HWSC treatments did not manage Italian ryegrass populations or provide better wheat yield when compared to the no-tillage program. Effective herbicidal weed control in these treatments reduced Italian ryegrass populations to <1 plant m^-2, negating any need for extra weed control from HWSC. The conventional tillage program actually boosted Italian ryegrass populations and reduced wheat yield compared to other programs. Appropriate and effective implementation of HWSC in double crop production systems depends on an integrated approach that considers the demands of the whole cropping system. Further research will evaluate the many factors that impact HWSC’s value across cropping systems and work to understand the impact of repeated HWSC use on weed populations.

The objective of this study was to characterize the emergence of natural weeds during winter cover crops growing cycle (june-november) as well as during the decomposition of their residues under a soybean crop (december-march). The species used as cover crops were oats (Avena sativa), triticale (Triticosecale) and vetch (Vicia villosa). These were sown in monocultures and double mixtures (oats/triticale, oats/vetch and triticale/vetch) and triple ones (oats/triticale/vetch), with 250 pl.m^-2. A sector was left with no cover crops as a negative control. Frames of 0.25.m^-2 were established in each treatment, with three repetitions. In each experimental unit, total number and species of emerged weeds were quantified fortnightly. During the growing cycle of cover crops, oats, oats/triticale, oats/vetch and triticale/vetch showed the lowest number of emerged weeds while during the fallow a highest number of emerged weeds were observed (p<0.01). During december-march period, triticale, vetch, oats/triticale, oats/vetch and triticale/vetch showed significant differences with fallow (p<0.01). Adoption of cover crops would reduce emergence of winter and spring weeds, reducing the possible interference in soybean crop.

The weed suppressive ability is the capacity of a crop plant to reduce weed growth through a greater resources uptake. Weed suppressive ability in wheat was related to plant height, leaf width and length, foliar insertion angle and total foliar area. According to this, the objective of the present work was to determine what seedling morphological characteristics allowed to predict early suppressive capacity of wheat genotypes.

The objective of the present work was to evaluate the early vigor of commercial genotypes of wheat. The experiment was carried out in a greenhouse at the Pergamino Experimental Station of the National Institute of Agricultural Technology (33° S, 60° W, Argentina). Eight commercial varieties of wheat were used, that present a differential weed suppressive ability. Five morphological characteristics were evaluated: height, length and width of the leaf blades, foliar insertion angle, and leaf aboveground dry matter (ADM). The determinations were made 10, 20 and 30 days after the transplanting. There was significant variation in height of seedling, foliar insertion angle and length of first and second leaf, and weight of the first leaf. Only length and ADM of first leaf coincide whit previous weed suppressive capacity ranking of varieties. The results let us conclude that length and ADM of the first leaf, are the traits of early vigor that most closely correlate with wheat competitiveness against weeds at late growing stage.

At the northern fringes of the North American soybean (Glycine max (L.) Merr.) growing region lies the Canadian Prairies.  Short-season soybean variety development has enabled producers in the eastern prairies to eagerly adopt the crop for primary grain production.  During the last decade, the prairie region has seen over a six-fold increase in soybean production area.  Current production recommendations have largely been appropriated from the warmer, long-season soybean growing regions of North America.  Soybean production in these long-season areas have contributed to the selection of many herbicide-resistant (HR) weed biotypes.  As part of a responsible, integrated weed management strategy, soybean production in the prairie region must adopt good agronomic practices to reduce selection pressure for HR weeds.  Cultural weed management tools used to interfere with weeds and reduce selection pressure for HR weed biotypes include narrow row widths, high population densities, and competitive cultivars.  This study evaluated the influence of row width (19 cm vs. 76 cm), population density (0.75, 1, and 1.5 times a recommended target density) and cultivar (erect, intermediate, and bushy) on the critical weed free period (CWFP) (i.e. the duration which a crop must be kept weed free to reach maximum yield potential) of soybean grown in the northern Great Plains region.  Data from three experimental sites revealed that choosing narrow row widths or competitive cultivars can shorten the duration of the CWFP, while low population densities can lengthen the CWFP of soybean grown in the northern Great Plains region.

Weedy (red) rice re-emerged as an important pest of California rice during the 2016 rice season. Managing weedy rice in CA rice is difficult, as there are no currently-registered herbicides that can be used to control it, either for selective control of weedy rice (without injuring rice) or for spot-treatment. Five weedy rice ecotypes were identified by the end of the 2016 season. Since CA is still in the preliminary stages of researching control stratgies for this pest, the rice industry was interested to know if there are: 1) any currently-registered rice herbicides impact the growth of weedy rice; 2) any potential herbicides that may be effective as a spot-treatment; and 3) if there are differences in control amongst the 5 weedy rice ecotypes. Thus, a series of experiments was carried out starting in the summer of 2017. Preliminary evaluations of currently registered pre-emergent rice herbicides and all post-emergent granular and foliar-applied registered rice herbicides were conducted to determine if there were any that might have a negative effect on weedy rice growth. Oxyfluorfen was also included in the experiments, due to the ongoing research on Roxy® rice (a medium-grain variety resistant to oxyfluorfen). Herbicides known to cause mortality to rice were selected to test for their efficacy on weedy rice for spot-treatments.

All weedy rice ecotypes (1-5), as well as Roxy® (oxyfluorfen-resistant rice), and M-206 (a California medium-grain variety) were evaluated. For all tests, pre-germinated rice seeds were planted to a depth of approximately 1 cm in clay soil, at a rate of 5 seeds per pot. Foliar-applied herbicides were applied using a cabinet track sprayer with an 8001-EVS nozzle delivering 40 gallons of spray solution per acre, one day after planting for pre-emergent herbicides, at the 2-leaf stage of rice (for post-emergent herbicides), and at tillering stage of rice (for potential spot-treatment herbicides). Plants were flooded to 10 cm above the soil surface at 48 hours after application. Granular herbicides were applied into the water at 10 days after planting (1-2 leaf stage of rice). Water depth for granular herbicide application was 10 cm above the soil surface, and that depth was maintained throughout the experiment.  Visual evaluations were taken between 14-23 days after application, depending on the application method. All currently-registered rice herbicides were applied at the highest labeled field rate. Pre-emergent herbicides tested were pendimethalin and thiobencarb. Post-emergent granular herbicides tested were thiobencarb, clomazone, benzobicyclon + halosulfuron, and penoxsulam. Post-emergent foliar herbicides tested were propanil and bispyribac-sodium. Oxyfluorfen, which is not currently registered in rice, was applied both as a pre- and post-emergent. Potential spot-treatment herbicides tested were clethodim, paraquat, imazethapyr, glufosinate and glyphosate. Spot-treatment herbicides were applied at the highest labeled rate for a labeled crop.  

Visual evaluations indicate that there are no currently registered rice herbicides that can be used to manage weedy rice, irrespective of ecotype (0% control). Several potential spot-treatment herbicides controlled all weedy rice ecotypes (100% control) including clethodim, glufosinate, and paraquat. Oxyfluorfen does not control weedy rice when applied as a pre-emergent herbicide (0% control). It may be effective in controlling weedy rice when applied post-emergent. It only controls types 1, 2, 3, and 4 (100% control). It does not control type 5 (0% control). 

The development and implementation of 2,4-D- and dicamba-resistant soybean and cotton has been driven by the increasing spread of herbicide resistance in weed species. Off-target movement of 2,4-D and dicamba is a major concern, especially for neighbors with sensitive crop or plant species. A study was conducted in 2017 to determine the sensitivity of driftable fractions of 2,4-D and dicamba with or without glyphosate on common ornamental, shade, fruit, and nut trees, and berry species.  Three driftable fractions corresponding to 1/2, 1/20 and 1/200 of the manufacture’s full labeled rate (1X rate) of 2,4-D choline, 2,4-D choline plus glyphosate, dicamba, and dicamba plus glyphosate were applied to apple, crabapple, dogwood, elderberry, elm, grape, hydrangea, maple, oak, peach, pecan, redbud, rose, raspberry, strawberry, sweetgum, viburnum, and black walnut plants that were contained in 10 to 20 L pots. The experimental design was arranged as a split plot with five replications. Main plots consisted of plant species, while the subplots consisted of the herbicide treatments. Data were analyzed using the PROC GLIMMIX procedure in SAS, and means were separated using Fisher’s Protected LSD.  There was a significant overall species by treatment by rate interaction. The 1/2X rates of all four herbicide treatments caused the greatest visual injury across all species tested 28 days after treatment (DAT). When averaged across all species evaluated, the 1/2X rate of 2,4-D choline plus glyphosate resulted in 57% injury 28DAT, while the 1/2X rate of dicamba plus glyphosate resulted in 45% injury. However, there were substantial differences between species in sensitivity to 2,4-D or dicamba.  Based on the 1/20X rate of 2,4-D choline and dicamba alone, apple, elderberry, maple, peach, redbud, and viburnum were more sensitive to dicamba than 2,4-D; and there were no differences in the sensitivity of crabapple, dogwood, elm, grape, hydrangea, oak, pecan, raspberry rose, strawberry, sweetgum, and walnut  to either herbicide at the 1/20X rate. Results from this experiment indicate that there can be substantial injury to common ornamental, shade, fruit, and nut trees, and berry species, and that there are differences in the sensitivity of most of these species between 2,4-D and dicamba.

Exposure of crop plants to stress or injury, such as soybean injury by PPO-inhibitor herbicide, may stimulate the upregulation of Systemic Acquired Resistance (SAR) and reduce plant susceptibility to other stressors, such as disease-causing pathogens. Field and laboratory studies were initiated to evaluate the upregulation of SAR, examining the effects of PPO-inhibiting herbicide treatment on Sudden Death Syndrome incidence and severity in soybean and the relationship of disease incidence and severity related to stand count and yield with various population densities. A two-year field study was established in Shawneetown, IL to evaluate grain yield and disease potential of soybean cultivars which are either sensitive or tolerant to protoporphyrinogen oxidase (PPO)-inhibitor herbicides, with seed either treated with Upshot (insecticide + fungicide) and Avonni (biological fungicide) or non-treated. The seeds were planted at six different seeding rates: 197,684; 247,105; 296,526; 345,947; 395,368; 444,789; with the controls planted at a density of 345,947 seeds ha^-1 in a 2x2x7 factorial study design. Field experiments were planted on April 25, 2016 and May 6, 2017 in 4-row plots measuring 3m by 7m, and herbicide was applied to treated plots over the center 2 rows. Data collection included stand count at 14 and 28 days after treatment (DAT), plant height at end-of-season (EOS), and disease incidence and severity ratings beginning at the onset of symptomology. Yield data was collected from the center two treated rows. All plots, except the non-treated controls, received an application of sulfentrazone + cloransulam-methyl (316 g ai ha^-1). There were differences in stand count by seeding rate and seed variety at 17 and 28 DAT, but no interactive effects between the factors in 2016; however, in 2017, there were differences in stand count by seeding rate and seed treatment at 14 and 28 DAT, but again, no interactive effects between factors. Relationships between stand count and seeding rate indicated a threshold at which the environment cannot sustain higher planting densities. Environmental conditions were more favorable for crop growth in 2016 than 2017. Rainfall 10 days following planting was recorded at 67 mm and 290 mm in 2016 and 2017, respectively. Disease incidence (scale of 0 to 100%) in 2016 ranged from 0.75 to 2.35 across rating dates, while severity (scale of 0 to 9 based on leaf symptomology) ranged from 0.8 to 2.1 across rating dates. In 2017 disease incidence ranged from 1.0 to 2.0 across all rating dates, and disease severity ranged from 1.0 to 2.2 across all rating dates. Yield in 2016 ranged from 3,652.32 kg ha^-1 to 3,942.49 kg ha^-1 with the highest yield in the sulfentrazone-tolerant variety and the lowest in the -sensitive variety. However, in 2017, yield was lowest in the 197,684 plants ha^-1 treatments at 2,309.44 kg ha^-1 and highest in the 444,789 plants ha^-1 treatments at 3,466.35 kg ha^-1. Significant varietal and seed treatment differences were also noted in 2017. Disease was more prominent in the high-density plots than in the low-density plots, as would be expected because of the effects of competitive stress on plant susceptibility to pathogens. Lab experimentation is still being conducted to determine if sulfentrazone injury induces an upregulation of SAR for a single or multiple genes.

Few herbicides are registered for use in sweetpotato plant production beds.  However, weeds can reduce quantity and quality of sweetpotato plants in plant production beds. Limited research has been conducted on the tolerance of sweetpotato plants produced in production beds to herbicides. Thus, field and greenhouse studies were conducted in 2016 and 2017 to determine sweetpotato tolerance to herbicide treatments applied to plant production beds and then transplanted.   After plants were clipped just above the soil surface in the plant production bed, the non-rooted cuttings (slips) from the beds were transplanted to containers and then placed either in the greenhouse or on an outdoor pad to determine any affects from the herbicide treatments on sweetpotato growth.  Herbicide treatments included PRE application (immediately after covering seed roots with soil) of flumioxazin (107 g ai ha^-1), S-metolachlor (800 g ai ha^-1), fomesafen (280 g ai ha^-1), flumioxazin plus S-metolachlor (107 g ai ha^-1 + 800 g ai ha^-1), fomesafen plus S-metolachlor at (280 g ai ha^-1 + 800 g ai ha^-1), fluridone (1120, 2240 g ai ha^-1), fluridone plus S-metolachlor (1120 g ai ha^-1 + 800 g ai ha^-1), napropamide (1120 g ai ha^-1), clomazone (420 g ai ha^-1), linuron (560 g ai ha^-1), linuron plus S-metolachlor (560 g ai ha^-1 + 800 g ai ha^-1), bicylcopyrone (38, 49.7 g ai ha^-1), pyroxasulfone (149 g ai ha^-1), pre-mix of flumioxazin plus pyroxasulfone (81.8 g ai ha^-1  + 104.2 g ai ha^-1), or metribuzin (294 g ai ha^-1). Paraquat plus non-ionic surfactant (280 g ai ha^-1 + 0.25% v/v) applied POST was also included as a treatment. Initial root growth of cuttings after transplanting was affected by fluridone (2240 g ai ha-1) and fluridone plus S-metolachlor. However, by 5 weeks after transplanting few differences were observed between treatments.  Fluridone with and without S-metolachlor and fomesafen plus S-metolachlor decreased sweetpotato plant quality at slip harvest in both years. 

Organic weed management in pulse crops 

Weed management is one of the biggest challenges for organic producers worldwide. Organic producers tend to rely heavily on mechanical weed control (MWC) and cultural methods as they are the only option for weed control in organic systems. Currently, there are sufficient recommendations for use of mechanical weed control tools such as rotary hoe, harrow, inter-row cultivation separately. Additionally, there have been several studies examining the cultural weed control practice of increased crop seeding rate. However, these mechanical and cultural weed control methods have not been directly compared or tested in combination.

The study was conducted during the 2016 – 2017 field season at three locations: Kernen Crop Research Farm, Goodale Research Farm near Saskatoon and producers land at Hunter, SK. Mechanical weed control with rotary hoe, flex tine harrow and steerable inter-row cultivator was applied as single, paired and multiple combination treatments in field pea and lentil grown under recommended conventional and organic seeding rates for Western Canada. Extension videos were recorded during the summer of 2017 to visually supplement research findings at the final stage of the project.

First year results of the study are positive as they demonstrated high potential for weed control when utilizing optimal cultural and mechanical weed control methods. Single use of the rotary hoe, the harrow and the steerable inter row cultivation exhibited insufficient weed suppression, while multiple MWC application with all three tools together resulted in crop injury overcoming the weed control benefit. However, combining either rotary hoe or harrow with inter-row cultivation reduced weed biomass by 54% to 93% in field pea and by 51% to 90% in lentil. Increasing seeding rate increased yield by 27-32% in lentil and by 13% in pea. Integrated weed management with harrow + inter-row cultivation grown at organic seeding rate resulted in 77% more yield in peas. Rotary hoe + inter-row at organic seeding rate resulted in 80% yield increase in lentil. Combining two weed control tools (rotary hoe or harrow + inter-row) and high seeding rate maximized weed suppression and crop yield in both crops. Best results were achieved when MWC was performed according to appropriate crop and weed stage. For rotary hoe: weeds at white thread stage (just cracking through the soil). For harrow: up to cotyledon stage for weeds and crop up to 4 node stage. For inter-row: from 4 to 10 node stage in crop (prior to crop canopy closure).

Preliminary results from 2017 field season correspond to 2016 findings. Combined analyses for all 4 site years (2016 and 2017 field season) for both field pea and lentil will be conducted to gain more statistical power and reinforce abovementioned results. These results could potentially benefit organic and conventional producer’s profit margins by enhancing their weed management strategies. Economic analyses will be conducted to advocate the return on the MWC machinery purchase investment and investigate the most economical MWC combination with the highest economic return on acre basis. Final results will be presented at scientific conferences, published to weed science journals, distributed among organic producers, posted on YouTube and Vimeo in a form of extension video as well as will be available on the University of Saskatchewan website.

There are limited herbicide options for weed control in sugarcane making evaluation of new herbicides for the crop critical. Topramezone was recently registered for use in sugarcane for postemergence control of annual and perennial weeds. The key to successful use of topramezone in Florida sugarcane will depend on its efficacy on weed control and crop safety. Field studies were conducted on organic soils in Belle Glade, FL in 2016 to 2017 to evaluate sugarcane tolerance to topramezone applied alone or in combination with triazine herbicides (atrazine, metribuzin, ametryn) currently used in sugarcane, and to determine their efficacy on weed control. Tolerance of new plant cane sugarcane varieties (‘CPCL 05-1201’, ‘CP96-1252’, ‘CPCL 02-0926’, ‘CPCL 00-4111’) to topramezone (25 and 50 g/ha) applied alone or in combination with atrazine (2240 g/ha), metribuzin (2240 g/ha), and ametryn (440 g/ha) was evaluated using a randomized complete block design with a split-plot arrangement and replicated four times. Whole plots consisted of the four sugarcane varieties and sub-plots consisted of 11 herbicide treatments and a weed-free control. Sugarcane tolerance to topramezone and triazine combinations was evaluated by determining chlorophyll fluorescence (Fv/Fm), chlorophyll content, and carotenoid content using the top visible dewlap leaf at 7, 14, 21, and 28 days after treatment (DAT). The herbicide efficacy study set up as a randomized complete block design with four replications was conducted on first ratoon sugarcane variety ‘CP96-1252’ using similar herbicide treatments as the tolerance study. Asulam at 3740 g/ha and an untreated control were included. Weed control was evaluated at 14, 28, 42, 56, and 70 DAT. All data were subjected to ANOVA using a mixed linear model and treatment means were separated using Tukey`s test at the 0.05 level of significance. Chlorophyll fluorescence for ‘CPCL 05-1201’ and ‘CPCL 00-4111’ were affected by topramezone (50 g/ha) + ametryn at 7 DAT while that of ‘CP96-1252’ was affected by the tank-mix with atrazine compared to the untreated control. At 14 DAT, all herbicide treatments across all varieties had no effect on chlorophyll fluorescence with the exception of topramezone (50 g/ha) + ametryn. Herbicide effect on chlorophyll fluorescence was not observed 21 DAT. Carotenoid content of all varieties were affected by topramezone (25 and 50 g/ha) + metribuzin or topramezone (50 g/ha) + ametryn at 7 and 14 DAT compared to the untreated control. No herbicide effect on carotenoid content was observed 21 DAT. Similarly, topramezone (50 g/ha) + atrazine or ametryn affected chlorophyll a and b contents compared to the untreated control at 7 and 14 DAT with no effect observed at 21 DAT. Sugarcane was able to fully recover by 21 DAT indicating that it was probably able to metabolize the herbicides after this time period. Fall panicum, the predominant weed species at the study sites was controlled 85 to 94% by topramezone and triazine tank-mixes compared to 34% by asulam at 14 DAT. At 28 DAT, topramezone at 25 and 50 g/ha + metribuzin provided 78 and 88% fall panicum control, respectively compared to other topramezone treatments that provided 50 to 68% control, and 87% control by asulam. By canopy closure (56 to 70 DAT), fall panicum control was 84% and 89% by topramezone (50 g/ha) + metribuzin and asulam, respectively indicating that the tank-mix provided fall panicum control that was not significantly different from asulam which is commonly used for grass control in Florida sugarcane. More studies are ongoing to evaluate effect of timing of application on the efficacy of topramezone and triazine tank-mixes on fall panicum and control of other weed species in Florida sugarcane.

There is increasing interest in use of remote sensing in agriculture, with specific applications for weed mapping and monitoring. As imagery and sensor technology continue to improve, understanding the information that can be extracted from these systems will be imperative to using the data for making management decisions. Therefore, field studies were conducted in 2016 and 2017 in Clinton, NC to assess the effect of Palmer amaranth (Amaranthus palmeri S. Wats.) and large crabgrass (Digitaria sanguinalis (L.) Scop.) phenology and density on weed species differentiation using hyperspectral reflectance properties in ‘Covington’ sweetpotato.  Weed densities were established 1 d after sweetpotato transplanting and maintained season-long with and without the presence of a sweetpotato. Palmer amaranth and large crabgrass were established at densities of 0, 1, 2, 4, 8 and 0, 1, 2, 4, and 16 plants m-1 row, respectively. Weed phenology is an important factor for both species differentiation and weed density detection. Weed differentiation occurred later in the season when weeds were grown with sweetpotato compared to no sweetpotato. Areas of spectral interest for differentiation include reflectance wavelengths isolated to shortwave infrared (SWIR) portions of the spectrum when grown with sweetpotato, and included visible light (VIS) and SWIR when no crop was present.  Weed densities were differentiated within species during weed growth, using spectral areas in the VIS, near infrared (NIR) and SWIR regions. Weed densities among each weed were difficult to distinguish from one another early in the season and late in the season once weeds developed large canopies within sweetpotato.  Spectral characteristics of high weed densities were greater absorption in wavelengths associated with chlorophyll and leaf water content, and greater foliar reflectance values when compared to low weed densities. 

Herbicide resistance is a global problem that calls into question the present and future utility of herbicides. This is especially true when considering acetolactate synthase (ALS)-inhibiting (group #2) herbicides, which account for more cases of herbicide-resistant weed biotypes globally than any other mechanism of action group. Many instances of resistance to group #2 herbicides involve multiple resistance with at least one other mechanism of action. Such is the case with horseweed (Conyza canadensis), where some biotypes have developed resistance to glyphosate (group #9) and group #2 herbicides. Herbicides from other mechanisms of action with weed resistance have been shown to still provide some level of field efficacy, especially if the resistance mechanism has been characterized to enable low- to moderate-level resistance. For example, soil residual applications of some PPO-, HPPD-, and photosystem II-inhibiting herbicides have been reported to contribute significantly to field-level management of some weed biotypes classified as resistant to postemergence applications of these herbicides. Of group #2-resistant weeds found in Indiana, horseweed has the lowest magnitude of resistance to these herbicides and may have the potential to be controlled with ALS-inhibiting herbicides.

Field research was conducted to determine if the efficacy of ALS-inhibiting herbicides is influenced by the application method (PRE vs. POST), the specific active ingredient, or the application rate of the group #2 herbicide in a population of horseweed containing the Pro-197-Leu mutation conferring group #2 resistance. This is the first report of the P197L mutation conferring group #2 resistance in horseweed. Greenhouse dose-response experiments demonstrated that the P197L mutation has a resistant/susceptible ratio of 14.5 and 8.7 to chlorimuron-methyl and cloransulam-ethyl, respectively, in foliar applications. Field experiment treatments included chlorimuron (11.2, 22.4, 44.8 g ai ha^-1) and cloransulam (35.3 g ai ha^-1) applied both PRE and POST. Visual estimates of control for each plot were recorded at 7, 14, 21, 28, and 35 d after treatment (DAT). Cloransulam applied PRE was more efficacious (89% control) than the two lower rates of chlorimuron applied PRE and POST. Cloransulam applied PRE resulted in plots with fewer flowering plants than plots treated with chlorimuron at the 11.2 and 22.4 g ha^-1 rates. In general, no differences were observed between PRE and POST applications of either cloransulam or chlorimuron based on visual control estimates at 35 DAT, stand counts, and inflorescent plant density. Tissue samples were analyzed for the presence of P197L alleles to determine differences in the presence of the P197L allele among plants surviving cloransulam or chlorimuron treatments and the nontreated control. All chlorimuron and cloransulam treatments increased the frequency of P197L alleles in surviving individuals compared to the nontreated control. No difference in selection was observed between chlorimuron and cloransulam, despite an increased reduction in flowering plants and increased visual control with cloransulam. Increased control of group #2-resistant horseweed with cloransulam compared to chlorimuron may be caused by the ability of cloransulam to control some individuals with the P197L mutation. In summary, PRE applications of cloransulam and chlorimuron on a population of group #2-resistant horseweed showed the same response as POST applications. However, PRE and POST cloransulam applications may still contribute to weed control in horseweed, despite the presence of group #2 resistance via the P197L mutation.

While the release of dicamba tolerant soybeans by Monsanto will aid growers in weed control, it will also present several challenges. Glyphosate is very water soluble, allowing it to be easily removed from spray tanks through three rinses with water alone. Synthetic auxin herbicides however, are not as water soluble and therefore can be difficult to completely remove from sprayer components. Synthetic auxins are also highly active on some plant species at very low concentrations. The objective of this study is to determine the effects of increasing rinse volumes during a triple rinse cleanout procedure, for the removal of dicamba from contaminated sprayers. Field experiments were conducted in 2017 in Brooksville and Starkville, MS. Four different rinse volumes, 10%, 20%, 40%, and 60% of the tanks volume, were evaluated. The addition of a tank cleaner (Wipeout, Helena Chemical®) during the second rinse was also evaluated at each rinse timing, compared to utilizing three water rinses alone. A no cleanout treatment was also included at each rinse volume. A small scale sprayer was designed to replicate the cleanout procedures used on commercial sprayers. This system was first contaminated with dicamba (Xtendimax, Monsanto®) and rhodamine dye. The sprayer then underwent a 3 rinse cleanout, utilizing one of the four rinse volumes during each rinse step. During each rinse, the solution was recirculated through the system for 15 minutes and samples were collected for both field and lab analysis. Once the sprayer was cleaned using the triple rinse procedure it was filled with a labeled rate of glyphosate, and another sample was collected. All samples were sprayed over actively growing soybeans at the R1 growth stage. Visual rating for phytotoxicity were taken 7, 14, 21, and 28 DAT (days after treatment) and plant heights were taken 14, 21, and 28 DAT. Samples collected during each rinse were analyzed using HPLC to determine auxin herbicide concentrations as a means to evaluate cleaner efficacy. Plants were harvested at end of the growing season for yields (KG/HA). Data reveal treatments utilizing rinse volumes greater than 10 percent of the tanks volume produce less visual injury 28 days after treatment (DAT). Visual injury was 21, 18, 17, and 17% for rinse volumes of 10, 20, 40, and 60%, respectively. Plant height reduction and yield reductions were different with rinsates from the first rinse but did not differ from the untreated check following the first rinse. HPLC analysis confirmed differences existed only in the first rinse with no significant difference compared to the untreated check in subsequent rinses. A fourfold reduction in dicamba PPM is observed following each rinse step.

Linuron resistant pigweed is becoming an increasingly difficult weed for growers to control as there are few herbicides registered for use on carrots. If left uncontrolled, a 92-100% yield loss could occur. The objective of this research was to test the effectiveness of herbicides not yet registered for use on carrots and to test alternative timings of herbicides already registered to create a comprehensive weed control program for carrot growers. Field trials were conducted on high organic matter soils in 2016 and 2017 to test 14 herbicide combinations. Excellent control of linuron resistant pigweed was obtained with rates of pyroxasulfone (89 g ai/ha), prometryne (3400 g ai/ha), pendimethalin (3000 g ai/ha), s-metolachlor (1373 g ai/ha), glufosinate (500 g ai/ha), bicyclopyrone (50 g ai/ha) and metribuzin (140 g ai/ha) or the biological effective dose of acifluorfen (18.75 g ai/ha) oxyfluorfen (60 g ai/ha), fluthiacet-methyl (3.75 g ai/ha), and fomesafen (10 g ai/ha). During the 2017 season, which had significantly more rainfall than the 2016 season, treatments that included fomesafen (240 g ai/ha) or carfentrazone (28.1 g ai/ha) applied PRE severely reduced carrot emergence when compared to the control. 

Many residual herbicides are strongly adsorbed to organic matter. This can cause reduced efficacy when residual herbicides are applied to cover crop residue resulting in reduced control. Multiple field experiments were conducted in 2017 at two locations in Mississippi to investigate the relationships of nozzle type, carrier volume, and cover crop residue on efficacy of various residual herbicides. Annual grass and common waterhemp control with pendimethalin was approximately 7 and 12% higher 28 DAT when applied to bare soil compared to wheat residue regardless of nozzle or carrier volume, respectively. Pendimethalin efficacy as affected by carrier volume alone was not reduced in either grass or waterhemp until volumes of 10 GPA or lower were used regardless of nozzle or wheat residue. Likewise, grass and waterhemp control 28 DAT with S-metolachlor applied to wheat residue was reduced by lower carrier volumes except for when at least 20 and 15 GPA was used, respectively. Residual flumioxazin efficacy was not influenced by nozzle, volume, or wheat residue. Grass control with acetochlor was reduced by approximately 10% and 45% when volumes of 10 GPA or less were used to apply to bare soil or wheat residue, respectively. Results of these studies indicate that carrier volumes of more than 10 GPA should be used when applying residual herbicides to wheat residue. Future research will investigate differential levels of residue and other techniques to overcome adsorption.

          Shifts toward herbicide-resistant weed populations in row-crop agriculture are a widespread epidemic. Sequential and untimely applications of glyphosate, acetolactate synthase-inhibitors, and other herbicide site-of-action groups, have led to the selection and spread of herbicide-resistant weed biotypes. New soybean systems with resistance to auxinic herbicides, along with proprietary herbicide formulations, have been developed to combat resistance issues in soybean production. These new technologies were assessed for weed control efficacy in standard herbicide programs and grain yield over two years, at two sites in both conventional- and no-tillage systems. New technologies were assessed alongside technologies which have been available for several years. At site one, where conventional-tillage was used, there were few differences in weed control when a preemergence (PRE) followed by (fb) postemergence (POST) herbicide program was used. All soybean systems provided 87% or higher control of Ambrosia trifida, Amaranthus tuberculatus, Setaria faberi, Xanthium strumarium, and Abutilon theophrasti; and with the exception of conventional soybean, all soybean systems provided 88% or more control of Ipomoea hederacea. The results of the orthogonal contrasts analyses for 2016 grain yield suggested that there was no difference between soybean systems; only herbicide program was significant. In 2017, there were significant differences for both herbicide program and soybean system. TIR1 auxin inhibitor- and glyphosate-resistant soybean systems provided an average yield of 2538 kg/ha^-1, while glufosinate-resistant and conventional soybean provided average yields of 1820 kg/ha^-1 and 1748 kg/ha^-1, respectively. Further, 2,4-D-resistant soybean provided higher yield than dicamba-resistant soybean. At the no-tillage site, when using PRE fb POST herbicide programs, there was 85% or higher control of Amaranthus tuberculatus and Panicum dichotomiflorum provided by all soybean systems. In 2017, the glufosinate-resistant soybean system provided less control of Panicum dichotimoflorum than all other soybean systems. Amaranthus tuberculatus and Ipomoea lacunosa control was equal for all soybean systems when using PRE fb POST programs. Orthogonal contrasts for grain yield indicate there was a significant herbicide program interaction for both years. In 2016, there were no differences between soybean systems for grain yield, with the exception of increased yield provided by 2,4-D-resistant soybean relative to dicamba-resistant soybean. In 2017, first- and second-generation glyphosate-resistant varieties provided higher yield than TIR1 auxin inhibitor-resistant varieties. The conventional soybean system provided an average yield of 2868 ka/ha^-1, while glufosinate-, T1R1 auxin inhibitor-, and glyphosate-resistant soybean systems provided average yields of 2516 to 2538 kg/ha^-1. Although soybean system was significant for the control of Ipomoea hederacea in 2016 and Panicum dichotomiflorum in 2017 when using PRE fb POST herbicide programs, herbicide program interactions suggest that soybean system choice may be of less importance than using broad spectrum PRE herbicides followed by timely POST applications. Grain yield data indicate that while proper weed control is important, optimum grain yield is achieved when soybean variety selection is based upon yield potential in addition to herbicide-resistance trait.

In 2011, Zollinger et al. published a paper titled “A test method for evaluating water conditioning adjuvants” as a standard test method for evaluating water conditioning adjuvants. In the test method, artificial hard water is mixed to 1000 mg L^-1 of calcium chloride and magnesium chloride. Water conditioning adjuvants are mixed with herbicides and evaluated for increased weed control efficacy. While this is an effective test method, a comparison of water antagonism of different salt types used to make the hard water has never been conducted. The objective of this research was to validate the standard test method in comparison to additional salt types. Three hard water samples of 500 mg L^-1 calcium were evaluated using calcium chloride, calcium formate, and calcium nitrate. Field trials were conducted near Hillsboro, ND in 2016 and 2017. In separate trials, glyphosate and mesotrione were applied at 342 and 70 g ai ha^-1, respectively. Three types of water conditioners were evaluated: diammonium sulfate (AMS), an AMS replacement, and monocarbamide di hydrogen sulfate (AMADS). AMADS was not included in the mesotrione study. Indicator species included flax (Linum usitatissimum L.), amaranth (Amaranthus cruentus L.), sunflower (Helianthus annuus L.), and conventional corn (Zea mays L.) for the glyphosate study. Amaranth, foxtail millet (Setaria italica (L.) P. Beauvois), quinoa (Chenopodium quinoa Willd.), and sunflower were used for the mesotrione study. Herbicide antagonism was similar between all hard water samples. Within each type of water conditioning adjuvant, antagonism was overcome similarly in all water types. The results of these studies validate the test method established by Zollinger et al. (2011). 

Barnyardgrass (Echinochloa crus-galli) is a herbicide resistance-prone species prevalent in most cropping systems in the midsouthern United States. Prior field experiments have determined tank-mixtures of glyphosate + glufosinate and glyphosate + dicamba are antagonistic when applied to barnyardgrass and raises the concern of evolving glyphosate or glufosinate resistance. To investigate a potential mechanism for the antagonism observed in the field, herbicide absorption and translocation of ¹⁴C-labeled glyphosate and ¹⁴C-labeled glufosinate was evaluated in Frankfurt, Germany. In one experiment, ¹⁴C-glyphosate was applied alone and in combination with nonradiolabeled (cold) glufosinate or dicamba. In the second experiment, ¹⁴C-glufosinate was applied alone and in combination with cold glyphosate or dicamba. The radiolabeled herbicide solution was applied to the second-youngest fully expanded leaf of barnyardgrass at the 4- to 5-leaf stage. At 48 h after treatment, plant tissue was harvested and sectioned. The treated leaf was washed to remove any radioactive material that was not absorbed. Plant tissues were dried and radioactivity for each section was quantified in a liquid scintillation counter following oxidation. Applying a solution of glufosinate + glyphosate reduced uptake of ¹⁴C-glyphosate by 10 percentage points compared to glyphosate alone (22% and 32% of applied ¹⁴C glyphosate, respectively). Furthermore, the mixture of glyphosate + glufosinate reduced translocation of ¹⁴C-glyphosate out of the treated leaf, with 50% of absorbed radioactivity moving out of the treated leaf for glyphosate alone compared to 28% for the tank-mixture. Applying glyphosate with dicamba did not affect the uptake of ¹⁴C-glyphosate compared to glyphosate alone, nor was translocation different between these two treatments. In the ¹⁴C-glufosinate experiment, the plant absorbed 68% of the applied ¹⁴C-glufosinate when applied alone compared to only 44% when applied in mixture with cold glyphosate. These data suggest that altered absorption or translocation of both glufosinate and glyphosate in mixture could be the source of antagonism observed in the field. To mitigate the likelihood of evolving resistance to both herbicides, it may be necessary to increase use rates or use sequential applications to mitigate antagonism. However, for the mixture of glyphosate + dicamba, the mechanism causing antagonism does not appear to be altered herbicide uptake or translocation and requires further investigation to identify a potential source of antagonism.

Time of Day Effects on Peanut Weed Control Programs. O.W. Carter* and E.P. Prostko,

            Department of Crop and Soil Sciences, The University of Georgia, Tifton, GA 31793-0748

Reductions in weed control performance at the farm level have caused extension weed science programs to focus on potential causes of the differences observed between small-plot research and commercial applications.  One possible explanation for these differences in control may be related to the application time of day (TOD). Herbicide recommendations made by weed scientists are often  based on research conducted during the working hours of 6 am to 9 am -  However, growers very typically spray as early as 6 am and as late as 10 pm.  Recent research on the herbicide glufosinate and several PPO-inhibiting herbicides has shown reduced performance in low light intensity.  Consequently, research was conducted in 2015, 2016, and 2017 to determine if TOD  influences the performance of peanut weed control programs.  Two University of Georgia recommended peanut weed control programs were chosen and the entire program was applied at four different TOD (7 am, 12 pm, 5 pm, and 10 pm) .  The herbicide treatments consisted of the following:  paraquat (0.21 kg ai/ha ) + acifluorfen (0.19 kg ai/ha) + bentazon (0.37 kg ai/ha) + s-metolachlor (1.23 kg ai/ha) (EPOST) followed by either imazapic (0.07 kg ai/ha) + s-metolachlor (1.23 kg ai/ha) + 2,4-DB (0.25 kg ai/ha) or lactofen (0.22 kg ai/ha) + s-metolachlor (1.23 kg ai/ha) + 2,4-DB (0.25 kg ai/ha) (POST).  Treatments were applied when weed species were approximately 5-7 cm.  Crop injury ratings were obtained 1 week after the EPOST and POST applications.  Weed control ratings were obtained twice after both the EPOST and POST applications  There was no interaction between time of day or herbicide treatment.  Peanut injury was significantly lower at 7 am and 10 pm when compared to the other timings after both herbicide applications.  Generally, lactofen was more injurious than imazapic..  Palmer amaranth control was not influenced by timing or treatment.  When averaged over treatments, annual grass control was significantly lower at the 7 am application timing when compared to the other timings.  When averaged over timing, annual grass control  was also significantly reduced with the lactofen treatment.  A significant reduction in control of sicklepod was observed  at the 10 pm timing and with the lactofen treatment.  While TOD  did influence peanut injury and weed control,  peanut yield was not affected. 

Incorporation of a legume, such as red clover (Trifolium pratense), into grass-based pasture systems, offers many benefits. However, available red clover lines are highly susceptible to herbicides, in particular, 2,4-D (2,4-dichlorophenoxyacetic acid), which has been widely used for broadleaf weed management in pastures. A novel red clover line, UK2014, was developed at the University of Kentucky through conventional breeding and progressive introgression of the 2,4-D tolerance trait into Kenland, a common variety used by Kentucky’s forage producers. Adopting this new tolerant line would broaden weed management options in a legume-grass mixed pasture. The novel line UK2014 displays an increased tolerance level to 2,4-D herbicide than its counterpart Kenland. However, the basis for the increased tolerance of UK2014 to the herbicide is not yet characterized. Moreover, application of exogenous auxins such as 2,4-D is often responsible for triggering hormone interactions and complex response pathways. Therefore, the examination of the genetic basis of the tolerance in UK2014 provides a unique opportunity to study the differential molecular response of red clover to 2,4-D. An RNA-Seq transcriptome analysis was utilized to search candidate genes that confer increase 2,4-D tolerance in UK2014 compared to Kenland. Both lines were field-grown at University of Kentucky’s Spindletop research farm in Lexington, KY. In August 2015, plots of each line were sprayed with 2,4-D at 0 and 1.12 kg ae / ha. Ten leaflets of each plot were extracted at 4, 24 and 72 hours after treatment (HAT) and those were utilized for mRNA extraction. Sequencing was performed on the Illumina Hiseq 2500 platform. A reference cDNA transcriptome was assembled and assigned putative annotations on CLC genomics. Differential expression in this preliminary study was considered at a 5-fold change cutoff for the comparisons between normalized reads. At 4 HAT, 223 contigs, or 7% of the total differentially expressed genes, were associated with 2,4-D treatment in both varieties. Among these, GH3 and IAA auxin-responsive genes, and glutathione-transferase genes were prevalent. At 24 and 72 HAT, respectively, 356 (38%) and 101 (11%) genes were differentially expressed due to 2,4-D treatment in both varieties. A similar pattern was observed on the prevalence of auxin-responsive genes. Genes encoding cytochrome P450s, an herbicide metabolizing great protein family, were overexpressed in both varieties with 2,4-D treatment. At 4 HAT, in special, 44 differentially expressed p450 genes were found. However, none were unique to treated UK2014. Gene Ontology of the biological processes displayed response to chemical stimulus, response to stress and cellular amino acid derivative metabolic process. This study is a preliminary analysis of the transcriptome profile of the UK2014 and Kenland following 2,4-D herbicide treatment. Further research shall include qPCR validation of the overexpressed genes. Metabolic and translocation analysis in these same lines in association with p450 inhibitors will are planned in this project and shall give a better insight on the differential activity of these cytochromes in metabolizing the herbicide molecule.

Palmer Amaranth (Amaranthus palmeri S. Wats.) Control Using Various Droplet Sizes of Acifluorfen and Lactofen. L. X. Franca^*1, D. M. Dodds¹, C. A. Samples¹, G. R. Kruger², T. Butts²; ¹Mississippi State University, Mississippi State, MS, ²University of Nebraska, Lincoln, NE.

ABSTRACT

Widespread occurrence of glyphosate and ALS-resistant Palmer amaranth has led to increased use of protoporthyrinogen oxidase (PPO) inhibiting herbicides. Lactofen and acifluorfen are non-systemic, PPO-inhibiting herbicides used to control several annual broadleaf species in soybeans, cotton, and peanuts. Concerns exist with regard to the dissemination Palmer amaranth populations resistant to PPO-inhibiting herbicides across the Midwestern and southern United States. Palmer amaranth populations resistant to PPO-inhibiting herbicides have been reported in Arkansas, Tennessee, Illinois, and Mississippi. Therefore, efficacious and cost effective means of application are needed to maximize lactofen and acifluorfen effectiveness.

Experiments were conducted in 2016 and 2017 at Hood Farms in Dundee, MS, and the West Central Research and Extension Center in North Platte, NE to evaluate the influence of droplet size on lactofen and acifluorfen effectiveness for Palmer amaranth control. Lactofen (Cobra^®, Valent U.S.A., Walnut Creek, CA 94596-8025) was applied at 0.21 kg ai ha^-1 + Crop Oil Concentrate (COC) at 1% v/v and acifluorfen (Ultra Blazer^®, United Phosphorus Inc., King of Prussia, PA 19406) at 0.42 kg ai ha^-1 + Crop Oil Concentrate (COC) at 1% v/v using the following droplet sizes: 150 μm, 300 μm, 450 μm, 600 μm, 750 μm, and 900 μm. Prior to experiment initiation, droplet size spectra for each herbicide was characterized in a low speed wind tunnel at the Pesticide Application Technology Laboratory at University of Nebraska, North Platte, NE. Treatments were POST applied to 10 cm to 15 cm Palmer amaranth using a tractor mounted sprayer equipped with a CAPSTAN^® AG Pulse Modulated Sprayer (Capstan Ag Systems, Inc., Topeka, KS) and Wilger Precision Spray Technology Tips (Wilger Inc., Lexington, TN 38351-6538) at 4.8 km per hour using a spray volume of 140 L ha^-1. Visual Palmer amaranth control was collected at 7, 14, 21, and 28 days after application. Fifteen plants per plot were tagged and used for dry biomass calculation at the end of the experiment. Data were subjected to analysis of variance using PROC GLM procedure in SAS^® Software v. 9.4 (SAS Institute Inc., Cary, NC 27513-2414) and means were separated using Fischer’s Protected LSD at α=0.05.

Different droplet sizes of lactofen did not result in different Palmer amaranth control, regardless of rating period. Acifluorfen applied using 300 μm droplets significantly provided the greatest Palmer amaranth control at 14 and 28 days after application. Furthermore, Palmer amaranth control was significantly greater with acifluorfen applied using 300 μm at 7 days after application. Compared to 450 μm, acifluorfen applied with 300 μm provided a 25% increase in Palmer amaranth control, regardless of rating period. All droplet sizes of lactofen provided significant dry biomass reduction of Palmer amaranth. Acifluorfen applied using 300 μm, 150 μm, and 750 μm droplet sizes resulted in significant reductions of Palmer amaranth biomass. These data suggest that the use of small or large droplet sizes does not affect lactofen effectiveness on 10 to 15 cm Palmer amaranth. Additionally, the use of 300 μm droplets is recommended to optimize Palmer amaranth control from acifluorfen.

Lxf3@msstate.edu

Roundup Ready (RR) sugar beet is resistant to glyphosate via the insertion of an insensitive form of 5-enolpyruvoylshikimate-3-phosphate synthase (EPSPS). RR sugar beet has been available in the United States since 2005, and makes up nearly 100% of the US sugar beet crop. At a national level, sugar beet accounts for 50% of domestic sugar production. Recent anti-GMO movements have been questioning the safety of the product derived from sugar beet that has been sprayed multiple times with glyphosate. This study investigated the fate of the glyphosate that is taken up by the plant following post-emergence application of the herbicide during the growing season. Glyphosate levels were quantified via LC-MS/MS after derivatization of the extracts with FMOC. Sugar beet samples were collected from two Colorado fields over the course of the 2017 season, as well as from various steps in the post-harvest process. There is a metabolite of sugar beet that could be mistaken for glyphosate, but is distinctly measured in our study. Glyphosate is evident in the beet root and shoot, but is removed during processing. We conclude the glyphosate concentration in the final product reaching the consumer is far below the 1.75 ppm limit set by the EPA.

Though novel quizalofop-resistant acetyl-CoA carboxylase (ACCase) in wheat was previously associated with a missense mutation, the molecular interactions between the enzyme and herbicide have been unexplored. Quizalofop-resistant wheat accessions were developed by the Colorado State University Wheat Breeding Program using ethyl methanesulfonate mutagenesis and selection. Resistant accessions are characterized by an alanine to valine amino acid substitution. A heterozygous or homozygous mutant genotype for ACCase in one of three diploid wheat genomes (A, B, or D) is associated with resistance, yet degree of resistance varies by the genome with the mutation. Protein simulations were conducted to examine molecular interaction differences between wildtype wheat and quizalofop-resistant wheat. Because a crystal structure of wheat ACCase is unavailable, both wheat ACCase sequences were aligned to a yeast ACCase crystal structure using Modeller 9.19. Molecular dynamics of both ACCase types were calculated and optimized using GROMACS 5.1.4. A comparison of wildtype and quizalofop-resistant wheat ACCase models primarily reveals a conformational difference in the carboxyltransferase domain.

Harvest weed seed control (HWSC) destroys weed seeds retained at the time of crop harvest, which would typically be spread by the harvester along with other field residues. Improvement of HWSC techniques could make them viable for adoption in organic cropping systems, where growers seek tools for improved weed management in the absence of herbicides. Field experiments in organically-managed, double crop soybean (Glycine max (L.) Merr.) and winter wheat (Triticum aestivum L.) fields infested with Italian ryegrass (Lolium perenne L. ssp. multiflorum (Lam.) Husnot) compared weed management programs featuring two HWSC techniques, windrow burning and field residue removal, to a grower-standard, tillage-based weed management program without HWSC. Wheat yield and Italian ryegrass populations were measured in the years before and after HWSC use. Neither HWSC treatment affected Italian ryegrass populations or wheat yield at different levels than the grower-standard treatment. Across treatments, high Italian ryegrass populations, typically >250 plants m^-2 at harvest time, led to a failed harvest in the year after HWSC application. HWSC’s use in organic cropping systems must be viewed in context with other organic management practices that may alter HWSC efficacy. Researchers must work to characterize the economic and logistic realities of HWSC use within organic crop production, specifically relating to the standards of the National Organic Program. Future research will improve HWSC efficacy at extreme levels of weed infestation and compare HWSC to the effects of organic practices, such as tillage, that directly affect the soil seed bank.

Resistance to PPO-inhibiting herbicides in both tall waterhemp and Palmer amaranth has been attributed to the deletion of a glycine codon at the 210^th position of the PPX2 gene, which codes for production of PPO, referred to as the ΔG210 mutation. In addition to the ΔG210 mutation, substitutions at the 98^th position of PPX2 have been reported to confer resistance in Palmer amaranth. These substitution mutations result in the production of either a glycine or methionine at the 98^th position, instead of the wild-type arginine, and are referred to as R98G and R98M, respectively. Recently, the R98G mutation was documented in a tall waterhemp population from Gibson County, Indiana. As a result, laboratory research was conducted in the fall of 2017 to characterize the new mutation in tall waterhemp. As part of this research, the R98 region from plants within the Gibson County population, including plants testing positive and negative for the R98G mutation, were sequenced via Sanger sequencing. The sequenced regions from Gibson County plants were aligned with a wild-type tall waterhemp sequence, as well as two Palmer amaranth sequences from Indiana and Alabama using the BLASTN program from NCBI. Within the Gibson County population several polymorphisms were discovered between plants containing the R98G allele, and those with the susceptible allele. Sequenced fragments containing the susceptible allele aligned nearly identically with the wild-type waterhemp sequence, whereas those containing the R98G allele shared numerous polymorphisms with both Palmer amaranth populations. A quantitative PCR (qPCR) assay, designed to detect Palmer amaranth specific nucleotide polymorphisms in the internal transcribed spacer of the ribosomal coding region, indicated that Gibson R98G plants were not Palmer amaranth. Gibson R98G plants were further identified as tall waterhemp by examination of an EcoRV restriction site found on the ALS allele of tall waterhemp. Because the wild-type sequence of tall waterhemp at the R98 position (AGA) would theoretically require two separate substitutions to acquire the R98G (GGG) mutation, it appears more likely that the mutation was transferred from Palmer amaranth, where the wild-type sequence (AGG) only requires a single nucleotide substitution and has already been documented in Palmer amaranth biotypes. These results provide evidence that the R98 mechanism of resistance in this tall waterhemp population from Gibson County was likely transferred from Palmer amaranth via horizontal gene flow. Transfer of other herbicide resistance genes between Amaranthus species has been previously documented, and this research provides evidence that resistance to PPO-inhibiting herbicides can be spread similarly.

Common ragweed (Ambrosia artemisiifolia L.) has been reported to be resistant to glyphosate and ALS inhibiting herbicides in Virginia, resulting in many soybean producers struggling to control this weed.  Additionally, resistance to PPO-inhibiting herbicides has been reported in neighboring North Carolina; Virginia producers have reported poor results with these herbicides.  A field study was initiated to determine postemergence herbicide options that effectively control suspected glyphosate and ALS resistant common ragweed in 2017 in Lawrenceville, VA.  Roundup Ready 2 Xtend soybeans were planted on May 9, 2017.  Treatments consisted of glyphosate (1260 g ae ha^-1), cloransulam-methyl (17.6 g ai ha^-1), chlorimuron-ethyl (8.7 g ai ha^-1), imazethapyr (70 g ai ha^-1), bentazon (1120 g ai ha^-1), bentazon + acifluorfen (560.7 + 279.3 g ai ha^-1), lactofen (218.8 g ai ha^-1), fomesafen (420 g ai ha^-1), acifluorfen (420 g ai ha^-1), glufosinate (593.8 g ai ha^-1), dicamba (558.3 g ai ha^-1), 2,4-D (1120 g ai ha^-1), fluthiacet-methyl + fomesafen (9.9 + 245.9 g ai ha^-1), and a nontreated check.  Treatments were arranged in a randomized complete block design with 4 replications.  Treatments were applied when weeds in the nontreated checks were 5 to 10 cm in height with a handheld spray boom calibrated to deliver 140 L ha^-1 of spray solution.  Soybean plots were 4 rows wide on 76 cm centers with whole plots measuring 3 by 6 m.  Plots were assessed for visible common ragweed control and soybean injury biweekly for 6 wks on a 0 (no control or injury) to 100% (complete control or necrosis) scale.  Yield data were also collected. All data were subjected to ANOVA and subsequent means separation using Fisher’s Protected LSD (α=0.05) using JMP Pro 13.  The nontreated check was excluded from the statistical analysis.  At 2 wk after treatment (WAT) bentazon + acifluorfen, fomesafen, acifluorfen, dicamba, 2,4-D, and fluthiacet-methyl + fomesafen provided >96% common ragweed control.  Six WAT the treatments resulting in the greatest common ragweed control were fomesafen (99%), dicamba (94%), 2,4-D (96%), and fluthiacet-methyl + fomesafen (98%).  Other treatments that provided adequate control of common ragweed 6 WAT included cloransulam-methyl (88%), chlorimuron-ethyl (83%), bentazon (85%), bentazon + acifluorfen (84%), and acifluorfen (84%).   Glyphosate performed poorly (<40%) indicating that the population of common ragweed population was glyphosate resistant.  Glufosinate and 2,4-D were excluded from the injury analysis since the crop was completely necrotic by these treatments.  Soybean injury was observed in treatments containing diphenyl ether herbicides with the maximum injury observed being 14% 2 WAT.  By 4 WAT no injury was observed in these treatments.  Overall, there are multiple postemergence herbicide options including PPO inhibiting herbicides that are highly effective at controlling common ragweed when applied at appropriate timings.  To better steward these chemistries, herbicides should be used in conjunction with effective preemergence herbicides.  Further research may evaluate the effectiveness of these herbicide options over different populations of common ragweed that are known to be ALS resistant across the commonwealth of Virginia.

Use of Metam Potassium for Purple Nutsedge (Cyperus rotundus) Control in Strawberry (Fragaria ananassa). K.Khatri* and N.S. Boyd, Gulf Coast Research and Education Center, University of Florida, Wimauma, FL

Strawberry production in Florida is highly dependent on soil fumigation for the control of soil borne pest and weeds. Teloneâ C-35 (1,3- dichloropropene 63.4 % plus chloropicrin 34.7%) is the common fumigant used at the time of bed formation in strawberry fields. Teloneâ C-35  does not have herbicidal component,  therefore, supplemental application of metam potassium, which has strong herbicidal capacity is imperative to achieve adequate control of  purple nutsedge. The goal of the project is to compare the effectiveness of two metam potassium application methods: mini coulters and drip irrigation injection, and compare three timings of drip irrigation injection: at the time of bed formation, after a week of bed formation, and after two weeks of bed formation for purple nutsedge control. Field experiment was conducted in 2016-17 growing season at the Gulf Coast Research and Education Center, Wimauma, FL and different different application methods and timings were tested. Purple nutsedge population was measured three times during the growing season; at the time of transplant, midseason and at the final harvest. Thirteen strawberry harvests were done during the season. Results indicated that there is no difference (p>0.05) in weed control with mini coulters and drip irrigation injection. However, better weed control was achieved with metam potassium injection after a week and two weeks of bed formation (p<0.05) as compared to at the time of bed formation. There was no difference in strawberry yield (p>0.05) with any of the treatments tested.kkhatri@ufl.edu

Key words: metam potassium, mini coulters, purple nutsedge

Time constraints limit the opportunity to establish cover crops in the fall following corn harvest in the upper Midwest. By interseeding cover crops into corn at the early vegetative growth stages, farmers could benefit from increased cover crop biomass, improved soil health, and weed suppression; however, cover crops may compete with corn if interseeded too early. Furthermore, the effects that interseeded cover crops may have on the following year’s crop is not understood. The objectives of this research were to evaluate the competitiveness of cover crops when interseeded into corn at the V1-V7 growth stages, and to determine the impacts of these cover crops on the following year’s cash crop. In 2015 (1 site), 2016 (1 site), and 2017 (2 sites), annual ryegrass, Tillage Radish®, and crimson clover were broadcast interseeded at the V1 (2015 only), V2, V3, V4, V5, V6, and V7 corn growth stages. Seeding rates were 18 kg/ha, 9 kg/ha, 18 kg/ha, respectively. Glyphosate was applied immediately prior to interseeding the cover crops at each timing. Cover crop and weed density were measured 30 days after interseeding and just prior to corn harvest. Cover crop and weed biomass were also measured just prior to corn harvest. In the following spring of each year, cover crop and weed density and biomass were measured, and corn was planted as an indicator crop. Soil samples were collected in the spring and at VT in the indicator crop. Tissue samples were taken in the indicator crop at VT. Annual ryegrass produced more biomass compared with Tillage Radish® and crimson clover when combined over site years; biomass was greatest at the V4-V7 interseeding timings. Tillage Radish® biomass was greatest at the V2 and V5 timings, and crimson clover biomass was greatest at the V2, V4, and V5 timings. High weed biomass caused corn grain yield loss at the V1 interseeding timing in 2015, and the V2 and V3 interseeding timings at one location in 2017. Soil from annual ryegrass plots had less NO₃ and NH₄ compared with the Tillage Radish® and crimson clover plots in April of 2016 following the 2015 interseeding; annual ryegrass was the only cover crop that overwintered. There were no differences in the C:N ration in the corn tissue samples from the 2016 indicator crop, indicating no immediate effect of cover cropping on plant N content in the year following interseeding. Annual ryegrass, crimson clover, and Tillage Radish® were not competitive with corn at any interseeding timing; however, research on weed control options for early interseeded cover crops is necessary to prevent corn yield loss. With a better understanding of weed control in corn interseeded in cover crops, farmers will have additional options for establishing cover crops in corn rotations.

Reflected light from plant canopies has a reduced red (R) to far-red (FR) ratio. Plants can sense changes in R:FR and modify their morphology and physiology which can affect growth and yield even in the absence of competition. This study evaluated effects of reflected FR from grass (Kentucky bluegrass) on growth and non-structural carbohydrate (NSC) partitioning of sugarbeet. There were two weed treatments (grass and no grass) and four harvest timings (15, 32, 50, and 77 days after panting (DAP)) in 2016, two weed treatments and five harvest timings (15, 26, 35, 48, and 55 DAP) in 2017. Treatments were arranged in a randomized complete block with 15 replicates in 2016 and 15 to 30 replicates in 2017. The study methods ensured no resource competition. The grass treatment significantly increased cotyledon length (2.2 vs 1.5 cm) and width (0.6 vs 0.5 cm) in 2016, and cotyledon length to width ratio in 2016 (3.8 vs 2.8) and 2017 (4.7 vs 4.3). The grass significantly reduced number of leaves such that there were three less leaves in the grass treatment at final harvest in both years. The grass treatment significantly reduced leaf area, shoot dry weight root diameter, and root dry weight in 2016. Generally, weed treatments did not significantly affect root and shoot NSC concentrations. Root soluble CHO increased with increasing DAP and ranged from 77 to 653 mg g^-1 while root starch decreased with increasing DAP and ranged from 34 to 88 mg g^-1. Same trends were observed in shoot NSC. Shoot soluble CHO ranged from 52 mg g^-1 to 197 mg g^-1 while starch ranged from 39 mg g^-1 at 50 DAP to 164 mg g^-1. Thus, NSC allocation in sugarbeet is maintained even under perceived competition. Therefore, the opportunity cost for optimal NSC allocation is reduced growth.

Paraquat is highly effective in controlling broadleaf weeds, but does not consistently control summer annual grass weeds. Paraquat use in burndown and fallow systems has increased in recent years to help control glyphosate- and ALS-resistant biotypes of kochia, Palmer amaranth and waterhemp. Producers prefer to control both grass and broadleaf weeds with a single herbicide application rather than multiple, sequential treatments in a burndown or fallow situation. To improve grass weed control efficacy of paraquat treatments, it may be necessary to add a tank-mix partner. Field experiments were conducted in 2017 in Riley and Kingman Counties in Kansas to evaluate POST grass weed control of paraquat tank-mixes. Atrazine (1121 g ai ha^-1), linuron (560 g ai ha^-1), metribuzin (420 g ai ha^-1), clethodim (85 g ai ha^-1), glufosinate (738 g ai ha^-1), and glyphosate (867 g ae ha^-1) were applied alone and in combination with paraquat (560 g ai ha^-1) using a 144 L ha^-1 carrier volume with the appropriate adjuvants. In addition, two three-way mixtures of paraquat plus glufosinate plus metribuzin or atrazine, and a 24 h sequential treatment of glyphosate followed by paraquat were included for a total of 16 treatments. Herbicide treatments were applied when giant foxtail (SETFA) and large crabgrass (DIGSA) reached 10-cm in height and grain sorghum (SORVU) reached 30-cm in height and visually evaluated for percent control three weeks after treatment.

Paraquat applied alone provided 43% control of SETFA, 53% control of DIGSA, and no control of SORVU. Glufosinate alone provided greater than 98% control of SETFA and DIGSA and 79% control of SORVU. Clethodim alone provided greater than 90% control of SETFA and SORVU and 55% control of DIGSA. Photosystem II (PSII)-inhibiting herbicides provided no control of grass species when applied alone. Glyphosate provided excellent control (100%) of all grass species when applied alone.

All two- and three-way mixtures provided at least 91% control of SETFA except for paraquat plus glyphosate (79% control). Three-way mixtures provided at least 93% control of DIGSA, whereas two-way mixtures provided less than 65% control. Two-way mixtures containing a PSII-inhibitor or glyphosate offered no control of SORVU; whereas paraquat plus clethodim resulted in 53% control, and paraquat plus glufosinate plus metribuzin provided 56% control of SORVU. The highest level of SORVU control was observed with paraquat plus glufosinate (88% control) or paraquat plus glufosinate plus atrazine (80% control).

Glyphosate fb paraquat resulted in a superior level of control (99% for SETFA and DIGSA and 69% for SORVU) when compared to the tank-mix of paraquat plus glyphosate across all species, which indicates the potential for antagonism in the paraquat plus glyphosate tank-mix.

Preemergence and postemergence herbicide options were evaluated to control perilla mint (Perilla frutescens (L.) Britton), a potentially deadly plant for livestock. The germination requirements of seed from weedy populations were also investigated to better understand and predict emergence timing. Postemergence applications of aminocyclopyrachlor blends, glyphosate, picloram + 2,4-D, aminopyralid + 2,4-D, and 2,4-D alone provided superior control of perilla mint when applied in the early reproductive growth stage. Picloram + 2,4-D and aminocyclopyrachlor + chlorsulfuron also provided soil residual activity and the most effective preemergence control, followed by, pendimethalin and aminopyralid + 2,4-D for at least 141 DAT. Seed from weedy populations tend to germinate in a range of night/day soil temperatures from 10/15 C to 25/30 C. Therefore, application and activation of the most effective preemergence treatments should be made before these temperatures occur in areas where weedy perilla mint populations are found.

A Palmer amaranth (Amaranthus palmeri S. Watson) biotype has evolved resistance to photosystem (PS) II- (atrazine) and 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicides (mesotrione, tembotrione, and topramezone) in seed corn production field in Nebraska. The labeled rates of atrazine and mesotrione were not able to control resistant Palmer amaranth when applied individually; however, their tank-mixture applied POST resulted in greater control under greenhouse and field conditions. A study was conducted under growth chamber conditions to investigate the physiological basis of synergism of atrazine and mesotrione tank-mixtures by determining the absorption and translocation of ¹⁴C-atrazine or ¹⁴C-mesotrione applied alone or in a tank-mixture. Absorption of ¹⁴C-mesotrione tank-mixed with atrazine ranged from 25 to 55% of applied compared to 27 to 37% % when ¹⁴C-mesotrione was applied alone at 4 to 72 h after application in resistant plants. Similarly, in susceptible plants, absorption of ¹⁴C-mesotrione tank-mixed with atrazine ranged from 24 to 51% of applied compared to 26 to 38% when ¹⁴C-mesotrione was applied alone. Maximum absorption of ¹⁴C-mesotrione tank-mixed with atrazine was greater compared to ¹⁴C-mesotrione applied alone in resistant as well as susceptible Palmer amaranth. However, the time required for 90% of maximum absorption of ¹⁴C-mesotrione tank-mixed with atrazine was not different than ¹⁴C-mesotrione applied alone in resistant as well as susceptible plants. Additionally, mesotrione in tank-mixture with ¹⁴C-atrazine did not affect the maximum absorption and time required for 90% of the maximum absorption of ¹⁴C-atrazine compared to ¹⁴C-atrazine applied alone in resistant and susceptible plants. Increased absorption of mesotrione with addition of atrazine in tank-mixture appears to be the basis of synergism for Palmer amaranth control in Nebraska.

Few studies have been conducted to understand the extent to which newly-labeled dicamba formulations are present in the air following application.  The objectives of this research are to determine the effects of time of application, surface temperature inversions and new formulations on the concentration of dicamba detected in the air following application.  A series of field experiments were conducted near Columbia, Missouri during the summer of 2017.  Air samplers were placed equidistantly within 6 x 31 m plots and 31 cm above the canopy prior to dicamba applications to obtain background levels of dicamba. Air samplers were removed immediately prior to dicamba application, and then returned to the treated field 30 minutes following application.  Applications were made at the 1X rate for each product, and plots were a minimum of 480 meters apart. Glass fiber filters and polyurethane foam substrates (PUF plugs) from the air sampling machines were replaced at set intervals throughout the experiments, which extended up to 72 or 96 hours following application.  A methanol wash was used to extract dicamba from the filter paper and PUF plugs, and HPLC-UV was utilized to detect dicamba. Preliminary results from two experiments in which Xtendimax plus VaporGrip and Engenia were applied at the same time on the same evening showed the majority of dicamba, regardless of formulation, was detected in the first 0.5 to 8 hours after treatment (HAT).  The average concentration of dicamba for the Xtendimax treatment was 26.5 to 31.1 ng/m³ while that for Engenia was 18.4 to 20.5 ng/m³. By 24 to 48 HAT, dicamba levels had declined to less than 3 ng/m³ for each treatment.  In a separate experiment, dicamba concentrations of at least 20 ng/m³ and 12 ng/m³ were detected following Engenia and Xtendimax plus VaporGrip applications made at the same time on the same afternoon, respectively. The majority of dicamba from the Engenia treatment was detected 16 to 24 HAT while the majority of Xtendimax plus VaporGrip was detected 8 to 16 HAT.  Results from two experiments in which both formulations were applied without inversion conditions present and with the addition of glyphosate revealed the majority of dicamba was detected 0.5 to 8 HAT regardless of formulation at concentrations of 21.6 and 20.5 ng/m³ for Xtendimax plus VaporGrip and Engenia, respectively. Across four experiments in which Xtendimax plus VaporGrip was applied during inversion conditions with glyphosate, dicamba concentrations were 34.2 ng/m³ 0.5 to 8 HAT, which was higher than any other sampling interval. Spearman’s correlation was used to study relationships between dicamba concentrations, lapse rates, and maximum wind speeds for 0.5 to 8 HAT, 8 to 16 HAT, and 16 to 24 HAT samples in which Xtendimax was applied alone or with the addition of glyphosate (n=83).  A correlation coefficient of -0.72031 (P < 0.0001) was observed between maximum air temperature and dicamba concentration for all samples, suggesting a trend between environmental conditions and dicamba concentration.  Wind speed can be an indicator of atmospheric stability.  The correlation coefficient between maximum wind speed and dicamba concentration was -0.72265 (P < 0.0001).  These preliminary results indicate that dicamba can be detected in the air following applications and to an extent, concentrations are likely influenced by atmospheric stability. Dicamba detected at 9.25 ng/m³ in the afternoon following application also suggests that volatilization of the chemical is a contributing factor in off-site movement. 

A native acetolactate synthase (ALS)-inhibiting herbicide-resistance gene has been introduced in grain sorghum [Sorghum bicolor (L.) Moench] through conventional breeding (Inzen^TM sorghum–DuPont Pioneer). ALS-resistance in grain sorghum will provide a new herbicide option for post-emergence grass weed control, but commercialization of this trait raises concerns of the transfer of resistance alleles to its weedy relatives through pollen-mediated gene flow. Upon commercialization, this nuclear trait may transfer to closely related species of sorghum, thus, an opportunity exists to empirically monitor gene flow at a regional scale and document reproductive success under different cropping systems that include Inzen^TM sorghum. High-throughput screening of Sorghum weed populations for Inzen^TM alleles will be needed to support large-scale ecological monitoring. Molecular inversion probes (MIPs) were constructed to target five DNA sequences known to confer resistance within ALS as well as thirty published Sorghum simple sequence repeats (SSRs). The MIPs constitute a robust toolkit for high-throughput, simultaneous detection of resistance alleles present in shattercane [Sorghum bicolor (L.) Moench ssp. drummondii (Nees ex Steud.) de Wet ex Davidse] and johnsongrass [Sorghum halepense (L.) Pers]. Use of MIPs alongside other developed markers can be used to evaluate the possibilities of crop-to-weed gene flow and provide insight into the baseline genetic diversity of many wild shattercane and johnsongrass populations before ALS-resistant grain sorghum is introduced into the market.

Organic small grain production is experiencing a resurgence in the northeastern United States, but weeds continue to be the foremost production problem facing current and potential growers. A planting strategy called band sowing is being developed and tested in Europe and is hypothesized to provide superior weed control. In band sowing, grains are planted in wide bands instead of rows, intending to maximize crop competitiveness and weed suppression intra-band, while weeds in the inter-band zone are managed with aggressive cultivation using sweeps. To assess the performance of band sowing, field experiments were conducted at sites in Maine and Vermont using spring barley as a test crop and condiment mustard (Sinapis arvensis) as a surrogate weed. Three treatments were planted in rows, at spacings of 11, 17, and 23 cm, and target crop populations of 500, 325, and 325 plants m^-2, respectively. Two band sown treatments were tested, both planted in 13 cm bands, with 15 cm between bands, and target crop populations of 325 plants m^-2. All treatments received tine harrowing, and the 23 cm row treatment and one band sown treatment received inter-row and inter-band cultivation. Systems were evaluated based on crop yield and quality, and weed density and biomass. We expected the band sowing strategy to provide superior weed control, however, results varied among the two experimental years and sites. In year one at the Maine site, inter-band cultivation was performed late, after surrogate weeds were well established. In this site-year the band sown treatment receiving inter-band cultivation had the same surrogate weed biomass as the region’s standard practice (17 cm rows, 325 plants m^-2), and the narrow-row, high-density treatment (11 cm rows, 500 plants m^-2) had the lowest surrogate weed biomass and greatest yield. In year two at the Maine site, post-emergence tine harrowing, and inter-row and inter-band cultivation events were performed optimally, resulting in lesser surrogate weed biomass across all treatments when compared with the year one Maine site, but differences among treatments were insignificant. A single sowing density was tested with the band sown treatment in this study, which may have been suboptimal. Despite the promising theoretical rationale behind the practice of band sowing, there remains little information on optimal seeding rates, band width, and inter-band spacing; further experimentation is needed to determine the effects of alterations made to these variables.

The recent registrations of soybeans resistant to 2,4-D and dicamba will likely increase the use of these herbicides in Michigan. The expanded use of these herbicides has caused some concerns for sugarbeet growers due to their susceptibility to 2,4-D and dicamba. Sugarbeet exposure to these herbicides may occur from tank-contamination, particle drift, or volatility. While sugarbeet injury and yield reductions are a concern for growers, the potential for 2,4-D or dicamba residues in the harvested crop are of greater concern.In 2016 and 2017, research was conducted at two locations in Michigan to: 1) determine the impact of sub-lethal doses of 2,4-D and dicamba on sugarbeet injury and yield, and 2) quantify the levels of 2,4-D and dicamba residues in the harvested crop. Dicamba and 2,4-D were applied at rates ranging from 0.0625-2% of the field use rates when sugarbeet were at the 2-leaf, 6-leaf, and 14-leaf growth stages, assuming a field use rate of 1.1 kg a.e. ha^-1 for both herbicides. Sugarbeets were evaluated for injury for 3 weeks following application and at harvest, and harvested for yield. Sugarbeets were sampled and tested for residues 2 weeks after treatment (WAT) and at harvest. At harvest, samples were split into above and belowground tissue. Sugarbeet injury from 2,4-D and dicamba was greatest 2 WAT. Rates as low as 0.25% of 2,4-D and 0.5% of dicamba caused 10% injury. Regardless of application timing, sugarbeet injury ranged from 34-40% and 34-43% from a 2% rate of 2,4-D and dicamba, respectively. Averaged across application timings, the 2% rate of 2,4-D and dicamba reduced sugarbeet yields 20-31% and 17%, respectively. This was also reflected in recoverable white sucrose per hectare. Averaged over all rates, yield was reduced 8-17% by 2,4-D at the 14-leaf timing at both locations.From the 2016 samples, at harvest 2,4-D residues were less than 0.0012 and 0.0037 ppm in above and belowground tissue, respectively, for the 2% rate, regardless of application timing. Dicamba residues at the 2% rate were less than 0.0497and 0.081 ppm in above and belowground tissue, respectively.While these residues appear to be low, currently there are no maximum residue limits (MRLs) set for 2,4-D or dicamba in sugarbeet. As a comparison, the current MRLs for 2,4-D and dicamba in sugarcane are 0.05 and 0.3 ppm, respectively. We are currently analyzing the 2017 samples for 2,4-D and dicamba residues.

False-green kyllinga (Kyllinga gracillima) is a perennial sedge and pervasive weed of cool season turfgrass. Two greenhouse experiments and four field experiments were conducted to test the efficacy of various postemergence herbicides against false-green kyllinga in 2017.

Field treatments included both single and sequential applications of: imazosulfuron (420 g/ha), imazosulfuron (740 g/ha), halosulfuron-methyl (70 g/ha), sulfentrazone (110 g/ha), a single application of sulfentrazone (110 g/ha) + imazosulfuron (420 g/ha) and a non-treated control. Treatments were arranged in a randomized block design. False-green kyllinga injury was rated visually from 0-100% (with 0% being no injury and 100% completely necrotic). ‘Control’ was determined by assessing the percent cover of false-green kyllinga in each plot compared to the non-treated control at 4, 8, and 12 weeks after initial treatment (WAIT). Grid intersect counts were also conducted at 12 WAIT. For each experiment, data were analyzed in SAS (v9.4) using a single factor RCBD and Fisher’s Protected LSD (α=0.05).

Two experiments were conducted in IN, one at Bloomington Country Club (Bloomington, IN) and Victoria National Golf Club (Newburgh, IN) on creeping bentgrass (Agrostis stolonifera) fairways. Applications were made on May 30th and June 30th. Another two experiments were conducted at Stone Harbor Golf Club in Cape May, New Jersey. Site 1 was a driving range maintained at 6.35cm and Site 2 maintained as a fairway. Both sites consisted of creeping bentgrass and natural populations of false-green kyllinga.  Initial and sequential applications were made on 13 June and 13 July, respectively.

At 12 WAIT all applications of imazosulfuron (regardless of rate of number of applications) provided >97% false-green kyllinga control. Two sequential applications of halosulfuron-methyl provided >95% control but the single application only provided 64% control. Sulfentrazone provided 50% in site 2 and no control in site 1 at 12 WAIT. In IN, sequential applications of imazosulfuron reduced kyllinga cover by 100% at 12 WAIT. Sulfentrazone + imazosulfuron reduced kyllinga cover by >89% at 12 WAIT, while sulfentrazone alone reduced cover by 35%.

A greenhouse experiment was conducted to further explore efficacy of single applications of imazosulfuron, halosulfuron-methyl, imazosulfuron and imazosulfuron + carfentrazone-ethyl on false-green kyllinga. Herbicides were applied at 0.1, 0.25, 0.5, 0.75, 1, 2, 4 and 10 times registered use rates for each herbicide. Treatments included:  a nontreated control, imazosulfuron (42, 105, 210, 315, 420, 840, 1680 and 4200 g/ha), halosulfuron-methyl (6.8, 17.3 34, 52.5, 70, 140, 280 and 700 g/ha), sulfentrazone (28, 70, 140, 210, 280, 560, 1120 and 2800 g/ha) and sulfentrazone + carfentrazone-ethyl (77, 153, 230, 306, 640, 1230, and 3060 g/ha). Treatments were replicated four times and arranged in a completely random design; the experiment was repeated in time. Treatments were applied to 12.7x12.7cm pots of false-green kyllinga in peat-based growing medium. Visual injury (0-100%) was measured weekly and clipping yield was collected biweekly.

Greenhouse results supported field findings. Regression analyzes were conducted in SAS (v9.4) to characterize the dose response curve for each herbicide. 

Cover crops have increased in popularity in Midwest corn and soybean production systems in recent years. However, little research has been conducted to evaluate how cover crops and pre-emergence, residual herbicides are most appropriately integrated together in a soybean production system.  Field studies were conducted in 2016 and 2017 to evaluate summer annual weed control in response to six different cover crops combined with herbicide applications, which consisted of pre-plant applications of glyphosate plus 2,4-D with or without sulfentrazone plus chlorimuron.  Pre-plant applications were made at two different timings, 21 and 7 days prior to planting (DPP).  The cover crops evaluated included hairy vetch, cereal rye, Italian ryegrass, oats, Austrian winter pea, wheat and a mixture of hairy vetch and cereal rye.  These same herbicide treatments were applied to tilled and non-tilled soil without any cover crop for comparison.  Visual ratings of weed control, groundcover and cover crop control were taken at regular intervals after planting. Weed density counts were conducted when soybean reached R5.  Soil samples were taken 0, 14, 28, 56 and 84 days after the two pre-plant timings to quantify sulfentrazone residue levels.  Data was subjected to analysis using the PROC GLIMMIX procedure in SAS, and means were separated using Fisher’s Protected LSD (P≤0.05). Greater than 73% control of waterhemp was achieved across all cover crops that included a residual herbicide treatment, which was higher than the control achieved by any of the treatments that included glyphosate plus 2,4-D alone.  When applied PRE, herbicide treatments with sulfentrazone had significantly higher weed control 21 DPP than 7 DPP.  When applied POST, herbicide treatments with sulfentrazone had significantly higher weed control 7 DPP than 21 DPP.  Across all sampling time points in 2016, soil samples from the tillage treatment contained the highest concentration of sulfentrazone (181.09 parts per billion), which was significantly higher than all other treatments.  For all cover crop and tillage treatments in 2016, sulfentrazone was detected 84 days after application.  Across all tillage and cover crop treatments in 2016, sulfentrazone applied 21 DPP was detected in higher concentrations 0 and 28 days after application compared to 7 DPP. Results from this research will provide useful information on the integration of cover crops and residual herbicides in soybean production systems.  

Palmer amaranth (Amaranthus palmeri) is a troublesome weed with widespread herbicide resistance that has become difficult for growers to control. A population of Palmer amaranth (SF-R) was identified in Stafford County, KS, as resistant to POST applications of mesotrione, a 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitor herbicide,and to atrazine, a photosystem II (PSII) inhibiting herbicide. Anecdotal evidence from the field has indicated that PRE applications of mesotrione provided modest control of HPPD resistant populations. Greenhouse studies were conducted to evaluate the PRE efficacy of two HPPD-inhibiting herbicides (mesotrione and isoxaflutole) and atrazine on SF-R population and an additional susceptible population from Riley County, KS (RL-S). Reduced sensitivity and greater seedling survival were observed in the SF-R population for all three herbicides at recommended field use rates. Resistant-to-sensitive (R/S) ratios for mesotrione, isoxaflutole and atrazine were 8.4, 4.4, and 6.7 respectively. Field experiments were conducted in 2017 at a producer’s field in Barton County, KS with reported HPPD-inhibitor and atrazine resistant population and in a confirmed HPPD-inhibitor susceptible and atrazine resistant population in Reno County, KS. The experiment was a randomized complete block design with 18 treatments applied PRE into a non-crop scenario in Reno County and in grain sorghum in Barton County. Three HPPD-inhibiting herbicides: mesotrione, isoxaflutole, and bicyclopyrone plus bromoxynil were applied at multiple rates with and without 2240 g ha^-1 of atrazine. Palmer amaranth control was visually evaluated 4 weeks after treatment. At both sites, mesotrione (89%) provided better control than isoxaflutole (81%) on average across both rates. Bicyclopyrone was the least effective HPPD-inhibiting herbicide across both rates providing 55 and 65% control in Barton County and Reno County respectively. Higher rates of 210 g ha^-1 of mesotrione and isoxaflutole performed better than 105 g ha^-1 providing 89% compared to 81% control. For all herbicides, the higher rate provided better control than the lower rate. The addition of atrazine increased weed control from 82 to 88% when added to all HPPD-inhibiting herbicides. For mesotrione treatments across both sites, Palmer amaranth control was reduced to less than 90% when rates lower than 210 g ha^-1 were applied. When comparing sites, weed control efficacy was reduced from 92 to 79% in the resistant population compared to the susceptible populations for applications of mesotrione. Overall reduction of weed control in Barton County demonstrated reduced sensitivity to soil applied HPPD-inhibiting herbicides compared to Reno County, but the same trends were observed. HPPD-inhibiting herbicides should be used at maximum labelled use rates and tank-mixed with atrazine for best residual control of all Palmer amaranth populations as part of an integrated weed management plan.

Cotton and Peanut Response to Fluridone. D. L. Teeter*¹ , T. A. Baughman¹ , P. A. Dotray² , R. W. Peterson¹ ; ¹Oklahoma State University, Ardmore, OK, ²Texas Tech University, Lubbock, TX

Renewed interest in fluridone can be correlated with the increase in resistant weed populations throughout the Southern Great Plains.  Palmer amaranth (Amaranthus palmeri) resistance to glyphosate in crops grown in this area is the leading front runner in this issue.  Studies conducted in cotton and peanut were done to evaluate tolerance to differing rates of fluridone applied alone and in combination with other herbicides preemergence.

Cotton was planted during the 2016 and 2017 growing season at the Oklahoma State University research stations near Fort Cobb and Tipton and the Texas Agricultural Experiment Station near Lubbock. The trials were irrigated at Fort Cobb and Lubbock and dryland at Tipton.  Fluridone was applied preemergence at 0.168 (1X) and 0.337 (2X) kg ai/ha^-1 alone and in combination with fluometuron at 0.84 (1X) and 1.68 (2X) kg ai/ha^-1.  Peanut trials were conducted during the 2015, 2016, and 2017 growing season near Fort Cobb.  Fluridone was applied preemergence at 0.168 (1X) and 0.337 (2X) kg ai/ha^-1 alone and in combination with flumioxazin at 0.107 kg ai/ha^-1 and s-metolachlor at 1.42 kg ai/ha^-1.  All treatments were applied with a CO₂ backpack sprayer in 94 L/ha^-1.  The entire trial area for all studies were kept weed free and standard production practices were used throughout the growing season.  Cotton and peanuts were evaluated for visual injury throughout the growing season.  Cotton plant stand counts and plant height were recorded during the season.  All trials were harvested with commercial harvesting equipment modified for small plot harvest to determine cotton and peanut yields and grade.

Cotton injury evaluated at 2, 4, and 8 WAP was less than or equal to 10%, except for treatments sprayed with a 2X rate of fluridone + fluometuron at all 3 locations in 2016.  End of season injury was less than 5% at both Oklahoma locations for 2016 and 2017.  Cotton stand counts and heights were not affected by the 1X rate of fluridone applied alone or in combination with fluometuron either year.  Fluridone + fluometuron applied at the 2X rate reduced cotton stand counts and plant height at both Oklahoma locations but did not affect cotton at Lubbock in 2016. Cotton yield was not affected by any treatment regardless of location for both years, except with fluridone + fluometuron at the 2X rate at Fort Cobb in 2017.

Due to a significant amount of rainfall near planting peanut injury in 2015 at Fort Cobb was 15% or greater when fluridone was applied alone or in combination with flumioxazin and s-metolachlor.  Injury was 40% or greater when fluridone was applied at a 2X rate 8 and 12 WAP.  In 2015, yields were impacted by all fluridone treatments.  In 2016 no significant injury was observed except 4 WAP in treatments applied with a 2X rate of fluridone.  Yields were not affected by any treatment in 2016.  Peanut injury for 2017 was 13% or greater for all treatments early season.  This injury was 1% or less late season.  There were no differences in peanut yields between any fluridone treatments in 2017.  This research indicates that at the 2X rate and under extreme weather conditions injury can occur with fluridone.  However, under normal weather conditions and at the labeled use rate it can potentially be an effective tool for managing resistant weeds in Southern Great Plains cropping systems

Waterhemp (Amaranthus tuberculatus var. rudis) is a small-seeded broadleaf weed, which emerges throughout the growing season. Glyphosate resistant (GR) waterhemp was discovered in Ontario in 2014. If left uncontrolled, yield decreases of up to 73% have been observed. Dicamba and glyphosate-resistant soybean (Roundup Ready 2 Xtend soybean) allow for dicamba to be applied pre-plant, pre-emergent (PRE) and/or post-emergent (POST). The objective of this study was to determine the control of GR waterhemp in dicamba-resistant soybean with more than one herbicide mode of action applied PRE or in a two-pass system (PRE fb POST), with glyphosate/dicamba (Roundup Xtend) applied POST. At 56 days after application (DAA), glyphosate/dicamba, pyroxasulfone, s-metolachlor/metribuzin, pyroxasulfone/sulfentrazone and flumioxazin/pyroxasulfone controlled GR waterhemp 44, 80, 87, 91 and 96%, respectively. The addition of glyphosate/dicamba to pyroxasulfone, s-metolachlor/metribuzin, pyroxasulfone/sulfentrazone and flumioxazin/pyroxasulfone PRE controlled GR waterhemp 84, 89, 91 and 92%, respectively. In a two-pass program, pyroxasulfone, s-metolachlor/metribuzin, pyroxasulfone/sulfentrazone and flumioxazin/pyroxasulfone PRE controlled GR waterhemp 68, 80, 77 and 84%, respectively. The same PRE herbicides, followed by glyphosate/dicamba POST, improved control of GR waterhemp to 93, 99, 98 and 99%, respectively. In conclusion, the addition of glyphosate/dicamba pyroxasulfone, s-metolachlor/metribuzin and pyroxasulfone/sulfentrazone applied PRE, resulted in a small increase in GR waterhemp control. Additionally, a two-pass program of an effective soil applied herbicide followed by glyphosate/dicamba POST controlled GR waterhemp >85%.

Herbicide resistant weeds are a problem that likely every cotton (Gossypium hirsutum) producer encounters in West Texas.  Heavy reliance of a single herbicide mode of action to control troublesome weeds in previous years has led to the development of glyphosate-resistant Palmer amaranth (Amaranthus palmeri S. Wats).  Two recently released herbicide resistant traits in cotton (XtendFlex^TM and Enlist^TM) provide producers additional options to control troublesome weeds including Palmer amaranth.  Prior to the release of these traits, group O herbicides could not be applied during the cotton growing season and some counties have calendar restrictions.  To minimize the impact of the development of herbicide resistance to these new products, it will be critical to utilize weed management strategies that include multiple herbicide modes of action as well as mechanical weed control.  The objective of this research was to evaluate season-long weed control in XtendFlex^TM and Enlist^TM cotton using several different weed management systems that include the use of dicamba in XtendFlex^TM cotton and 2,4-D choline in Enlist^TM cotton.  A field study was established in a randomized complete block design in Lubbock, Texas using a variety of herbicides at different application timings to best manage Palmer amaranth.  All treatments include bed listing followed by rod weeding to ensure no weeds were emerged prior to the initiation of the trial.  Weed management treatments included one or more of the following:  trifluralin at 1.12 kg/ha applied preplant; prometryn at 1.35 kg/ha applied preemergence; S-metolachlor at 1.36 kg/ha, dicamba at 0.56 kg/ha, glyphosate at 1.12 kg/ha, 2,4-D choline at 1.06 kg/ha, and glyphosate at 1.13 kg/ha applied early and mid-postemergence; and interrow cultivation.  The greatest end-of-season control of Palmer amaranth was observed from systems that included more than two weed management inputs with a residual herbicide.  Differences in Palmer amaranth control varied from 91% when two weed management practices were utilized to 100% on systems that used five or more inputs. 

Watermelon grafting was initially adopted for management of diseases caused by soilborne pathogens, and there is evidence that grafted plants confer additional benefits such as tolerance to drought, salinity, and suboptimal temperatures. However, there is little research available to compare weed-competitive ability of grafted and nongrafted watermelon. Field studies were conducted at the Horticultural Crops Research Station in Clinton, North Carolina in 2015 and 2017. ‘Exclamation’ triploid (seedless) watermelon was used as the scion for all grafted plants. Grafting treatments included two interspecific hybrid squash rootstocks (ISH) rootstocks ‘Carnvor’ and ‘Kazako’, as well as nongrafted Exclamation as the control. Weed treatments included Palmer amaranth at densities of 1, 2, 3, and 4 Palmer amaranth per triploid watermelon vine (0.76 m^-1 row) as well as a weed-free control. Both grafting treatment and Palmer amaranth density had a significant effect (P < 0.05) on marketable yield and fruit number. Watermelon yield reduction was described as a rectangular hyperbola model, and 4 Palmer amaranth 0.76 m^-1 row reduced marketable yield 41, 38 and 65% for Exclamation, Carnivor and Kazako, respectively. Neither grafting treatment nor Palmer amaranth density had a significant effect on SSC or on the incidence of hollow heart in watermelon fruit. Total Palmer amaranth seed number was similar across weed population densities, but seed number per female Palmer amaranth decreased according to an exponential decay curve. Thus, increasing weed population densities resulted in increased intraspecific competition among Palmer amaranth. While grafting may offer benefits regarding disease resistance and tolerance to abiotic stresses, there is no immediate benefit regarding weed-competitive ability and a consistent yield penalty was associated with grafting, even in weed-free treatments.

Tolpyralate is a 4-hydroxyphenyl-pyruvate dioxygenase (HPPD)-inhibiting herbicide under evaluation for post-emergence (POST) weed management in corn. A total of six field studies were conducted in Ontario, Canada over a three-year period (2015, 2016 and 2017), to determine the biologically-effective dose of tolpyralate for the control of seven annual weed species. Tolpyralate was applied POST alone at doses ranging from 3.75-120 g a.i. ha^-1 or tank-mixed at a 1:33.3 ratio with atrazine at doses ranging from 125-4000 g ai ha^-1. Regression analysis was conducted to determine the predicted tolpyralate, and tolpyralate + atrazine doses required to achieve 90% control of each species, 8 weeks after application. The required dose of tolpyralate [g ai ha^-1] for 90% control is presented in parenthesis for the following species: velvetleaf (Abutilon theophrasti (Medik.)) [3.2], common lambsquarters (Chenopodium album (L.)) [5.6], common ragweed (Ambrosia artemisiifolia (L.)) [7.3], green/redroot pigweed ((Amaranthus powellii (S.) Wats.) + A. retroflexus (L.)) [8.5], green foxtail (Setaria viridis (L.) Beauv.) [15.5], wild mustard (Sinapis arvensis (L.)) [>120] and ladysthumb (Polygonum persicaria (L.)) [>120]. The required dose of tolpyralate plus atrazine [g a.i. ha^-1] for 90% control is presented in parenthesis for the following species: velvetleaf [2.4 + 81], common lambsquarters [2.5 + 82], common ragweed [5.8 + 194], green/redroot pigweed [5.9 + 198], wild mustard [7.4 + 245], green foxtail [11.3 + 377] and ladysthumb [13.1 + 436]. Based on these studies, tolpyralate + atrazine, applied POST, at the proposed label rate range of 30-40 + 500-1000 g a.i. ha^-1 provides excellent broad-spectrum weed control in corn in Ontario. 

Italian ryegrass (Lolium perenne L. spp. multiflorum) is a troublesome weedy species spread worldwide. Its control has been chiefly dependent on herbicides and, as result of the selection pressure, herbicide-resistant populations are relatively common. Recently, Italian ryegrass biotypes from Oregon were reported with resistance levels up to 2.4-fold the field rate of glufosinate compared to a known-susceptible (S). Interestingly, while one glufosinate-resistant biotype exhibited an altered target site (MG), another biotype (OR1) did not. The objective of this research was to evaluate the absorption, translocation and metabolism of glufosinate in these three Italian ryegrass biotypes from Oregon. Response to glufosinate was determined using an ammonia accumulation bioassay. The experiment was conducted in a completely randomized 3x7 factorial design (biotype x timepoint) by incubating leaf segments in glufosinate solutions at increasing concentrations for 24 hours under constant light (PAR = 1000 µmol m² s^-1) at 16/10 °C (day/night). Herbicide absorption, translocation, and root exudation were assessed with ¹⁴C-glufosinate as a tracer and the experimental design was a 3x5 factorial arrangement of biotype x timepoint.  ¹⁴C-glufosinate was applied to the youngest fully-expanded leaf of plants at the 3-leaf stage.  Plants were destructively sectioned into upper treated leaf, treated section, lower treated leaf, culm and roots and ¹⁴C quantified. Herbicide metabolism was evaluated using HPLC-RAD to determine the relative amounts of ¹⁴C-glufosinate and metabolites at 6, 24, 48 and 72 hours after treatment in a completely randomized 3x4 factorial (biotype x timepoint) design. Biotypes MG and OR1 accumulated less ammonia compared to S (P<0.05), confirming resistance. More ¹⁴C-glufosinate translocated out of the treated leaves of S (P<0.05), whereas OR1 retained the greatest amount of ¹⁴C in the treated leaf, notably towards the leaf apex. Glufosinate metabolism did not differ between S and MG; however, OR1 metabolized more ¹⁴C-glufosinate compared to S at 48 and 72 hours after treatment (P<0.05). These results confirm that both target site and non-target-site mechanisms of resistance exist in Italian ryegrass populations from Oregon.  Because ryegrass is an obligate outcrossing species, it is likely that some populations could accumulate both target- and non-target-site mechanisms which will further complicate management.

Evaluating the Relative Contributions of Crop Rotation, Tillage, and Herbicide Diversity for Proactive Herbicide Resistant Kochia (Kochia scoparia) Management. E.G. Mosqueda*¹, A. Kniss¹, P. Jha², N.C. Lawrence³, G.M. Sbatella¹, ¹University of Wyoming, Laramie WY, ²Montana State University, Huntley, MO, ³University of Nebraska-Lincoln, Scottsbluff, NE.

Cultural and mechanical practices are often recommended in agronomic settings to impede the buildup of weeds, however, there is a lack of knowledge about their impact on herbicide resistant weed evolution. Kochia (Kochia scoparia) is a summer annual tumbleweed which has become problematic for growers throughout western United States in part because of evolved resistance to numerous herbicides. Therefore, research on non-chemical forms of weed control is critical and should be continuously investigated. Field sites were established in 2014 in Wyoming, Nebraska, and Montana to quantify the impact of varying crop rotation, tillage, and herbicide regimes on kochia populations. A known proportion of ALS-resistant kochia was established summer of 2013 prior to imposition of treatments. Four crop rotations consisted of continuous corn, corn-sugarbeet, corn-dry bean-corn-sugarbeet, and corn-dry bean-wheat-sugarbeet. Herbicide treatments included complete reliance on ALS inhibitors, mixtures including ALS inhibitors, or an ALS herbicide rotation. Tillage treatments included annual intensive tillage or minimum tillage. Kochia densities were estimated in summer 2016 by counting the number of kochia plants within a randomly placed m² quadrant per plot. Data was analyzed using a generalized linear mixed effects model. The least diverse crop rotations (continuous corn, and corn-sugarbeet) on average usually contained higher kochia densities than plots with more diverse crop rotations (corn-dry bean-corn-sugarbeet, and corn-dry bean-wheat-sugarbeet). Within these plots, those treated with either a mixture including ALS inhibitors, or an ALS herbicide rotation usually contained less kochia on average than plots treated with complete reliance on ALS inhibitors. In addition, plots which were annually intensively tilled usually contained lower kochia densities on average compared to plots that were minimally tilled.  

Injury and yield loss in sensitive cotton and soybeans can occur from exposure to dilute concentrations of 2,4-D and dicamba. The availability of older formulations not labeled for use in new weed control systems complicates crop injury diagnosis.  Crop response from an event involving a legally applied auxin herbicide does not differ visually from that of older, non-labeled herbicides.  Fourier-Transform Infrared spectroscopy (FTIR) is an accurate and inexpensive way to analyze samples for the presence of different chemical functional groups. FTIR has great potential to differentiate plant tissue damaged by herbicides with identical active ingredients that differ only in the molecular structure of the additives they are formulated with. An experiment was conducted to develop a FTIR method to identify various auxin herbicides present in cotton and soybean tissue damaged by a dilute herbicide rate. Principal component analysis (PCA) and linear discriminant analysis (LDA) were used to model sample FTIR spectra. LDA models for soybean samples taken 0, 3, 7, 14, and 28 DAT identified auxin formulation with 89, 92, 84, 91, and 93% accuracy, respectively. Cotton samples taken at the same timings yielded herbicide identification with 90, 87, 90, 84, and 89% accuracy, respectively. IR spectrum peaks at 1687-1560 cm^-1 and 1633-1556/1395-1350 cm^-1 were critical for sample identification in soybeans and cotton, respectively. Future efforts will seek to develop models for more herbicide formulations and multiple rates and tissue types.

Despite improved technologies, the large-scale adaptation of conservation tillage, increasing area under herbicide-tolerant crops, over reliance on herbicides, lack of diversity in herbicide-based weed management in combination with prevailing climate change scenario intensifying the weed problems in Australia. Increased dependence on herbicides resulted in the frequent and widespread evolution of herbicide-resistant weeds as well as weed population shifts in Australian cropping systems. Still, plentiful effective and tactical weed management options are available that will increase the crop yield and profitability in the country, although these options are not widely adopted. Ensuring diversity in weed management through the integration of improved weed management techniques (e.g., harvest weed seed control, weed seed catchment, burning) will reduce the selection pressure on weeds, which minimizes the evolution of herbicide resistance and will prevent their shifts towards dominance or difficult-to-control weeds. Increasing crop competitiveness through altering crop management practices need to be explored more broadly as a potential option for sustainable weed management. In future, development of precision weed management technologies for herbicide spraying and targeted tillage might help in reducing herbicide usage and increase profitability. In addition, formation of a fully integrated weed-activated mapping and spraying system is in progress and is advancing rapidly. Significance of allelopathy for weed management in agricultural systems, either in the form of allelopathic cultivars or plant-derived chemicals, cannot be neglected; however, the use of allelopathy is still in an infant stage in Australian agriculture. In addition, weed seed predation through insect pests and pathogens will open up diverse ways for the biological weed management and the development of bioherbicides or mycoherbicides. Furthermore, a nationally coordinated approach involving all government levels in collaboration with industries, landholders, and the community is required to establish appropriate legislative, educational, and coordinated frameworks against weeds.

Determining Genetic Diversity of Glyphosate Resistant Giant Ragweed (Ambrosia trifida) Using Molecular Markers

Abstract

Giant Ragweed (Ambrosia trifida) is a problematic weed that occurs throughout North America. Significant crop losses may occur where infestations are not properly controlled. The weed is very competitive due to its aggressive growth habits, and, in recent years, has developed resistance to several herbicides including glyphosate. The objectives of this study were to 1) confirm glyphosate resistance in several biotypes from MS (MS-R1, MS-R2, MS-R3), TN (TN-R), and OH (OH-R) having survived field doses of glyphosate 2) characterize levels of glyphosate resistance through dose rate response study, and, 3) measure genetic diversity among and within resistant and susceptible biotypes using microsatellite markers. The whole plant bioassay confirmed glyphosate resistance among all suspected resistant biotypes and provided complete control of the susceptible MS biotype at 14 DAT. Dose rate response study identified glyphosate resistance levels to be 2.2 and 6X in MS-R and TN-R biotypes, respectively. Seven simple sequence (SSR) repeat markers were used to determine levels of genetic diversity among six different biotypes. Plant tissues were harvested from 10 plants per biotype, DNA extracted, and PCR analysis conducted using seven SSR primers. PCR products were then visualized in 1% acrylamide gel. Gel images were scored using Cross Checker, and Nei's gene diversity, genetic distance, Shannon’s index, and cluster dendrogram were generated using POPGEN. Total genetic diversity and Shannon’s index among populations were 0.5324 and 0.8616, respectively. However, genetic diversity and Shannon’s index within the populations were similar: MS-S 0.5086/0.8188, MS-R1 0.5184/0.8433, TN-R 0.5018/0.8065, MS-R2 0.5033/0.8212, MS-R3 0.5264/0.8523, and OH-R 0.5249/0.8490. About 85% of the seven markers used in this study were polymorphic. Cluster analysis revealed the susceptible biotype to be distinct from the resistant biotypes. These results were consistent with our hypothesis that resistant biotypes are more genetically diverse. However, to obtain greater accuracy, additional primers will need to be incorporated in follow-up studies.

Organic mulch can reduce weed growth, moderate soil temperature, and increase soil moisture levels of landscape and container-grown plants. It is not clear how physical properties of various mulch materials impact weed seed germination and growth. A greenhouse and an outdoor container experiment were conducted to determine how mulch properties would affect weed seed germination and growth. For the greenhouse experiment, nursery containers (3.8 L) were filled with standard substrate and divided into two equal halves using plastic corrugated sheets and 20 seeds of crabgrass or garden spurge were surface sown to one-half of each container either above or below pinestraw, pinebark or hardwood chip mulch applied at depths 0, 1.3, 2.5, 5.1, or 10.2 cm. For the outdoor container experiment, nursery containers (11.4 L) were filled, mulched, and seeded in a similar manner. A square transparent plastic tube (30.5 cm × 3.8 cm × 3.8 cm) was inserted in the center of each container, below the mulch layer. Data collection included biweekly weed counts in both experiments and light intensity measurements (photosynthetic photon flux density) under mulch layers using a light measuring sensor (Li-Cor LI-250A) for 12 weeks seeding in the outdoor container experiment. In the greenhouse experiment, weed germination decreased 37 to 90% when seeds were placed below the mulch layer in all three mulch types for both weed species compared with seeds placed above mulch. When seeds were placed above the mulch layer, pots mulched with pinestraw or pinebark had 63 to 83% fewer weeds compared with pots mulched with hardwood and were similar to non-treated controls in both weed species. Results from the outdoor container experiment were similar in that germination increased when seeds were sown on top of the mulch layer and pinebark and pinestraw provided a 69 to 74% reduction in seed germination compared to hardwood across all mulch depths. However, there was no difference in mulch type at depths of 5.1 cm or greater. Mulch depths 2.5 cm or greater excluded 99.5% of light and there was no difference in light readings at higher mulch depths. Results from these trials suggest that for the species evaluated, weed germination will likely increase when seeds are present (are introduced) above mulch layers compared with seeds already present in the soil. Pinebark and pinestraw may provide greater weed control compared with hardwood mulch when less mulch is applied but mulch type will have less influence on seed germination as depths increase.

Contact: marblesc@ufl.edu

Glufosinate inhibits glutamine synthetase (GS) by stopping the amination of glutamate into glutamine, causing a rapid accumulation of ammonia within leaf tissue. Although the inhibition of GS is the main glufosinate mode of action, the reason why plants show rapid injury after glufosinate exposure might be associated with inhibition of photosynthesis. Therefore, the objective of this research is to understand the relationship between glufosinate phytotoxicity, inhibition of GS, accumulation of ammonia, inhibition of photosynthesis, and levels of glufosinate, glutamate and glutamine, which may help to provide opportunities to enhance the herbicidal effect of glufosinate. For all experiments, five weed species with different carbon assimilation pathways were utilized: Conyza canadensis (C3), Lolium rigidum (C3), Kochia scoparia (C4), Amaranthus palmeri (C4), and Sorghum halepense (C4). A dose response experiment was conducted in the greenhouse using 0, 9, 28, 93, 280, 560, 1120 and 2240 g ai ha^-1 of glufosinate, and evaluating visual phytotoxicity over time. Leaf tissue samples were collected from the same plants and analyzed for enzyme activity. The accumulation of ammonia was measured by a leaf disc assay in the presence of increasing glufosinate concentrations. Glutamine, glutamate and glufosinate levels before and after treatment were evaluated by liquid chromatography with mass spectrometry. Carbon assimilation was also measured over time after treatment, using an infrared gas analyzer. C. canadensis was the most sensitive species, followed by A. palmeri, K. scoparia, S. halepense, and L. rigidum. A lower herbicide concentration was found in leaves of L. rigidum, which also reflected in less inhibition of GS. Besides the inhibition of GS and accumulation of ammonia, glufosinate also affected photosynthesis and glutamate and glutamine content for all species, reaching up to 94% of inhibition in C. canadendis and 59% in L. rigidum, which confirms that its phytotoxic effect is beyond the enzyme inhibition. The mechanism of photosynthesis inhibition by glufosinate is still unknown.

Integrated weed management (IWM) tools of winter annual grasses (WAG) are required to extend herbicide effectiveness and provide more alternatives in wheat fields. Feral rye (Secale cereale; FR), downy brome (Bromus tectorum; DB), and jointed goatgrass (Aegilops cylindrica; JGG) are problematic WAG in Colorado. Harvest weed seed control (HWSC) methods aim to remove or destroy weed seeds, thereby preventing seed bank enrichment at crop harvest. FR, DB, and JGG have a potential to be controlled with HWSC due to similarities in growth habits with wheat. Post-emergence control of WAG in wheat is limited to imazamox (Clearfield^® wheat) and quizalofop (CoAXium^® wheat). Currently, there is no information on the imazamox and quizalofop resistance status for FR, DB, and JGG in Colorado. Our main objectives were to assess the seed retention at harvest and destruction percentage as efficacy indicators of HWSC and to conduct an herbicide resistance survey for FR, DB, and JGG. During 2015 and 2016, 40 wheat fields in eastern Colorado were visited for collections. Four samples were collected in each field. Seed retention was quantified and compared per weed species by counting the seed above the 15 cm fraction of the wheat canopy and on the soil surface. A Harrington seed destructor (HSD) prototype was used to determine the seed destruction percentage per species. Each site was screened for quizalofop (62 g ai ha^-1) and imazamox (31 g ai ha^-1) resistance. Averaging across both years seed retention was DB 74.5%, FR 90.3%, and JGG 76.2%. Weed seed destruction percentages were ≥98% for the three species. No resistance cases were found for quizalofop. Seven samples for which imazamox control was less than expected are currently under further study. HWSC showed potential as an effective IWM tool for weed control. Early detection of herbicide resistance in weeds is crucial for the successful implementation of IWM.

While synthetic auxin herbicides have been used for over 70 years, the precise mode of action that leads to plant death has yet to be clearly characterized. As new chemical families are discovered and the use of these herbicides continues to increase, it is imperative to understand how each active ingredient works within the plant to cause an herbicidal effect. The objective of this research was to employ a transcriptomic approach to study the differences and similarities in plant response to exogenous applications of different synthetic auxin compounds in horseweed (Conyza canadensis). In this experiment, greenhouse-grown horseweed rosettes were sprayed with halauxifen-methyl, dicamba, and 2,4-D at 5, 280, and 560 g ae ha^-1, respectively. Leaf tissue was collected from treated plants at 1 and 6 hours after herbicide treatment (HAT) for analysis and RNA sequencing was conducted using 2X100 base pair reads on the Illumina-HiSeq 2500 platform. The horseweed draft genome was annotated using GeneMark and transcript reads were mapped back to this genome using STAR aligner. Differential gene expression analysis included edgeR, DESeq2, and Cufflinks; transcripts significant in two of the three analyses were included in the results. The Cytoscape plug-in ClueGO was used for functional interpretation of differential gene expression lists via gene ontology (GO) term enrichment.

At 6 HAT, 1236 genes representing 29% of the total number of differentially expressed genes were consistent among all herbicide treatments; 735 of which were upregulated and 501 which were downregulated. The GO term enrichment of upregulated genes included response to hormones including auxin, abscisic acid (ABA), and jasmonic acid as expected. The enzyme involved in the rate-limiting step of ABA biosynthesis, NCED, was upregulated in all herbicide treatments at 1 and 6 HAT. Increased ABA concentration in leaf tissue at 6 HAT supported RNA-Seq results and qRT-PCR validation of NCED expression. Downregulation of genes associated with photosynthesis, response to light, and the electron transport chain further supports the hypothesis that the hormone cross-talk between auxin and ABA is involved in the auxin herbicide mode of action. Halauxifen-methyl elicited upregulation of genes involved in plant defense response, calcium ion transport, and response to reactive oxygen species at 6 HAT that were unique from 2,4-D or dicamba treatment.

This research presents a first look into the differential gene expression profiles in horseweed following a foliar application of synthetic auxin compounds that represent three unique chemical families. While there is an abundance of transcriptome similarities induced by each herbicide that account for the general auxin herbicide response, distinct gene expression changes exclusive to each compound cannot be ignored as a contributor to the mode of action. Future research will include a fine-tuned pathway analysis and identification of key genes involved in upstream transcriptome regulation to further differentiate auxin herbicide action by active ingredient in horseweed. 

Understanding the critical period of weed control is vital for designing effective weed management programs in popcorn production. A field study was conducted at the University of Nebraska—Lincoln South Central Agricultural Lab near Clay Center, Nebraska in 2017. The objective of this study was to determine the duration of weed infestation after planting of popcorn that does not affect popcorn growth and yield with and without the utilization of a pre-emergence herbicide. Main plots included no pre-emergence and atrazine + S-metolachlor (2470 g ai ha^-1) as a pre-emergence. Subplot treatments included weed-free, untreated, and weed removal at corn V3, V6, V9, V15, and R1 growth stages. A four-parameter log-logistic function was fitted to popcorn yield loss (%) and growing degree days (GDD) separately to each main plot. Growing degree days when 2.5, 5, and 10% yield loss was reached was extracted from the model and compared between main plots. With no PRE applied, 2.5, 5, and 10% yield loss were reached at 79, 101, and 130 GDD, respectively. This correlated to the V3, V4, V5 growth stages, respectively.  With PRE-emergence of Bicep II Magnum applied, 2.5, 5, and 10% yield loss were reached at 386 (V9), 431 (V11), and 483 (V15) GDD, respectively. The results of this study indicate that weeds must be controlled before the V3 popcorn growth stage when no PRE is applied to avoid substantial yield loss and that the utilization of a PRE-emergence herbicide such as atrazine + S-metolachlor can delay the critical time for weed removal until the V9 stage in popcorn. 

Herbicide behavior in soil is often characterized using two parameters, Kd and Koc. These parameters can be used to understand the influence of soil properties on herbicide bioavailability and movement. The objective of this research was to determine Kd, Koc, and desorption of radiolabeled indaziflam, imazapic and amicarbazone in 16 North American soils. These selected soils represented a wide range of organic matter, texture, and pH. Three grams of soil were combined with a range of herbicide concentrations (0.125, 0.25, 0.50, 0.75 and 1.00 ppm) plus 0.24 KBq indaziflam, 0.26 KBq imazapic, or 0.20 KBq amicarbazone. Tubes were shaken horizontally on a table-shaker for 24 hours. After shaking, tubes were centrifuged at 1500 rpm for 15 minutes and a 1 mL aliquot of the supernatant was transferred to a scintillation vial followed by the addition of 10 mL of scintillation cocktail. Herbicide concentration in the supernatant was determined by liquid scintillation spectroscopy (LSS) and by difference used to determine adsorption. Desorption was determined by adding 8 ml of 0.02 M CaCl₂: 0.5 mM HgCl₂, shaking for 24 h, and centrifuging as previously described. A 1 ml aliquot of the solution was analyzed. This process was repeated four times. For imazapic and amicarbazone, Kd values were low due to their high water solubility; however, for imazapic adsorption and desorption were strongly influenced by soil pH. For indaziflam, Kd was negatively correlated with clay and sand content, but was positively correlated with organic matter. Indaziflam adsorption was high for all soils and the Kd values were not correlated with soil properties. Desorption was influenced by the organic matter, higher organic matter resulted in lower desorption. Since indazilfam is a relatively new herbicide, this research provides an opportunity to understand its behavior in soil relative to other well-researched herbicides. 

The East Maui Watershed (EMW) is nearly 50,000 ha of forested landscape on the windward slope of the Haleakala Volcano. This area provides critical habitat to over one hundred threatened and endangered species and produces over 200 billion liters of fresh surface water annually; vital ecosystem services to life and agriculture on Maui. The EMW is climatically diverse, with wide mean annual gradients for peak temperature (9-26°C) and precipitation (652-10,271 mm). Miconia (Miconia calvescens) is a mid-canopy species native to South and Central America and a highly invasive, ecosystem modifier in many tropical regions of the Pacific Rim. Miconia threatens the EMW as it is a highly fecund, autogamous species, with propagules dispersed by avian frugivory. Introduced to Hana, Maui in the early 1970s, miconia was not realized as a major forest invasive until two decades later, when active management commenced. Since 1991, over 300,000 miconia plants have been removed from the EMW. Despite these efforts, miconia has spread to over 1000 ha of “infestation” with multiple incipient invasions spread across the area. With the extent of this problem beyond eradication, the current strategy seeks to contain the incipient invasions from dispersing further into novel areas. Thus, there is a critical need for intelligence of suitable habitat to determine where miconia might succeed. Herein, we report on an ensemble habitat suitability model (EHSM) developed with five techniques (boosted regression trees, generalized linear models, multivariate adaptive regression splines, MaxEnt and random forests), using historical, georeferenced miconia occurrence data, and four variables of the physical environment (precipitation, temperature, aspect and slope). In addition, we empirically derived a propagule dispersal kernel (kernel) from the same occurrence data that estimated a maximum, non-stochastic range for miconia at 1980 m, where a single mature plant could impact up to 1232 ha of the vicinity. The EHSM calculated a total area of 37,503 ha, containing over 99% of the occurrence data. The total impact area of the kernel for all mature occurrences was calculated at 24,598 ha, with 80% of this area overlapping suitable habitat. As management techniques are costly within these remote areas, interest in optimizing search and management efforts exist. We compared the area occupied by the overlapping kernel and EHSM to a complete search of the EMW. When evaluated across 10 watershed units, we found, on average, a 50% decrease in required search area. These results indicate that the miconia invasion now occupies a majority of its suitable habitat and that considerable cost reduction can be achieved by utilizing the aggregated EHSM and kernel models to target areas for monitoring and management, with an assumption that dispersal into unsuitable habitat will be less consequential.

Impact of Palmer amaranth size on yield in LibertyLink® cotton. Michael Plumblee*¹, Darrin Dodds¹, Sam Garris², Lucas Franca¹, and Bradley Wilson¹; ¹Mississippi State University, Mississippi State, MS, ²Bayer CropScience, Bentonia, MS.

ABSTRACT

Glufosinate-resistant cotton (LibertyLink^®) was commercialized in 2004 by Bayer Crop Science. LibertyLink^® cotton was developed through the insertion of the bialaphos resistance (BAR) gene, which provides resistance to glufosinate. In 2011 GlyTol^® was commercialized by Bayer Crop Sciences which provided season-long, in plant tolerance to glyphosate herbicide which is the first Roundup Ready^® alternative to be commercialized. Due to the popularity of cotton varieties with these traits and the ongoing battle with resistant weed species, applications of single post-emergence herbicides are becoming uncommon. Therefore, the objective of this research was to evaluate various glufosinate-based weed control programs and their efficacy on different sized Palmer amaranth.

This experiment was conducted in 2016 and 2017 at Hood Farms in Dundee, MS to evaluate the efficacy of various glufosinate-based weed control programs on Palmer amaranth size. Deltapine 1646 B2XF was planted on May 7, 2016 and May 10, 2017 in 4-row plots 3.86 m wide x 12.2 m long. Applications of glufosinate (Liberty) at 0.65 kg ai ha^-1, S-metolachlor (Dual Magnum) at 2.13 kg ai ha^-1, and Ammonium Sulfate (AMS) at 10.2 g L^-1 were made to cotyledon cotton on May 19, 2016 and May 15, 2017 and were followed by applications of glufosinate (Liberty) at 0.65 kg ai ha^-1 and Ammonium Sulfate (AMS) at 10.2 g L^-1 as needed. Other treatments consisted of applications of glufosinate (Liberty) at 0.65 kg ai ha^-1, S-metolachlor (Dual Magnum) at 2.13 kg ai ha^-1, and Ammonium Sulfate (AMS) at 10.2 g L^-1 when the average height of Palmer amaranth species in plots reached heights of 7.62, 15.24, and 22.86 cm followed by glufosinate (Liberty) at 0.65 kg ai ha^-1 and Ammonium Sulfate (AMS) at 10.2 g L^-1 7 to 10 d after application. Visual weed control ratings were collected at 7, 14, 21, 35, and 42 days after application. End of season data collected included lint yield. Data were subjected to analysis of variance using PROC Mixed procedure in SAS 9.2 and means were separated using Fisher’s protected LSD at p = 0.05.

The treatment with the highest visual control of Palmer amaranth at the 42 d rating after all applications had been made was glufosinate + s-metolachlor + AMS applied to cotton at the cotyledon growth stage following a PRE. Visual control at this timing was 20-29% greater than either the application made at 15.4 cm (6”) tall Palmer amaranth or  22.8 cm (9”) tall Palmer amaranth, respectively. The treatment with the highest seedcotton yield (kg/ha), similarly to visual weed control ratings at 42 d was where applications were made at the cotyledon growth stage in cotton following a PRE application. Seedcotton yield at this timing was 21% greater than applications made at the 15.4 cm (6”) tall Palmer amaranth timing, 36% greater than applications made at the 22.8 cm (9”) tall Palmer amaranth timing, and 100% greater than untreated plots. Overall, as weed size of Palmer amaranth increased above 7.6 cm (3”) in height both visual control and seedcotton yield decreased. Based on these results the optimum timing for Palmer amaranth control in a glufosinate-based weed control system is when weeds are 7.6 cm (3”) in height or less.

Palmer amaranth (Amaranthus palmeri) is a major weed in U.S. cotton and soybean production systems. Originally native to the Southwest, the species has spread throughout the country. In 2004 a population of A. palmeri was identified with resistance to glyphosate, a herbicide heavily relied on in modern no-tillage and transgenic glyphosate-resistant crop systems. This project aims to determine the degree of genetic relatedness among eight different populations of glyphosate-resistant (GR) and –susceptible (GS) A. palmeri from various geographic regions in USA by analyzing patterns of phylogeography and diversity to ascertain whether resistance evolved independently or spread from outside to an Arizona locality (AZ-R). Shikimic acid accumulation and EPSPS genomic copy assays confirmed resistance or susceptibility. With a set of 1,351 single nucleotide polymorphisms (SNPs), discovered by genotyping-by-sequencing (GBS), UPGMA phylogenetic analysis, principal component analysis, Bayesian model-based clustering, and pairwise comparisons of genetic distances were conducted. A GR population from Tennessee and two GS populations from Georgia and Arizona were identified as genetically distinct while the remaining GS populations from Kansas, Arizona, and Nebraska clustered together with two GR populations from Arizona and Georgia. Within the latter group, AZ-R was most closely related to the GS populations from Kansas and Arizona followed by the GR population from Georgia. GR populations from Georgia and Tennessee were genetically distinct from each other. The data suggest the following two possible scenarios: either glyphosate resistance was introduced to the Arizona locality from the east, or resistance evolved independently in Arizona. Glyphosate resistance in the Georgia and Tennessee localities most likely evolved separately. Thus, modern farmers need to continue to diversify weed management practices and prevent seed dispersal to mitigate herbicide resistance evolution in A. palmeri.

Three Year Evaluation of Herbicide Programs in XtendFlex^TM Cotton (Gossypium hirsutum) on Growth, Development, and Yield

C.A. Samples

D.M. Dodds

M.P. Plumblee

L.X. Franca

Abstract

Due to the continued spread of glyphosate resistant Palmer amaranth (Amaranthus palmeri), technologies have been developed allowing growers to apply auxin-type herbicides post emergence. The XtendFlex^® technology from Monsanto will allow growers to apply glypohsate, glufosinate, and dicamba over the top of cotton (Gossypium hirsutum L.).  Dicamba applied at 1.1 kg ae ha^-1 provided up to 90 percent Palmer amaranth control. Dicamba tank mixed with glufosinate increased Palmer amaranth control over dicamba alone. Dicamba has also been observed to control other glyphosate resistant species 79 to 100 percent 14 days after application. 

Experiments were conducted in 2015, 2016, and 2017 in Starkville, MS at the R. R. Foil Plant Science Research Center and in Brooksville, MS at the Black Belt Branch Experiment Station. Plots consisted of 4-1 m spaced rows that where 12.2 m in length. Each plot was replicated four times. DP 1522 B2XF was planted in Starkville and Brooksville. Applications were made on 2-4 leaf cotton with a CO₂-powered backpack sprayer calibrated to apply 140 L ha^-1 @ 317 kpa while walking 4.8 kph. Treatments applied to DP 1522 B2XF included glyphosate @ 1.1 kg ae ha^-1, glufosinate @ 0.6 kg ai ha^-1, S-metolachlor @ 1.07 kg ai ha^-1, dicamba (Engenia) @ 0.6 kg ae ha^-1, dicamba (Clarity) @ 0.6 kg ae ha^-1, and dicamba (MON 119096) @ 0.6 kg ae ha^-1 either alone or in combination. Visual injury ratings were made 3, 7, 14, 21, and 28 days after applications. Other data collected included height at 1^st bloom, height at the end of the season and lint yield. Data were analyzed using the PROC MIXED procedure in SAS version 9.4 and means were separated using Fisher’s protected LSD at p=0.05. 

All six of the highest injury levels 3 days after application on DP 1522 B2XF were from treatments containing glufosinate and S-metolachlor in which visual injury ranged from 36- 42 percent. The highest level of injury came from treatments containing dicamba (Engenia) + glyphosate + glufosinate + S-metolachlor. Similar to 3 days after application, five of the six treatments with the highest level of injury seven days after application contained glufosinate and S-metolachlor with injury levels ranging from 26-31 percent. At 14 days after application injury to DP 1522 B2XF had dissipated however, five of the six highest levels of injury contained glufosinate + S-metolachlor and there injury ranged from 12-14 percent. At 21 Days after application, cotton injury had further dissipated but significant differences persisted with the highest levels of injury still being attributed to treatments containing glufosinate+S-metolachlor. Significant differences persisted through bloom with height of cotton treated with the most injurious treatments being significantly shorter than the untreated check. Heights ranged from 63 cm -70 cm. Similarly, height at the end of the season was affected. Height of cotton that was treated with the most injurious treatments was reduced compared to the untreated control. However, there were no significant differences in lint yield at the end of the season with yields ranging from 1,589-1,777 kg lint ha^-1.

Two separate field experiments were conducted in 2015/2016 at the Montana State University-Southern Agricultural Research Center, Huntley, MT to determine the reproductive fitness of glyphosate-resistant (GR) and dicamba-resistant (DR) kochia [Kochia scoparia (L.) Schrad] in the presence or absence of glyphosate and dicamba, respectively. Seeds for the two studies were collected in 2012 and 2013 from a segregating GR and a segregating DR kochia population, respectively, each originating from a single production field in MT. Homogenous GR:GS (glyphosate susceptible) and DR:DS (dicamba susceptible) lines were obtained after 3 generations of recurrent group selection with a 2X rate of the herbicide. Each study was set up in a randomized complete block design, with a factorial arrangement of treatments with six replications, and repeated in time (2016/2017). Kochia seedlings with predetermined EPSPS genomic copy numbers (1 = susceptible, 3 to 4 = low resistance, 5 to 6 = moderate resistance; 8 to 15 EPSPS copies = high resistance) were transplanted into the field. Glyphosate was applied to 10-cm kochia seedlings at 0; 870; 870 followed by (-) 870 (1,740 g ha^-1); 1,265-949 (2,214 g ha^-1); 1,265-949-870 (3,084 g ha^-1); and 1,265-949-870-870 (3,954 g ha^-1 total). Sequential treatments were applied 10 d apart, simulating POST applications in GR sugar beet. No differences were observed in the time of flowering, seed set, pollen viability, seed viability, and 1000-seed weight between GS and GR (3 to 15 EPSPS gene copies) individuals. However, GR kochia with 3 to 4 and 5 to 6 EPSPS copies failed to produce seed at 1,265-949 g ha^-1 or higher rates of glyphosate applied sequentially. GR kochia with 8 to 15 EPSPS copies survived a total of 3,954 g ha-1 glyphosate, with minimal seed reduction. In separate experiments, kochia seedlings with known resistance to dicamba (DS, DR1 = 1.5-fold, DR2 = 2.5-fold, DR3 = 6.8-fold) were transplanted into the field. Plants (10-cm tall) were treated with dicamba at 0, 35, 70, 140, 280, 560, 840, 1120, and 2240 g ha^-1. The ED₉₀ values for seed reduction ranged from 1,545 to 4,202 g ha^-1 for DR lines compared to 227 g ha^-1 for the DS line. Dicamba applied at the highest rate reduced fecundity of DR1 line by 270-fold (108,000 to 400 seeds plant^-1). In the absence of dicamba, DR lines produced 24 to 53% less seeds compared to DS. Although no differences in pollen viability and seed viability, DS kochia took less days to reach 50% flowering and seed set, and had higher 1000-seed weight compared to DR lines, averaged across dicamba doses. These results indicate a fitness cost in DR kochia in the presence or absence of dicamba. Although glyphosate at the high rates (applied sequentially) should be utilized to prevent further development of GR kochia with low to moderate levels of resistance (2 to 6 EPSPS copies) in GR sugar beet, no reproductive penalties were observed in the absence of glyphosate. This study is the first to possibly explain the slow rate of spread of dicamba-resistant compared with glyphosate-resistant kochia in the western US cropping systems.

With many weeds resistant to common herbicide active ingredients (PSII-, ALS-, and ACCase-inhibitors), farmers around the world are seeking new weed management options. For example, control of resistant grasses in winter cereals has seen a shift away from reliance on post-emergence graminicides to an increased emphasis on pre-emergence applications.

In a continuous effort to identify new weed control solutions, BASF investigated the utility of cinmethylin, applied pre-emergence, to control key grasses in cereals. Cinmethylin demonstrated excellent efficacy against blackgrass (Alopecurus myosuroides) and ryegrass (Lolium rigidum, Lolium multiflorum), including resistant biotypes, while maintaining crop selectivity. The molecule targets a novel site of action (fatty acid thioesterase), which makes it the ideal tool for integrated resistance management.

Metribuzin has a long history of weed control in the United States. Introduced in 1973, metribuzin was initially widely used in many crops. The use of metribuzin decreased with the introduction of bentazon, PPO herbicides, ALS herbicides and then the introduction of glyphosate tolerant soybeans.  The development of herbicide resistant weeds has caused a large increase in the interest and use of metribuzin in weed control programs. 

Metribuzin can be made into dry or liquid formulations.  Older liquid formulations of metribuzin had a reputation for many issues (i.e. short storage shelf life, mixing/compatibility difficulties and handling problems including difficulty getting the product out of containers and screen plugging). Much of the industry switched to less expensive dry formulations over time even though dry metribuzin formulations need ample time and water to disperse adequately.  Applicators desired a liquid metribuzin formulation that did not have the storage and handling issues of the older liquid formulations.

Introducing Dimetric® Liquid (AGH15003): a new liquid formulation of metribuzin that is easier to mix and apply than dry formulations. Dimetric® Liquid contains 33% metribuzin active ingredient (0.36 kilograms per liter) or 3lb per gallon. Labeled use rates of Dimetric® Liquid are based on the same amount of active ingredient as that of other dry metribuzin products. Key to the Dimetric® Liquid formulation is its longer shelf life and easy mixing with less handling and compatibility issues than with older liquid metribuzin formulations. Weed control with Dimetric® Liquid was greater than or equal to that of other metribuzin formulations when compared at equal amounts of active ingredient.

The necessity of a new mode of action to combat herbicide resistant weeds raised the interest in the dormant molecule cinmethylin as a new solution for the control of weedy grasses in cereals. To further evaluate its mode of action we applied a chemoproteomic pull-down approach in Lemna protein extracts. Interestingly, three potential targets belonging to the same protein family of fatty acid thioesterases (FAT) bound to cinmethylin with high affinity. FAT proteins play a crucial role in plant lipid biosynthesis by mediating the release of fatty acids (FA) from the plastids for subsequent synthesis of glycerolipids and very long chain fatty acids at the endoplasmic reticulum. By GC-MS analysis we could show that cinmethylin treatment leads to deprivation of both saturated and unsaturated free FAs in the plant, undermining that FA release for subsequent lipid biosynthesis is the primary target of cinmethylin. Furthermore, we proved that physiological effects and downstream metabolic changes induced by cinmethylin differ substantially from other lipid biosynthesis inhibitors. Our results therefore suggest FAT inhibition by cinmethylin as a new mode of action and powerful tool for weed resistance management.

Trifludimoxazin [1,5-dimethyl-6-thioxo-3-(2,2,7-trifluoro-3,4-dihydro-3-oxo-4-prop-2-ynyl-2H-1,4-benzoxazin-6-yl)-1,3,5-triazinane-2,4-dione] is a potent, novel inhibitor of protoporphyrinogen IX oxidase (PPO or Protox).  This herbicide has shown the unique ability to control PPO resistant weed biotypes such as the Amaranthus spp. selected by the continuous use of diphenyl ether herbicides in soybeans.  Given its unique ability to control several resistant weed biotypes, trifludimoxazin will be an important tool for resistant weed management in multiple crops including soybean, corn, cereals, peanuts, and various permanent (i.e., citrus, oil palm, etc.) and pulse crops (i.e., lentils).  In addition to Amaranthus spp., trifludimoxazin alone or in mixtures with saflufenacil provides outstanding burndown and/or residual control of key agronomically important weeds like Conyza spp., Ambrosia spp., Chenopodium spp., Euphorbia spp., Kochia spp., Raphanus spp. Brassica spp., Artotheca spp., Portulaca spp., Stellaria spp., Fumaria spp., Abutilon spp., Galium spp., Lamium spp., Eleusine spp., Echinochloa spp., and Lolium spp. Trifludimoxazin is primarily targeted for use in North America, South America, and Australia.

An estimated 3.6 million acres of soybean were injured by off-target dicamba movement in 2017.  State departments of agriculture are collectively investigating more than 2,700 dicamba-related injury claims. In many incidences, the cause(s) of dicamba movement have been identified as factors related to physical drift (wind speed, improper nozzles, boom height, etc.), but in other instances further investigations have revealed that label guidelines were followed yet off-target movement of dicamba still occurred.  The objective of this ongoing research is to assess weather and environmental factors surrounding dicamba applications in order to identify any consistencies that may explain off-site dicamba movement.  A data set is being assembled that contains information surrounding successful dicamba applications where the herbicide stayed on-site, and incidences in which dicamba moved off-target for unknown reasons. Presently 66 cases of unexplained dicamba movement have been identified and compared to 54 applications deemed successful by the lack of observable dicamba injury in the surrounding region.  Weather data were retrieved from university-maintained weather stations.  Data included maximum wind speed, maximum air temperature, and precipitation.  For Missouri-specific incidences, inversion data was added to test for correlations between atmosphere stability and dicamba movement.  Soil pH estimates for each location were retrieved from the National Resources Conservation Service web soil survey.  The estimated total soybean acreage within each county was also determined from USDA agricultural census data. The current data set includes information from four states and two countries.  A second data set was developed that included the above incidences as well as incidences in which the location is known but application date is in question.  This second set includes information from six states and 124 incidences of off-target movement, and is being utilized to further explore soil properties and the percent of county in soybean production.  A logistic regression model was utilized to identify which weather variables increased the likelihood of a successful application; however, presently, none have been identified.   The average air temperature on the day of application was 23 degrees Celsius for successful applications and 24 degrees Celsius for non-successful applications.  Maximum air temperatures were 30 degrees Celsius for each of the scenarios.  Precipitation and maximum wind speeds were also similar between successful and off-target events. From the larger data set of the 124 incidences, soil pH was identified as a possible indicator of the likelihood of a successful application.  Preliminary results of the logistic regression model suggested that an increase in soil pH increases the likelihood that the dicamba application will remain on-target.   Average soil pH was 6.31 for successful applications and 6.18 when applications moved off-target. Cases continue to be added to the data set, and the latest results will be presented.

Managing tough-to-control and herbicide-resistant weeds economically has been and continues to be a challenge for soybean growers. Weeds impact soybean yield potential by competing for limited light, water, and nutrient resources. Successful integrated weed management systems require an understanding of crop and weed interactions. Nearly complete weed control is needed during the first weeks after soybean emergence to avoid potential yield losses due to early emerging weeds. The Roundup Ready 2 Xtend^® soybean is the industry's first biotech-stacked soybean trait with both dicamba and glyphosate herbicide tolerance. The Roundup Ready^® Xtend Crop System is designed to help maximize weed control and increase yield potential by enabling the use of herbicides with different sites of action on soybean crops built on proven high yielding Roundup Ready 2 Yield^® technology. The LibertyLink^® System, which provides tolerance to Liberty^® herbicide, is a current commercial system that some growers are using. With the objective to compare the agronomic and economic benefits of the Roundup Ready^® Xtend Crop System and the LibertyLink^® System, field studies were conducted in 2017 at 30 locations across multiple states. All sites were sown at 140,000 seeds/ac and 30 in row spacing, with a split plot design, in which main plot being trait and herbicide input being the subplot. Both traits had three herbicide input levels (low, medium, and high), as well as one untreated check. Crop injury, weed control, and yield were measured.

Performance of DiFlexx and DiFlexx Duo for Weed Management in Texas Corn. M.E. Matocha¹, S.A. Nolte², ¹Extension Program Specialist, ²Assistant Professor and Extension Weed Specialist, ^1,2Texas A&M AgriLife Extension Service, College Station, TX.

 Management of glyphosate-resistant broadleaf weeds in corn has been challenging since traditional synthetic auxin herbicides often result in undesirable crop safety issues.  To address this Bayer CropScience has developed DiFlexx (dicamba) and DiFlexx DUO (dicamba + tembotrione) herbicides which both contain the new safener cyprosulfamide (CSI).  This safener provides minimized crop response and a wider application window (burndown to V10), safens the use of MSO or COC in late POST timings, and safens additional HPPD and ALS products as well. Field studies were conducted in 2016 and 2017 at the Texas A&M AgriLIfe Research Farm near College Station, TX to evaluate DiFlexx and DiFlexx DUO for efficacy in corn.  Species evaluated included, Palmer amaranth (Amaranthus palmeri S. Wats), and junglerice (echinochloa colonum), and others.  Treatments in 2016 consisted of Balance Flexx + atrazine preemergence (PRE) followed by late postemergence (LPOST), or a stand alone mid postemergence (MPOST).  MPOST treatments included various combinations of Capreno, atrazine, Roundup Powermax, Laudis, DiFlexx, DiFlexx Duo and Halex GT.  LPOST treatments were similar except for Status used as a standard.  In 2017, the same products were used but all were applied at the MPOST timing.  Herbicides and rates are as follows; Balance Flexx at 0.07 kg ai/ha, atrazine at 1.12 kg ai/ha, Roundup Powermax at 1.26 kg ae/ha, DiFlexx at 0.35 kg ae/ha, DiFlexx Duo at 0.26 kg ae/ha, Status at 0.196 kg ae/ha, Capreno at 0.09 kg ai/ha, Laudis at 0.09 kg ai/ha, and Halex GT at 2.21 kg ai/ha.

 In 2016, early season Palmer amaranth control was fairly high (91-100%) with all treatments that included dicamba and/or atrazine at either timing.  These same treatments provided from 86 to 92% control of Palmer amaranth by late season.  In 2017, excellent control of Palmer amaranth was observed early season with the single MPOST timing that included dicamba, atrazine, or both.  Late season control still exceeded 92% with these same treatments.  Yields in 2016 were variable with the dicamba or atrazine containing treatments not significantly out yielding Roundup Powermax.  Only numerical differences were observed in 2017.  

Introduction of cotton varieties with tolerance to dicamba and glufosinate herbicides has provided additional sites of action (SOA) for the management of herbicide resistant and tough-to-control weeds in Texas.  Field experiments were conducted in 2015 and 2017 in College Station, TX to assess the control of Palmer amaranth (Amaranthus palmeri) and other weed species with various systems utilizing preemergence (PRE) and postemergent (POST) herbicides with multiple SOA and overlapping residuals.  The experiment was a randomized complete block design with three replications and plots were 4 m wide x 9 m long.  Weed species evaluated included Palmer amaranth, common waterhemp (Amaranthus rudis) and annual grasses. In 2015, PRE treatments were made on May 4, early-postemergence (EPOST) treatments were made on May 28, mid-postemergence (MPOST) treatments were made on June 24 and post-directed (PDIR) treatments were made on July 23.  In 2017, pre-plant incorporated (PPI) treatments were made on April 10, PRE treatments were made on May 1, EPOST treatments were made on May 30 and LPOST treatments were made on July 7. Herbicides and rates included the following; Prowl H20 at 1.6 kg ai/ha, Roundup PowerMax at 1.26 kg ae/ha, Engenia at 0.56 kg ae/ha, Outlook at 0.74 kg ai/ha, Caparol at 1.4 kg ai/ha, Zidua at 0.09 kg ai/ha, Liberty at 0.65 kg ai/ha, Cotoran at 0.84 kg ai/ha, Warrant at 1.26 kg ai/ha, and Dual Magnum at 2.13 kg ai/ha. Treatments containing dicamba or Liberty and a soil residual herbicide in either the EPOST or MPOST timings provided 100% control of palmer at the end of season rating and resulted in the numerically highest lint yield.  The highest season long control of palmer and annual grass was achieved when Outlook was applied with Engenia and Roudup PowerMax at the EPOST timing compared to being applied at the MPOST timing.

No Observable Effect Level for Dicamba in Soybean and Cotton

Greg R. Kruger, Debora Latorre, Bob Bruss, Chad Sayer, Richard Shaw

Abstract 

Innovative advances in biotechnology have produced soybean and cotton cultivars resistant to dicamba, allowing dicamba herbicide to be sprayed on these crops. Crop injury resulting from exposure to dicamba is dependent upon the rate, environmental conditions at the time of application, for how long plants were exposed, and the species and crop stage of the plants involved. The objective of this study was to determine the no observable effect level for soybean and cotton to dicamba. Field studies were conducted as a randomized complete block factorial design with three replications in three different states to each crop: North Carolina, Missouri, and Iowa to soybean, and Arkansas, Mississippi and North Carolina to the cotton. Each plot was four rows of soybean or cotton wide and nine meters long. Treatments were composed of five dicamba doses: 8.75, 1.09, 0.0683, 0.0170, 0.001708 g ae ha^-1 (480 g ae of dicamba L^-1), an experimental blank, and an untreated control plot. Treatments in soybean were applied when plants were in VC, V1, V3 and R1 growth stage. Treatments in cotton were applied when plants were with two leaves, four leaves, first square, and first blossom. Visual estimation of injury (%) ratings and plant height (cm) were collected at 7, 14, 21, and 28 d after plants were treated, and yield was obtained for plots. The visual estimation of injury, plant height, and yield were analyzed using a mixed model ANOVA. Soybean plants treated with 8.75 g ae ha^-1 of dicamba had yield reduction across applications timings in North Carolina and Iowa. Treatments with 8.75 g ae ha^-1 of dicamba reduced plant growth in 40% and more injury was observed across growth stages and evaluation periods at the three locations. Cotton plants treated with 8.75 and 1.09 g ae ha^-1 of dicamba showed more plant injury and growth reduction. Minor effects of dicamba were observed when applied to cotton plants with two true leaves. Cotton plants treated at the four leaf growth stage showed the highest percentage of injury across growth stages evaluated in North Carolina, however 28 DAT herbicide injury was no longer observed. In Mississippi cotton plants showed more injury when dicamba was applied to plants at the four leaf growth stage up to the last evaluation. In Arkansas cotton plants showed injury to 8.75, 1.09, 0.0683, and 0.0170 g ae ha^-1 dicamba doses with four leaves, but the highest injury and yield reduction was observed at the first square stage when treated with 8.75 g ae ha^-1. Overall, soybean and cotton growth, visual injury and yield were impacted by dicamba rates and application timing. Soybean plants were more sensitive to dicamba and plants presented injury to dicamba at all growth stages tested in these trials. The highest dicamba dose resulting in no observable effect level in soybean and cotton was 0.0683 g ae ha^-1, representing 0.014% of the label dose. Injury based on visual observations of dicamba in soybean and cotton should cause caution to applicators who are applying dicamba around areas with sensitive crops and other sensitive plant species. This study also illustrated the importance of looking at yield, as foliar response of soybean and cotton were not good indicators of yield loss. 

The extent of injury from dicamba to soybean and other off-target vegetation in 2017 led to many complaints by producers about the herbicide.  Injury to non-target plants was speculated to be caused by primary movement of dicamba because of applicators not properly following label application guidelines, spray tank and/or product contamination, and misidentification of symptoms caused by other herbicides. Others noted that secondary movement of the herbicide via rainfall, irrigation, dust, and volatility was involved in injury.  A large-plot field trial using non-Xtend soybean (dicamba-resistant) was conducted in Arkansas, Tennessee, Indiana, Missouri, and Nebraska in 2017 to evaluate primary + secondary and secondary movement of the dicamba formulations Engenia and XtendiMax following labeled guidelines. Engenia and XtendiMax were simultaneously applied to separate fields of 0.8 to 1.4 ha using two similarly equipped sprayers. To determine the extent of secondary dicamba movement, 19-L buckets were placed over three to four soybean plants every 3.3 m from 3.3 to 30.3 m and every 6.6 m from 30.3 to 60.7 m along four transects in the downwind direction of spraying.  Buckets were removed 30 minutes after application.  Soybean plants beneath the buckets and plants not covered were rated for injury 2 to 4 weeks after treatment, with covered plots evaluated for injury from potential secondary dicamba movement and plots not covered evaluated for primary + secondary movement.  Soybean was injured from secondary movement of dicamba at all field sites, albeit the extent of movement differed slightly among locations.  Off-target movement of Engenia and XtendiMax appeared similar across the field sites evaluated.  At the Arkansas and Tennessee field sites, soybean injury occurred at locations in the field other than those that were downwind at the time of application, indicative of additional secondary movement.  Changes in wind direction a few hours after application explained the off-target movement at both locations.  Furthermore, at the Arkansas site, 20 greenhouse-grown soybean plants in pots and plastic trays were placed at 10 locations in the Engenia and XtendiMax treated areas beginning 0.5 and 24 hours after application.  All potted soybean plants were removed 36 hours after application.  Dicamba symptoms resulted on the greenhouse-grown soybean plants that were removed from the Engenia and XtendiMax treated areas for both periods, with no difference between formulations within a period.  The occurrence of injury to plants covered by buckets at all sites, injury to soybean in multiple directions at two sites, and injury to potted soybean placed in the field following application would indicate that secondary movement, which is out of the control of the applicator, at least partially contributes to the injury to soybean observed in these field trials.  For these reasons, it should be recognized that both of the newer formulations of dicamba move via secondary movement at appreciable levels to injure soybean and other sensitive vegetation when applied under field conditions typical of summer month applications.  Relying solely on a downwind buffer during application may not protect soybean or other sensitive vegetation from secondary movement following application.  Future research should focus on factors contributing to secondary movement caused by volatilization in hopes of understanding ways to minimize its occurrence and ways to best utilize the new dicamba formulations for resistance management without incurring off-target movement.    

Commercial release of 2,4-D and dicamba-tolerant crops has led to increasing concerns of tank contamination causing non-target exposure of sensitive crops to harmful herbicide residues. To test retention of 2,4-D and dicamba in commercial sprayers following common tank cleaning procedures, field and laboratory experiments were conducted in 2017. Hagie STS 10, John Deere 6700, and SpraCoupe 4660 with tank capacities of 3570 L, 1590 L, and 1580 L respectively, were used to apply 2,4-D at 1.06 kg ai ha^-1 and dicamba at 1.12 kg ai ha^-1. Following applications, sprayer tanks were cleaned using four protocols. One cleaning method was triple rinse with water and the remaining three included a first rinse of 3% v/v ammonium and third rinse of water and the second rinses were either glyphosate, Fimco, or Protank detergent at 5.11 kg ai, 0.90 kg, and 0.95 L per 378 L water, respectively. For each rinse, 378 L of water and assigned cleaning agent were added. Half of the cleaning solution was sprayed out of the tank before rinsate samples were collected from the left, middle, and right sections of the boom simultaneously for each individual rinse. A fourth rinse using only water was conducted to demonstrate the cleaning efficacy of each triple-rinse protocol. All cleaning protocols were repeated three times in field. Herbicide residues were evaluated using HPLC with a detection limit of 0.05 ppm. Higher 2,4-D and dicamba concentrations were detected in the Hagie Upfront STS 10; however, herbicide residual concentrations after the completion of any cleaning protocols tested ranged from 1-2 ppm and did not result in cotton and soybean yield loss in field bioassay after actual rinsates from last rinse were sprayed.  Meanwhile, different doses of 2,4-D and dicamba (6.84 g ai ha-¹, 35.07 g ai ha^-1, and 140.00 g ai ha^-1) were then sprayed on field bioassays in 2017 to evaluate injury of cotton and soybean resulting from 2,4-D and dicamba herbicide residues, respectively.  Leaf tissues were collected at 1, 7, and 21 days after treatment (DAT) for analysis of 2,4-D and dicamba foliage concentrations. Results suggested foliage concentrations were only 1-3% at 21 DAT as compared to those observed at 1 DAT for all doses, which indicates majority of 2,4-D and dicamba absorbed by crop foliage has been metabolized. Soybean and cotton injury increased through 21 DAT. Cotton injury resulted from 2,4-D exposure was less severe than soybean response to dicamba. At 21 DAT, cotton visual injury after exposure to 140.00 g ai ha^-1 of 2,4-D was 50% while soybean exposed to 140.00 g ai ha^-1 of dicamba resulted in 90% visual injury. Soybean yield after exposure to 140.00 g ai ha^-1 was reduced by 90% while cotton exposed to 140.00 g ai ha^-1 of 2,4-D only resulted in 30% yield reductions. As a summary, sprayers with large tanks may retain more herbicide residues and additional measures may be required for removal. Cleaning performance of triple rinse with water is sufficient to prevent crop injury and comparable to protocols with glyphosate and commercial cleaning agents. Exposure to higher rates of 2,4-D and dicamba will result greater concentration in crop foliage, thus leading to more injury and yield loss. 

A study consisting of six field experiments (three with DeKalb 52-61 and three with Pioneer 0094AM corn hybrids) was conducted at Ridgetown, Ontario over a two-year period (2015 and 2016) to determine the tolerance of two corn hybrids to 2,4-D (560 and 1120 g ai ha^-1) and glyphosate (1800 g ae ha^-1) applied alone or in combination at V1, V3 or V5. In DeKalb 52-61 corn, 2,4-D caused as much as 24, 16, 11 and 11% visible injury at 1 WAT, 2 WAT, 4 WA-C and 8 WA-C, respectively. Plant stand was not affected, but plant height decreased 5 cm at 560 g ai ha^-1 and 7% at 1120 g ai ha^-1. Comparing injury, goosenecking and brace root malformation as a function of 2,4-D rate and application timings, averaged across two glyphosate rates, indicated that as the application timing was delayed from V1 to V5, there was a trend to increase injury at both 2,4-D rates. At 1 WAT, the addition of glyphosate (1800 g ae ha^-1) to 2,4-D increased injury as much as 22% at 560 g ai ha^-1 and 35% at 1120 g ai ha^-1. DeKalb 52-61 corn yield decreased 8% with 2,4-D applied at 560 g ai ha^-1 and 12% at 1120 g ai ha^-1. In Pioneer 0094AM corn, 2,4-D caused as much as 16, 9, 7 and 7% visible injury at 1 WAT, 2 WAT, 4 WA-C and 89 WA-C, respectively. Plant height was not affected, but goosenecking and brace root malformation were increased as the rate of 2,4-D was increased. There was generally no difference between glyphosate rates (1800 vs 0 g ae ha^-1) at V1 corn stage but visible injury, goosenecking and brace root malformation at other application timings was as much as 15, 3 and 19% greater when 2,4-D was applied in a tankmixture with glyphosate (1800 g ae ha^-1), respectively. Yield was reduced 12% when 2,4-D (1120 g ai ha^-1) was applied with glyphosate in the tankmixture. Based on these results, both DeKalb 52-61 and Pioneer 0094AM corn hybrids are sensitive to 2,4-D amine applied alone or in combination with glyphosate at V1, V3 and V5 application timings. The co-application of 2,4-D with glyphosate (1800 g ae ha^-1) increased corn injury.

Field trials were conducted in 2011 and 2012 cropping seasons at the Institute for Agricultural Research, Samaru, Zaria in the Northern Guinea Savanna ecological zone Nigeria to evaluate performance of upland rice as affected by weed control treatments, poultry manure and stand density. The treatments consisted of three rates of five weed control treatments (0.6 +0.4, 1.2 + 0.8, 1.8 + 1.2 kg a.i ha-1 propanil+2,4-D and poultry manure (0, 5 and 10t ha-1) factorially combined in the main plot while there were three stand density (2, 4 and 6 plants per hill) in the sub-plot given a total of 45 treatments. The treatments were laid out in a split-plot design with three replications. The result revealed that application of 1.2 + 0.8 kg.a.i ha-1 of (propanil+ 2-4 D) produced significantly larger leaf area, high leaf area index, higher crop growth rate, relative growth rate, net assimilation rate and grain yield of rice than the other rates but were comparable with the hoe weeded control. The application of 10 t ha-1 of poultry manure gave significantly larger leaf area, high leaf area index, higher crop growth rate, relative growth rate, net assimilation rate and grain yield of rice than the lowest rates and the control (0 and 5 t ha-1). The four plants per hill resulted in significant increase in leaf area, high leaf area index, higher crop growth rate, relative growth rate, net assimilatory rate and grain yield of rice and higher yield of rice in both locations. The study showed that application of 10 t ha-1 of manure, 1.2 + 0.8 kg a.i ha-1 of propanil+2, 4-D and four plants per hill gave the best yield of rice.

Keywords: Rice, poultry manure, herbicide, plant density and grain yield

Demands for the non-GE (genetically engineered) food products are high in the United States and with the depressed soybean price in recent few years, growers have shown interest in no-tillage conventional (non-GE) soybean production in Nebraska. The objectives of this study were to compare the efficacy of PRE followed by (fb) POST herbicide programs containing foliar-active and soil-residual POST herbicides with the PRE-only, and PRE fb foliar-active POST herbicide programs for Palmer amaranth (Amaranthus palmeri S. Wats.) control, and evaluate the effect of herbicide programs for conventional soybean injury, yield, and economics. Field experiments were conducted in 2016 and 2017 at the South Central Agricultural Laboratory of the University of Nebraska-Lincoln located near Clay Center, NE. Herbicide treatments were arranged in a randomized complete block design with four replications. Data were subjected to ANOVA using SAS. A priori orthogonal contrasts were performed to determine the relative efficacy of herbicide programs (PRE-only vs. PRE fb POST; and PRE fb foliar-active POST vs. PRE fb foliar-active POST tank-mixed with soil-residual herbicides) for Palmer amaranth control and soybean yield. The PRE application of herbicide premixes resulted in ≥ 97% control and density reduction of Palmer amaranth up to 28 d after PRE (DAPRE). Averaged across all herbicide treatments, the PRE fb POST herbicide programs including the tank-mixture of foliar-active and soil-residual POST herbicides provided ≥ 92% Palmer amaranth control throughout the season and reduced the density to 3 plants m^–2 at 28 d after POST (DAPOST) compared to the nontreated control (110 plants m^–2). Tank-mixing soil-residual herbicides to the foliar-active POST herbicides in PRE fb POST herbicide programs improved the aboveground biomass reduction of Palmer amaranth (98%) at 56 DAPOST compared to the PRE fb foliar-active POST (84%) and PRE-only (66%) herbicide programs. The PRE fb POST herbicide programs improved conventional soybean yield (4,050 and 2,015 kg ha^–1 in 2016 and 2017, respectively) over the PRE-only herbicide programs (3,552 and 1,192 kg ha^–1 in 2016 and 2017, respectively). Flumioxazin plus pyroxasulfone applied PRE fb fluthiacet-methyl plus S-metolachlor plus fomesafen as POST resulted in higher gross profit margin (US$1,184.3) and the benefit-cost ratio (5.2). Tank-mixing soil-residual and foliar-active POST herbicides in a PRE fb POST herbicide program improved the season-long Palmer amaranth control and resulted in higher yield compared to the PRE fb foliar-active POST and PRE-only herbicide programs in no-till conventional soybean.

The benefit of precision agriculture must be defined both in terms of profitability as well as environmental enhancement.  Maintaining biodiversity within the landscape is central to the protection of ecosystem services.  In this presentation, I will explore the diversity-stability hypothesis and illustrate how precision agriculture can play a pivotal role in identifying opportunities to enhance biodiversity within our agricultural production system.

Torpedograss (Panicum repens) is an invasive grass that is very difficult to manage in aquatic systems. Its growth and spread are primarily driven by asexual means including rhizomes in the sediment and shoots that grow in the water column. Variable control of torpedograss has been reported from multiple sites and water depth is believed to be an important contributing factor. However, this not well understood. To address this, in the spring of 2016, torpedograss was established in 60 (100 liter) tubs in both saturated soil conditions and in water 30 cm deep. Plants were established for six weeks before treatment.  Herbicide treatments included glyphosate, imazapyr, sethoxydim, and fluazifop-p-butyl. All herbicide treatment and water depth combinations had five replicate tubs each and the entire study was repeated two weeks later. Above water, below water, and below ground (root plus rhizome) biomass were harvested at 0 (baseline) and 90 days after herbicide treatment.  

Prior to treatment, plants in saturated conditions produced approximately 54% higher above water biomass compared to plants grown at the 30 cm depth. Total shoot (above + below water) biomass, below ground biomass, and total plant (shoot + below ground) biomass did not differ between plants grown at either water depth.

At ninety days after treatment, across depths, all herbicides reduced total shoot biomass by 95 to 100% compared to the untreated control and were not different. However, glyphosate reduced belowground biomass by 78% which was significantly different than fluazifop but not imazapyr or sethoxydim. Across herbicide treatments, water depth did not influence final shoot biomass. However, final root biomass was 35% lower for plants grown at the 30 cm depth compared to plants grown in saturated conditions. This study indicates that higher emergent shoot biomass in saturated conditions compared to the 30 cm water depth did not influence control of torpedograss shoots. However, it may have differentially influenced herbicide impacts on belowground biomass. Future studies examining herbicide translocation in torpedograss grown in differing water depths could help to clarify this effect.     

Terrestrial invasive plants are a widespread and pervasive problem in the United States. In the state of Wisconsin alone, over 140 plant species are regulated. The large number of species make it challenging for individuals to efficiently monitor for all species. Recently land managers have requested tools to assist in prioritizing monitoring efforts. Using data through 2016, we developed ensemble habitat suitability models (HSMs) for 20 species in the state of Wisconsin at a 30m spatial resolution using common environmental, climatic and topographic predictor layers. The ensemble included boosted regression trees, generalized linear models, multivariate adaptive regression splines, MaxEnt, and random forests. Of the 20 species considered, six are classified as prohibited (control required) in all or parts of Wisconsin which we labeled as “early detection” species; the remaining fourteen are restricted (not requiring control) and were labeled “widespread.” To validate the accuracy of these models to predict suitable habitat, we engaged stakeholders in 2017 to report invasive plants to the online database EDDMapS (Early Detection and Distribution Mapping System). As a result, 3,916 reports for these 20 species were submitted from 89% of Wisconsin counties. We used this independent dataset to evaluate how well each model predicted these occurrences. Models of suitable habitat for detection of both early detection and widespread species were correct more often than expected values at an 80% threshold. While individual modeling approaches provided, on average, 61-69% correct classifications, the ensemble model had a higher predictive value (82%) than any of the individual models alone. This held true for both widespread (52-63% for individual models; 77% for ensemble) and early detection species (79-87% for individual models; 96% for ensemble). These data confirm that the ensemble approach to modeling suitable habitat for invasive species is a robust method for predicting suitable habitat for a variety of species across an array of habitat types. The ensemble models will be used to assist land managers in prioritizing monitoring efforts for modeled species.

Detecting and mapping invasive species requires a culmination of monitoring techniques, field observation strategies, and specialized knowledge of weed ecology. The use of remote sensing systems has provided mangers with important toolsets to aid in the early detection and rapid response of invasive species dynamics among natural and cultivated systems. However, the economic hindrance and recurrent application of these systems is often confounded by the limited functionality of spatial, temporal, or spectral requirements needed to make timely management decisions. The more recent popularity and affordable growth among unmanned aircraft has generated multiple avenues for researchers to peruse. Unmanned aerial vehicles (UAVs) provide a platform for small optical imagers and other remote sensing devices for weed detection with decreased economic, environmental, and radiometric limitations amid larger airborne or satellite sensors. Therefore, understanding the application of UAV platforms established through research is essential for the continual use of remotely sensed data delivered to invasive species managers for prompt field-based decisions. This study describes the use of a consumer available UAV platform to summarize varying components among aquatic and noncropland environments to map and monitor the spatial abundance, and temporal presence of invasive weed species. Specifically, this research concentrates on the identification and management of Berberis thunbergii in pasture setting, and the detection of a recently discovered federal noxious aquatic species, Salvinia molesta, found among several southeastern waterbodies in 2017. 

Doveweed is a summer annual monocot weed in the spiderwort family (Commelinaceae). Doveweed has become a common weed in southeastern United States nursery crops, cotton, soybean, and turfgrass, ranging from Texas to North Carolina, and has since moved into Virginia. Doveweed is a difficult weed to control, as it has a degree of tolerance to glyphosate and many preemergence herbicides will not control this weed, allowing it to dominate treated areas.  Experiments were conducted to evaluate potential postemergence control options for this weed in cool- and warm-season turfgrass.  Single applications of various 3- and 4-way turf broadleaf herbicides [Triplet (2,4-D + MCPP + dicamba), 4-Speed XT (2,4-D + triclopyr + dicamba + pyraflufen), and Surge (2,4-D + MCPP + dicamba + sulfentrazone)] did not provide acceptable doveweed control in a 2016 trial.  Celsius (thiencarbazone-methyl + iodosulfuron-methyl + dicamba) caused 53% injury to doveweed at 25 days after treatment (DAT) but plants outgrew the injury over time.  Change Up (MCPA + fluroxypyr + dicamba) applied either alone or in combination with metsulfuron did not provide acceptable control.  Sulfosulfuron, Celsius, and Avenue South (penoxsulam + sulfentrazone + 2,4-D + dicamba) also did not provide acceptable control in that trial after one application. In a 2017 trial, at 20 days after the first application, atrazine, Surge, Tribute Total (thiencarbazone-methyl + foramsulfuron + halosulfuron) and Celsius, all applied with 1% v/v methylated seed oil, were providing good control.  A higher rate of Surge was used in this trial compared to the 2016 study – this could explain the better results seen in 2017.  Also, a methylated seed oil was added to Surge in 2017 but was not in 2016 so that also could be a reason for the better control in 2017.  After two applications, excellent doveweed control was seen with mesotrione, atrazine, Surge, and Tribute Total.  Doveweed shoot fresh weight at 27 days after the second application confirmed control ratings taken visually.  Two applications of mesotrione, atrazine, Surge, and Tribute Total all reduced doveweed shoot fresh weight by 99 to 100%.  Topramezone and Celsius also significantly reduced doveweed shoot weight, with less reductions seen with foramsulfuron and imazaquin. In cool-season turfgrass, two applications of mesotrione should be an effective treatment for doveweed control.  In bermudgrass, Tribute Total applied twice would be a control option.  Atrazine would be an option in St. Augustine.  Repeat applications of Surge would be an option in both cool- and warm-season turfgrass species.  Addition of a methylated seed oil may improve doveweed control with these products.  Additional data is needed on doveweed control using repeated applications of 3- and 4-way turf broadleaf herbicides, including adjuvant addition.

Turfgrass renovations commonly involve changing cultivars or species that are better suited for a given setting. Common bermudagrass [Cynodon dactylon (L.) Pers.] is a perennial turfgrass that is difficult to eradicate before renovations, and poses contaminant concerns for the subsequent stand. Basamid (dazomet) is a granular soil fumigant that has activity on various pests, including common bermudagrass. Field research was conducted from 2015 to 2016 in Raleigh, NC and College Station, TX to evaluate basamid treatments including various combinations of soil incorporation (irrigation- or tillage-incorporated) and sealing (tarp or no tarp) methods, application rates [291, 291 followed by (fb) 291, 468, or 583 kg ai ha-1], and fluazifop-P [fluazifop (0.4 kg ha-1)] + glyphosate (2.8 kg ae ha-1) application(s) for established common bermudagrass control. Overall, treatments required fluazifop + glyphosate before basamid application for acceptable control (>90% cover reduction) at 42 and 46 weeks after initial treatment (WAIT) in Texas and North Carolina, respectively. Soil-incorporation results varied by location, with basamid application (583 kg ai ha-1) fb tillage resulting in ≥88% cover reduction across locations, while acceptable control from irrigation incorporation was only observed in North Carolina. Tarping did not improve efficacy when tillage incorporation at the maximum label application rate provided acceptable control, suggesting practitioners may eliminate this procedure. Information from this research will aid turfgrass managers in developing cost-effective, ecologically sound common bermudagrass eradication programs before renovations.

Field research was conducted in 2016 and 2017 to evaluate the response of ‘Capri’ colonial bentgrass (Agrostis capillaris) to various pre-emergence herbicides. Experiments were conducted on stand of mature colonial bentgrass maintained as a simulated golf course fairway on a sandy loam soil at the Rutgers Horticulture Farm No. 2 in North Brunswick, NJ. Treatments were arranged in a randomized complete block design with four replications and applied to 0.9 by 2.0 m plots using a CO2-powered single nozzle boom with 374 L ha^-1 of water carrier though an AI9504EVS nozzle. Applications were made on 14 April 2016 and 9 May 2017. In 2016, colonial bentgrass injury was evaluated on a 1 (i.e., complete injury or death) to 9 (i.e., no injury) scale and turfgrass quality was evaluated on a 1 (i.e., lowest quality) to 9 (i.e., highest quality) scale. In 2017, bentgrass injury was evaluated on a 0 (i.e., no injury) to 100 (i.e., complete injury or death) percent scale. Treatments were evaluated until September in each year.

Treatments consisted of various pre-emergence herbicides applied at one (low rate) and two (high rate) times the maximum registered use rates in creeping bentgrass (Agrostis stolonifera). Treatments consisted of the following: dithiopyr at 0.56 and 1.1 kg ha^-1; pendimethalin at 1.7 and 3.3 kg ha^-1; prodiamine at 0.48 and 0.96 kg ha^-1; DCPA at 11.8 and 23.6 kg ha^-1; dimethenamid-P at 1.1 and 2.2 kg ha^-1; bensulide at 11.2 and 22.5 kg ha^-1; bensulide + oxadiazon at 6.7 + 1.7 and 13.4 + 3.4 kg ha^-1. Treatments were irrigated with 60 mm of water within 4 hours of application.

In 2016, bensulide did not reduce turf quality compared to the non-treated control on any rating date. By 5 weeks after treatment (WAT) bensulide + oxadiazon at 13.4 + 3.4 kg ha^-1 caused turfgrass quality reductions; these reductions were still apparent at 15 WAT where turfgrass quality was < 3. At 15 WAT, both rates of DCPA also reduced turfgrass quality to < 4. By 20 WAT, turfgrass quality was ≥ 5 for all treatments except DCPA at 24 kg ha^-1 and bensulide + oxadiazon at 13.4 + 3.4 kg ha^-1.

In 2017, injury was observed at 3 weeks after treatment for all treatments except low rates of bensulide and bensulide + oxadiazon as well as both rates of prodiamine. By 9 WAT all treatments except both rates of bensulide, the low rates of dimethenamid-P and bensulide + oxadiazon caused colonial bentgrass injury. Both rates of DCPA, the high rate of dithiopyr and pendimethalin caused more than injury than all other treatments (>65%) at 9 WAT. By 20 WAT, < 15% injury was observed in all treatments except the high rates of dithiopyr, pendimethalin, and prodiamine as well as both rates of DCPA. High rates of DCPA and prodiamine cause more injury than all other treatments (>75%). 

Goosegrass (Eleusine indica) is a problematic summer annual weed in turfgrass. Economical and effective management programs for goosegrass depend on predicting seedling emergence patterns. The objective of this study was to determine goosegrass seedling emergence patterns in bare cool season turfgrass. Experiments were initiated in April 2017 at the Rutgers Horticultural Research Farm No. 2 in North Brunswick, NJ and the Tamarack Golf Course in East Brunswick, NJ. Plots at the Research Farm site were arranged in a randomized complete block design with one factor and four replications. Plots were subjected to three different ground cover treatments; bare ground, perennial ryegrass (PRG; Lolium perenne) mowed at 1.25 cm, and PRG mowed at 6.4 cm. Plots measured 1.0 by 1.0 m and goosegrass seedlings were counted and removed on a weekly basis from fixed circles measuring 1000 cm² within each plot. Soil temperature and soil volumetric water content were monitored at a depth of 5.0 cm. At the Tamarack location, the experiment was performed on two sites with four replications each. Plots measured 1.0 by 1.0 m and goosegrass seedlings were counted and removed on a weekly basis from fixed circles measuring 500 cm² within each plot. Plots were maintained as a golf course fairway at a 1.25 cm mowing height. Soil temperature was monitored at a depth of 5.0 cm. When applicable, data were subjected to analysis of variance in SAS. Regression analysis was performed on cumulative percent emergence data in PRISM.

At the Rutgers Horticultural Research Farm No. 2 site, goosegrass seedling emergence was first observed on May 26 and last observed on October 20. Ground cover treatment had an effect on the rate of seedling emergence. Season long seedling emergence rates were 8520, 2470, and 1560 seedlings per M² in bare soil, 1.25 cm mowing height, and 6.4 cm mowing height, respectively. Regression analysis on cumulative percent emergence data showed that 50% of total emergence occurred on June 24, July 2, and June 28 in bare soil, 1.25 cm mowing height, and 6.4 cm mowing height, respectively. In addition, 90% of total emergence occurred on August 12, August 28, and August 22 in bare soil, 1.25 cm mowing height, and 6.4 cm mowing height, respectively.

At both Tamarack Golf Course sites, goosegrass seedling emergence was first observed on May 10 and last observed on September 14. The season long seedling emergence rates were 11500 and 25200 seedlings per M² at site 1 and site 2, respectively. Regression analysis on cumulative percent emergence data showed that 50% of total emergence occurred on May 31 and May 21 at site 1 and site 2, respectively. In addition, 90% of total emergence occurred on July 4 and June 19 at site 1 and site 2, respectively.

Tolerance of multiple container-grown ornamentals to pre-packaged granular herbicides, Biathlon^® (oxyfluorfen 2% plus prodiamine 0.75%) or Freehand^® 1.75 G (dimethenamid-P 0.75% plus pendimethalin 1%), was evaluated at the Valley Laboratory of the Connecticut Agricultural Experiment Station and Tennessee State University Nursery Research Center. The experimental design was a completely randomized block (CRD) with 12 to 15 replications. Biathlon rates were 0, 100, 200, and 400 lb/A, and Freehand® 1.75 G rates were 0, 150, 300, and 600 lb/A. Ornamental species included: chocolate flower (Berlandiera lyrata), corn flag (Gladiolus spp.‘Firecracker’), flag iris (Iris versicolor ‘Blue Flag’), Japanese spurge (Pachysandra terminalis ‘Green Carpet’), lily-of-the-valley (Agapanthus africanus ‘Getty White’), periwinkle (Vinca major ‘Bowles’), Shasta daisy (Leucantheum maximum ‘Superbum’), and stonecrop (Crasssula radicans ‘Red Carpet’). Treatments were applied with a shaker bottle over-the-top of dry foliage of newly transplanted ornamentals (dormancy broken) and again six to seven weeks after initial application. Overhead irrigation (0.5 inch) was applied 10 to 30 mins after application of granules and plants were watered for a 30-min period. Periodic visual observations of plant injury (necrosis, chlorosis, and stunting) were made on a scale of 0 to 10 (0 = no injury, 10 = dead) at 7, 14, and 28 d after first and second application. corn flag, flag iris, Japanese spurge, and lily-of-the-valley were tolerant to Biathlon rates up to 400 lb/A and two sequential applications made at 6 to 7 wk intervals. All rates of Biathlon resulted in commercially unacceptable injury to Shasta daisy. At 28 d following first application, necrotic injury was rated 2.3, 2.3, and 3.3 with 100, 200, and 400 lb/A, respectively. The second application did not cause injury greater than 0.7 with any of the rates tested. Periwinkle was significantly stunted at Biathlon rates of ≥ 200 lb/A despite no necrotic or chlorotic injury. At 28 d following the first application, stunting injury was rated 2.3 and 4.4 with 200 and 400 lb/A, respectively. Following the second application, stunting injury ranged from 1.8 with 200 lb/A to 3.2 with 400 lb/A. For chocolate flower, Freehand 1.75 G rates up to 300 lb/A were safe with an injury rating of < 2.0 at 28 d after first treatment.  All Freehand 1.75 G rates following second application only produced stunting injury which ranged from 1.2 to 3.0 during the evaluation period. For stonecrop, Freehand 1.75 G at ≥300 lb/A produced significant injury, loss of pigmentation, and stunting injury. However, two applications of Freehand 1.75 G at 150 lb/A were safe on stonecrop without significant discoloration and stunting injury.

The response of green antelopehorn (Asclepias viridis) to herbicides used as part of an integrated vegetation management program for rights of way and forage systems was evaluated. Field populations near Starkville, MS received single applications of herbicides at labeled application rates late spring of 2016 and 2017. Four replicates of treatments were applied with a CO₂ pressurized sprayer that delivered 15 gpa. Nonionic surfactant was added per label recommendations at 0.25% v/v. Visual response, plant height and stem counts were used to evaluate herbicide activity up to 1 year after application. Effect of treatments on the number of emerged green antelopehorn stems 1 year after 2016 application is reported. Twelve treatments reduced the number of green antelopehorn plants in plots the year following treatment. These treatments would be useful to livestock producers that want to reduce the potential exposure of livestock to this toxic plant in hay fields or pastures. Compared to the number of green antelopehorn stems in the untreated control 1 year after treatment, aminopyralid+metsulfuron (Chaparral/Opensight 0.71% ai) applied at 1.1 and 2.1 oz ai/A, fluroxypyr (Vista 2.8 lb ae/gal) at 0.48 lb ae/A, glyphosate (Roundup PowerMax 4.5 lb ae/gal) applied at 1.12, 2.25, and 4.5 lb ae/A, imazapyr (Arsenal 2 lb ae/gal) applied at 0.25 and 0.5 lb ae/A, picloram+2,4-D choline (Graslan 3.81 lb ae/gal) applied at 1.3 lb ae/A, triclopyr ester (Remedy Ultra/Garlon 4 lb ae/gal) applied at 1.25 and 2.5 lb ae/A, and triclopyr choline (Vastlan 4 lb ae/gal) applied at 2 lb ae/A reduced the number of emerged stems. Plots treated with 2,4-D+dicamba (Weedmaster 3.87 lb ae/gal) at 0.65 and 1.3 lb ae/A, aminocyclopyrachlor (Method 2 lb ae/gal) at 0.03, 0.06, 0.13,and 0.19 lb ae/A, aminocyclopyrachlor+chlorsulfuron (Perspective 55.3 DF) at 1.4 and 2.8 oz ai/A, aminocyclopyrachlor+imazapyr+metsulfuron-methyl (Viewpoint 61.7 DF) at 6.2 and 12.3 oz ai/A,  aminopyralid (Milestone 2 lb ae/gal) at 0.05 and 0.1 lb ae/A, aminopyralid+2,4-D (GrazonNext HL 3.74 lb ae/gal) at 0.46 and 0.93 lb ae/A, chlorsulfuron+metsulfuron-methyl (Cimarron Plus 63% ai) at 0.38 and 0.76 oz ai/A, fluroxypyr (Vista 2.8 lb ae/gal) at 0.24 lb ae/A, foramsulfuron+iodosulfuron-methyl+thiencarbazone-methyl (Derigo 36.4 WDG) at 1.1 and 2.2 oz ai/A, hexazinone (Velpar 2L) at 0.38 or 0.75 lb ai/A, metsulfuron-methyl (Escort 60 DF) at 0.15 and 0.3 oz ai/A, nicosulfuron+metsufluron-methyl (Pastora 71DF) at 0.53 and 1.1 oz ai/A, picloram+2,4-D choline (Graslan 3.81 lb ae/A) at 0.62 lb ae/A, sulfometuron (Oust 75 DF) at 0.38 and 0.75 oz ai/A, sulfosulfuron (Outrider 75DF) at 0.49 and 0.98 oz ai/A, and triclopyr choline (Vastlan 4 lb ae/gal) at 1 lb ae/A had numbers of green antelopehorn plants no different than the untreated control. Highway, electric utility, and other right of way vegetation managers that need to selectively control other weeds, but maintain green antelopehorn milkweed for monarch butterfly and other pollinators can incorporate these herbicides into their management plans to maintain suitable habitat.

A survey of 43 mixed tall fescue and legume pastures was conducted during the 2015 and 2016 growing seasons to determine the effects of selected soil and forage parameters on the density of individual weed species and overall weed density. Seasonal variation in forage quality of 15 common pasture weeds was also investigated in 22 mixed tall fescue and legume pastures. The soil and forage parameters investigated included soil phosphorus (P), potassium (K), magnesium (Mg) and calcium (Ca) concentration, soil pH, cation exchange capacity (CEC), cattle grazing density, total forage groundcover density, tall fescue density, and beneficial legume density (comprised of white clover, red clover and annual lespedeza). Survey locations were sampled at 14-day intervals throughout the season from April to September. Weed density was categorized into total, annual broadleaves, perennial broadleaves, and annual grasses and most common individual weed species encountered. These datasets were further divided by 3 timings, spring (April-May), summer (June-July), and fall (August-September), and then analyzed using regression tree models. Across all weed types and species, forage groundcover density was the main parameter that affected weed density. Common ragweed was the most common weed species found in Missouri pastures, and soil K level was the primary parameter that affected its density. Similarly, perennial broadleaf weed density in the summer and fall timeframes was reduced by soil P level (66 and 59% reduction with P > 1.5 ppm). Cattle grazing densities less than 1.2 units per acre also resulted in fewer annual grass weeds in pastures. Analysis of forage nutritive values of common pasture weed species revealed that most weed species declined linearly (P<0.05) in neutral detergent fiber digestibility (NDFD) but increased linearly in neutral detergent fiber (NDF) content as the season progressed. Crude protein (CP) concentration of common ragweed, lanceleaf ragweed, horsenettle, and dandelion were often higher than the representative forage sample for the majority of sampling timings throughout the season while yellow foxtail and ironweed species were lower in CP concentration than the representative forage samples from July 26 to August 23 (P<0.05). Digestibility of common ragweed, lanceleaf ragweed, broadleaf plantain, Pennsylvania smartweed, dandelion, and common cocklebur was greater than that of the representative forage sample for the majority of sampling periods throughout the season (P<0.05).  Results indicate that maximizing the groundcover of beneficial forage species is the most important factor that contributes to reductions in weed density in mixed tall fescue and legume pastures. Analysis of the nutritive value of common weed species also revealed specific times at which certain weeds have the potential to influence the overall forage quality of mixed tall fescue and legume pastures in Missouri.

Seven herbaceous weed control (HWC) treatmentswere applied to recently planted loblolly pine seedlings on forest industry land. All treatments were over-the-top applicationsand were  replicated three times. Initial height and groundline diameter (GLD) were recorded prior to the start of the growing season. Competition control was recorded at 30, 60,90,120,and 150 days after treatment (DAT). Height and GLD were measured fter the end of the first growing season. Results from competition control,survival, height growth, and GLD growth will be compared amng all treatments.

In an effort to determine crop tolerance of recently planted loblolly pine seedlings to treatments using products with potential for operational application in forestry, two studies were established-- one involving applications of Frequency and one with applications of Cleartraxx. Both studies were installed soon after the seedlingswere planted but beore they had broken dormancy. All treatmenrts were replicated three times. Competition control and crop tolerance ratings were recorded at 30, 60, 90, 120, 1nd 150 DAT. Results from both evaluatiuons will be presented.

Herbicide resistance (HR) has become one of the most important threats to agricultural production. Effective training and easy access to educational material about how to deal with this problem is of great importance to help farmers prevent, delay and manage HR. A large number of academic institutions, agribusiness companies and individuals have generated educational materials about HR. However, those materials are either useful but difficult to locate using regular search engines, or can be easily found but the information/recommendations presented are not supported by scientific data or meet the basic principles of HR management as endorsed by the Weed Science Society of America (WSSA). For this reason, the WSSA Herbicide Resistance Education Committee (E12b) initiated a project to develop an on-line portal that will provide the general public with a fast and easy tool to find information sources (e.g., websites, pdf, videos, podcasts, etc.) that properly address HR management and that are based on sound scientific data. The WSSA Herbicide Resistance Portal is a website-interface connected to a database that is populated by submission of documents and media. All documents will be reviewed by volunteered WSSA members to make sure that they meet established standards before inclusion. Users will be able to search the database with multiple filter options including crop, weed species, region/state, and herbicide. The impact of the WSSA Herbicide Resistance Portal will depend on the members of WSSA and affiliated regional societies actively submitting suitable documents and recommending this tool to end-users.

Maleza en Foco: A Herbicide Resistance Training Program for Argentinian Weed Managers.

 C. Rubione*; Claudio Rubione R&D

Maleza en Foco® is a Weed Resistance program designed to cover an information and knowledge gap between Countries who actively develop strategies or technologies to manage the issue, and those who are a step behind them.

There are many reasons which contribute to this gap being worldwide. After attending many different conferences, this gap seems to be bigger in the area of communication. For instance, while in the US researchers are working very hard to find out why dicamba, as a new tool when applied to tolerant crops, became a nightmare for farmers and the Industry, farmers in Australia are debating the best option to manage weed seed bank at harvest. In addition, countries such as Argentina and Uruguay are trying to rapidly learn how to manage cover crops to suppress weeds.

At the center of this picture is the fact that herbicide weed resistance information from researchers or institutes is quite difficult to get, as well as time consuming, regardless of whether it comes from the private or public sector.  The reason for this is that there is a lot of information spread out among many sources.

The Maleza en Foco Program started three years ago, as a result of trying to understand which were the main factors that contributed to Argentina´s weed resistance problem, and therefore, explore the different options available to stop it.

Due to this, a couple of papers were written, and a local survey was done. In addition, Maleza en Foco presented the Argentinian weed resistant status at the past two WSSA annual meetings. First, in the 2016^th edition: “Update in Argentinian weed resistance situation”, and second, in the 2017^th  edition: “How does Argentinian situation compares to countries with similar weed problems, like Australia and USA, as regards weed resistance management”. Maleza en Foco concluded that increasing communication and education were the main measures to be taken in order to make Argentinian growers and professionals aware of weed resistance and how to manage it. With this, Maleza en Foco shifted its focus from diagnosing weed resistance status and management to extension and education.

Maleza en Foco® started diagnosing a case study on Argentina weed resistance with a survey and without any sponsorship. As a result, and after visiting five of the most resistant weed infested areas, one of the main topics to consider appeared to be the lack of information on weed resistance management alternatives to chemistry.

Following that, many ideas came to the table, in order to reduce the knowledge gap between Argentina and other countries with a large history of weed resistance research, such as the US and Australia.

The first step taken was to design a trip to the US for Argentinian technicians. Its purpose was to provide an in depth training on weed resistance. The trip started at the GHRC, held in Denver. After attending lectures and workshops, the group was led to different Universities, such as Colorado State, Arkansas, Missouri, Illinois, Penn State, North Carolina and Delaware, as well as Industry research farms, and the central USDA-ARS site, in Beltsville, Maryland.

Many of the most outstanding weed scientists of the US gave specific training, either in class lectures or within field trials.

The next step was to get in touch with AHRI´s team (Australian Herbicide Resistance Initiative) and focus on two different teaching options: a webinar, and a visit to Australia´s farms and scientists.

The webinar objective was to listen and interact through the web with the people who developed six different ways to reduce the weed seed bank. A good number of attendees from Australia, USA, Argentina and Uruguay joined the webinar.

The second objective focuses on training in Australia´s strategy to defeat weed resistance in-situ. This country has a similar past history to Argentina´s, having switched from the cattle business (beef or wool) to the agriculture business, where weed resistance spread very fast as a consequence, and now both countries battle it, using different resources.

In addition, Maleza en Foco ® has uploaded lots of information to different alternative media, like Facebook, Tweeter, Blogger, and LinkedIn. The dissemination of this issue on TV, radio, and the Internet was instrumental in reaching the audience.

The third step, and most challenging one, is to develop a specific Weed Resistance site, under the Maleza en Foco® domain. This site aims to get the participation of every country in the world, with a main objective being to gather all the information available in just one place.

As a web page, Maleza en Foco® will be a unique site containing Weed Resistance information gathered from as many countries as are willing to participate. The site focuses on eight different weed resistance subjects, addressed in news articles from different media, technological posts, news, interviews with scientists and farmers, and a Q&A section.

With this tool, we hope to offer our target audience a faster and easier access to what is leading in the world, in weed resistance management. Topics such as chemistry discovery, R&D news, cover crops, machinery, robotics, weed seed bank management, amongst others, will be part of the web site, which will provide updated information to subscribers in both Spanish and English

The website idea is not to impose, but that users be able to find the necessary information, or links, searching according to cases similar to theirs. Much of the information will be from very important or International renowned sources, which will be cited to ensure transparency.

Complementary to the web site, many related educational programs are offered to professionals who need an update or just need new sources of information.

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Key-words:  communication,  extension, web site, media. 

From time to time over the past 30 years, I have been called upon to provide expert testimony (affidavit, deposition or trial) on various types of investigations I’ve conducted.  I learned the process of how and why things are done in a certain way and in this presentation, I will summarize some of these things.  Areas to be covered are: 1) rules defining the qualifications of an expert witness and what the expert’s opinion(s) may be based upon; 2) the difference between in a retained and non-retained expert; 3) the difference between a consulting and testifying expert; 4) definition of a subpoena; 5) the types of testimony that an expert may be asked to give, specifically affidavit, deposition, and trial; 6) how an expert witness may be challenged; and 7) things to consider when preparing to give expert testimony. 

WEEDucator is a game-like digital tool designed for beginning farmers and agriculture students and developed in partnership with new media artists; a project goal was to create a tool that users found ‘fun.’  The current iteration of WEEDucator contains three learning modules: 1) an interactive guidebook illustrating weed lifecycles, 2) a gallery of videos and infographics describing organic weed management best practices, and 3) a simple management simulator that lets the user build a season-long weed management plan, and see potential impacts on weed density and the seedbank.  We convened a focus group of established organic farmers to provide input on the tool’s design and help prioritize content for inclusion.  To test WEEDucator’s efficacy as a teaching tool and collect user feedback for improvement, anonymous Pre- and Post- surveys were administered to students (N=19) recruited from two University of Maine sustainable agriculture courses during the 2017 fall semester.  Mean knowledge scores increased by 58% after exposure to WEEDucator, and respondents’ confidence in their answers increased by 107%.  A majority (>90%) indicated that they found WEEDucator helpful and fun to use.  Respondents were asked to rank in order of preference methods through which they might learn this content.  WEEDucator was tied with lab and classroom activities as most preferred, significantly exceeding YouTube videos, classroom lectures, and reading a textbook.  Overall, WEEDucator is a promising proof-of-concept for how computer games and new media can contribute to student learning and engagement.       

Dicamba/glyphosate resistant soybean, also known as Roundup Ready 2 Xtend soybean was available commercially in 2017 growing season. About 500,000 acres were planted with Xtend soybean in Nebraska in 2017 growing season. Three dicamba based herbicides, including XtendiMax, FeXapan, and Engenia are labeled for application in Xtend soybean. They can be applied pre-plant, pre-emergence, or post-emergence until R1 soybean growth stage (beginning of flowering). In the first week of July 2017, dicamba off-target injury issues started and continued until the end of Aug. Non-Xtend soybean is very sensitive to dicamba. Upward leaf cupping is a typical symptom of dicamba in soybean. Nebraska Extension received 348 dicamba related non Xtend soybean injury complains (Figure 2), primarily in eastern half of the state. Nebraska Department of Agriculture (NDA) received 93 off-target complains in non-Xtend soybean. The United States Environmental Protection Agency (USEPA) has declared new dicamba products (XtendiMax, FeXapan, and Engenia) as a Restricted-Use Pesticide – for use only by Certified Applicators. The new label adds requirements for dicamba spray application training, record keeping, wind speed limitations (3 to 10 miles per hour), application timing restrictions, and more. The 2018 season is the second in the two-year temporary registration granted by the USEPA to Engenia, FeXapan, and XtendiMax. A recent survey of Nebraska soybean growers reported that more Xtend soybean will be planted in 2018 growing season. Therefore, soybean growers in Nebraska will be required to consider dicamba training and follow new label requirements. 

With the advancement of technology, teachers have a vast number of ways to integrate new tools as teaching strategies. Students have access to unlimited information with electronic devices that can be utilized in the classroom to facilitate learning, rather than as distractions. One popular approach is the use of polling applications to allow students to interact in class while providing quick assessment for the educator. In two different courses over the past year, the Top Hat polling software application was used in AGRON 650 - Integrated Weed Management and in AGRON 330 - Weed Science at Kansas State University. The objective of using polling applications was to increase student involvement in the subject matter presented during each class. Outcomes demonstrated that using this tool as a key teaching method created a more inclusive environment for students, and complemented subsequent discussion, especially in larger enrollment classes. The polling application allows and requires that all students participate when they might not otherwise. Systems differ in functionality and cost, and each polling application is unique in the user interface. However, all facilitate a common goal of stimulating critical thinking and increasing student involvement. Challenges about developing effective questions to meet learning objectives will be described. The use of polling software is not a stand-alone method of engaging students, but is an innovative tool that provides another form of student interaction if implemented properly.

Weedy (red) rice was first found in California in the early 20^th century. Due to the flooded agroecosystem of California rice, along with a strong seed certification program, it disappeared from the radar in the 1950’s. In the early 2000’s, it was reported in a county in the northern end of the Sacramento valley, and an official survey by in 2006 found it in four rice fields in two counties. An expanded survey by the University of California Cooperative Extension Rice Advisors in 2008 found no additional fields beyond the 2006 survey. In the winter of 2015, the California Crop Improvement Association (CCIA), which certifies California rice seed fields, reported many suspected infestations. By the end of the 2016 season, weedy rice was found to be infesting 10,000 acres in eight out of nine rice-growing counties. Five distinct populations were identified, distinguishable both morphologically and genetically. To best address this emerging pest, a research and extension program was implemented over the winter of 2016-2017. Over 750 Pest Control Advisers (PCAs), County Agricultural Biologists, and rice growers were trained (in meetings) to identify weedy rice in the field, to properly submit samples for suspected infestations, and to manage weedy rice in the field. A website was launched, and brochures with best management practices were mailed to every rice grower and PCA.

Results of the extension program were encouraging, showing a concerted effort to address the pest. By the end of the 2017 season, 22 samples were confirmed to be weedy rice out of the 53 samples submitted for genetic testing. Eight potential and established seed fields were found to be infested with weedy rice and were rejected: 3 were new medium-grain certified seed fields, 1 was an established medium-grain certified seed field, and 4 were specialty-variety seed fields. Likewise, by the end of the 2017 season, research results yielded improved best management practices, including specific recommendations for each of the five weedy rice ecotypes. Genetic testing revealed several possible routes of weedy rice introduction into CA. The culmination of these efforts resulted in changes to the CA certified seed law to prevent the spread of weedy rice through seed.

Overall, research and extension efforts over the past 2 years have resulted in improved participation by the rice industry in efforts to eradicate weedy rice. To continue to combat the pest, research and extension efforts will need to be maintained at similar levels over the next few years. 

Safeners protect cereal crops from herbicide injury by inducing proteins involved in detoxification pathways, such as glutathione S-transferases (GSTs) and cytochrome P450s. Although the precise signaling pathway(s) that regulate gene expression within these detoxification pathways is not yet clear, our previous research found that GST proteins involved in herbicide detoxification were highly expressed in the outermost cells of wheat seedling coleoptiles after safener treatment. Other research examining stress-responsive gene expression in Arabidopsis cell cultures showed that oxylipins (such as phytoprostanes (PPA-1) or the jasmonate precursor OPDA) triggered detoxification and defense response in a manner similar to what occurs following safener treatment. As a result, experiments were designed to test the hypothesis that safeners and PPA-1 induce GST activity and the expression of many genes related to plant defense and detoxification in sorghum shoot coleoptiles in an analogous manner. A novel cryostat-microtome sectioning method was developed to extract high-quality RNA from the outermost cells of frozen coleoptiles (excluding leaf tissues) for transcript profiling to enrich for safener- and PPA-1-responsive mRNAs. A total of 63 cDNA libraries from different sorghum genotypes were then constructed for RNAseq. RNAseq results identified >10-fold increases in transcripts of several detoxification genes, including multiple GSTs, P450s, and UDP-glycosyltransferases, in safener- or PPA-1-treated seedlings compared with untreated controls. Moreover, transcripts encoding proteins related to plant development and defense were upregulated >8-fold by safener, including enzymes involved in lipid signaling, kinases, and hormone-related processes such as synthesis of benzoic acid, salicylic acid, or auxin metabolism/homeostasis. An interesting finding was that one sorghum genotype displayed about 6-fold more transcripts regulated by safener (4 and 12 h after treatment) compared with the sorghum genotype BTx623, and also showed dramatically different patterns of up- or down-regulated genes. This implies that certain genotypes might lack the ability to tightly control and regulate a specific, coordinated defense and detoxification response immediately following safener treatment. Ongoing analyses using bioinformatics and comparative gene expression approaches are aimed at further mining these RNAseq data to provide additional insight into how transcriptional responses are reprogrammed in sorghum coleoptiles following safener or PPA-1 treatment. This research is helping to elucidate the unknown mechanisms that trigger specific detoxification responses related to safener-regulated protection of cereal crops.

When clomazone was initially released, its mechanism of action was a puzzle.  While clearly a bleaching herbicide, new growth was devoid of green color after plants were treated with clomazone, the mechanism of bleaching was illusive.  Attempts to tie the bleaching to inhibition of phytoene desaturase or steps in the cytoplasmic isoprenoid biosynthesis pathway were unsuccessful.  These failures, combined with a small amount of experimental evidence, caused some to suspect a bioactivation of clomazone was necessary for activity.  This line of inquiry was the basis for the PhD dissertation of Yurdagul Ferhatoglu (2002) and subsequent publications from her dissertation (Pestic.  Biochem. Physiol. 81:59-70 and 85:7-14).  The research was initiated at the request of FMC who had labelled clomazone for use in cotton as long as the insecticides disulfoton or phorate were applied as safeners in-furrow with the cotton seed.  FMC was interested in understanding the basis for the safening.  Whole plant studies with cotton showed that, indeed, phorate reduced the bleaching effects of clomazone.  Phorate treatment also reduced clomazone metabolism in both cotton plants and excised cotton shoots.  This reduction was across several of the clomazone metabolites that were separated on HPLC.  Because organophosphate insecticides such as phorate are known inhibitors of P450 mediated herbicide metabolism, we focused on clomazone metabolism by P450.  To do this, we utilized microsomes prepared from etiolated corn seedlings grown from naphthalic anhydride treated seed.  The microsomes formed three metabolites from clomazone, two that were NADPH (P450) dependent and one that was not.  Of the two NADPH dependent metabolites, formation of one was inhibited by phorate while the other was not.  We also showed that several other known P450 inhibitors safened both cotton and corn from clomazone and reduced clomazone metabolism in cotton.  Interestingly, a suspected P450 clomazone metabolite, 5-OH clomazone, was also phytotoxic to cotton but with lower unit activity than clomazone.  Phorate did not safen 5-OH clomazone.  Further elucidation of the clomazone bioactivation and mechanism of action required the discovery of the chloroplastic isoprenoid biosynthesis pathway.  With this knowledge, we were able to show that 5-keto clomazone, but not clomazone or 5-OH clomazone, inhibits DXP synthase.  Clomazone must be bioactivated to 5-keto clomazone.  

Glyphosate is metabolized by many microbes by either C-P lyase, producing sarcosine and inorganic phosphorous, or a glyphosate oxidoreductase (GOX), producing aminomethylphosphonic acid (AMPA) and glyoxylate.  Some species of  both crops and weeds metabolize glyphosate via a GOX-type enzyme, whereas other species either do not metabolize glyphosate or metabolize it very little.  Plant GOX has not been isolated, nor has the gene for it been identified. How much of plant metabolism of glyphosate to AMPA is due to endophyte GOX is unknown. Of the major crops, soybeans metabolize glyphosate more than others that have been studied, with significant amounts of AMPA accumulating in seed of glyphosate-resistant (GR) soybean seeds. AMPA is a weak phytotoxin, and, and under some circumstances, enough AMPA can accumulate in glyphosate-treated, GR soybeans to cause mild phytotoxicity. This is not the case with GR canola with a GOX transgene, but canola is less sensitive to AMPA than soybean. There are many patents for using transgenes for glyphosate metabolism (e.g., C-P lyase, GOX, D-amino acid oxidase, and glyphosate acyl transferase) to generate GR crops, but GOX in canola was the only use of such genes in a commercial GR crop, and its use has been discontinued. There are no rigorous studies showing enhanced glyphosate metabolism as a mechanism of evolved resistance to glyphosate in weeds. This is an enigma, as such a mechanism should be successful with such a slow-acting herbicide.

Natural phytotoxins are small bioactive molecules produced by living organisms.  Many of these chemicals are toxic to the organisms producing them.  Consequently, these molecules are often synthesized and/or stored as inactive protoxins that are bioactivated in the target organisms.  Bioactivation takes different forms, such as cleaving extraneous portions of the protoxin or by adding important components to the molecule. 

The conversion of the protoxin bialaphos to the herbicidal L-phosphinothricin, the active enantiomer of glufosinate, is perhaps the most relevant example of bioactivation via cleavage of protecting groups.  Bialaphos is a tripeptide L-alanyl-L-alanyl-phosphinothricin produced by Streptomyces hygroscopicus and S. viridochromogenes.  This metabolite is bioactivated in planta by removing two alanine residues to release L-phosphinothricin, a potent inhibitor of glutamine synthetase.  Organisms producing bialaphos also have specific acetylases that rapidly deactivate L-phosphinothricin as another mechanism of protection against the toxic effect of this bioactive natural product.  Another common bioactivation by cleavage involves the removal of glycosides by glucosidases to release herbicidal aglycones, such as that observed with podophyllotoxin produced by the plant Podophyllum peltatum and ascaulitoxin produced by the pathogenic fungus Ascochyta caulina.

Metabolic activation of natural phytotoxins adding relevant groups to protoxins is often achieved via phosphorylation.  Most often, these toxins are inactive substrate analogues that serve as substrate mimics for native kinases.  Upon phosphorylation, these chemicals become potent inhibitors.  Certain strains of S. hygroscopicus produce hydantocidin.  Upon phosphorylation, hydantocidin forms an analog of inosine monophosphate (IPM), which is a potent inhibitor of adenylosuccinate synthetase, an enzyme involved in purine biosynthesis. Similarly, carbocyclic coformycin is a potent protoxin whose primary MOA involves the irreversible inhibition of AMP deaminase following phosphorylation of its 5’-hydroxy group. The microbial metabolite 2,5-anhydro-D-glucitol produced by an isolate of Fusarium solani is bioactivated by the action of two glycolytic kinase (hexokinase and phosphofructokinase) to yield a diphosphate substrate analog that inhibits fructose-1,6-diphosphate aldolase, a key step in glycolysis. 

Bioactivation may also be the result of more subtle changes such as the reduction of a disulfide bridge of a depsipeptide produced by Bukholderia sp. A396.  This step is catalyzed in planta by native enzymes and increases the potency of this phytotoxin on plant histone deacetylases.

Not all natural phytotoxins require bioactivation.  In these cases, organisms often protect themselves from the toxic effect of a compound they produce by compartmentalization or exudation.  For example, the phytotoxin leptospermone, a b-triketone that served as a template for the development of HPPD-inhibiting herbicides, is produced exclusively in schizogenous glands.  This allows the production of a potent toxin in a cellular compartment isolated from the rest of the physiologically active portion of the plant.  Alternatively, some phytotoxins like the photosystem II inhibitor sorgoleone, are produced in specialized root hair cells and rapidly exuded from the root into the rhizosphere. 

Non-target site resistance (NTSR) to herbicides is now widespread in wild grasses and in the United Kingdom (UK) and other northern European countries this has resulted in major reductions in the efficacy of chemical weed control, notably in winter wheat.  As part of a national initiative, we have been studying the mechanistic basis of NTSR in blackgrass (Alopecurus myosuroides), which as a result of widespread herbicide resistance  in populations of this weed, is currently causing major losses in wheat production in the UK.   While NTSR is associated with changes in the expression of 100s of genes, only a small group of soluble proteins have been found to be functionally associated with NTSR.  One of these, the phi class glutathione transferase (GST), termed  AmGSTF1, is consistently upregulated in NTSR populations.  GSTs are well known to detoxify herbicides in both crops and weeds by catalysing their conjugation with glutathione.  However, AmGSTF1 has little conjugating activity toward herbicides and a direct role in detoxification seems to be a minor part of its NTSR-promoting activity.  Instead it appears that AmGSTF1 has an alternative  central regulatory role in controlling the toxicity of herbicides in blackgrass, with its orthologs contributing to NTSR in other wild grasses.  This role is quantitative, with the relative levels of expression of AmGSTF1 positively correlated with the enhanced detoxification of herbicides by both GSTs and glutathione-independent pathways involving cytochromes P450.  This in turn is functionally linked to the degree of herbicide resistance in NTSR black-grass.  Practical use of this link between AmGSTF1 expression and the degree of NTSR exhibited in different weed populations has been made by developing a practical in field diagnostic for resistance based on measuring the expression of the protein in planta.  Practical benefit has also been made of defining a causative role for AmGSTF1 in NTSR through the use of GST inhibitors that can be targeted to the protein for selective covalent modification.  Following such inhibition, NTSR blackgrass becomes more sensitive to several post-emergence herbicides, suggesting that it may be possible to develop commercial synergists to counteract resistance.  Using transgenic Arabidopsis expressing AmGSTF1 as a model system, we are currently conducting studies to further understand the mechanisms by which this protein and related orthologs regulate herbicide detoxification and NTSR in a range of weeds. 

Herbicide resistance due to enhanced herbicide metabolism (EMR) in weeds is a threat which can confer broad spectrum herbicide resistance. EMR is not genetically well characterized. An RNA-Seq transcriptome analysis was used to identify genes conferring EMR in a population (R) of a major global weed (Lolium rigidum Gaud.), in which herbicide resistance to diclofop-methyl was experimentally evolved through recurrent selection from a susceptible (S) progenitor population. A reference transcriptome of 19,623 contigs was assembled (454 and MiSeq sequencing). Transcriptomic-level gene expression was measured using Illumina 100 bp reads. In a forward genetics validation experiment, nine contigs, found overexpressed in R vs S plants, co-segregated with the resistance phenotype in an F2 population, including CytP450s, GSTs, and GTs. Four of these were found over-expressed in resistant populations collected in fields. Further studies to characterize the physiological and biochemical function of these genes showed that one of the 9 genes overexpressed in the R population confers resistance by detoxification of at least 3 herbicides representing two modes of action (ALS and ACCase). In addition, it appears that not all chemicals belonging to a given chemical class (e.g. FOPs, or SUs) can be detoxified by the same gene product. This basic knowledge will allow us to develop metabolic resistance diagnostics based on molecular markers. This will be a valuable element in the integrated weed management tool box for choosing the best strategy to control resistant weed populations.

Multiple-herbicide resistant Echinochloa phyllopogon has been found in California paddy fields. The resistant line exhibits resistance to herbicides from at least five modes of action including acetolactate synthase (ALS) inhibitors and acetyl-CoA carboxylase (ACCase) inhibitors. Previously, we identified two ALS-herbicide-metabolizing cytochrome P450 genes (CYP81A12 and CYP81A21) whose overexpressions are associated with the ALS inhibitor resistance. Meanwhile, the genes involved in the resistance to ACCase inhibitors remain to be identified. To better understand resistance mechanism to ACCase inhibitors, we examined susceptibility of the resistant line to four ACCase inhibitors from three chemical classes such as fenoxaprop-P-ethyl and diclofop-methyl (FOPs), tralkoxydim (DIM) and pinoxaden (DEN). The resistant line exhibited reduced susceptibility to all the four herbicides compared to the susceptible line. Rice transformation of CYP81A12 or CYP81A21 was performed to evaluate its possible involvement in the reduced susceptibility to ACCase inhibitors. Transgenic rice expressing either of the genes grew vigorously in media containing diclofop-methyl, tralkoxydim or pinoxaden while the growth of wild type rice was severely suppressed. Interestingly, fenoxaprop-P-ethyl resistance was not observed in the P450 expressing rice. The results strongly suggest that the overexpressions of the P450 genes cause metabolic cross-resistance to ALS inhibitors and some ACCase inhibitors, diclofop-methyl, tralkoxydim and pinoxaden, while other mechanisms are suggested for fenoxaprop-P-ethyl resistance.

An Echinochloa population, MS1, collected from a rice field in Sunflower County, Mississippi was confirmed to be resistant to acetolactate synthase (ALS)-inhibiting imazethapyr and cross-resistant to other ALS inhibitors such as imazamox (3.3-fold), penoxsulam (9.4-fold), and bispyribac-sodium (7.2-fold). Addition of malathion to penoxsulam and imazethapyr reduced shoot dry weight and/or increased mortality compared to the respective herbicides applied alone, indicating possible involvement of herbicide metabolism driven by cytochrome P450 monoxygenase enzymes (CYP) as a mechanism of resistance. ALS enzyme assays or ALS gene sequencing analysis did not indicate a modified target-site based resistance. Lower levels of transocation of ¹⁴C-bispyribac and ¹⁴C-imazamox in the MS1 population compared to a susceptible population were recorded, perhaps, indicating a result of metabolism. RNA-seq analysis of gene expression before and after herbicide treatment revealed that the MS1 population exhibited a stress response upon exposure to imazamox. Additional studies revealed that the MS1 population was multiple resistant to fenoxaprop-P-ethyl, an acetyl coenzyme A carboxylase (ACCase) inhibitor, (11-fold; but susceptible to sethoxydim and clethodim, also ACCase inhibitors), propanil, and quinclorac.  Sequencing of ACCase of MS1 did not reveal the presence of any known resistance-conferring point mutations. An enzyme assay confirmed that the ACCase in the MSI population was herbicide sensitive. Further investigations with two CYP inhibitors, malathion and piperonyl butoxide, and a glutathione-S-transferase inhibitor, 4-chloro-7-nitrobenzofurazan, did not indicate involvement of any metabolic enzymes inhibited by these compounds. Treatment of MS1 plants with quinclorac in combination with malathion indicated increase in susceptibility to the herbicide, implicating a role for CYP. 

Multiple herbicide-resistant Palmer amaranth (Amaranthus palmeri) is widespread and poses a major challenge for sustainable crop production throughout the US. A Palmer amaranth population from Kansas (KSR) was found resistant to photosystem II (PS II)-, acetolactate synthase (ALS)- and hydroxyphenylpyruvate dioxygenase (HPPD)-, inhibitors. The widespread occurrence of resistance to PS-II (atrazine) and/or ALS- (chlorsulfuron) inhibitors in Palmer amaranth is known in KS. However, it is important to note that the KSR population was never exposed to HPDD-inhibitors (mesotrione or tembotrione) selection. Whole plant dose-response assays suggest that KSR was 178-237; >275 and 10-18, fold more resistant to atrazine, chlorsulfuron, and mesotrione, respectively, compared to two known susceptible populations (MSS or KSS). Upon investigation, it was found that rapid detoxification via glutathione-S-transferase (GST)-mediated conjugation confers atrazine resistance, while the known mutation in ALS gene (proline197serine) associated with chlorsulfuron resistance was found in only 30% of KSR, suggesting ~70% of plants might have a non-target-site, possibly cytochrome P450-mediated metabolism based resistance in this population. Also, metabolism of mesotrione mediated by P450 enzyme activity was also found. Interestingly, qPCR and immunoblotting showed a 4-12 fold increase in the HPPD transcript with a corresponding increase in HPPD protein expression. We hypothesize that the metabolism-based resistance to PS II and ALS-inhibitors via GST or P450 enzyme activity may have predisposed the KSR population to evolve resistance to HPPD-inhibitors, although there was no history of HPPD-inhibitor selection on this population. This is a classic example of metabolism-based resistance may confer resistance to other herbicides and even those that are yet to be discovered.

Waterhemp [Amaranthus tuberculatus (Moq) Sauer] is a problematic weed severely affecting corn, soybean, and cotton production in the U.S.  Previous research reported resistance to 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicides in a waterhemp population from central Illinois (named MCR), which is also multiple resistant to s-triazines and ALS inhibitors via rapid metabolism. Experiments were conducted with the objective of determining underlying resistance mechanism(s) and identifying gene candidates conferring multiple resistances to mesotrione, topramezone, atrazine, primisulfuron-methyl, and carfentrazone-ethyl in MCR.  Biochemical studies using excised leaves and whole-plant methods demonstrated that elevated rates of metabolism, through distinct detoxification pathways, contributes to resistance; these pathways include (1) oxidative metabolism (presumably via P450s) for mesotrione, topramezone, and primisulfuron-methyl, and (2) glutathione conjugation via glutathione S-transferase (GSTs) for atrazine.  Constitutively-expressed transcript levels of one phi-class GST (named AtuGSTF2) correlated strongly with metabolic atrazine resistance in two waterhemp populations (MCR and ACR) and an F₂ population that segregates for metabolic atrazine resistance. This correlation suggests that AtuGSTF2 is the predominant GST protein that confers atrazine resistance in waterhemp.  MCR also demonstrated resistance to topramezone, an HPPD inhibitor having a distinct chemical structure (pyrazole) than mesotrione (triketone). Biochemical studies indicated that an elevated rate of oxidative metabolism confers topramezone resistance in MCR relative to two HPPD-sensitive waterhemp populations. However, the metabolic route determined in MCR is different than the rapid N-demethylation reaction that occurs in tolerant corn. Finally, the MCR population also displayed foliar resistance to carfentrazone-ethyl, which was not due to a glycine codon deletion or arginine substitution in the PPO2 enzyme.  These findings indicate that waterhemp possesses multiple genes encoding diverse metabolic enzymes that confer complex, herbicide-dependent, cross- or multiple resistance patterns

The Generation Z (born after 1995) are entering colleges and universities and just as the Millennials before them, this digitally innate generation is changing the learning styles in higher education. They grew up in a world where the internet, social media and mobile technology always existed. Technology in classroom has slowly evolved towards digital media, providing student’s information through sound, photos, videos, and using interactive 360° virtual media. For a teacher to successfully convey the message to students, the correct tools need to be selected. The use of virtual media was put to test in the Herbicide Physiology & Biochemistry course at MSU in Fall 2017. Survey from students at the end of the course indicated that the use of virtual media in classroom increased their learning experience in the course and highly recommend to continue using virtual media in this course. Because resources were limited, especially in relation to the number of virtual reality videos available online, only a few video samples were used in the Fall 2017 course. For the Fall 2018 course, we intend to develop our own interactive virtual media videos that will cover content more closely related to course topics. The virtual media will be used as a teaching and assessment tool, and will allow students to explore a virtual environment using a computer, keyboard, and mouse, or through a virtual reality headset to get a fully immersive experience. Students will also have access to the Mechdyne/Fakespace FLEX virtual reality environment system (VERTEX) installed in the High Performance Computing Collaboratory at MSU. The VERTEX system projects 10 x 7.5 ft stereoscopic images onto the front, left, and right walls as well as the floor, and inertial-ultrasonic motion tracking provides the user with an immersive virtual reality experience. Examples of virtual environment will be a weed habitat that may not be physically accessible to students, a 3D image of a weed species to help with weed identification, or a plant cell interior, where students can move around the cell and learn about the different plant cell organelles such as chloroplast, mitochondria, nucleus, ribosomes, etc. This approach can be adopted by different courses in weed science, and will also benefit students and faculty beyond the weed science community. 

The Generation Z (born after 1995) are entering colleges and universities and just as the Millennials before them, this digitally innate generation is changing the learning styles in higher education. They grew up in a world where the internet, social media and mobile technology always existed. Technology in classroom has slowly evolved towards digital media, providing student’s information through sound, photos, videos, and using interactive 360° virtual media. For a teacher to successfully convey the message to students, the correct tools need to be selected. The use of virtual media was put to test in the Herbicide Physiology & Biochemistry course at MSU in Fall 2017. Survey from students at the end of the course indicated that the use of virtual media in classroom increased their learning experience in the course and highly recommend to continue using virtual media in this course. Because resources were limited, especially in relation to the number of virtual reality videos available online, only a few video samples were used in the Fall 2017 course. For the Fall 2018 course, we intend to develop our own interactive virtual media videos that will cover content more closely related to course topics. The virtual media will be used as a teaching and assessment tool, and will allow students to explore a virtual environment using a computer, keyboard, and mouse, or through a virtual reality headset to get a fully immersive experience. Students will also have access to the Mechdyne/Fakespace FLEX virtual reality environment system (VERTEX) installed in the High Performance Computing Collaboratory at MSU. The VERTEX system projects 10 x 7.5 ft stereoscopic images onto the front, left, and right walls as well as the floor, and inertial-ultrasonic motion tracking provides the user with an immersive virtual reality experience. Examples of virtual environment will be a weed habitat that may not be physically accessible to students, a 3D image of a weed species to help with weed identification, or a plant cell interior, where students can move around the cell and learn about the different plant cell organelles such as chloroplast, mitochondria, nucleus, ribosomes, etc. This approach can be adopted by different courses in weed science, and will also benefit students and faculty beyond the weed science community.

Plagiarism, along with falsification and fabrication, (together FFP) are the sanctionable research misconduct offenses as defined by the Federal Policy on Research Misconduct (https://ori.hhs.gov/federal-research-misconduct-policy). By far, plagiarism is the most frequently-occurring research misconduct offense.  It is often not reported, at least in science, given its high prevalence and, relative to fabrication and falsification, its low impact on science. Plagiarism is claiming others’ ideas, phrases, and sentences as one’s own.  These days self-plagiarism, especially of peer-reviewed papers-to peer-reviewed submissions, is also considered plagiarism.  Plagiarism-detecting software, such as iThenticate, is now commonly used to ‘police’ grant proposal and manuscript submissions.  Such software compares a submitted text, i.e., submitted manuscript, with the universe of published text.  The software output is the degree of similarity (match) of phrases and sentences with the best-matched public work. Plagiarism, self-plagiarism, and some examples of plagiarism via iThenticate analysis will be presented in the workshop to show the range of severity and forms of plagiarism.  Also discussed will be ‘allowable plagiarism’ such as seen in the recycling of one’s own text that appears first in non-peer-reviewed sources, such as conference abstracts, posters, grant proposals and teaching materials.    

Cover crops can modify soil nutrient dynamics in a way that can influence the performance and yield of the following crop. While grass cover crops tend to immobilize nutrients because of the high C:N ratio of their residues, the residues of nitrogen fixing leguminous cover crop have a lower C:N ratio and can increase nutrient availability and enhance crop growth. Differential nutrient dynamics may also influence weedy plants growing in a crop thereby mediating the crop – weed interaction.  To investigate the influence of cover crops on crop – weed interaction we followed 360 pairs of corn – common lambsquarter (Chenopodium album L.) plants in corn plots following triticale, pea and no cover crop (fallow) through the summer of 2017 in an organic systems experiment in central PA. All plots received manure before corn planting. Weed and corn height and weed volume was measured in July and August and corn and weed biomass were measured at harvest. Corn yield was also measured at harvest. At the first measurement period in July the height and size distribution of the C. album and of the maize was similar across the three cover crop treatments. A month later (mid-August) weedy plant height and canopy volume and the final biomass of plants grown in soils preceded by a pea or fallow treatment were larger than those grown in soils preceded by triticale. Corn plant height and biomass was largest when grown in soils preceded by pea, then fallow and smallest in soils preceded by triticale.  Distribution of corn sizes also differed across treatments. Neither corn height nor corn biomass influenced weed height, volume or biomass in any of the treatments. Plots following pea cover crops had both the largest corn and weed plants and also the highest yields. High nutrient availability following pea cover crops may have buffered corn competition on C. album plants, allowing the growth of the two. In contrast, corn and weed plants grown in soils preceded by triticale were significantly smaller in height and biomass. Lower nutrient availability following triticale may have increased competition for resources between corn and C. album plants resulting in smaller weed plants, but also decreased corn plant growth and yield. These results provide preliminary evidence of the role cover crops play mediating weed – crop interactions through influencing nutrient resource availability.

Dock (Rumex spp.) is a persistent problem weed in clover grown for seed. Currently registered herbicides provide poor control of dock. Trials were designed to evaluate rates and timings of herbicides known to control dock and to evaluate other possible candidate herbicides for label development including carfentrazone , fluthiacet and saflufenacil. Initial clover injury was observed with the contact herbicides paraquat, carfentrazone and saflufenacil, however, injury from these treatments was no longer visible by late June. No clover injury was observed in plots treated with fluthiacet. Clover injury from 2,4-DB consisted of cupping, thickening, and elongation of the leaves with a bluish tint as compared to untreated plants. The injury did not affect the vigor of the plants and did not result in a reduction in clover seed yield. The addition of NIS in 2,4-DB treatments did not result in increased injury. Injury from asulam was still visible by late July, but none of the treatments resulted in a significantly reduced seed yield at p-value 0.05.  Asulam and 2,4-DB applied in March provided 85-95% control of dock. Control was not improved by split applications or higher rates for either product. January applications of asulam and 2,4-DB did not provide acceptable control of dock. A oxyfluorfen + diuron + paraquat treatment included as a grower standard provided minimal control of dock and fluthiacet had no activity on dock. Applications of carfentrazone and saflufenacil, which removed all growing tissue of both red clover and dock, increased the relative competitiveness of dock and the number of seed heads/plot was significantly higher than the untreated plot at p-value 0.05.

Canada is the world’s leading producer and exporter of lentils, and more than 95% of Canada’s lentil production takes place in the province of Saskatchewan.  Locally developed Clearfield lentil varieties tolerant of imidazolinone (acetolactase synthase (ALS) inhibitor) herbicides initially provided excellent weed control, however selection pressure has led to increasing populations of weeds such as wild mustard, kochia, and cleavers that survive ALS inhibitor herbicide application.  While there are other limited broadleaf herbicide options, all pose a risk of crop damage in lentil.  A four site-year study was conducted in central Saskatchewan to develop a strategy to selectively apply non-ALS inhibitor herbicides to herbicide resistant wild mustard with weed wipers to reduce weed seed production in lentil.  The study used a factorial treatment structure to test two factors, herbicide and application timing, on a randomized complete block design.  The four herbicide treatments included 2,4-D amine, dicamba, glyphosate, and untreated control, while the four application timings began in the first week of wild mustard inflorescence emergence and were conducted weekly either once or twice (week 1, week 2, week 3, weeks 1 & 3).  All tested herbicides reduced wild mustard seed production by an average of 54% compared with the untreated control.  Weed wiping with glyphosate additionally reduced emergence of wild mustard seedlings the following spring by more than 50% in the site-year for which data are available.  In all site-years yield of lentil was reduced by more than 60% by wiping weeds with dicamba, despite use of a low volatility dicamba formulation in three of the four site-years.  No herbicide significantly increased lentil yield compared with the control.  The optimal timing of weed wiper herbicide application to maximize weed control and lentil yield varied among herbicides and site-years, however weed wiping twice was equally effective as weed wiping once at the optimal timing.  Weed wiping with 2,4-D amine or glyphosate twice in alternating weeks beginning at weed inflorescence emergence is an effective strategy to minimize seed production of ALS inhibitor herbicide resistant wild mustard in lentil.

Common lambsquarters (Chenopodium album L.) is among the most problematic, summer annual broadleaf weed species in agronomic crops across the northwestern United States. During summer/fall of 2016, two common lambsquarters biotypes (ID-13 and ID-14) surviving the field-use rates (870 g ae ha^-1) of glyphosate were identified from sugarbeet fields in southcentral Idaho. These fields were under continuous corn–sugarbeet rotations, with frequent use of glyphosate for weed control. In spring 2017, a common lambsquarters biotype surviving glyphosate treatment was also collected from a corn field near Laurel city, Montana. The main objectives of this research were (1) to confirm and characterize the response of those selected common lambsquarters biotypes to glyphosate, and (2) investigate the underlying mechanism of enhanced glyphosate tolerance in those selected biotypes. For this research, seeds of a known susceptible common lambsquarters biotype (GS) were collected from a field near Huntley, MT, with no history of glyphosate use. Seedlings from each selected biotype were grown in a greenhouse at the MSU Southern Agriculture Research Center, near Huntley, MT. Whole-plant glyphosate dose-response experiments were conducted in randomized complete block design, with 5 to 8 replications and repeated. Glyphosate doses ranging from 0, 281, 562, 870, 1125, 2250, and 4500 g ae ha^-1 were tested. Ammonium sulfate at 2% wt/v was also included in each treatment. Visual assessment on percent injury were made and individual plants were harvested to determine the shoot dry weights at 21 d after treatment (DAT). To determine the underlying mechanism of enhanced tolerance in selected biotypes, the EPSPS (5-enolpyruvylshikimate-3-phosphate synthase) gene was analyzed for known target-site mutations and increase in gene copy numbers. Based on the visible injury and shoot dry weight response (LD₉₀ and GR₉₀ values), three selected biotypes showed 2.5- to 3.0-fold tolerance to glyphosate relative to the GS biotype. Sequencing of partial EPSPS gene at threonine 102 and proline 106 codons did not differ between the tolerant and susceptible biotypes. Furthermore, no differences in the EPSPS gene copy number were observed between glyphosate tolerant and GS biotypes. Further studies on [¹⁴C]-glyphosate uptake and translocation pattern among tolerant vs. GS biotypes are under progress. These results confirm the evolution of common lambsquarters biotypes with enhanced tolerance to glyphosate in the predominant corn-sugar beet rotation in this region. Growers should diversify weed control tactics in conjunction with alternative and effective sites-of-action herbicides to prevent further evolution of elevated levels of glyphosate tolerance in common lambsquarters in corn-sugar beet rotation.

Weeds cause significant crop yield and economic losses in agriculture. Treatment with herbicides is by far the most effective means of controlling weeds as modern herbicides can control 90 to >99% of the weeds targeted. Recently, there is growing concern over the potential impacts of global climate change, in particular, increasing temperatures and carbon dioxide (CO₂) concentrations, on reducing the response and sensitivity of weeds to herbicides. Winter annual weed species, Conyza canadensis and Chenopodium album, were tested for their response to glyphosate under different environmental conditions. Plants from the same populations were grown under ambient and enriched CO₂ environments in combination with control (12/18⁰C), current maximum (19/25⁰C) and future predicted maximum (26/32⁰C) temperatures. Plants were sprayed with glyphosate (Roundup PowerMax, Monsanto, St. Louis, MO, USA) at the recommended field rate of 870 g ae ha^-1. Reduced herbicide sensitivity, as measured by plant mortality and fresh weight, was found in both C. canadensis and C. album in response to glyphosate under elevated temperatures, enriched CO₂ levels and the combination of both treatments. C. album plants that survived glyphosate treatment exhibited the loss of apical dominance and initiation of the growth of multiple lateral buds. When enriched CO₂ was applied alone, reduced sensitivity was not always sufficient for plant survival; however, the combination of both high temperatures and enriched CO₂ levels resulted in much higher survival rates. Glyphosate translocation was also examined using ¹⁴C-glyphosate. In plants that were subjected to the enriched CO₂ environment combined with high temperatures, glyphosate was translocated faster out of the treated leaf compared to control conditions. These results suggest that glyphosate translocation and sequestration away from shoot meristems may be the basis of the reduced glyphosate efficiency. Exploring this mechanism of plant survival to glyphosate treatment at the transcriptional and metabolic levels can shed light on the way weeds will respond to glyphosate under future environmental conditions.

Sugarbeet, grown for biofuel, is being considered as an alternate cool-season crop in the southeastern U. S. coastal plain. Previous research identified ethofumesate PRE and phenmedipham + desmedipham POST as herbicides that controlled troublesome cool-season weeds in the region, specifically cutleaf eveningprimrose.  Ethofumesate and phenmedipham + desmedipham are used on sugarbeet grown for edible sugar, a subsidized specialty crop, and the herbicides are priced accordingly.  Since sugarbeet would be grown as a biofuel in the southeastern U. S., production budgets would likely be based on less costly forms of weed control compared to sugarbeet grown for edible sugar.  Research trials were conducted from 2014 through 2016 to evaluate sweep cultivation and reduced rates of ethofumesate PRE and/or phenmedipham + desmedipham POST for cost-effective weed control in sugarbeet grown for biofuel.  There were no interactions between the main-effects of cultivation and herbicides for control of cutleaf eveningprimrose and other cool-season species two years of three.  Cultivation improved control of cool-season weeds, but the effect was independent of control provided by herbicides.  Of the herbicide combinations evaluated, the best cool-season weed control was from a sequential application of the full-rate of ethofumesate PRE followed by the full-rate of phenmedipham + desmedipham POST, which was the most costly treatment combination.  Reduced rates of ethofumesate PRE or phenmedipham + desmedipham POST, applied alone or sequentially, were not as effective as full rates of either herbicide.  Surprisingly, despite improved weed control, sugarbeet yield was not affected by cultivation two years of three.  In contrast, sugarbeet yield response to herbicides was similar to weed control response; full rates of sequentially applied herbicides were needed for maximum sugarbeet yield.  These results indicate that for cool-season weed control in sugarbeet grown for biofuel, cultivation has a very limited role.  While weed control is improved by cultivation, that benefit does not compensate for inferior control provided by reduced rates of herbicides.  The premise of cost-effective weed control based on cultivation combined with reduced herbicide rates does not appear to be viable for sugarbeet grown for biofuel.

In 2017, Brake Herbicide was federally registered for preplant and preemergence applications in U.S. cotton. Brake contains the active ingredient fluridone which inhibits the phytoene desaturase (PDS) enzyme involved in carotenoid biosynthesis. Currently, there are no other PDS inhibiting herbicides used in U.S. cotton. Thus, the use of Brake may reduce selection pressure caused by repeated use of the same herbicide families. Experiments were conducted in 2016 and 2017 to evaluate efficacy and selectivity of Brake and Brake tank-mixes with other residual herbicides. Treatments containing Brake did not cause significant cotton phytotoxicity. Further, control of annual grass and small seeded broadleaf weeds including Palmer amaranth (Amaranthus palmeri) was similar or greater than other residual herbicides used in cotton. These results suggest Brake provides a new and effective mode of action for preemergence weed control in cotton.  

Burndown programs for cotton in the southeast frequently include 2,4-D and dicamba for control of glyphosate and ALS inhibitor resistant weeds. Sensitive cotton varieties planted into soil treated with 2,4-D and dicamba in burndowns can result in crop stunting and stand loss if the plant back interval is too short. With the introduction of 2,4-D and dicamba tolerant cotton varieties, producers can apply 2,4-D or dicamba in burndowns very close to planting or use them with preemergence (PRE) treatment. However, with this program, if the cotton stand fails and a susceptible shorter season cotton has to be replanted, injury may be seen if intervals between applications and replanting are short and this injury may further delay maturity when the grow season is already short. Therefore, the objective of this study was to evaluate cotton injury and yield responses as resulted from 2,4-D and dicamba residues in the soil. In 2016, field studies were conducted in Macon and Baldwin counties in Alabama and Santa Rosa county in Florida.  In 2017, cotton at Henry, Macon, and Baldwin counties were also evaluated. Treatments of 2,4-D included 532 and 1063 g ai h^-1 applied 3 weeks before planting and 53, 160, 266, 532, 1063 g ai h^-1 applied at planting as PRE. Treatments of dicamba included 560 and 1120 g ai h^-1 applied 3 weeks before planting and 56, 168, 280, 560, 1120 g ai h^-1 applied at planting as PRE. Stand counts and plant heights were collected 3 weeks and 7 weeks after planting, with a final yield collected to assess herbicide residual injury on the cotton. 2,4-D and dicamba treatments applied 3 weeks before planting did not have any adverse effects on cotton establishment or yield. The only significant height reduction observed over all locations for both years was in the 2016 Macon county study for dicamba 560 g ai h^-1, with a stunting of 27% at 21 days after planting (DAP) and 38% at 51 DAP, respectively.  In 2016, 560 g ai h^-1 of dicamba showed significantly reduced stands by 36 % at 21-24 DAP which did not recover at 50-51 DAP over all locations. In 2017, stands were reduced by 2,4-D at 266 g ai h^-1 by 26% and by 168 g ai h^-1 dicamba by 17% at 20-23 DAP respectively but recovered by 47-48 DAP. Dicamba at 280, 560 and 1120 g ai h^-1 produced significantly reduced stands by 17-25% also 532 and 1063 g ai h^-1 of 2,4-D had stand reductions of 29-36% at 20-23 DAP over all locations in 2017. None of these stand losses recovered by 47-48 DAP but they did not cause any significant yield loss at harvest. Our data suggests cotton stunting and stand reduction may occur if susceptible cotton varieties are planted too close to a burndown application with 2, 4-D and dicamba, but final yields may not be affected after a full growing season. 

Glyphosate-resistant (GR) horseweed (Conyza canadensis (L.) Cronq) infests a majority of the row crop production in Tennessee. Repeated burndown applications are very common for controlling horseweed prior to planting. New herbicide modes of action are needed for the management of this problematic, winter annual broadleaf weed. In 2017, Dow AgroSciences registered Elevore^TM herbicide with Arylex^TM active (halauxifen-methyl), a synthetic auxin herbicide (WSSA group 4) from the arylpicolinate chemical class. Research was conducted in west Tennessee in 2016 and 2017 to evaluate Elevore for crop safety and control of GR horseweed. The recommended use rate is 1.0 fl. oz. product/acre [5.0 g ae ha^-1 of Arylex] up to 14 days prior to planting soybeans and 30 days prior to cotton planting. Elevore was compared to competitive standards when applied with glyphosate and in tank mixes with 2,4-D  to GR horseweed at 10 and 20 cm growth stages. Elevore applied at 5.0 g ae ha^-1 + glyphosate at 1120 g ae ha^-1 provided better control of 20 cm horseweed when compared to 2,4-D at 560 g ae ha^-1 + glyphosate at 1120 g ae ha^-1. Crop injury evaluations indicated that soybeans and cotton can be planted according to plant-back restrictions provided on the Elevore label without significant crop injury. Elevore will provide growers with an additional tool for control of GR horseweed.

Monsanto Company has developed a new premix corn herbicide of acetochlor with safener and mesotrione.  The product is branded as Harness® MAX and will be available to growers for the 2018 growing season.  Harness® MAX offers excellent residual benefits of acetochlor with the added post-emergence and residual activity of mesotrione for a broadened range of control against tough to control weeds in corn such as amaranths (Amaranthus sp.), common lambsquarters (Chenopodium album), morningglories (Ipomoea sp.) and foxtails (Setaria sp.).  Field studies were conducted in 2017 to evaluate weed efficacy and crop response following Harness® MAX alone and with tank-mix combinations applied at planting (pre-emergence) and post-emergence on 5 and 11-inch corn.  Results from these studies indicate that Harness® MAX alone and in combination with tank-mix partners can provide excellent weed efficacy with minimal crop response compared to competitive offerings.  For post-emergence weed control, the addition of Roundup® branded agricultural glyphosate only herbicides will be recommended for improved control of emerged weeds.  Harness® MAX will be a valuable product for weed management in corn.

Divine nightshade has become an increasingly problematic weed in Louisiana sugarcane production and has been identified in 17 of the 24 sugarcane producing parishes. After final cultivation, the current standard layby herbicide program is to tank-mix the synthetic auxin herbicides 2,4-D and/or dicamba plus pendimethalin plus metribuzin and broadcast-direct apply under the crop canopy for control of emerged morningglory spp. and provide residual grass and broadleaf weed control. The first objective of this research was to evaluate the HPPD-inhibiting herbicides: 105 g ha^-1 of mesotrione, 2.9 kg ha^-1 of a premix formulation of S-metolachlor plus atrazine plus mesotrione plus bicyclopyrone, and 24.5 g ha^-1 of topramezone and the synthetic auxin herbicides: 1.1 kg ha^-1 of 2,4-D, 87 g ha^-1 of aminopyralid, 0.56 kg ha^-1 of dicamba, 157 g ha^-1 of fluroxypyr, 0.56 kg ha^-1 of picloram, and 1.1 kg ha^-1 of triclopyr applied alone. Herbicide tank-mixture treatments were also applied. A single growth regulator herbicide was tank-mixed with an HPPD-inhibiting herbicide (mesotrione, premix formulation of S-metolachlor plus atrazine plus mesotrione plus bicyclopyrone, or topramezone) for control of 5-10, 11-20, and 21-30 cm tall divine nightshade. The second objective was to determine if herbicide treatments reduce theoretical recoverable sucrose (TRS), sugarcane yield, and sucrose yield when treated to the sugarcane cultivar L 01-299. Results from the data showed spring applied herbicide treatments did not reduce TRS, sugarcane yield, or sucrose yield when compared to the nontreated. A tank-mixture of 2,4-D plus a HPPD-inhibiting herbicide resulted in 43, 63, and 53% greater control of 5-10, 11-20, and 21-30 cm tall plants as compared to 2,4-D alone, respectively. Increased control of smaller plants (5-10 cm) was marginal with tank-mixtures of HPPD-inhibiting herbicides plus dicamba when compared to dicamba alone; however, the former tank-mixture resulted in 25% more control of larger plants (11-20 cm) compared to smaller plants (5-10 cm). Overall, control with synthetic auxin herbicides applied alone generally followed: picloram > triclopyr > aminopyralid > fluroxypyr > dicamba > 2,4-D. Although not currently labeled for use in sugarcane, the results of this study show early-season treatment with synthetic auxin herbicides picloram, aminopyralid, and fluroxypyr and the premix formulation of S-metolachlor plus atrazine plus mesotrione plus bicyclopyrone resulted in greater divine nightshade control than 2,4-D and did not negatively affect L 01-299 yield.

Successful integrated weed management programs need to be developed for herbicide-resistant Palmer amaranth and other difficult to control weeds, as well as avoid the selection of additional herbicide-resistant biotypes.  Using cover crops as a tool for weed management is often mentioned, but additional research is needed on how best to utilize cover crops in a program approach.  The research objective was to evaluate the management of cereal rye cover crop for Palmer amaranth control.

This study included the combination of three factors:  level of rye biomass, timing of spring burndown application, and the benefit for residual herbicides.  Rye biomass levels were none, low, and high; low and high biomass levels were achieved by 0 or 20 kg/ha spring nitrogen application.  Timing of burndown (glyphosate + 2,4-D applications) was either 10 or 20 days before planting.  Treatments either had a residual herbicide applied with the burndown or had no residual herbicide.  Residual herbicide with this trial was a mixture of flumioxazin, chlorimuron, plus thifensulfuron, which also helped with overall burndown weed control.  This was a three-factor factorial, with 3 or 4 replications, and the study was conducted for 3 years.  All plots received a postemergence application of glyphosate plus fomesafen at 5 weeks after planting.  Visual weed control and soybean yield were collected.

Winter annual weed control was influenced by the main effects.  Both low and high rye resulted in better control than no rye.  Treatments with the residual herbicide had better control than no residual, because of the postemergence control provided by flumioxazin, chlorimuron, plus thifensulfuron.  Winter annual control was better with the 20 days early preplant because weeds were small and more susceptible.

After the POST application of glyphosate plus fomesafen, Palmer amaranth control was best with plots treated with residual herbicide (at least 93% control, regardless of level of rye).  Low and high levels of rye with no residual herbicide resulted in 86 and 91%, respectively.  If no rye was present, the residual herbicide resulted in 72% control.  Ivyleaf morningglory control was at least 93% control with low and high levels of rye.  In addition, rye termination at 10 days before planting provided 95% control while 20 days before planting averaged 89% control.

Soybean yield was highest with low and high levels of rye, averaging 2828 kg/ha and no rye yield was 2353 kg/ha.  Residual herbicide treated plots averaged 2825 kg/ha while no residual herbicides yielded 2515 kg/ha.

Rye cover crops improved control of Palmer amaranth and ivyleaf morningglory, but there was little difference between high and low rye levels.  In a few cases, the high level of rye provided early-season weed suppression and improved the control if rye was terminated 20 days before planting.  Yield was higher in the presence of a rye cover crop, presumably due to moisture conservation.

Farmer Attitudes Toward Cooperative Approaches to Herbicide Resistance Management

David Ervin, Elise Breshears, George Frisvold, Katherine Dentzman, Wesley Everman, Jeffrey Gunsolus, Terrance Hurley, Raymond Jussaume, Jason Norsworthy  and Micheal Owen

When herbicide resistant (HR) weeds move across farm boundaries, individual farmer actions to control HR weeds on their lands become less than fully effective.  Recent evidence indicates HR weed mobility is more significant than previously thought, although the risk varies by species (Beckie et al. 2015, Shaner and Beckie 2014). Mobility can occur through pollen drift, water movement, migratory bird flights; interstate livestock feed shipments, and the transportation of machinery. If HR traits are indeed mobile, the susceptibility of weeds to herbicides becomes an open access common pool resource (CPR) shared by all operators in the affected community (Ervin and Jussaume 2014). In cases of significant mobility, it can be in the collective interest of farmers to pursue cooperative approaches to delay resistance and conserve the efficacy of the herbicide (Ervin and Frisvold 2016).

 It is one thing to recognize the need for cooperative approaches to control HR weeds when significant mobility occurs and another to achieve cooperation amongst farmers.  Farmers are notoriously independent as a group and usually prefer to address production problems on their own terms.  That preference is understandable as significant costs, including sometimes uncomfortable interactions with neighbors, must be incurred to build cooperation. Understandably, farmers may avoid engaging in such efforts unless assured that their neighbors will reciprocate their actions.

Public organizations, including the Extension Service, and private firms have struggled to achieve farmer cooperation in addressing HR weed issues.  A notable exception is the Zero Tolerance program in Arkansas (Barber et al 2016).  However, those small cases do not lend general insights into the personal and community factors that are associated with receptivity to cooperation.  Some small-scale studies have begun to fill the void, contributing valuable information to the nascent literature on the topic (Graham and Rogers 2017, Stallman and James 2015).  Their findings generally indicate that a significant proportion of farmers are receptive to cooperative approaches, but they must be tailored to actively engage those most supportive.  Broader scale studies are needed to affirm or question the findings from these local and state-level investigations.

In this paper, we argue that the satisfaction of three preconditions affect the likelihood of implementing cooperative HR weed management approaches.  First, farmers must be aware of the threat that HR weeds can migrate from their neighbors’ lands.  Second, farmers must believe that HR weeds cannot be managed effectively without cooperation amongst farmers in a community.  Finally, farmers must be willing to communicate with their neighbors about HR weeds and their control.  We report the responses from the first national survey of farmers on HR weed management to assess the overall likelihood these three preconditions are met.  We then use logistic regression models to identify the farmer and other conditions that are statistically significantly related to the satisfaction of the three preconditions.  Our findings affirm the significance of some findings from the smaller scale studies but add new evidence on other factors that have not been analyzed with multivariate methods to date.  With the benefit of these evidence-based insights, educational programs and other strategies to implement cooperative approaches to manage HR weeds can be envisaged.

References

Barber, L.T., K.L. Smith, R.C. Scott, J.K. Norsworthy, A. M. VanGilder. (2016). “Zero Tolerance: A Community-Based Program for Glyphosate- Resistant Palmer Amaranth Management.” Agriculture and Natural Resources. FSA 2177. Fayetteville: U. Arkansas Division of Agriculture, Research and Extension.

 Beckie, H. J., et al,., , Brenzil, C. A., ... & Ford, G. (2015). Glyphosate-resistant kochia (Kochia scoparia L. Schrad.) in Saskatchewan and Manitoba. Canadian Journal of Plant Science, 95(2), 345-349.

 Ervin D, Jussaume R (2014) Herbicide Resistance: Integrating Social Science into Understanding and Managing Weed Resistance and Associated Environmental Impacts, Weed Sci 62:403-414.

 Ervin DE, Frisvold GB. (2016). Community-based approaches to herbicide-resistant weed management: lessons from science and practice. Weed Sci 64(Special Issue): 609-626

 Graham, M and S Rogers (2017) How Local Landholder Groups Collectively Manage Weeds in Southeastern Australia. Environmental Management. 60(3): 396-408.

 Shaner, D. L., & Beckie, H. J. (2014). The futurę for weed control and technology. Pest management science, 70(9), 1329-1339.

 Stallman, H. and H. James Jr. (2015) Determinants affecting farmers’ willingness to cooperate to control pests. Ecological Economics 117:182-192

This study reports on results from a large-scale national survey of cropping practices on U.S. farm fields. Up to twelve weed management practices were compared across fields that were owner-operated versus those that were rented.  It has been frequently argued that growers have less incentive to prevent herbicide resistant weeds on leased than owned land.  Survey data analyzed at both the national and regional level for both corn and for soybeans do not show strong evidence to support this hypothesis.  In most instances, there were no statistically significant differences in practice adoption or herbicide use. In cases where there were significant differences, more often than not, practices associated with greater resistance management were more prevalent on rented than owned land.    

Farmers collect and report data on a large number of production variables as part of risk management, food safety and marketing programs required by supply chain partners, but until now this vast resource hasn't been widely used to optimize production.  In this project, we explored the use of data mining (often called "big data") and machine learning to move precision agriculture from a decision support tool to a decision-making tool, including weed management. "Big data" has become a popular term to simply refer to a large volume of structured and/or unstructured (unorganized) data, often first attributed to the financial and health care sectors. In the current form "big data" is used to describe what has happened, but the addition of machine learning adds predictive ability by understanding the complex relationships among inputs and outputs.  Machine learning refers to the algorithms that are used to tease out the combinations of variables that consistently and reliably predict outcomes.  In a pilot study we applied these tools to data on 41 variables collected by cranberry farmers on over 500 cranberry "beds" or fields in 2016 as well as small-plot replicated weed management research from about 10 growing seasons.  More specifically, the implications of herbicide choice and rate relative to cranberry yield quantity and variability were explored.  In general, optimal herbicides and rates were similar between small-plot research and the data submitted from commercial production beds.  Interestingly, for a couple of herbicides a slight negative yield impact was noted in commercial beds in one growing season particularly as herbicide rate increased.  We hypothesize that the benefit from controlling minor weed infestations was outweighed by cranberry crop injury in these cases.  The methodology developed here offers an insightful new way to study weed management beyond conventional small plot research and with vast libraries of existing data.

Application of herbicides can control members of the broomrape family, which parasitized major crops like sunflower, tomato, carrot and potato. However, adoption of precise control management practices, which detect and treat infested areas exclusively, can mitigate environmental and human health issues that associate with herbicide usage. The main challenge in developing such control practices for parasitic weeds like the broomrape lies in the fact that by the time the shoots of this root parasite emerge, the weed has already damaged the crop and detection of their impact in a pre-emergent stage is vital. In this study, we show that use of 3D morphological parameters, which can be extracted by simple sensors, has the potential to detect broomrape infection at a very early stage. For that purpose, controls and O. cumana-infested sunflower plants were grown in pots and imaged weekly over a 45-day period. Three-dimensional plant models were reconstructed using image-based multi-view stereo, followed by extraction of their morphological parameters. All of the parameters showed differences between the infected and control plants; height and first internode length were the first to show that in a distinct manner. Furthermore, changes in both parameters were detected early enough for herbicide pre-emergence application. Notably, plant width, a 2D related parameter which was also evaluated, failed to exhibit any difference. Additionally, such ability to detect changes was not demonstrated by any other color-based methodology thus far. Our 3D morphological modeling is based on commercially available sensors and is applicable under field conditions; it can be used for precise management of parasitic weeds on larger scales.

Modern precision agriculture relies on site-specific weed management tactics to maximize yield and resource use efficiency. Traditionally, scouting for weeds is carried out by trained specialists through extensive and routine visual examination of fields. However, recent advancements in sensor-based applications have shown enormous potential for facilitating efficient weed infestation assessments as well as site-specific weed management. Field and greenhouse studies were carried out during 2016 and 2017 at College Station, TX to identify various weed species and determine their density in soybean, sorghum, corn and cotton fields. Hyperspectral imaging at 700 - 1150 nm wavelength could easily distinguish Palmer amaranth (Amaranthus palmeri), soybean, cotton and corn. However, differences between barnyardgrass (Echinochloa crus-galli), cotton and johnsongrass (Sorghum halepense) were more evident at the 1400 - 1750 nm range. The object-segmentation in RGB image analysis could also discriminate among certain crop and weed species. Broadleaf weeds could be identified with 80-95% efficiency whereas grasses were identified with only 60% efficiency. Integration of spatial features along with hyperspectral and RGB segmentation may improve the discrimination among the grass weeds. 

Analytical Hierarchy processing (AHP) can be described as a mathematical model of multi-criteria decision making (MCDM) that uses pairwise comparisons, in conjunction with a ratio scale, to indicate the strength of preference or importance of various criteria. AHP enables users to set priorities and make decisions when both quantitative and qualitative elements need to be considered.  Grant monies (EPA Region 1, PE-0-96156701) were obtained to evaluate if MCDM could be integrated into cranberry weed management.

We chose dodder (Cuscuta spp.) to be our case study as its management is not straight-forward; no single tactic provides adequate control across years and locations.  An integrated approach must be used and often, a different approach may be needed for the same grower in one year as compared to the next.  Currently, no process in available to help growers determine which criteria are important and which tactic(s) should be used in any given year.

We worked with stakeholders and evaluation specialists to see if/how AHP might be a useful tool for making decisions about dodder management. We held multiple stakeholder focus groups with small groups (ca. 20-25 growers) to: introduce and discuss AHP, identify the critical criteria to include in the AHP, and develop a survey that would be “grower-friendly”.  We used SimpleMind mapping software to illustrate the relationships of the identified dodder management criteria in a visual flow-chart type diagram.  The maps were useful in developing the prototype ratio-scaled survey. The survey was designed to allow the results to be entered into a spreadsheet, which contained the formulas to prioritize the selected criteria. Growers took the developed survey and gave feedback on format and content.  Using a ratio scale format is very different from typical surveys. The major take-home messages were that instruction prior to the survey is needed and that success would be improved if distributed to small groups as one-on-one help may be needed to insure growers correctly answer the survey questions. Growers were highly receptive to using AHP, providing the survey process could be simplified and results would be easily accessible and interpretable.

hsandler@umass.edu

Pesticide regulation in the U.S. is governed by multiple statutes. This presentation will provide an overview of these laws, what makes something a pesticide, and how pesticide regulatory activities are coordinated between the federal and state governments.

Modern biotechnology is providing a number of tools for the agricultural sector. Amongst these tools can be found products to enhance farmer’s ability to control weeds, as well as products for insect control. This presentation will provide an overview of how the US government regulates products of modern biotechnology for agricultural uses with a particular focus on the role played by the Environmental Protection Agency’s Office of Pesticide Programs (OPP).

The Registration Division is responsible for project oversight and regulatory decision-making for new products, uses, and chemicals that are proposed for conventional pesticide use in the United States, including herbicides.  Much of this work focuses on risk management strategies designed to minimize risks while still achieving product efficacy.  This presentation will provide a brief overview of the scope of the Registration Division’s work and strategies used to register safe and efficacious herbicides in the U.S. 

FIFRA Section 3(g) requires the Agency to review each registered pesticide every 15 years to ensure the products continue to meet the FIFRA standard. The current registration review program has 725 “cases” of which 457 are conventional pesticides.  EPA must complete registration review by October 1, 2022, for all pesticides registered as of October 1, 2007.    This presentation will describe the current status of the registration review program, the challenges EPA faces in working towards the 2022 goal, and discuss areas for stakeholder input specifically on herbicides working through the registration review process.

Staff from the Office of Pesticide Programs Health Effects Division (HED) will provide an overview of how human health risk assessments of pesticide active ingredients are completed.  The overview will include a summary of hazard identification and endpoint selection; and a summary of conducting dietary, residential, occupational, aggregate, and cumulative exposure and risk assessments.  This overview presentation will also highlight topical issues in HED that pertain to herbicides.

The Environmental Fate and Effects Division (EFED) of the Office of Pesticide Programs conducts drinking water assessments and assesses, characterizes, and communicates the potential risks from pesticide applications to nontarget aquatic and terrestrial organisms, including federally threatened and endangered species. A brief overview of the risk assessment process employed by EFED will be presented.

Under Section 7 of the Endangered Species Act (ESA), federal agencies consult with the Services (i.e., National Marine Fisheries Service and the United States Fish and Wildlife Service) on actions that may affect a federally listed species.  Following the recommendations of the 2013 National Academy of Sciences’ (NAS) (National Research Council) report on assessing risks to endangered and threatened species from pesticides, the EPA has worked with the Services to develop shared interim scientific methods for use in pesticide consultations.  A brief overview of the interim methodology will be presented.  

The anthropogenic-mediated introduction of exotics has brought into contact taxa that have been otherwise separated by geographical barriers for millennia. This novel encounter, contingent on the degree of reproductive isolation, can result in hybridization between introduced and resident species (either native or another colonizer). While the stimulating effect of hybridization in driving biological invasions has almost exclusively been attributed to genetic mechanisms enhancing “invasiveness” of hybrid lineages, here we propose a novel role for hybridization in species invasions and range expansion, a purely demographic mechanism without the requirement for any local adaptations. As the founding population in most new species introductions is likely to be small, the colonizer is susceptible to demographic Allee effects driven by pollen or mate limitations. Using a plant population simulation model that incorporates demography, mating systems, quantitative genetics, and pollinators, we show that Allee effects can potentially be overcome by neutral hybridization with a resident species. Our model showed that hybridization can be completely neutral (i.e. without invocation of adaptive changes) and transient (i.e. hybrids are ephemeral) and yet allow the establishment of an otherwise failed plant invasion. That is, when a cross-compatible resident species co-exists in the habitat, the colonizing species can exploit the individuals of the resident species as its potential mates and overcome the problem of mate limitation. Pure colonizer-type individuals can subsequently be reconstituted through crossing among hybrids and backcrossing with the colonizer parents. The spatial version of our model showed that neutral hybridization can also accelerate the rate of invasions either with or without Allee effects. Conservation programs should therefore account for this cryptic role that hybridization could play in biological invasions.

Microstegium vimineum is a C₄, shade-tolerant, annual grass which has invaded disturbed and forested areas throughout the northeastern US and is now found as far west as Texas.  There are regional patterns of genetic variation detected between populations located in the Ridge and Valley province (RV, relatively low annual precipitation) and the Appalachian Plateau province (AP, relatively high annual precipitation) of West Virginia.  This grass has demonstrated rapid evolution of phenology during invasive range expansion, suggesting sufficient genetic diversity to drive adapatively significant evolution, such as expanding westward into even drier environments or adapting to drier conditions resulting from climate change.  We evaluated the effects of drought on the growth and reproduction of M. vimineum under light conditions typical of disturbed areas that this species is known to invade and within both the RV and AP provinces.

Seeds were collected from three populations in each province and germinated in plug trays.  After the first true leaves developed the plants were transplanted into1-gallon pots in media with slow release fertilizer.  Four replicates from each province were grown in two greenhouse rooms in which light levels were set for conditions similar to (1) a first-year shelterwood harvest or high light (HL) and (2) a thinned forest or low light (LL).  Drought levels included (1) maintaining field capacity close to 35% volumetric water-content (VWC) or C, (2) half the VWC at field capacity, close to 18% VWC or M, and (3) one-fourth field capacity, close to 8% VWC or D.  After 30 days of growth under C-level of moisture, the drought treatments were applied and maintained for two weeks.  The plants experiencing D-level drought were allowed to wilt for 24 hours after which watering was resumed and plants recovered over three days.  Plants were then kept at the same drought levels with occasional short-term wilting in the D-level drought treatment for another three weeks when the plants flowered and formed seeds.  Data were analyzed using a generalized linear mixed model with a normal distribution for shoot biomass, number of tillers, and seed weight, and lognormal distribution for root biomass, number of flowers, seed-to-empty-floret ratio, seed count, and viability (using a tetrazolium dye test).  A Kruskal-Wallis test was used to evaluate percent germination.

HL plants produced more shoot and root biomass, tillers, flowers, and seed than LL plants, but the seed number did not differ significantly (p < 0.05).  Light, drought, and province had a significant three-way interaction for number of tillers, but there were no significant 2- or 3-way interactions for any other measured variables. The LL and RV plants had significantly higher seed-to-empty-floret ratios than the HL and AP plants, respectively.  The D-level drought treatment had nearly twice as large seed-to-empty-floret ratios compared to C and M, but were only marginally significant (p = 0.090).  Seed weight and viability did not differ significantly by light, drought treatment, or province, but there was a trend for the RV. LL, and D-level plants to have the most viable and largest seeds.  Percent germination was higher for LL, D-level, and RV plants, but was not significant for province.  LL, RV, and D-level drought plants appear to produce fewer flowers that do not produce seeds and more viable seeds that are ready to germinate under optimal conditions than HL, AP, and C or M-level drought plants.  This strategy of using high seed production and seed germination to combat stressful environmental conditions (low light and drought) once conditions improve appears more likely to occur with RV province plants.

Growth, physiological and biochemical responses of two Australian biotypes of African turnip weed (Sisymbrium thellungii L.)   to varied soil moisture regimes

Gulshan Mahajan^1,5, Barbara George-Jaeggli ^2,3, Michael Walsh⁴, Bhagirath S. Chauhan¹

¹Queensland Alliance for Agriculture and Food Innovation (QAAFI), The University of Queensland, Gatton, Queensland 4343, Australia.

²Queensland Alliance for Agriculture and Food Innovation (QAAFI), The University of Queensland, Warwick, Queensland 4370, Australia.

³Agri-Science Queensland, Department of Agriculture and Fisheries, Warwick, Queensland 4370, Australia.

⁴ Sydney Institute of Agriculture, School of Life and Environmental Sciences, The University of Sydney, Narrabri, NSW 2390, Australia.

⁵ Punjab Agricultural University, Ludhiana 141 004, India

Abstract: African turnip is an emerging problematic weed of the northern grain region of Australia. Several different biotypes have been identified across this region and not all biotypes exhibit the same weed competitive ability. This might be due to local adaptation to the different agro-ecologies the biotypes originated from, however, information on this aspect is limited. To determine whether adaptation to water stress was a factor in biotype demographic behaviour and potentially competitive ability, we evaluated the physiological response of two Australian African turnip weed biotypes, selected from high (Dalby) and low (St George) rainfall areas, to differing pot soil moisture levels corresponding to 100, 75, 50 and 25% of soil water holding capacity (WHC). When averaged across all moisture levels, the St George biotype (low rainfall area) had 89% greater biomass, and produced 321% more seeds than the Dalby biotype. The St George biotype seemed to be less affected by increasing levels of water stress than the Dalby biotype. While at 100% WHC the Dalby biotype produced 4787seeds plant^-1, it only produced28 seeds plant^-1at 25% WHC. The St George biotype still produced 4061 seeds plant^-1at 25% WHC, down from 9834seeds plant^-1 at 100%. The St George biotype also tended to have lower conductance across all levels of water treatments, which especially at higher moisture levels was associated with lower net photosynthetic rates, possibly further indicating its specific adaptation to water stress conditions. Across the moisture levels, the St George biotype had higher total soluble sugar and phenolics, but free proline content was higher in the Dalby biotype compared with the St George biotype. Like total soluble sugar, proline content increased with water stress in both biotypes, but it increased to a greater extent in the Dalby biotype, particularly at 25% of WHC. Branching, flowering and maturity occurred earlier in the St George compared with the Dalby biotype, indicating relatively fast growth of the St George biotype, which again seems to be an adaptation to water-limited environments. In conclusion, the St George biotype of African turnip weed reproduced better than the Dalby biotype across all the moisture regimes, which suggests its enhanced invasiveness. Overall, the ability of African turnip weed  from the low rainfall area to maintain good growth, physiological response and seed production under water stress may enable it to spread to a wide range of Australian environments.

Carbon dioxide, CO2, in addition to water, sunlight and nutrients is a resource necessary for plants to grow. As CO2 increases it will therefore increase the growth of plants. But as a resource increases, plant species may respond differently.   This is especially relevant in agriculture as weeds can compete with crops for resources. To determine if the recent increase in atmospheric CO2 may may have already resulted in changes in weed:crop competition, two populations of wild oat from the same location, one collected in 1967 when the atmospheric CO2 concentration was 320 ppm; the other collected in 2014 when the CO2 concentration was 403 ppm were grown and compared relative to the recent CO2 increase.  Monocultures of each wild oat population differed in their response to recent increases in atmospheric CO2 with the newer population showing a signficantly higher response for all vegetative parameters relative to the population from the 1960s.  Assessment of outcomes relative to a cultivated oat line which has been genetically unchanged since its release in the 1960s indicated that at current CO2 levels, the newer wild oat population had increased competitive abilities with respect to leaf area and above ground biomass.  Overall, these differences are consistent, but not conclusive, of improved evolutionary fitness and increased early competitive ability of wild relative to cultivated oat in response to recent increases in atmospheric CO2.  Although additional empirical studies are needed, these preliminary results are consistent with the idea that weeds, not crops, may be adapting more rapidly to rising levels of atmospheric CO2.

Heterogeneity in the distribution of soil resources is a universal characteristic of the soil environment. In agricultural systems, soil heterogeneity is unintentionally generated via the patchiness of incorporated decomposing organic matter (e.g. tilling in a preceding cover crop), or intentionally generated via banding of N fertilizers. ‘Root foraging’ encompasses the various traits exhibited by roots to find and acquire soil resources. Root foraging traits vary considerably both within and between species, and understanding how crop and weed foraging strategies differ may have profound impacts on determining which root traits enhance crop competitive ability and productivity. Despite the important role that belowground competition has on crop productivity and yield, research on common weed root foraging traits and their relationship to plant productivity is limited. We examined root foraging traits of maize and four common agricultural weeds (Abutilon theophrasti, Echinocloa crus-galli, Setaria faberi, and Sinapis arvensis) within soil environments with both heterogeneously and homogenously distributed resources. We utilized four maize recombinant inbred lines (RILs) from the IBM (B73 X Mo17) population that had previously been shown to have high (M0063 and M0167) and low (M0005 and M0013) root plasticity in response to the heterogeneous distributions of soil resources. At the University of New Hampshire MacFarlane Greenhouse, we utilized rhizo-boxes as growing containers to facilitate the visualization and measurement of root foraging traits. While both the maize cultivars and weed species utilized within this study produced greater shoot biomass within heterogenous compared to homogenous distribution of soil resources, this increase in shoot biomass was overall greater among the weed species. The increase in shoot biomass in response to heterogenous soil resources was not correlated with root proliferation into resource patches (measured as the percent of root length, surface area, or biomass within the nutrient patch) and instead was influenced more by the increase in total root length and surface area in heterogenous compared to homogenous soil environments. These results support previous work which demonstrates that maximizing soil volume occupancy with greater root length and surface area throughout the soil greatly enhances productivity, regardless of the distribution of resources within the soil environment. Therefore, maize breeders can enhance crop competitiveness against weeds by selecting for traits that enable crop roots to disseminate throughout the soil quickly. Additionally, the rhizo-box method used in this research can be used for studying root traits and has applications for evaluating the mechanisms of belowground competition.  

Italian ryegrass (Lolium multiflorum Lam.) is a persistent grass weed in annual and perennial crops in California. In 2005, 118 populations from a diversity of crops and roadsides across the Central Valley were sampled for seeds. Seedlings were treated with glyphosate at the labeled field rate (867 g ae ha^-1), and glyphosate-resistant individuals were confirmed in 46% of the sampled populations. To determine whether the distribution and frequencies of glyphosate-resistant plants and populations have changed over the past 10 years, populations were resampled in 2015 using the 2005 GPS coordinates. Mature panicles were collected randomly from 30 individuals at each site and bagged separately. One hundred seeds from each individual were bulked to produce a population to be used for resistance testing. Populations were retested for resistance to glyphosate and also evaluated for resistance to glufosinate (984 g ai ha^-1), sethoxydim (515 g ai ha^-1), and paraquat (560 g ai ha^-1), herbicides commonly used to combat glyphosate-resistant ryegrass in the region. Seeds were germinated and 35 seedlings of each population were treated with these herbicides at the 3- to 4-leaf stage using an automated sprayer calibrated to deliver 20 GPA of the herbicides. Plant mortality data were collected 21 days after treatment (DAT) and subjected to ANOVA using PROC MIXED in SAS. To test for relationships between changes in the frequencies of herbicide-resistant individuals within populations and counties over the last ten years and the weed management practices used, herbicide use data were obtained from the California Department of Pesticide Regulation (DPR) database. The total number of herbicide applications, number of glyphosate applications, and number of herbicides differing in mechanism of action (MOA) were collected for as many populations and years (from 2005-2015) as possible. Herbicide use data were subjected to Principal Component Analyses (PCA) and t-tests. Relative to 2005, the percentage of glyphosate-resistant populations increased significantly from 46% to 79%. Of the 83 populations treated with glyphosate, only 28 (21%) were completely controlled. Multiple resistance (MR) to glyphosate, sethoxydim and/or paraquat (in at least two or three combinations) was confirmed in 20 populations. Of these MR populations, resistance to glyphosate, paraquat and sethoxydim was confirmed in four populations from counties in the northern part of the Central Valley. Only eight populations contained individuals that survived glufosinate treatment and five populations showed multiple resistance to glufosinate, glyphosate and/or paraquat. Significantly greater (a = 0.05) numbers of applications of glyphosate and all herbicides were observed in orchards with glyphosate-resistant (GR) than -susceptible (Sus) populations. Surprisingly, numbers of MOAs used in an orchard did not differ between GR and Sus populations suggesting that resistance status was not significantly associated with the number of different herbicide MOAs applied. To date, relationships between herbicide use practices and resistance has only been analyzed for orchards.

Weedy red rice is considered the most important weed of rice. It is morphologically similar to cultivated rice and is also characterized under the same species Oryza sativa.  After several years of weedy rice free rice production, the problem of weedy red rice resurfaced recently in California. For this reason, we conducted a genome-wide profiling of weedy red rice collected in 2006 and 2016 to understand and infer their possible origins. A total of 40 weedy red rice along with 20 weedy red rice from the Southern USA, 8 wild rice, and 7 temperate japonica, 3 tropical japonica, 5 indica, 5 aus and 4 specialty rice were grown in the greenhouse. The DNA from each entry was extracted and used in DNA profiling using 99 SSR markers and 1 primer pair for Rc1 gene distributed in the 12 chromosomes of rice. The PCR products were resolved in 6% polyacrylamide gel using the ABI 377 DNA sequencer. A total of 521 alleles were generated and scored as 1 or 0 for the presence or absence of alleles, respectively. Clustering and phylogenetic analysis was employed in DARwin v.6 using the UPGMA hierarchical cluster analysis with 1000 bootstrap iterations. The resulting genetic tree indicated two distinct main groups that separated the indica from japonica rice. Moreover, the markers clearly differentiated the tropical and temperate japonica. In general, CA weedy red rice were grouped into four clusters with a general pattern of grouping based on hull color and grain type. Hierarchical grouping indicated that bronzehull weedy red rice (type 2) entries were genetically distinct but closely related to the strawhull weedy red rice from the Southern US, from which, the CA medium grain strawhull (type 3) and short grain blackhull types (type 4) with very long awn likely evolved. The short grain strawhull weedy rice (type 1) clustered with Southern blackhull weedy rice and aus rice while the medium-long grain strawhull ecotype (type 5) clustered with japonica rice, thus demonstrating possible recent hybridization of weedy rice with cultivated rice. Additionally, specialty rice entries grouped in type 2 and type 5 indicating multiple possible origins of CA weedy red rice. Therefore, the results of our study suggest a stricter policy to be adopted in California to prevent and stop the spread of weedy red rice. It is also recommended that all incoming rice seeds to California should be checked for red pericarp to prevent unintentional spread of voluntary weedy rice.

Glyphosate resistant Palmer amaranth (Amaranthus palmeri) is a major threat to the sustainability of Roundup-Ready cropping systems in southern US. Control of these weeds with glyphosate has therefore been rendered inadequate. Moreover, the variability in the genetic makeup of the different biotypes of Palmer amaranth populations makes generalization of their management strategies more challenging.  In the current study, the metabolic perturbations following glyphosate application was compared across two susceptible (S-) and three resistant (R-) biotypes of Palmer amaranth with varying resistance (GR₅₀) to glyphosate. Comparative metabolic profiling of the different biotypes indicated that the most resistant biotype (C1B1) was innately abundant in several metabolites derived from phenylpropanoid pathway. Upon treatment with glyphosate, the metabolic pool dynamics of all biotypes correlated with the respective GR₅₀ levels, with the most resistant biotype having a higher pool of metabolites known to have anti-oxidant potential. Compared to the most resistant biotype, the S-biotypes had relatively low levels of both primary and secondary metabolites, indicating glyphosate induced metabolic inhibition. After glyphosate treatment, the content of total phenolic and flavonoids decreased in S-biotypes, whereas the abundance of these metabolites either remained the same, or increased in the R-biotypes. These results indicate that the phytochemistry and the antioxidant capacity that might play a complementary role in glyphosate resistance is partly induced after glyphosate application, rather than being constitutively expressed.

The continuity of life under changing circumstances is maintained by natural selection acting on genetic diversity, and weeds are not an exception.  When an herbicide application targets a weed population, adaptive alleles are selected allowing the population to survive. These alleles conferring resistance may have their origin in the standing genetic variation of the population prior to when the selection process started, or they may have migrated from another population via pollen or seeds. Additionally, resistant mutants may arise spontaneously in a weed population as a product of the constant load of de novo mutations in plant genomes. Understanding the relative input that each of these three sources adds to herbicide-resistance evolution is crucial to design effective resistance-mitigation strategies. Therefore, in an effort to provide empirical information to the resistance evolutionary framework, this study aims to calculate the de novo mutation rate conferring herbicide resistance in plant populations and to test the hypothesis that the rate is increased when plants are exposed to sub-lethal doses of herbicides. Because spontaneous mutations are very low-frequency events, we developed a model system using grain amaranth and ALS inhibitors, which robustly detects resistant individuals among millions of susceptible plants. From the first 70 million screened plants, we have not recovered any resistant mutants, suggesting the mutation rate providing resistant genotypes is lower than expected based on theoretical calculations. In addition, we found no evidence that herbicide stress increased the mutation rate.

Cases of differentiation in life-history traits among Palmer amaranth have been documented, but it is not clear which evolutionary processes are responsible. We conducted a Q_ST-F_ST analysis using microsatellite data to determine whether differences in life-history traits between ten Palmer amaranth populations could be attributed to selection. The results confirmed previous reports that genetic differentiation is small likely due to high rates of gene flow among Palmer amaranth populations and that most of the genetic variability is found within populations. Using Q_ST-F_ST analysis, we confirmed that most life-history traits studied followed the same pattern as neutral markers and were not under selection, but several traits affecting weediness such as plant height and dry weight, and nitrogen use efficiency were under selection (P=0.001). These results provided evidence that selection forces in agroecosystems have the potential to increase weediness in Palmer amaranth, and these evolutionary processes should be considered when designing strategies that are aimed at ensuring long lasting weed management.

Advanced genomic techniques for investigating non-model weedy species were historically unavailable to most weed scientists due to its high cost and the intensive technical training needed for these types of research. Moore’s Law, paired with the wide-spread adoption and commercialization of cutting-edge technologies and necessary instrumentation, have reduced the hurdles in this field of research. Genomic research related to all aspects of weed biology, physiology, or even management are now available to any researcher wanting to probe deeper into understanding weediness traits. Our research team used transcriptomics to better understand multiple herbicide resistance in Echinochloa colona from Arkansas; specifically, to identify key genes in xenobiotic detoxification and protection from herbicide damage. In the process, we were able to evaluate the transcriptomic responses to various herbicides of interest and deduce putative linkage of abiotic stress responses to herbicide action. Our goal is to provide novice researchers, such as ourselves, with a basic understanding of the process we used and to encourage further research using this technology. To generate the necessary data, our team collaborated with the Clemson University Genomics Institute (CUGI), which provides for-hire services including RNA-sequencing and bioinformatics analysis. This service included all data processing and reports necessary to generate a de novo transcriptome, differential gene expression (DGE) analysis, and gene ontology (GO) analysis. We then converted these reports to Microsoft Excel® files, which we used for further data processing. In the bioinformatics analysis, genes were identified using several databases. CUGI uses the UniProt (http://www.uniprot.org/) database to assign gene annotations; this database also provides some basic information about the genes.  Our team used the reports to assign genes to functional families as informed by the DGE analysis. Relevant terms such as ‘auxin’, ‘calcium’, ‘glycosyltransferase’ and others can be used to search similarly annotated genes in the file. These families were then assigned categories, which broadly classified them into pathways or processes including ‘photosynthesis’, ‘respiration’, ‘xenobiotic detoxification’, and others. Using JMP Pro 13 with the Venn Diagram add-in, we further narrowed down the gene groups using these categories and the experimental ‘conditions’ we set up to answer our research questions. Using Venn diagrams of more than two categories and the ‘sort’ function in Excel allows for a more granular approach to interpreting both the DGE and GO outputs. We used the KEGG database (http://www.kegg.jp/) to visualize relevant pathways, genes, and proteins. Our approach is simple and provides basic information on the response of a weed species to a treatment of interest by analyzing differential transcriptomic response. Deeper bioinformatics analysis can be conducted; however, this simplified approach can be easily adopted by applied science research teams with minimal or no genomic backgrounds.

Crop Characteristics and Weed Interactions of Diverse Recurrent Inbred Lines (RILs) from a Weed-Suppressive x Non-Suppressive Rice Mapping Population




Gealy, D.R. USDA-ARS, DBNRRC, Stuttgart, AR

Indica rice genotypes with enhanced weed suppression traits have been previously identified as potentially useful in supplementing weed control efforts in drill-seeded systems in the southern USA.  A particularly weed-suppressive indica genotype (PI 312777) that was also high tillering and high yielding was crossed with a non-weed-suppressive tropical japonica genotype (Katy) to produce a mapping population of ~330 recombinant inbred lines (RILs).  In 2016, the two parents and 27 F9 RILs with diverse growth traits were selected from the full population of 330 RILs, and evaluated in the field.  In 2017 an additional ~21 F10 RILs were evaluated for a total of 48 RILs.  Crop growth, yield, and weed suppression were evaluated, with the objective of identifying RILs possessing both yield and weed suppression potential that meet or exceed those of the parents (particularly the tropical japonica).  The experiment was designed as a split plot with four replications, and main plots as rice genotypes, and subplots as weed control (weedy or weed-free).  Rice seed was planted 2 cm-deep in plots 3-m-long with 6, 18-cm-wide rows on May 9-13, and emerged on May 23-24 in both years.  Weeds in weed-free plots were controlled using commercially-recommended rates of pre-emergence (clomazone plus quinclorac) or postemergence (fenoxaprop) herbicides.  Propanil was applied postemergence to weedy plots at the 1/2X use rate in late June as needed (2016 only) to mildly suppress barnyardgrass. Plots were fertilized with 110 kg/ha N as urea and flooded on June 21-27, and harvested Oct. 3-26 in both years. Agronomic traits, including rice tiller number, biomass, plant height, yield, and weed suppression, were recorded.  In 2016 yield averaged over all genotypes was ~50% lower in weedy compared with weed-free plots.  Several RILs, however, maintained relatively greater yields under weed pressure.  These included RIL90, RIL122, RIL274, RIL144, and RIL100 (2017 yield data not yet available).  Averaged over all genotypes in 2016 and 2017, respectively, heights were 11% and 8% lower, tiller numbers were 25% and 22% lower, and midseason rice biomass was 33% and 30% lower in weedy compared with weed-free plots.  RILs that were among the most weed-suppressive  in both years based on visual control ratings and weed biomass reduction included RIL392, RIL90, RIL244, RIL122, RIL404, RIL291, RIL274, and RIL33. Among the RILs that were evaluated for the first time in 2017, RIL109, RIL394, RIL306, and RIL44 also exhibited high levels of weed suppression.  Various RIL traits appeared to be associated with weed suppression in these studies.  RIL90 is very tall and was shown to maintain yields well under ‘alternating wetting and drying’ (AWD) moisture stress in previous research.  RIL122, RIL291, and RIL244 are high tillering lines (similar to PI 312777). RIL274 is also very tall and was shown to produce high root mass in previous research.  Thus, many of the promising RILs appeared to benefit from advantageous traits such as tall plant height, high tillering, high root mass, high seedling emergence rates, and tolerance to AWD moisture stress as determined in these or in previous studies, and might be using several strategies to reduce the impacts of weeds.  Overall, RIL90 and RIL122 appear to have an excellent combination of high yield and weed suppression ability that equals or exceeds that of the parents, and may be potential candidates for release as improved germplasm.

During the summer and fall of 2017, weed species were documented in the District of Columbia, Maryland, and Virginia (DMV) area.  The area surveyed was in the Arlington/Alexandria Virginia and District of Columbia Capitol Mall areas.  Ninety-two distinct species were identified and photographed.  Of these, 87 were terrestrial and 5 were aquatic; 58 were annuals, 27 were perennials and 7 were biennials; and 75 were dicots and 17 were monocots.  The most common areas to find weedy species were in undeveloped vegetation islands within the developed areas of Virginia; poorly maintained landscape areas including turf, ornamental beds and planters, and along riparian areas of Four Mile Run and the Potomac River.  The most common species observed were: giant foxtail (Setaria faberi), poison ivy (Toxicodendron radicans), goosegrass (Elusine indica), horseweed (Erigeron canadensis) and Amaranthus ssp [there were five documented with smooth pigweed (Amaranthus hybridus L.) and slender amaranth (Amaranthus viridis) the most commonly found].  Giant foxtail was most commonly found in natural areas, landscapes and planters.  Poison ivy was found mostly in undeveloped vegetation islands.  Goosegrass was found mostly in poorly maintained turf and associated areas.  The Amaranthus ssp were mostly found in ornamental and flower beds.  There is some evidence these species may have been introduced to the areas in transplant ornamentals over the years.  Horseweed was found in almost all types of areas that were not regularly mowed.  One population of horseweed was documented in the median area at the underground Metro station at Pentagon City.   This population has been observed at this location since 2014 but may have existed here for some time before that.  Two species that were not commonly found but of interest were purple loosestrife (Lythrum salicaria), found along Four Mile Run south of Reagan National Airport), and kudzu (Pueraria ssp) that was found in an area along the Mount Vernon Trail, north of Alexandria.   Purple loosestrife is a Tier 2 Noxious weed in Virginia defined as:  any noxious weed that (i) is not native to the Commonwealth, (ii) is not widely disseminated in the Commonwealth and (iii) for which successful suppression is feasible but eradication is unlikely.

Reduced- and no-tillage soil conservation practices provide many benefits, including decreased soil erosion and increased water infiltration and soil organic matter. Tillage practices may also cause weed species shifts over time, but there are relatively few long-term studies on the impacts of reduced- and no-tillage on weed communities. A long-term study was established in 1970 to examine the interactive effects of tillage and fertility treatments on grain yield and soil characteristics in St. Clair County, Illinois, at the Belleville Research Center. Now this study also provides the opportunity to test for differences in weed communities as a result of treatment over this 48-year period. The study was established on a Bethalto silt loam with four tillage regimes (continuous conventional tillage with a moldboard plow, continuous chisel plow, alternate-tillage treatment by year, and continuous no-till) and three fertility treatments (no fertilizer, N only, and NPK), and was planted in continuous corn from 1970 to 1990 and a corn-soybean rotation from 1991 to present. In order to assess the aboveground weed community composition, herbicides were not applied to half of each plot in 2017 to allow weeds to emerge in the field. Species were identified and quantified within a randomly-placed 1m² quadrat and terminated before seed production. In order to assess the belowground weed community assemblage present in the seedbank, 70-5 cm diameter soil cores were taken per plot to a depth of 20 cm; soil cores were sectioned by 5 cm increments and combined for one composite sample per plot at the depths of 0-5 cm, 6-10 cm, 11-15 cm, and 16-20 cm. Data were analyzed using Analysis of Variance, Non-metric Multidimensional Scaling Ordination, Analysis of Similarity (ANOSIM), and vector fitting of the species-associated variables of richness, evenness, and Shannon-Weiner diversity index. Seventeen species were present in the aboveground assessment, and 13 of these species had no relationship to tillage or fertility treatments: Abutilon theophrasti, Ambrosia trifida, Ampelamus albidus, Chenopodium album, Digitaria ischaemum, Digitaria sanguinalis, Ipomoea hederacea, Ipomoea lacunosa, Oxalis stricta, Poa annua, Polygonum pensylvanicum, Solanum carolinenese, and Xanthium strumarium. Panicum dichotomiflorum densities were greater in the NPK treatment than in the no-fertilizer treatment and lesser in continuous no-till than in alternate and continuous conventional tillage. Setaria faberi densities were greater in the continuous chisel and conventional treatments than in the no-till treatment. Densities of Amaranthus tuberculatus were greater in continuous chisel treatments than the alternate tillage treatment, although not significantly different than continuous no-till or conventional tillage treatments. Densities of Sida spinosa were greater in the continuous no-till treatment than in the continuous chisel and conventional treatments. There were differences in the weed communities by tillage but not fertility treatment, and ANOSIM indicated that communities in all tillage treatments were different from one another except in the continuous chisel and conventional treatments. Vector fitting indicated that there was increased species richness and diversity in no-till compared to all other treatments; however, there were no differences in species evenness. These results demonstrate potential species shifts which may occur over time as a result of tillage and fertility management programs. Belowground weed community assessments are currently being conducted in the greenhouse in a soil seed bank grow out study; preliminary data will be presented.

Compost for commercial mushroom production is a mixture of local materials that often includes hay, corn stover, and wheat straw.  After a two to three month production cycle, the spent mushroom compost is distributed or sold as a soil amendment.  The plant material used in compost may come from fields with weeds present and there has been concern about this serving as a source of weed infestation.  There is no information on the viability of weed seeds that go through the process. The process used in making mushroom compost is unique in that the composting material is exposed to several additional phases of high heat and humidity; the process is not analogous to standard composting of organic material.  Phase I is the mixing and conditioning of the compost outside in compost piles, known as ricks.  Phase II is a pasteurization phase after the compost has been placed into growing beds, during which compost is subjected to steam up to 71C and continued high humidity and temperatures for 10 days. Phase II is followed by mushroom production cycle and then the compost is subjected to a shorter pasteurization phase before it is removed from the mushroom houses.  We hypothesized that this process will have a negative impact on weed seed viability; allowing for the use of the spent mushroom compost without the unwanted spread of weed seeds.  The objective of this study was to examine weed seed viability following four phases of the mushroom growing process, and determine if various weed species respond differently.

The study was a two-factor factorial consisting of weed species and removal timing and was repeated using two separate mushroom houses.  Weed species consisted of annual ryegrass, hairy vetch, ivyleaf morningglory, Palmer amaranth, and velvetleaf.  Removal timings were at end of Phase II, two weeks after Phase II, after the final mushroom harvest, and following the compost sterilization phase. Fifty seeds of each species were placed in individual mesh bags, with five replications.  Seed bags of the individual species were placed in onion bags and placed in compost beds prior to the Phase II pasteurization process.  At each removal timing, onion bags were removed from each house for seed evaluation.  Seeds were pressure tested using forceps with those unable to withstand slight pressure (mushy) were considered non-viable.  Those seeds that resisted slight pressure were placed on petri dishes and allowed to germinate over a two-week period in the greenhouse.  As a control, fifty seeds of each species that were not placed in the mushroom houses were placed on petri dishes. Seeds that remained intact after the germination test were dried and a crush test was performed on each individual seed. Seeds were considered non-viable if they appeared powdery, black, or brown when crushed.  Remaining seeds were given an additional two-week germination period followed by a second crush test.  A seed was considered viable if it germinated in the petri dish or if it was determined intact after the crush tests.  Percent viable seed is expressed as a percent of the control seeds. 

There was a significant removal timing by species effect on weed seed viability.  Overall, a greater percentage of viable seed remained following Phase II (72 to 98%) compared to subsequent removal timings, and that number decreased as removal timing was delayed. Only one ivyleaf morningglory seed germinated following the final compost sterilization phase, while all other species had no viable seeds after the same removal timing.

In this study, we show that the indoor commercial mushroom production process will eliminate weed seed of annual ryegrass, hairy vetch, Palmer amaranth, and velvetleaf.  However, a low percentage of morningglory seed remained viable.  While this study did not examine the effects of Phase I composting, it may be necessary for the compost to go through the entire process have the greatest impact on viable seeds.  Further investigation is needed to determine if the entire mushroom composting cycle will eliminate the viability of all weed seeds.

The influence of temperature, light, solution pH, water stress, salt stress, and burial depth on seed germination and seedling emergence of catchweed bedstraw (Galium aparine) and the sensitivity of that weed to commonly available herbicides in China were studied in laboratory and greenhouse. Germination occurred at day/night temperatures from 5/0 C to 30/25 C, with optimum germination at 15/10 C. Catchweed bedstraw germinated equally well under a 12-h photoperiod and continuous darkness; however, a 24-h photoperiod inhibited seed germination. Catchweed bedstraw seed is moderately sensitive to osmotic potential and salt stress, with 15 and 3% germination rates at an osmotic potential of -0.5 Mpa and salinity level of 120 mM, respectively. Maximum seed germination was observed in near neutral pH; germination was greater than 80% over a broad pH range from 5 to 8. Seedling emergence of the seeds buried at a depth of 1 cm was higher (74%) than those placed on the soil surface (20%), but declined with burial depth increasing. Few (10%) seedlings emerged when seeds were placed at a depth of 5 cm. Bensulfuron-methyl, and ethametsulfuron-methyl applied PRE and tribenuron-methyl, fluroxypyr, and florasulam applied POST can be used to provide greater than 80% control of catchweed bedstraw. The results of this study have contributed to more complete understanding of the germination and emergence of catchweed bedstraw.

Hairy vetch is a leguminous winter cover crop species that is desirable for its ability to fix nitrogen, gain large amounts of biomass, and winter hardiness. However, this species is also known to become a weed in subsequent crops, which is often attributed to dormant seeds or seeds that germinate after the cover crop growing season.

Experiments were conducted in Blacksburg and Blackstone, Virginia from 2015 through 2017 to determine germination phenology and viability of two hairy vetch cultivars. Boxes were constructed, 500 seeds were sown in each box at the initiation of the experiments, and germination was tracked for the duration of the experiment. The fall-initiated experiment, started in October 2015, was set up as a 2 by 2 by 2 factorial with factors of cultivar, burial depth, and predation allowed or excluded. Cultivars were ‘Groff’ and ‘Purple Bounty’, a late flowering and early flowering cultivar, respectively. Seeds were either sown on the soil surface to simulate a cover crop being overseeded or buried to 2.54 cm, a drill depth. Lids were constructed and attached to half of the boxes to prevent vertebrate predation. The spring-initiated experiment, started in May 2016, was a 2 by 2 factorial with factors of cultivar and predation allowed and excluded. All seeds were sown on the soil surface to simulate seed shattering into no-till production. Both experiments were designed as a randomized complete block design with 6 replications. In June 2017, seeds were recovered and tested for viability using a tetrazolium assay. A companion experiment was also conducted to determine when these cultivars produce viable seed. In the seed set experiment, hairy vetch strips of ‘Groff’ and ‘Purple Bounty’ were planted in October 2015 and 2016. Pods and seeds were collected in the following spring and summer from both cultivars, starting when pods were beginning to fill and ending when seeds began shattering. A germination assay was performed to determine when seeds were first becoming viable. Data were subjected to ANOVA and effects were considered significant when p < 0.05, followed by a means comparison using Fisher’s Protected LSD (p < 0.05) in JMP Pro 12.

Groff had greater germination than Purple Bounty, 93% and 62%, respectively, in the initial cover crop growing period (October 2015 to May 2016) for fall-initiated experiment. In both experiments, cultivars had less than 2% of seed germinate after the initial planting period and less than 1% of seed recovered that was still viable at the end of the experiments. Both cultivars produced seed starting in late May, but most seed were not viable until mid-June. These results indicate that seed dormancy is not the primary cause of weediness in these two cultivars as nearly all germination occurred during the cover crop growing season, when germination is desirable. Also, if termination occurs before mid-June in Virginia, it is unlikely that viable seed will be added to the seedbank. Future research is needed to investigate alternative sources of weediness, specifically seedbank additions and timing of those additions from incomplete termination. 

Stakeholders from across the northern and central Great Plains of the US have identified kochia (Kochia scoparia) as one of the most problematic and economically damaging summer annual weeds. This tumbleweed is currently a threat to sustainable crop production due to a near lack of effective herbicide options, especially in sugar beet-based crop rotations. Widespread resistance to many different herbicides (including glyphosate, PS II inhibitors, ALS inhibitors, and dicamba) has increased the need and the desire for IWM-based solutions for managing this troublesome weed. For this multi-year (2017-2020) research conducted in Huntley, MT; Laramie/Lingle, WY; and Scottsbluff, NE; we propose: 1) quantifying temperature and moisture germination requirements of kochia populations collected from a north-south transect from Montana to Nebraska, and 2) using that information to evaluate the effectiveness of three ecologically-based, IWM strategies, including stale seedbed, cover crops, and diversified crop rotations. We will combine field-validated emergence data, hydrothermal time modeling, and climate data to evaluate non-herbicidal weed control strategies (stale seedbed, cover crops, and diversified crop rotations) that have a high likelihood of reducing kochia seed bank and exposure of this species to herbicide treatments, thereby reducing selection for herbicide resistance evolution across the three-state region. Implementation and adoption of these ecologically-based IWM strategies will reduce potential environmental impacts associated with increased herbicide use, apart from mitigating herbicide resistance. Results from this project (2019-2020) will be disseminated across geographic boundaries.

Black medic infestations are a problem for Florida strawberry production.  Dense populations hinder harvest and compete with the crop.  Control can be achieved with clopyralid but current practices provide suppression only.  Coordinating spray timings with black medic emergence is crucial to gain control.  The empirical approach to modelling black medic emergence has failed to be reliable over years so a reductionist approach is being developed to account for the three components that lead to field emergence: dormancy, germination, and pre-emergence growth.  The objective of this study was to develop the germination component and understand how black medic germination and emergence is influenced by temperature, osmotic potential, and burial depth.  A series of studies were initiated using Petri dishes in growth chambers where germination of black medic seed was monitored over time.  These experiments included the influence on germination of cooler temperatures (5, 10, 15, 20, and 25 C), warmer temperatures (20, 25, 30, 35, and 40 C), fluctuating temperatures (25/25, 35/35, 35/25, 35/20, 25/15, and 25/10 C), and osmotic potentials (0, -0.25, -0.5, and -1 MPa).  For emergence, the effect of burial was examined at various depths (0, 1, 2, 3, 4, 6, 8, and 10 cm) while grown in a greenhouse.  Black medic germination ranged from 5 to 35 C and reached an optimum germination between 10 and 20 C.  Cooler temperatures (5 and 10 C) delayed but did not limit germination.  Temperatures above 25 C reduced germination.  Germination was not effected by fluctuating temperatures compared to constant temperatures.  There was a reduction in germination from 43% to 14% when the osmotic potential was reduced from 0 to -0.25 MPa.  Black medic did not tolerate burial and only germinated on the surface.  The Florida population of black medic seed was able to germinate across a wide range of temperatures, germination was delayed by cooler temperatures (≤10 C), reduced by warmer temperatures (>25 C), and did not tolerate changes in osmotic potential or burial.  Results establish the necessary information for the germination component for field emergence.  Further study is necessary to account for the seed softening and pre-emergence growth components for black medic field emergence in Florida strawberry fields.

Vinasse is a by-product of bio-ethanol production that has been reported with potential herbicidal activity. Laboratory experiments were conducted to characterize vinasse effects on seed germination and seedling growth. Seed viability was predominantly reduced by vinasse after the imbibition phase. Susceptibility to vinasse was species dependent. For most species germinating seeds in vinasse solutions decreased germination and increased the number of non-germinated non-viable seeds. However, several species also exhibited reduced germination when they were imbibed in vinasse solutions and germinated in water. All evaluated species reduced radicle growth as vinasse concentration increased. The herbicidal activity was not due to osmotic effects, and it was likely present in the organic liquid phase of the vinasse solution. Palmer amaranth, sicklepod and southern crabgrass increased the proportion of dormant seed >2-fold when they were imbibed or imbibed and germinated in vinasse solutions. The results suggest that vinasse can be used in integrated weed management programs to increase seedling mortality and seed dormancy.

The Biological and Economic Analysis Division (BEAD) is responsible for providing use and usage information as inputs to risk assessments, preparing benefit assessments, and helping with risk benefit discussions by describing potential impacts from proposed mitigations.  This presentation will describe BEAD’s role within OPP, explain the purpose and methods of conducting a benefit assessment, and examples where members of the WSSA could provide valuable information for risk and benefit discussions. 

US EPA assesses the benefits of a pesticide as part of the registration and reevaluation of the pesticide.  These assessments rely on information and data from private and public sources to describe why, where, when and how pesticides are used.  The type of information needed to complete a benefits assessment and the sources of this information is described, as well as the ways in which pesticide industry experts and other stakeholders can participate in the process and provide crucial information and data to inform the process.

Most of the other presentations in this symposium deal with the process by which EPA registers and insures the safety of pesticides.  However, this presentation is about a discussion, hopefully with audience participation, about how EPA and WSSA can more effectively communicate with the end users of registered pesticides.  What information needs to be communicated?  Pesticide users need to understand how to safely and effectively use these materials.  How does EPA communicate that now?  The primary tool EPA has is language on the pesticide label.  But, most of us realize that labels can be confusing, long, and generally ignored.  As one industry person said, labels are the most expensive and least read literature in the world.  How can we change that?  Some aspects of labels are standardized but many are left up to the registrant.  Should there be more standardization?  The other means that EPA uses to communicate are notices through the Federal Register and on the EPA website.  These are important to explain the rationale behind registration decisions and actions but how visible and effective are they?  Users might be more willing, or at least understanding, to follow label guidelines if they understood why they are there.  How can EPA do better on this and how can WSSA help?  Some options and ideas will be presented for consideration.

Herbicides mixtures have commonly been used by applicators to broaden the spectrum of weed species controlled in a single application, improve the consistency of weed control, or as a critical component of Best Management Practices to mitigate herbicide-resistant weeds.  One of the greatest challenges in applying herbicide mixtures, especially foliar-active herbicides, is the potential for an interaction between the individual herbicides, which may result in a plant response that is different from expected.  The three types of plant responses from an herbicide mixture includes antagonism, additive, and synergism in which herbicide efficacy is less, equal, or greater, respectively, than predicted by a reference model from the herbicides applied individually.  Recent concerns in herbicide labeling and environmental stewardship have projected a greater emphasis on understanding the potential effect of herbicide mixtures on off-target plant species, especially when the plant response for the mixture is greater than expected. 

Analysis of the published literature reveals that herbicide synergism is the least common type of interaction, occurring in less than 20% of the interactions.  However, the literature may be biased by the most common research objectives outlined by the scientists conducting the research.  In most instances, the goal of the research was to document any potential unexpected change in herbicide efficacy conducted in field or greenhouse experiments to provide a practical result that could be used for developing weed management recommendations.  Thus, the herbicide interactions typically utilized the high end of the use range for the herbicide dose where near complete weed control was achieved with at least one of the herbicides and antagonism may have been the only possible biological response that could be observed for the combination. 

Indeed, herbicide synergy could have a positive impact on weed management.  However, the consistency with which a synergistic plant response is expressed is highly dependent on the specific plant species and environmental conditions.  Thus, the best approach to study synergistic herbicide combinations is to implement a full dose response of the herbicides under controlled conditions so that an increase in herbicide activity may be observed.  For instance, the interaction of two herbicides was classified as synergistic and antagonistic, depending on the target plants being grown in the greenhouse or outdoors.  Furthermore, the resulting herbicide interaction will frequently deviate at the lower end of the herbicide dose range from the plant response near full labeled rates.  Given these requirements and variances, conducting research on herbicide synergism may not provide a sound foundation for extension weed scientists to build management recommendations to growers. 

An academic investigation of herbicides interactions should require a full dose range of the herbicides since the response can vary with dose.  However, the statistical methods used to analyze specific points along the dose range are regarded as inferior to statistical methods that characterize the entire dose/response range for an herbicide mixture.  The lower end of the dose range (e.g. 1/500X) may have greater implications for predicting the impact of the herbicide interaction on off-target plants; whereas the higher end of the dose range (e.g. 1X) has been the most valuable to develop practical weed management recommendations.  Thus far, the literature has focused on the practical implications of herbicide interactions with dramatically less information on herbicide interactions at the lower end of the use range to inform individuals assessing environmental impact.

Herbicide tank-mixtures and premixtures have been important part of Midwest weed management. Midwest farmers use mixtures to increase the spectrum of weeds controlled, improve control of specific weeds, control herbicide-resistant weeds, and delay the development of herbicide-resistant weed populations. Herbicide mixtures have also been used to ‘safen’ crops from herbicide injury. For example, bentazon at rates as low as 0.28 kg ha^-1 is always added to imazamox in dry edible beans to reduce the crop injury that would be observed if imazamox was applied alone. In corn, mixtures of atrazine and one of the Group 15 herbicides (i.e., acetochlor, s-metolachlor, pyroxasulfone, etc.) improve the spectrum of weeds controlled by controlling both broadleaf and grass weed species. Additional herbicides with different sites of action (i.e., mesotrione a Group 27 herbicide) are often added to these mixtures to further broaden the weed control spectrum and to control weeds that have developed resistance to atrazine. The addition of atrazine to the Group 27 herbicides, mesotrione, topramezone, and tembotrione, has been shown to improve postemergence control of several weeds including atrazine-resistant Palmer amaranth. In soybean, farmers who were just using glyphosate for weed management are no longer able to accomplish this, since several species are now resistant to glyphosate and several other herbicide sites of action groups. Additional herbicide mixtures with glyphosate are commonly used to manage these glyphosate- and multiple-resistant weed species. For example, it is not uncommon for a farmer with multiple-resistant horseweed (glyphosate- and ALS-resistant) to tank-mix one or two different herbicides with glyphosate that have postemergence activity on horseweed and an addition to one or two soil-applied residual herbicides in the burndown weed control program to effectively control this species. Glyphosate is still included in these mixtures to help control other susceptible species in these field for which the tank-mix partners cannot control. Mixtures are also important to provide additional residual control in postemergence herbicide applications for weeds that continue to emerge throughout the season. For example, s-metolachlor and acetochlor are often added to an effective postemergence herbicide application to provide extended control of waterhemp and Palmer amaranth. Without the use of mixtures, effective weed control would not be possible without increased monetary and environmental costs, and the increased development of herbicide-resistant weeds.

Orchard and vineyard cropping systems in the Western US encompass a wide range of crops and environmental conditions and growers and land managers face several weed management challenges.  Like other crops, weed are managed in orchards to reduce the direct competition for resources; however, weed control also reduces frost risk, to minimizes physical disruption of harvest or other horticultural operations, and impacts management of other insect, pathogen, or vertebrate pests.  Weed control in these systems is an important management concern and can exceed $200 per acre per year including PRE and POST herbicides, mowing, and tillage operations.  Although glyphosate remains a cornerstone herbicide in most tree and vine crops, increasing problems with glyphosate-resistant weeds has led to a shift towards more complex programs that include both PRE and POST herbicides.  Adoption of PRE herbicide tank mix programs applied during the winter season has greatly improved control of winter-germinating weeds such as ryegrass, hairy fleabane, horseweed, and other glyphosate-resistant or –tolerant species.  However, very long growing seasons, intense irrigation, and warm soil conditions can lead to unsatisfactory control of summer-germinating weeds even with very good winter herbicide programs.  This challenge has been exacerbated in some areas by evolution of glyphosate-resistant species such as junglerice and other summer grasses.  To address these newer issues, recent efforts have focused on sequential programs that include PRE herbicides applied during the traditional winter timing plus a second PRE application of a dinitroanaline herbicide (usually pendimethalin or oryzalin) in the late winter.  The value of tree and vine crops in the western US varies greatly as do environmental and regulatory challenges that affect both chemical and physical weed control.  Tank mixes and sequential herbicide approaches represent key opportunities for growers and crop advisors to improve season long weed control efficacy, reduce reliance on glyphosate and other POST herbicides, and reduce the labor and machine costs of weed control in this crop sector.

Herbicide resistance is not new but the impact from resistant weeds has risen dramatically in recent years.  In Georgia, input costs managing glyphosate-resistant Palmer amaranth (Amaranthus palmeri S. Wats) exceeded $1 billion in the decade following discovery. Weed management programs are now more diverse, more complex, and more costly but are more effective. One critical component of the more effective weed management program has been grower adoption of herbicide tank mixtures including at least two active ingredients effective on Palmer amaranth.  In Georgia and North Carolina, cotton weed management programs often include four or more herbicide applications during the season with each application containing herbicide tank mixtures. 

In one experiment across two Georgia locations, the number of Palmer amaranth emerging through residual herbicides activated by rainfall demonstrated the value of tank mixtures. The number of plants per acre emerged by 3 wk after applying fomesafen alone at 0.25 lb ai/A, diuron alone at 0.5 lb ai/A, or acetochlor alone at 1.13 lb ai/A was of 39650, 26840, and 55510, respectively. Tank mixtures of these same herbicides at lower rates were much more effective. The number of plants per acre, at the same evaluation, with fomesafen at 0.19 lb plus diuron at 0.38 lb, fomesafen at 0.19 lb plus acetochlor at 0.75 lb, or diuron at 0.38 lb plus acetochlor at 0.75 lb was 2440 to 4575; in addition to reducing selection pressure on any one mechanism of action, this represented a reduction of at least 83% in Palmer amaranth emergence with tank mixes. 

Benefits for tank mixtures with postemergence applications may be even more critical. Although potential beneficial herbicide tank mixtures reach well into the hundreds, an example of two auxin herbicides and glufosinate will be shared.  A field study was conducted across six environments in Georgia, North Carolina, and Tennessee to determine the response of 13- to 20-cm weeds to 2,4-D amine or 2,4-DB applied alone or in mixture with glufosinate.  Fifteen- to 20-cm Palmer amaranth was controlled 59 to 78% and 68 to 80% by 2,4-DB (0.5, 0.75, 1.0 lb ai/A) and 2,4-D (0.5, 0.75, 1.0 lb ai/A), respectively, while glufosinate (0.43 lb ai/A) controlled Palmer amaranth 74%.  Control was improved (89 to 97%) with all auxin/glufosinate mixtures when compared to respective herbicides applied alone.  Glufosinate controlled tropical spiderwort (Commelina benghalensis L.) only 68%; 2,4-DB and 2,4-D controlled the weed 60 to 72% and 90 to 99%, respectively.  Combinations of glufosinate plus 2,4-DB were more effective than either herbicide applied alone (78 to 83%). 

Many tank mixtures can be used to improve weed control, lessen crop injury (most often soil-applied residual herbicides), and reduce herbicide selection pressure often without much additional cost. Of course, it is also critical to note a tremendous amount of research has shown tank mixtures can be antagonistic, can increase crop injury, and may not reduce selection pressure to resistance if the weed in question is resistant to one of the active ingredients in the mixture.  Thus, it is essential that academics, industry, and regulators cooperate in not only identifying sound effective tank mixtures but also cooperate in making these mixtures available for growers thereby improving farm sustainability. 

Agricultural aircraft have a number of technologies and techniques for making on-target applications and mitigating drift. The droplet size from agricultural aircraft can be controlled through nozzle type selection, orifice size selection, operating pressure, and deflection angle. Increasing the operating pressure for agricultural aircraft decreases the formation of fine spray droplets. Reducing the boom length relative to the wingspan for fixed-wing aircraft or rotor diameter for helicopters reduces the portion of spray trapped in the outer portion of the vortices, which reduces the risk of drift. A right-hand boom shut-off is used to create a fixed line from which no spray will pass and can be used to spray adjacent to sensitive areas. Agricultural aircraft have the ability to monitor weather conditions on site throughout the application. One method involves the use of an onboard smoker, which releases smoke that can be used to monitor wind direction and speed, as well as check for inversions. The other option is an onboard weather measurement system such as the Aircraft-Integrated Meteorological Measurement System (AIMMS), which measures wind speed and direction, temperature, and humidity. The AIMMS system is functional with GPS systems and provides both real time meteorological measurements and a record of the data. Aerial applicators commonly use temporary buffer zones, not required by the labels, to treat fields in multiple applications. This allows them to make applications of herbicides near sensitive crops by ensuring that the sensitive crop is always upwind from the aircraft while it is making the application. Once a section of the field is complete for a given wind direction, GPS systems and the speed of an aircraft allow it to return to a field to complete an application when the wind direction required for applying to the remainder of the field exists.

The United States Department of Agriculture (USDA) has multiple roles that can help growers manage herbicide resistance.  Scientists in the Agricultural Research Service (ARS) and Economic Research Service (ERS) conduct and publish research on basic biology, integrated management, and economics of herbicide resistance.  The National Institute for Food and Agriculture (NIFA) funds research and extension programs at Land Grant Universities and other institutions some of which relates to crop protection and pest management including herbicide resistance.  The results of this work informs the science community and provides information valuable to educational outreach efforts. The Animal and Plant Health Inspection Service (APHIS) Federal noxious weed program is designed to prevent the introduction into the United States of nonindigenous invasive plants. APHIS noxious weed activities include exclusion and permitting and (in cooperation with other agencies and state agencies) integrated management of introduced weeds, including biological control. APHIS Biotechnology Regulatory Services (APHIS BRS) implements separate regulations for certain genetically engineered organisms that may pose a risk to plant health, including many crops that have been engineered for herbicide resistance which may have sexually compatible weedy relatives.  Decisions by APHIS BRS to list such plants as Federal noxious weeds currently requires action under APHIS noxious weed regulations. APHIS coordinates biotechnology regulation efforts with other designated federal agencies and has provided guidance to developers under regulatory oversight to address proliferation of herbicide resistant weeds. APHIS has supported activities to identify and address the issue of herbicide resistance and support integrated weed management practices.  APHIS also supports cooperative agreements under Farm Bill Section 10007, including some related to weeds that meet one or more goals identified in the Farm Bill Implementation Plan for plant pest and disease management and disaster prevention.  The Natural Resources Conservation Services (NRCS) has programs that serve as incentives to help farmers initiate herbicide resistance management programs that also address soil and water conservation. The Risk Management Agency (RMA) assists growers with managing the risks they face in agricultural production, including the crop insurance program. The USDA’s multiple missions must include effective communication and coordination of programs among these units, with regional and state agencies and universities, and between the USDA and the farming community to maximize the potential for improving management of herbicide resistant weeds.

Pesticide product labels provide critical information about how to safely and legally handle and use pesticide products. Efforts to mitigate pesticide drift and pest resistance, protect pollinators, and improve water quality have led to increasingly complex pesticide labels.  Extension weed scientists and entomologists have been hearing a growing number of concerns from farmers and pesticide applicators about how difficult it is to gather and understand application directions from pesticide labels and then execute those directions.

University of Tennessee Extension surveyed farmers who personally apply pesticides on their farms to capture concerns regarding the complexity of pesticide labels. Over 80 individuals from 24 counties participated.  The applicators chosen represented a good subsection of farms from large (>4,000 ha) to smaller (<2,500) operations.  Soybean, cotton and corn were grown by most survey participates with a few vegetable and organic producers participating as well.

The questions asked:

1. Please rate your overall satisfaction level with pesticide labeling based on: a) ease of finding directions/information; b) ease of understanding directions; and c) uniformity among different labels.

2. Considering pesticide labels in general, how consistently are you able to comply with label directions with regards to: a) keeping the boom at recommended height; b) only making applications during recommended wind speed; c) using approved nozzles; d) “starting clean”; e) making timely applications; and f) establishing buffer zones?

3. How likely are you to: a) recognize a temperature inversion; or b) avoid spraying into an inversion?

4. How often do you use tank mixtures or pre-mixtures of different herbicides?

5. How important are tank mixtures or pre-mixtures for: a) resistant weed management; b) minimizing expenses; and c) improving control?

6. How important are the following products to managing herbicide resistance? a) new dicamba formulations; b) new 2,4-D formulations; c) Liberty; and d) Roundup.

7. How often do you consider bees and pollinators when making an insecticide application?

8. Do you agree or disagree with the following statement?: “It’s difficult to manage pests on my farm while also following recommendations and label restrictions to protect pollinators.”

9. On your farm, how often is it practical to spray insecticides at night to protect pollinators? a) evenings (5-9 PM); or b) night (after 9 PM)

10.  How effective are pesticide labels in curbing pest resistance? a) herbicides; b) insecticides; and c) fungicides

Survey results showed that 72% of Tennessee applicators were not satisfied with the uniformity across labels. Comments on how time consuming it was to find information across pesticide labels was a real disincentive to thoroughly read them. A standardized label format could greatly increase the reading and comprehension of pesticide labels.  For example, “section X” of every pesticide label contains information devoted to surfactant use.

Only 67% of applicators were “frequently” or “always” able to comply with label directions regarding boom height, which is likely due to the sharply rolling topography of the state. Applying pesticides at labeled wind speeds occurred at 78% of the time. Applicators were able to use labeled nozzles 98% of the time. Planting with no weeds emerged was achieved 93% of the time. Making timely applications to Palmer amaranth less than 10 cm tall occurred frequently or always 68% of the time. That 30% of the time weather or label parameters did not allow for timely applications is a real issue for weed management. These data would suggest that areas of a label like using correct nozzles and spraying in the wind speed window of the label were achievable by most applicators. However, not being able to comply with boom height requirements a third of the time has real ramifications on drift mitigation. Moreover, not being able to apply herbicides before weeds become to large greatly compromises weed management.  

Survey results showed that 77% of applicators felt they could recognize a temperature inversion.  This is very positive news and suggests that recent educational efforts by Extension and Industry personnel on how to detect and avoid spraying into temperature inversion conditions have been adequate.  It could also be at least partially due to recent first-hand experiences with temperature inversions.

Eighty-two percent of the survey respondents indicated they treated their fields with tank mixtures. Moreover they stated that tank mixtures or pre-mixtures of herbicides are very important to them for mitigating pesticide resistance, which suggests that most applicators in Tennessee are well educated in this area of weed resistance management.  

Fully 55% of Tennessee applicators consider pollinators when making an insecticide application. Though there is room for improvement, these results do show that a majority of applicators spray fields with a mindset of mitigating effects on pollinators.  Moreover, they put into action pollinator protection with 54% of those surveyed indicating they “occasionally, frequently or always” apply insecticides in the evening when bees are less active.  This is positive news that so many Tennessee producers have pollinators in mind when applying insecticides.  However 85% of the applicators indicated that tank-mixtures, and these may include insecticides, were very important for minimizing expenses. Label directions for new dicamba formulations dictate that they cannot be applied into a temperature inversion which frequently occurs in the evenings. Therefore from a practical standpoint to an applicator, the label directions to mitigate pollinator and drift issues clash. 

Finally 45% of applicators in this survey did not feel label directions could be overly effective in curbing the development of herbicide resistance.  The fact that most labels only address chemical control options which have not proven to be sustainable would be good data to support this thought.  However, adding in cultural weed management practices to a label which are often locally specific to be effective would be too cumbersome.  More local education is likely the most effective route to relay integrating cultural practices with pesticides for effective pest control.

In conclusion, these applicators indicate that labels across manufacturers are not uniform and therefore make it a struggle to quickly find needed information.  Uniform label formats for all pesticides could greatly help this issue and increase use and understanding of pesticide labels.  These data also suggest that registrants, when they draft labels, can essentially force non-compliance due to lack of practicality of regulations.  How does a grower who needs to tank mix to be efficient AND protect pollinators AND spray small weeds timely for good control AND stay on label? One suggestion is that registrants and EPA might consult with individuals applying pesticides to gain a more practical perspective as they construct labels. Finally, this survey would suggest that Tennessee applicators are striving to follow label directions to improve drift mitigation, improve pollinator health, and to achieve good weed control.  

New uses of dicamba on dicamba-tolerant soybean and cotton were registered in November/December of 2016. The 2017 growing season resulted in an unacceptably high number of cases of crop damage associated with off-target movement of dicamba. As the 2017 growing season progressed, reported incidents rapidly increased in frequency and geographic distribution across southern states and northern Missouri, eventually spreading into the Midwest, upper-Midwest and Dakotas. The EPA, in cooperation with university weed scientists and co-regulators in affected states, worked to develop reasonable changes to how these new dicamba products are used in 2018.

No new herbicide mechanisms of action (MoA) have been registered in the past 30 years or so.  New MoA’s are needed to help manage the widespread problem of weed resistance to existing chemistries. While business decisions drive the development of new herbicides, regulatory measures may be used to incentivize new MoA development. This presentation will describe current incentive programs at EPA’s Office of Pesticide Programs, some of the existing challenges, and invite the audience to suggest ways to incentivize the development of new herbicide MoA’s.

This presentation will provide an overview of how pesticide regulatory activities are coordinated between the federal and state governments.  EPA’s Office of Pesticide Programs has responsibilities under the Federal Insecticide, Fungicide, and Rodenticide Act to make decisions regarding the registration of pesticide products.  States have primacy for enforcement of pesticide labels.  As new dicamba labels are introduced for the 2018 use season, collaboration across all sectors of agriculture will be critical to monitoring the success of the revised labels and will help to inform whether the “over the top” use of dicamba can continue beyond the 2018 growing season.

Cogongrass (Imperata cylindrica) is a diploid C4 grass that is noxious weed in over 70 countries where it threatens global biodiversity and sustainable agriculture.  Recent genetic analyses identified four distinct non-hybridizing clonal lineages of cogongrass in the USA.  In Florida and other southeastern states, this invasive grass infests cattle pastures, pine plantations, and thrives in poor soil conditions such as ditch banks, roadside and railroad rights-of-way as well as reclaimed phosphate-mining areas.  Control of cogongrass relies primarily on mowing and the application of chemical herbicides.  For example, in 2009 the state of Alabama dedicated $6.3 million of federal stimulus funds exclusively for chemical control of this invasive weed. Biological control using natural enemies from the native range of cogongrass has received little attention and no biological control agents have been introduced anywhere in the world. The Indonesian gall midge Orseolia javanica Kieffer and van Leeuwen-Reijinvaan is a potential biological control agent of cogongrass.  Larval feeding induces the formation of hollow, sterile shoot gall in which one larva develops. These galls serve as nutrient “sinks” that divert rhizome resources away from normal shoot production.  According to literature, the only reported host plant for O. javanica is cogongrass. However, it is not known how  O. javanica will develop and reproduce on the Florida peninsula or Gulf Coast (Florida Panhandle) clones of cogongrass. We collected/propagated cogongrass from two different geographic locations in Florida and shipped healthy rhizomes under permit to Bogor Agricultural University, West Java, Indonesia, for clonal testing.  In Indonesia, rhizomes were propagated in black plastic polybags (25 cm diam x 14 cm ht) with eight drain holes per bag located in a secure greenhouse and allowed to sprout until leaves were ~ 14 cm in height.  In total, five plants from each source population (Cianjur, Indonesia [control], Avon Park, and Crestview, Florida) were placed in three separate rows (15 plants /cage).   The experimental design was randomized complete block with three replications; the positions of the three rows of five plants from each source population were changed in each replicate cage.  Mated female midges (10-20) were released inside each cage and allowed to oviposit.  Performance of O. javanica on each cogongrass clone (no. of galls and adults produced, development time to adult stage) was compared. Preliminary results indicate that gall production and development of O. javanica on the two Florida clones of cogongrass were comparable to Indonesian cogongrass.  These findings will validate/justify our request for importation permits to establish a colony of O. javanica in a Florida quarantine laboratory for detailed biological studies and extensive host range testing.

Cheatgrass (Bromus tectorum) and other invasive winter annual grasses are one of the greatest threats to rangeland ecosystem in the Western United States due to their ability to, for example, reduce biodiversity, decrease forage quality, reduce fire cycle intervals, and modify ecosystem function.  Unfortunately, there is a very limited suite of tools that may offer long-term control of cheatgrass.  Therefore, there has been significant interest in the recent production of bioherbicides labeled for annual grass control. Unfortunately, there are few field trials and none particularly in Wyoming’s many unique abiotic conditions that show the efficacy of these products.  This efficacy information is critically important particularly for a biological product that may be effected by abiotic conditions far more greatly than more common synthetics herbicides. The objective of this study is to determine if various bioherbicides alone or in combination with synthetic herbicides are a viable tool for cheatgrass management in Wyoming’s environmental conditions.

The study was established in the fall of 2016 as cheatgrass was germinating. Various rates and combinations of bioherbicides and common synthetic herbicides were utilized for cheatgrass control.  Ideally these applications would have occurred prior to germination, but due to the application requirements for bioherbicides of cool temperatures and imminent precipitation, there was no ideal application time prior to germination.  Cheatgrass cover was visually estimated in the summer of 2017 to determine herbicide efficacy.  In this first year, bioherbicides, regardless of their carrier, did not reduce cheatgrass cover.  Only synthetic herbicides were able to significantly reduce cheatgrass cover.  Specifically, imazapic (Plateau) was most effective at control and reduced cheatgrass cover by over 50%.  Additionally, combinations of synthetic and biological herbicides did not show any synergistic control and synthetic herbicides appear to drive the cheatgrass cover response. Because minor germination occurred at some sites prior to germination, better control may be observed in 2018.  Nonetheless, bioherbicides were not effective at first year control.

Air potato plant (Dioscorea bulbifera L.) is a perennial weedy vine of Afro-Asian origin, introduced into Florida, USA during 1905. It has spread and established throughout Florida, and parts of southern Georgia, Alabama, Louisiana and Texas. Air potato vine infestations have not only smothered and killed native vegetation but also have damaged physical structures in public and private properties. The fast growing attribute of this weed results in highly branched, long vines and causes various deleterious effects to the supporting structures underneath while producing a large quantity of highly potent vegetative propagules (bulbils) on the vines. Patches of air potato vines senescence during winter and drop bulbils. These new bulbils and perennial underground tubers sprout in spring, repeat annual cycles and keep spreading into the new areas. We hypothesized that any effective control method to be deployed to manage this weed ought to be able to suppress its invasive attributes (rapid growth and bulbil production) and eliminate smothering effects on native vegetation. In long-run, traditionally used chemical, mechanical and cultural methods were not deemed cost effective and environment-friendly due to their nonselective mode of action, and need for repeated site visits to administer applications. Hence, the biological control method was suggested to be more environment-compatible, self-sustaining and economically viable option, capable of suppressing existing infestations while reducing propagule production and further spread into the new areas. Invasive Plant Research Laboratory (IPRL) of the United States Department of Agriculture (USDA) /Agriculture Research Service (ARS), Fort Lauderdale, Florida discovered and imported two biotypes of an air potato foliage feeding beetle (Lilioceris cheni Gressit and Kimoto) from Nepal (in 2002) and China (in 2010), conducted thorough host-specificity tests, determined both biotypes to be highly specific to air potato vine growing in Florida, and initiated a field-release program during late 2011. During 2012, we established five field-research sites in four counties of Florida. Each research site contained three (5.0 m x 3.0 m) beetle restricted (=control, via quarterly application of insecticide Aloft ai Clothianidin 0.250% and Bifenthrin 0.125% by weight at the rate of 3.5 gm/m²) and three beetle unrestricted (beetle inoculated) treatment plots. In these plots, we documented beetle feeding-damage and its impact on the performance (vine damage and subsequent impact bulbil production) of air potato plants and the recovery of native vegetation during 2012–2016. The outcome of this 5-year study revealed: 1) a significant reduction (73%) in vegetation smothering effect of air potato vines and two-fold increase in plant species richness; 2) an increase or decrease of vine damage (% of total vine cover) with the progression of the growing seasons and availability of vane patches, and senescing of severely damaged vines prior to or shortly after the initiation of bulbils on leaf axils; and 3) a significant reduction in the bulbil biomass/size (about 85%) and bulbil densities (about 98%). We could not completely avoid some feeding damage in control plots due mainly to the mass migration of spilled over beetles from exploding population in the site, and this caused some vine damage in control, especially during the second-half of the growing season. These leaf feeding beetles are now established in almost all (65 out of 67) counties of Florida and more recently, they have also been introduced and used by neighboring states to manage their air potato infestations. So far, the leaf feeding beetles have proven to be efficacious and environment friendly alternate option for air potato management program in Florida and is expected to have comparable results in other states.  

Dicamba tolerant soybeans and cotton with specially designed herbicides and related technologies were introduced in 2017.  The tolerance of soybeans and cotton with the dicamba tolerance trait was remarkable.  New dicamba formulations were developed to provide excellent broadleaf weed control and minimize off target effects.  New nozzles, new adjuvant systems and new label methods were put into practice.  Buffer zones were a requirement for this technology. A large amount of training was conducted by manufacturers, distributors and other application experts. Volatility testing was done by manufacturers and others.  It was determined that Ammonium Sulfate (AMS), commonly used to improve glyphosate performance added to the risk of dicamba volatility and could not be used.  This caused many adjuvant suppliers to develop non-AMS adjuvants to improve herbicide performance but not increase potential dicamba volatility.

Much effort was made to minimize particle spray drift by greatly reducing driftable fines. Anything added to the spray mixture needed to be tested to determine its influence on the spray quality. Nozzles were also evaluated for their spray quality with the various herbicide mixtures.  Many nozzles were not allowed to be used to make the dicamba applications.

Many millions of acres of dicamba tolerant soybeans and cotton were sown and treated in 2017 with the new dicamba herbicide formulations.

The speakers at this symposium will discuss what happened prior to 2017, in 2017 and what needs to happen in 2018 to be successful.

New uses of dicamba on dicamba-tolerant soybean and cotton were registered in November/December of 2016. The 2017 growing season resulted in an unacceptably high number of cases of crop damage associated with off-target movement of dicamba. As the 2017 growing season progressed, reported incidents rapidly increased in frequency and geographic distribution across southern states and northern Missouri, eventually spreading into the Midwest, upper-Midwest and Dakotas. The EPA, in cooperation with university weed scientists and co-regulators in the affected states, worked to develop reasonable changes to how these new dicamba products are used in 2018. 

Drift and Getting Droplet Size Right

Greg R. Kruger

Abstract 

Pesticide drift, particularly with dicamba, has been a topic of major focus the last couple of years. Dicamba is more active than many of the other agricultural compounds that we use for weed control in row crop agriculture, especially on soybean, which makes it much more noticeable when off-target movement occurs. Off-target movement of dicamba can occur for a number of different reasons showing exactly how difficult applying this product can be. For example, in 2017, off-target movement of dicamba reports came from sprayer contamination, volatility, inversions, excessive boom heights, too windy of conditions, wrong tank-mixture partners, wrong nozzles, wrong pressure, large rainfall events following application (i.e. run-off), use of older formulations, inadequate buffers, and likely a myriad of other things. While sprayer contamination and volatility were without a doubt part of the equation, the focus of this presentation is making sure that the droplet size is right for the application. Droplet size is one of the major factors of off-target movement for any pesticide and is one of the most controllable aspects by any given applicator. Research going back for decades has shown that using coarser droplets will decrease the risk of off-target movement of pesticides. This general principle is true for dicamba as well. However, with every application made, there is a portion of that spray that is in fines that have a high propensity of move off target. Pesticide labels for the newly registered dicamba products over the top of soybean and cotton had specific nozzles and pressures which could be used. This is a direct result of understanding the basic principles of drift as it relates to droplet size. However, even with the strict guidelines on nozzle selection and pressure as well as tank-mixture partners, there was still a considerable number of drift incidences resulting from physical particle drift. That is because roughly 0.75% of the application volume being applied, even when adhering to the strict guidelines is in particles less than 150 microns in size. Given the current technologies, there are things that can be done to minimize the fines, such as using Ultra Coarse sprays, however, these never fully eliminate the small particles and conversely, in many cases, can cause problems with efficacy. In 2017, the use of Ultra Coarse spray nozzles coupled with drift reducing adjuvants (DRAs) in some situations caused reduced efficacy and in some cases even weed control failures. Applicators utilizing both Ultra Coarse spray nozzles and DRAs need to be careful not to let pressures drop below critical thresholds as that can cause pattern collapse and/or lead to inadequate coverage. Applicators should increase carrier volume or use adjuvants approved for application with these products which will lead to greater surface spreading of the droplets to ensure that applications remain as efficacious as possible. As a society we should take caution of not only off-target movement, but also the potential risk of weed species to evolve resistance to dicamba and other herbicides being co-applied with dicamba due to the use of dicamba with such stringent label requirements.

The prevalence of herbicide resistant weeds throughout the US continues to become a bigger management issue for growers each year. Engenia herbicide, the BAPMA salt of dicamba, was introduced for use in dicamba tolerant crops in the 2017 season to help growers manage these tough weed problems. With the launch of this innovative technology, Engenia herbicide was promoted in a two-pass herbicide system that included multiple, effective herbicide sites of action. Engenia herbicide can be applied preplant or preemergence but was most commonly used postemergence in combination with glyphosate following a strong residual preemergence herbicide. Overall, weed control performance of Engenia herbicide was excellent across soybean and cotton growing areas.
Because of the high sensitivity of dicot plants to dicamba, it is critical that product label requirements be followed to mitigate off target movement onto sensitive areas. BASF representatives investigated inquiries regarding potential off target movement of dicamba during the 2017 growing season. Based on information provided during the investigations the most common causes of off target movement were determined to be caused by improper application of Engenia. Attention to label requirements including spray buffer setback, wind speed, temperature inversion, boom height, nozzle selection, and mix system / sprayer hygiene are critical for on target application.
BASF worked with the federal EPA and local state regulatory agencies to help clarify the Engenia herbicide label for 2018. Nozzle and other off target mitigation equipment incentive programs have been expanded. And BASF is continuing applicator training and education efforts to ensure understanding and adherence to dicamba label requirements.

A personal history of dicamba use through the many years including application equipment, formulations, the benefits and the challenges faced is presented. 

The applicator and dealer experiences and challenges with the new dicamba technologies in 2017 and its performance and other results are reviewed.

Application methods and practices that can be changed to improve 2018 results and other improvements benefiting dealers, applicators and growers will be discussed.

Dicamba can be a great tool to control herbicide resistant weeds.  Dicamba must be managed right to get the desired results while avoiding potential problems.

The Good, the Bad, and the Ugly

In trying to manage Palmer amaranth, cotton and soybean growers in Arkansas, the Bootheel of Missouri, Mississippi, and Tennessee embraced the Xtend weed management system.  Roughly 85% of cotton and over 50% of soybean varieties planted were dicamba-tolerant varieties. The weed control, particularly Palmer amaranth, was very good.  Unfortunately, most growers in those states struggled to keep dicamba in the field. 

The Departments of Agriculture in each of these respective states were swamped with nearly 1,500 dicamba drift complaints to investigate. Weed scientists from those states estimated 1.9 million acres of non-Xtend soybeans alone were damaged by off-target dicamba.  This does not count service calls Extension ran on trees, vineyards, truck patches, gardens and homeowner landscaping exhibiting dicamba injury symptoms.

A survey of Tennessee Extension agents concerning the causes of the drift can be categorized into five basic reasons.  In listing from least frequent to most frequent cause of dicamba drift in their investigations, tank contamination was the least found cause followed by use of illegal dicamba formulations < dicamba misapplication < spraying into a temperature inversion < XtendiMax or Engenia volatilization.

Soybeans that were injured by off-target dicamba were at all different growth stages.  The ones that were still in the vegetative growth stages seemed to recover in a few weeks.  Soybean fields that were into flowering stages showed visual symptoms longer. In some cases, less fortunate fields that were drifted on multiple times never did completely recover.

The ramifications of all this off-target dicamba is still being assessed and probably will be on-going for years to come.  Many sensitive soybean fields that were damaged and showed significant visual symptoms recovered by harvest time and farmers reported little or no yield loss. Still other fields, particularly those drifted on multiple times, were reported by growers to have lost 10 to 20% of their expected yield.

Extensive dicamba stewardship training took place in all four states prior to the 2017 growing season.  For example, in Tennessee alone there were 5,523 applicators who took either a 60-minute in-person or a 30-minute dicamba stewardship training online module, there were 16 dicamba classroom training sessions that 2,300 applicators attended, over 20 blog posts on UTCrops.com that were accessed over 25,000 times, and 16 in-season YouTube training videos that were viewed over 13,500 times.  This plus all the education provided by Monsanto and BASF personnel would suggest that increased education alone cannot solve this issue.  

Xtend cotton was also used extensively in Alabama, Georgia, North Carolina, South Carolina and Texas.  Applicators in those states had much fewer issues with dicamba trespassing across the landscape.  A reason given for these fewer problems was the extensive applicator training conducted in Alabama, Georgia and North Carolina.  Perhaps, but applicator training in South Carolina and Texas was similar to what occurred in Tennessee.  Other reasons mentioned are the dramatically fewer soybean acres in most of those states.  Non-Xtend soybean are in harm’s way for drift for 3 months while most of the vegetable crops grown in Georgia have a much shorter growing season and are therefore less exposed temporally to drift. Other opinions such as differences in topography and environment maybe causes.  The bottom line is no one knows for sure.   

The US Environmental Protection Agency imposed new regulations for the use of dicamba in Xtend crops for the 2018 growing season in an effort to mitigate off-target dicamba.  These rules include that applicators must maintain specific records of product use, dicamba products can only be applied at wind speeds less than 10 mph, new tank clean-out procedures are mandated, and now Engenia and XtendiMax are restricted use herbicides.

The new EPA rules are similar to the State of Missouri emergency rules that went into place in early July 2017.  Based upon the fact that about half of the 310 official dicamba drift complaints reported to the Missouri Department of Agriculture came in after the emergency rules were implemented would suggest that off-target dicamba drift issues and complaints during the summer of 2018 will be significant again in many Southern States.

Dicamba drift occurred at different levels across the mid-west. Official dicamba-related injury investigations as reported by state departments of agriculture was 2,708 as of October 15, 2017. Estimates of dicamba-injured soybean acreage in the U.S. as reported by state extension weed specialists was approximately 3.6 million acres. Sources of dicamba drift were at least from particle drift, tank contamination, and dicamba volatility. Contamination of ammonia and/or nitrogen based additives from within the transfer process in dealerships may also increase dicamba volatility. Dicamba drift caused soybean symptoms at several days extending to 3 to 4 weeks after application. Soybean fields received two or more exposures to dicamba in 2017. In general, extension weed specialists recommend prohibiting dicamba application after June 21 or earlier because temperature is lower, soybean is in the vegetative stage, and more rainfall occurs to move free dicamba into the soil. Dicamba absorption in plants is 15 to 85%. Unabsorbed dicamba forms a crystal deposit on plant leaves. Dicamba has a Koc value of 2 and a Kd value of 0.05 to 0.13 which is the lowest of herbicides and which significantly decreases adsorption to soil and organic matter. Dicamba-acid has a high vapor pressure and dicamba-anion has a low volatility potential. Dicamba must dissociate to form dicamba-anion. Salt herbicides dissociate in water regardless of formulation. Subsequent acid association is a function of the pKa of the herbicide and the pH of the spray solution. The pKa of dicamba is 1.87. A proportion of herbicides with low pKa values dissociate and are anionic on plant and soil surfaces. Negative charged dicamba can bind with protons on soil and plant surfaces to form volatile dicamba-acid. Water evaporation in spray droplets increases proton concentration in the liquid deposit which lowers pH. Lower solution pH allows dissociated dicamba in spray droplets to bind with protons resulting in the formation of dicamba-acid. Ammonia significantly increases dicamba volatility by binding with dicamba and through an increased concentration of protons after the ammonia volatilizes. Two significant sources of ammonia are from the mineralization of organic carbon and from evolution of ammonia from all plants. Accumulated ammonia loss from soil is at least as high as 40 lbs/acre and 5.1 µg N m^-3 from plant leaves. Dissociated free dicamba can associate with protons and ammonia on plant leaves and soil resulting in dicamba volatility.

The Missouri Department of Agriculture (MDA) has been at the frontline on investigating complaints of the misuse of Dicamba on GMO crops for the past several years. The presentation will provide perspective from a State Lead Agency (SLA) on the challenges, obstacles and learning curve Missouri had to face including: difficulty of residue analysis, adjusting investigation protocols on short notice, dealing with a massive number of complaints, limited staff to handle workload (field and office), possible adulterated fruits and vegetables, working with other state and federal agencies, and new state legislation. The presentation closes on how MDA has handled this unprecedented event to become a national leader for SLAs on this subject.

The specialty crop industry is at the forefront of potentially huge crop losses with the widespread use of dicamba in the MidWest and MidSouth.  The  experiences of 2016 and 2017 have confirmed the likelihood that the long-known tendencies of dicamba, now proven with even the new formulations, to volatize and move to off-target locations is a clear and present danger to the specialty crop industry.  To further enhance the risks, there are currently no residue tolerances for dicamba on nearly all food crops raising the specter of financial losses far beyond what the commodity crops received in 2017 because any off-target movement would require crop destruction as opposed to yield reductions.  The liability insurance of applicators in few cases would be adequate to compensate a specialty crop producer for their losses, and that is even if liability was confirmed and a payment was forthcoming.  In 2017, many dicamba claims went unpaid as insurance companies took the stance that volatility was not indicative of applicator error and refused coverage.  In addition to the loss in any given year, the specialty crop industry problems are compounded if long term purchasing arrangements are compromised by the inability to fulfill orders or market demand.  For example, organic growers could lose certification that takes three years to achieve and would be without the premiums commanded and needed by organic producers.  Wine producers could lose multiple years of grape production and their specialty products and a processor that is unable to supply a vendor stands to lose that business for not only one year, but multiple years.  While all susceptible crops are at risk, specialty crops have much to be concerned about.

The U. S. cotton (Gossypium hirsutum) and soybean (Glycine max) industries need the auxin mechanism of herbicide action to manage Palmer amaranth (Amaranthus palmeri) populations that are resistant, singly and in all combinations, to acetolactate synthase (ALS) herbicides, glyphosate, and protoporphyrinogen (PPO) herbicides. All elements of U. S. agriculture need to support efforts to minimize off-target movement of the auxin herbicides in order to conserve the efficacy of our scarce remaining post emergence herbicides with broad-spectrum efficacy against broadleaf weeds. Off-target, movement of dicamba, applied in-season in conjunction with the Xtend® dicamba-resistance technology, was clearly an issue in 2016, when its use was all-together illegal, and again in 2017, when it was labelled for use in cotton and soybean. Use of dicamba differed among the three principal cotton-producing regions. In the High Plains of West Texas, dicamba is needed and was widely-used against glyphosate-resistant pigweeds (Amaranthus spp.) in stripper-harvested cotton where pigweeds not only interfere with the crop during the growing season, but also impede harvest, and can radically decrease fiber quality. In the Mid-South, where PPO resistance to Palmer amaranth is impelling use in soybean, dicamba is sorely needed to control multiply-resistant Palmer amaranth. In the Southeast, dicamba-resistant cotton cultivars were planted widely because of their competitive yields; however, dicamba herbicide was not widely used in the Southeast, because PPO herbicides, used pre-emergence in cotton, and post emergence in soybean and peanut (Arachis hypogaea), are still effective there. In addition, there is a long history of careful herbicide use in the Southeast because of the presence of many small (< 15 hectare) fields with high-value crops that are highly sensitive to auxins such as tobacco (Nicotiana tabacum), water melon (Citrullus vulgaris), transplant and staked tomatoes (Lycopersicon esculentum), peaches (Prunus persica), and pecan (Carya illinoensis). Very little dicamba was used in the Far West cotton producing region. The future of dicamba in 2019 may be determined by events on the ground in 2018. The key questions are: will the industry pull together to implement the stewardship that is necessary to apply dicamba properly, and will the formulations perform well enough that dicamba stays where it is applied.                

Questions and Answers

In plasticulture production vegetable crops are typically grown on raised beds covered with plastic mulch.  On the raised bed, broadleaf and grass weeds can only emerge in the transplant holes whereas nutsedge species can puncture the plastic mulch.  Growers frequently rely on bed top applications of preemergence (PRE) herbicides prior to laying the plastic mulch or post-transplant herbicide applications for nutsedge control.  A precision sprayer was developed that enables application of PRE herbicides only to the transplant hole which is the only location broadleaf weeds and grasses can emerge.  Adoption of this technology had no effect on weed control or crop yield.  Herbicide use was reduced by 90-92% in a bell pepper crop when using the precision applicator compared with a broadcast application.  Initial accuracy was low (55-86%) but equipment modifications resulted improvement to near 100% accuracy.  This approach is not effective for nutsedge management which can emerge anywhere on the raised bed.  To address this issue, deep learning convolutional neural networks (DL-CNN) were trained to accurately recognize purple and yellow nutsedge.  Integrating the DL-CNN with smart sprayers should reduce overall herbicide use.  Initial trials based on photos collected from commercial fields found that the technology detected nutsedge with 95-99% accuracy.  Herbicide use reductions were estimated to be 44%.  Precision application of herbicides in vegetable crops has the potential to significantly reduce overall herbicide use with no reduction in weed control. 

Control of yellow nutsedge (Cyperus esculentus) presents one of the major challenges in managing weeds in direct-seeded onions in the Treasure Valley of eastern Idaho and southwestern Idaho. Post emergence application of dimethenamid-p or s-metolachlor is not effective because chloroacetamides to do control emerged weed. We discovered that application of dimethenamid-p through the irrigation drip starting when onions are at the 2 leaf stage provided better yellow nutsedge control compared to post emergence (POST) broadcast applications at the same rate. The objectives of these studies were to evaluate onion response and yellow nutsedge control when dimethenamid-p solution was injected through the irrigation drip compared to standard POST broadcast application at the same rate. The solution containing dimethenamid-p at rates ranging from 368 g ai ha^-1 to 1,100 g ai ha^-1 was metered into the irrigation drip sequentially at weekly intervals or 2 weeks apart for a cumulative total amount of 1,100 g ai ha^-1 season^-1. The herbicide solution injection lasted 8 hours. All treatments were initiated when onions were at the 2-leaf stage and yellow nutsedge ranged from not emerged to 4 leaf stage. Onion injury was <5% and transient. Average yellow nutsedge control at 47 days after the last application (DALA) ranged from 70 to 95% for dimethenamid-p applied through the drip compared to 9% for the standard broadcast treatment. Evaluations at 70 DALA indicated 59 to 86% yellow nutsedge control across drip applied treatments compared to 3% for the standard broadcast treatment. Marketable onion yield was similar across treatments ranging from 86 to 103 T ha^-1 when dimethenamid was applied through the drip compared to 87 T ha^-1 for the standard treatment. Subsequent study in 2017 showed no adverse effects to onion when the herbicide solution was mixed with liquid nitrogen fertilizer. Special local needs labels were granted in Oregon and Idaho in 2016 to apply dimethenamid-p through irrigation drips at ≤ 365 g ai ha^-1 weekly sequential applications or ≥ 365 g ai ha^-1 at 14-day interval for a cumulative total amount of 1,100 g ai ha^-1 season^-1.

Onion weed control experiments have been conducted on organic (muck) and mineral soils for over 30 years.  Weed control is a major expense for onion growers because onion is a poor competitor and requires season-long weed-free conditions for maximum yield.

Onions emerge slowly after planting into cold soils in March and April.  Most onions are planted with a barley nurse crop which is killed in mid to late May with a graminicide.  Preemergence herbicides used on onion may reduce barley germination and stand, so application may be delayed to allow barley to emerge.  Onions are very susceptible to phytotoxicity between emergence (loop stage) and the 2 true leaf stage (LS).  An effective herbicide program must maintain the soil surface weed-free until late May, when additional herbicide treatments may be applied safely.

Pendimethalin is the primary preemergence herbicide for seeded onions.  It can be applied after seeding until emergence, and then after the 2 LS.  Pendimethalin normally is applied with bromoxynil 5-10 days after onions and barley are planted.  The delay allows barley to emerge but early broadleaves, eg. common chickweed(Stellaria media (L.) Cyrillo), ladysthumb (Polygonum persicaria L.), and shepherd’s purse (Capsella bursa-pastoris L. Medic.), also emerge.  Bromoxynil kills most small broadleaf weeds.  If the pendimethalin plus bromoxynil application is delayed until a few onions begin to emerge, it is more effective in achieving maximum broadleaf weed control. Barley and emerged grasses are killed with a graminicide in mid-late May.

Oxyfluorfen is the primary postemergence onion herbicide.  In some states (including Michigan) it may be applied at the onion full 1 LS.  In other states, it may be applied at the 2 or 3 LS and later.  The delay in application from the 1 to 2 LS results in more broadleaf weed germination and makes subsequent weed control more difficult.  Many broadleaf weeds, eg, ladysthumb, hairy nightshade (Solanum sarrachoides Sendtner), redroot pigweed (Amaranthus retroflexus L.), marsh yellowcress (Rorippa islandica (Oeder) Barbs), and common lambsquarters (Chenopodium album L.) may survive the first postemergence herbicide application because they have passed a critical growth stage, and are not controlled by subsequent applications.

Flumioxazin is labeled for use at the onion 3-6 LS, about mid-June to mid-July.  It may be tank mixed only with pendimethalin ACS.  Flumioxazin helps control several serious onion weeds, including ladysthumb, pigweeds (Amaranthus spp.), nightshades (Solanum spp.), spotted spurge (Euphorbia maculata L.), and common lambsquarters.  It has some postemergence activity at the labeled rate of 0.036-0.072 kg/ha, with an annual maximum of 0.108 kg/ha.  It has been effective and safe when applied preemergence at 0.036 kg/ha with pendimethalin ACS, and postemergence at the 2 LS with oxyfluorfen SC.

Pyroxasulfone at 0.15 kg/ha has been safe on onion on muck soil when applied preemergence.  Pyroxasulfone improved control of ladysthumb.  Bicyclopyrone applied delayed-preemergence at 0.037 or 0.05 kg/ha with pendimethalin resulted in greater onion yield and better ladysthumb control. 

On mineral soil, pendimethalin is labeled for application preemergence at 0.8-1.6 kg/ha.  It normally can be applied safely after onion emergence.   In Michigan, pendimethalin ACS has been safe on onions grown on sandy soil when applied at 0.50-1.07 kg/ha.  The excellent weed control achieved after preeemergence application has resulted in greater onion yield.  

Successful potato weed management requires season-long diligence to crop maintenance.  Early planting coupled with attacks to the canopy from insects, disease and maturation requires efficacious herbicides with long residual as part of the management plan.  Although currently successful, potato weed management has a long history of reliance on a very limited number of active ingredients from even fewer sites of action.  Furthermore, recent label changes have nearly eliminated the use of linuron herbicide in Wisconsin potato production due to organic matter and depth to groundwater restrictions.  To expand the toolbox of available herbicides, this research has evaluated currently unregistered actives with the intent of identifying herbicides that exhibit crop safety and as a result deserve further evaluation.  Multiple trials over several years were conducted on three soil types including low organic matter sand, silt loam and muck soils representative of Wisconsin potato production regions.  Evaluations included herbicides applied either PRE or POST and in some cases application of traditionally PRE herbicides early to mid-POST to extend residual control later into the growing season.  Chlorimuron-ethyl, flumetsulam, mesotrione, prometryn and pyroxasulfone all show promise at desirable weed control rates when applied PRE with minimal crop safety concern.  Bentazon and prometryn also exhibit excellent crop safety in POST applications.  Results suggest that additional work is needed in fine-tuning rates and soil types as these active ingredients vary greatly in progress towards registration on potato.

Nomenclature:     Bentazon; Chlorimuron-ethyl; Flumetsulam; Mesotrione; Potato, Solanum tuberosum L.; Prometryn; Pyroxasulfone.

Key Words:     Potato Weed Management, Future.       

A field study was conducted at the Northern Plains Potato Growers Association Irrigated research site near Inkster, ND to evaluate sub-lethal doses of glyphosate and/or dicamba on ‘Atlantic’, the most common chipping potato cultivar grown in North Dakota.  Plots were four rows arranged in a randomized complete block design with 4 replicates.  Seed pieces (57 g) were planted at 30.5 cm spacing with a 91.5 cm row spacing on June 5, 2017.  Potatoes were sprayed on July 18 at the tuber initiation stage with a CO₂ pressurized sprayer equipped with 8002 XR flat fan nozzles with a spray volume of 187 l/ha and a pressure of 276 kPa.  At 10 days after application (DAA) on the highest dose of glyphosate, dicamba, and glyphosate + dicamba (196 + 98 g ae ha^-1) caused greater visible injury than the untreated. By 20 DAA, only the highest dose of dicamba and glyphosate + dicamba caused greater visible injury than the untreated. Plots were harvested on October 17 and graded into various categories after harvest. Plants receiving the highest glyphosate +  dicamba dose produced the most undersized tubers (< 113 g) and had a greater cull yield compared to the untreated. Plants receiving the highest glyphosate +  dicamba dose also had the fewest marketable tubers even though marketable yield did not differ from the untreated. In previous simulated drift trials with ‘Russet Burbank’, environmental stresses just before and during the spray application greatly reduced visible injury and yield reducing effects from sub-lethal doses of glyphosate and/or dicamba.  

In the Sacramento Valley of California, English walnut and rice are grown in close proximity. The majority of rice herbicides are applied by air in a short window of time period in which walnuts have formed the current year’s nuts and are initiating and differentiating buds that will form flowers and leaves in the subsequent year. Therefore, if exposed to a sub-lethal dose of an herbicide due to off-target drift, walnut growth, nut quality and yield in the year of the exposure could be affected.  Additionally, if the herbicide affects initiation of reproductive structures, nut yield in the subsequent year could also be affected. Symptoms, growth and yield of walnut trees exposed to simulated drift of bispyribac-sodium, bensulfuron-methyl and propanil, three of the most commonly used rice herbicide in the Sacramento Valley, were evaluated. Bispyribac-sodium may damage nearby walnut orchards when exposed to substantial amounts of drift. Nevertheless, no yield effects were recorded even at rates as high as 10% of the rice use rate. In an additional study, symptoms, growth, yield and nut quality of walnut trees subjected to multiple exposure of simulated bispyribac-sodium drift were evaluated. While no yield reductions were observed either the year of drift exposure or the year following drift exposure, bispyribac-sodium negatively affected walnut kernel color, an important quality factor, in the year of drift exposure. Finally, field research was conducted to determine if bispyribac-sodium can generate visual symptoms without leaving detectable residues on walnut leaf tissues. Walnut trees exposed to low rates of bispyribac-sodium (1% of use rate in rice or lower) developed clear visual symptoms, but analytical analysis of leaf tissues collected seven days after application or later, were not able to detect any chemical residues (limit of detection was 10 ppb). The findings of this work show that bispyribac-sodium drift has the potential to damage walnut trees and can delay growth to some degree. However, the amount of drift necessary to cause significant damage or yield reductions is unlikely to happen frequently in a field situation. 

New Jersey produced 29.6 million kg of cranberries in 2016 at a farm value of $28 million (USDA 2017). New Jersey production is concentrated in the Pine Barrens coastal plain where sandy acidic soils are optimal for cranberry. The perennial nature of cranberry predisposes the crop to a diversity of weed species ranging from herbaceous weeds to woody perennial species. Carolina redroot [Lachnanthes caroliniana (Lam.) Dandy] is a perennial herbaceous weed species member of the Haemodoraceae family. Its proliferation and the lack of efficient control strategies has been an increasing source of concern for New Jersey cranberry growers. Information regarding herbicidal control of Carolina redroot is extremely limited and restricted to blueberry production (Myers et al. 2013). A study was initiated in 2017 in order to evaluate Carolina redroot control and crop response following a spring application of preemergence herbicide at various labelled rates with X equal to the maximum labeled rate. Evital 5G (norflurazon) was applied at 10 (1/16X), 20 (1/8X), 40 (1/4X), and 80 (1/2X) lb a^-1, Devrinol DF-XT (napropamide) at 12 (1X) lb a^-1, and Casoron 4G at 50 (1/2X) and 100 (1X) lb a^-1. Carolina redroot control 6 weeks after treatment (WAT) was 71% with Devrinol DF-XT, 97% and 99% with Casoron at 1/2X and 1X, respectively. Control 12 WAT remained above 70% with Devrinol, but decreased by 27% and 14% with Casoron at the 1/2X and 1X rate, respectively. Evital never provided more than 7% Carolina redroot control, regardless of rate applied. Carolina redroot dry biomass significantly decreased 15 WAT with applications of Devrinol (-62%) and Casoron at the 1/2X (-68%) and 1X (-90%) rates. No significant reduction was noted with Evital, regardless of applied rate. Even if some late season Carolina regrowth was noted, the significant decrease in weed biomass may interfere with the ability of the plant to build up nutritional reserves and restrict weed development the next season. Cranberry injury, mostly in the form of leaf chlorosis, was noted for all herbicide treatments, peaking 12 WAT with 12% for Casoron at the 1X, 8% for Casoron at the 1/2X rate, and less than 5% for Devrinol and Evital. Casoron, regardless of rate, caused significantly more vine stunting than Evital and Devrinol. However, higher crop damages with Casoron did not translate into significant yield reduction compared to the untreated check, Devrinol, or Evital. Fruit firmness, sugar content, titratable acidity, and rate of rotten fruits were not significantly different depending on the preemergence herbicide applied in spring. However, the total anthocyanin of the fruits increased significantly with Casoron at the 1/2X and 1X rates compared to Evital or the untreated check. Studies will again be conducted in 2018 to further evaluate the effects of fall or/and spring herbicide preemergence applications to control Carolina redroot as well as impact of herbicide on cranberry productivity and fruit quality.

In order to expand the known range of grape production and to cope with the challenging environmental conditions of North Dakota, specialized cropping practices may be required. North Dakota offers challenges both due to harsh winter conditions as well as low precipitation. These areas may be impacted by weed control options, especially those which influence the near-vine microclimate. Alternative weed control methods were tested in an experimental vineyard near Absaraka, ND for their ability to control annual weed species as well as for their effects on vine growth and production. The experiment was planted in 2012 and arranged in a randomized complete block design with a full factorial including four cold-climate white wine cultivars (‘Alpenglow’, ‘Brianna’, ‘Frontenac Gris’, and ‘La Crescent’) and six within-row weed control methods (woven landscape fabric, herbicide (glufosinate plus flumioxazin), black polyethylene film, straw mulch, tillage, and turfgrass) with four replications. Upon vine maturity, differences in overall production were observed. Across cultivars, yields were greater with the use of the polyethylene film or landscape fabric synthetic mulches and reduced with the use of turfgrass relative to an herbicide application. Differences were attributed both to impacts on cluster number as well as cluster weight. Differences in cluster weight were found to be due to differences in berry number, while berry weight was not altered by weed control method. Fruit quality was similar across treatments as berry brix, pH, and total titratable acidity were not found to significantly differ. Overall, the synthetic mulches provided high yield without fruit quality reduction. Taking into consideration the durability of each mulch, polyethylene film provides an adequate alternative to landscape fabric and had similar effect on fruit production. 

Biodegradable paper mulches play a role in increasing soil temperatures, retaining soil moisture, increasing organic matter content, and controlling weeds. Mustard seed meal (MSM) is a by-product of the oil processing process and pelleted mustard seed meal is being adopted by some growers in strawberry (Fragaria xananassa)  production as a preplant treatment, to control soil-borne pests. The objective of this study was to evaluate paper pellet mulch (PPM) at three dose rates, MSM, and PPM + MSM on weed control and crop yield in comparison with fumigated and a nontreated control in an annual plasticulture, raised bed, strawberry production.  A study was done in the 2015-16 and 2016-17 growing seasons at the Hampton Roads Agricultural Research and Extension Center in a randomized complete block design with four replicates. Treatments were PPM at 4882, 6103, 7324 kg/ha respectively, MSM at 2242 kg/ha, PPM 6103 kg/ha + MSM 2242 kg/ha, 1,3-dichloropropene + chloropicrin (39% 1,3-dichloropropene + 59.6% chloropicrin) that was shank-fumigated in beds at 152 kg ha^-1. All beds were covered with a 1.25 mil virtually impermeable film. Beds were 11.3 m in length, 0.7 m wide and 0.14 m high.  The center 4.6 m length of the bed, referred to as plots, was used for data collection. After the preplant treatment period, ‘Chandler’ strawberry plugs were transplanted in two rows, at 36 cm in-row spacing in the 4.6 m bed length. Weed data collection was done in 1.5 m bed length, and yield data were collected in the other 3.1 m. Dominant weeds in both seasons were white clover (Trifolium repens L.), common chickweed (Stellaria media (L.) Vill.), cudweed (Gnaphalium L.), yellow woodsorrel (Oxalis spp.) and Italian ryegrass (Lolium perenne L.). In the 2015-16 growing season, the fumigated plots had the lowest cumulative weed density for the season, followed by PPM + MSM plots. In the 2016-17 growing season, weed density was lowest in plots treated with PPM+MSM, and the highest density was in the fumigated plots, but overall, the treatment effect was marginal. Cumulative total yield (marketable and non-marketable) for the 2015-16 growing season was the highest in PPM+MSM plots, significantly higher than the nontreated, the fumigated treatment, and PPM at 4882 kg/ha. In the 2016-17 growing season, the fumigated plots had the highest total yield, followed by PPM+MSM, MSM, and PPM at 7324 kg/ha plots. PPM + MSM merits further evaluation as a bio-based alternative to fumigation.

Fumigation through drip-irrigation system is applicable to plasticulture vegetable production in Florida where many growers planted multiple crops in sequence using the same beds, drip tapes, and polyethylene films. Field experiments were conducted at the Gulf Coast Research and Education Center at Balm, Florida to determine the efficacy of emulsified concentrate formulations of dimethyl disulfide (DMDS) and DMDS + chloropicrin (Pic) for weed control in tomato (Solanum lycopersicum L.). Treatments included seven rates of DMDS ranging from 112 L ha^-1 to 440 L ha^-1, seven rates of DMDS + Pic ranging from 140 L ha^-1 to 560 L ha^-1, and a non-fumigant control. The effective fumigant rates required to provide 80% (ER₈₀) control of purple nutsedge (Cyperus rotundus L.) populations were 310 L ha^-1 and 350 L ha^-1 from DMDS and DMDS + Pic in spring experiment, respectively, while ER₈₀ values were >440 and 500 L ha^-1 in fall experiment, respectively. The DMDS + Pic is more effective than DMDS and provided consistent control of purple nutsedge across seasons. None of evaluated fumigant treatments damaged or stunted tomato growth. There was no difference in yield of all fruit size categories among fumigant treatments and non-fumigant control. Overall, drip-applied DMDS or DMDS + Pic is safe to tomato production and appears to be a viable technique for controlling purple nutsedge in tomato crops. However, supplemental weed management, such as herbicide applications, may be required to control weeds that emergence in the late growing season.     

The potential role of fall-seeded cover crops for weed management in edamame is unknown.  Field experiments were conducted over three edamame growing seasons to 1) determine the extent to which cover crop residue management systems influence edamame emergence while selectively suppressing weed density and biomass, and 2) determine if cultivars differed in emergence in cover crop residue management systems.  Cover crop treatments included a winter-killed oilseed radish, two canola treatments (early-killed and late-killed), two cereal rye treatments (early-killed and late-killed), and a bare soil control.  Two spring timings of a cover crop burndown application created the ‘early-killed’ and ‘late-killed’ treatments for canola and rye.  Twelve soybean cultivars were tested, including 11 edamame cultivars differing in seed size and a grain-type soybean control.  Spring residue biomass in cover crop treatments ranged from 438 kg ha^-1 for winter-killed radish to 9,003 kg ha^-1 for late-killed rye.  Cultivars responded similarly to cover crop treatments, and, with the exception of late-killed rye, cover crop treatments resulted in similar crop emergence as the bare soil control.  While all cover crop treatments reduced weed biomass 6 wk after planting compared to the bare soil, winter-killed radish and both canola treatments increased weed density.  Early-killed rye has potential for weed management in edamame, as evidenced by the fact that the treatment did not interfere with planting or crop establishment, yet reduced weed density 20% and suppressed early-season weed growth 85%.  

Lettuce is an important winter vegetable grown on organic soils in the Everglades Agricultural Area (EAA) in south Florida. Successful weed management, particularly use of chemical control is very important to sustain production of lettuce on organic soils of the EAA. Field experiments were conducted in 2016 to 2017 to determine the efficacy of preemergence imazethapyr, oxyfluorfen, pronamide, and bensulide alone or followed by postemergence application of imazethapy on weed control and lettuce (head and romaine) yield response. Preemergence oxyfluorfen (0.56 kg/ha) resulted in crop stand loss (>95%) while pronamide (4.44 and 6.66 kg/ha) , bensulide (5.6 and 10.1 kg/ha), and imazethapyr (0.035 and 0.07 kg/ha) resulted in crop stunting (<10) and no stand loss. Imazethapyr at 0.07 kg/ha resulted in 85% common lambsquarters control as a preemergence treatment at 35 days after treatment. Both rates of pronamide and bensulide provided 75 to 85% common lambsquarters control at 14 DAT, however control decreased to <60% at 35 DAT. Oxyfluorfen applied preemergence provide complete weed control at 14 DAT. Application of postemergence imazethapyr (0.035 kg/ha) at the two true-leaf stage of common lambsquarters following preemergence application of all the herbicides resulted in >85% common lambsquarters control at 35 DAT. Postemergence imazethapyr resulted in <8% injury of both romaine and head lettuce. However, these herbicides were not able to control late emerging common purslane which is now becoming very problematic for leafy greens on organic soils in the EAA. Preemergence imazethapyr, pronamide, and bensulide followed by postemergence imazethapyr resulted in better lettuce yield compared to preemergence only treatments indicating a yield benefit by having a preemergence and postemergence weed control program in lettuce grown on organic soils in the EAA.  

Organic vegetable farmers rely on tillage and cultivation as tools to control weeds, which can increase production costs and negatively impact soil health. Growing organic vegetables under reduced tillage is made even more difficult because of a lack of effective and affordable herbicides labelled for organic use. Organic mulches produced by winter cover crops can effectively suppress weeds in no-till agronomic crops, and can potentially allow vegetable crops to be grown with reduced tillage. Here, we present the first two years of data from a three-year project testing whether winter cover crops can provide season-long suppression of weeds in a bell pepper (Capsicum annuum) crop. Our experiment tests three treatments using an annual cover crop mixture of rye (Secale cereale) and crimson clover (Trifolium incarnatum) and one treatment using perennial red clover (Trifolium pratense). The rye/crimson clover treatments each received different amounts of tillage: a conventional tillage (CT) treatment was rototilled to incorporate all cover crop residue, a no-till (NT) treatment was flail mowed and pepper transplants were planted directly into the resulting residue, and a strip-till/roller crimp (ST-RC) treatment was rolled and tilled only within the strips where peppers were transplanted. The red clover treatment was also strip tilled, and the remaining clover was allowed to remain as a living mulch (ST-LM) between pepper rows. Data were collected on weed species and abundance, persistence of different types of mulch, plant performance, and yield quantity and quality. Results from this study will determine whether cover crops can offer an additional tool to for developing integrated weed management strategies in organic vegetables.

Vegetable legumes, including edamame, lima bean, and snap bean, constitute an important food group in human nutrition. One of the biggest obstacles of growing vegetable legumes is weed competition. Registered herbicides are limited, herbicide resistance is a growing concern, and hand weeding is effective but expensive. Therefore, further development of integrated weed management systems in vegetable legumes is needed. Field experiments were used to determine the effects of early-killed rye (EKR) and weed management on weed emergence and growth, hand weeding time, and crop yield in edamame, lima bean, and snap bean. Field experiments were conducted for each crop separately at the Vegetable Crop Research Farm near Urbana, IL in 2015, 2016 and 2017. Main plots consisted of EKR and stale seed bed (SSB) treatments within a split plot design. Cereal rye (Secale cereale) was sown at the rate of 162 kg ha^-1 in September and killed by broadcast glyphosate application the following April. Vegetable legumes were no-till planted in late-May.  Subplot treatments consisted of 1) standard PRE and POST herbicides (hereafter called ‘standard’), 2) standard treatment plus hand weeding (hereafter called ‘augmented’) and 3) a weedy control. Crops were machine harvested using the Oxbo BH100. At the time of rye burndown, cover crop biomass averaged 836 g m^-2. The EKR treatment reduced crop emergence 9 to 38%, with edamame least affected by EKR. Standard herbicides reduced weed density throughout the growing season, whereas EKR had little effect on weed density.  Nonetheless, EKR reduced at-harvest weed biomass 82% and 53% in snap bean and edamame, respectively.  The EKR treatment decreased hand weeding time 12% in edamame. The EKR treatment reduced yield of lima bean and snap bean 51% and 62%, respectively. In contrast, EKR had no effect on edamame yield and increased marketable pod recovery 3%. While EKR shows little promise for use in lima bean and snap bean, EKR has high potential as an additional weed management tactic in edamame production. 

Bicyclopyrone (BIR) is a Group 27 (F2) HPPD inhibitor currently registered for use on corn in combination with other active ingredients in the products Acuron® and Acuron Flexi®.  Research is ongoing for potential use of BIR as a stand-alone product in vegetable crops.  It will be most useful when tank mixed with other herbicides for broad spectrum weed control.  Bicyclopyrone was applied to various vegetable crops during five years to determine crop safety and weed control efficacy. 

Bicyclopryone at 0.037 kg/ha caused minimal or no crop injury when applied preemergence to asparagus (Asparagus officinalis L.), cucumber (Cucumis sativus L.), edamame (Glycine max (L.) Merr.), leek (Allium porrum L.), pumpkin (Cucurbita pepo L.), buttercup squash (Cucurbita maxima L.), butternut squash (Cucurbita moschata L.), golden hubbard squash, rhubarb (Rheum rhabarbarum L.), cauliflower (Brassica oleracea L.), hot banana pepper (Capsicum annuum L.), and hot cherry pepper.  In broccoli (Brassica oleracea L.) and cabbage (Brassica oleracea L.), bicyclopyrone was safe postemergence but not pre- or post-transplant.  Pre-transplant applications were safe on Chinese cabbage (Brassica rapa L.), but caused injury post-transplant and postemergence.  On jalapeno pepper, pre-transplant application was safe but post-transplant application caused crop injury.  Bicyclopyrone reduced stand of seeded chives (Allium schoenoprasum L.), but did not cause injury to established chives either pre- or post-emergence.  In carrots (Daucus carota L.) and onions (Allium cepa L.), crop safety differed by soil type; bicyclopyrone was safe pre- and post-emergence on muck soil for both crops, but reduced stand and caused significant injury on mineral soil.

Bicyclopyrone was not safe for any use on basil (Ocimum basilicum L.), red beets (Beta vulgaris L.), sugar beets, celery (Apium graveolens L.), dill (Anethum graveolens L.), fennel (Foeniculum vulgare L.), lettuce (Lactuca sativa L.), parsley (Petroselinum crispum (Mill.) Nyman ex A.W. Hill), bell pepper, spearmint (Mentha spicata L.), swiss chard (Beta vulgaris L.), or tomato (Solanum lycopersicum L.).

Preemergence applications of bicyclopyrone effectively controlled or improved control of barnyardgrass (Echinochloa crus-galli (L.) Beauv.), foxtails (Setaria spp.), large crabgrass (Digitaria sanguinalis (L.) Scop), fall panicum (Panicum dichotomiflorum Michx.), common ragweed (Ambrosia artemisiifolia L.), eastern black nightshade (Solanum ptycanthum Dun.), redroot pigweed (Amaranthus retroflexus L.), common lambsquarters (Chenopodium album L.), prostrate knotweed (Polygonum aviculare L.), wild radish (Raphanus raphanistrum L.), and hairy vetch (Vicia villosa Roth).  Bicyclopyrone was effective postemergence on barnyardgrass, foxtails, large crabgrass, common ragweed, eastern black nightshade, ladysthumb (Polygonum persicaria L.) and redroot pigweed.

Bicyclopryone could complement weed control programs in asparagus by tank mixing with diuron, pendimethalin, sulfentrazone, or halosulfuron to improve control of annual grasses, common ragweed, eastern black nightshade, and pigweeds.  In cucumber, it could be used with ethalfluralin, clomazone, halosulfuron, sethoxydim, and clethodim.  Pumpkin and winter squash weed control programs typically include the same herbicides as in cucumber, with the addition of S-metolachlor and fomesafen. Adding bicyclopyrone to a cucumber or pumpkin and squash weed control program would improve control of common ragweed, eastern black nightshade, redroot pigweed, and ladysthumb.  Onion weed control programs could use bicyclopyrone to complement pendimethalin, oxyfluorfen, flumioxazin, S-metolachlor, or dimethenamid-p, improving control of common lambsquarters, common ragweed, eastern black nightshade, and ladysthumb.  In all tolerant crops, bicyclopyrone could be used to control hairy vetch, for which there are few herbicide options available.  Tank mixing with NIS improves BIR activity but may cause crop injury.

Glufosinate is a nonselective contact herbicide and wtih potential for use as a directed basal treatment for burn-back of hops.  Due to some limited translocation of glufosinate in various plant species, there is potential for injury to hops above the treated spray zone.  In 2016 and 2017 studies, glufosinate applied at rates of 0.56, 0.84, and 1.12 kg ai ha^-1 to the base of hops that averaged at least 2.4 m tall did not significantly injure hops above the treated spray zone and did not reduce cone yields.  All application rates of glufosinate provided more complete burn-back of basal growth of hops and prevented regrowth of hops for a longer period than standard rates of paraquat or carfentrazone.  Glufosinate injured hops above the treated zone when applied to hops 1.2 to 1.8 m tall and reduced cone yields.  Hop varieties Cascade, Willamette, Columbus, Mt Hood, Chinook, and Warrior tolerated directed basal applications of glufosinate similarly when plants averaged 2.4 m tall when applied and cone yields were not significantly reduced.

Camellia oleifera Abel is a major kind of woody oil plant in the southern part of China. At present, only single herbicide formulation such as glyphosate or glufosinate ammonium is applied for weeds control in C. oleifera forest, whereas most regularly adopted are still weeds pulling and soil digging especially when the forest is during its young stage under three years, which are time-consuming and uneconomic. In this paper, the weeds species and occurring regularities of C. oleifera forest in Hunan were surveyed, and ten tests of herbicides mixed in barrels by using glyphosate, glufosinate ammonium, fomesafen, or quizalofop-p-ethyl were carried out in  C. oleifera forest with an area of 10 m² each. The results indicated that there were 80 weeds species belonging to 31 families in C. oleifera forest in Hunan, China. Among them, there were 13, 9, 6, and 5 species in the Asteraceae, Gramineae, Rosaceae, and Leguminosae families, respectively. The main weeds communities in C. oleifera forest belonged to the Gramineae family. The optimal herbicide formulation mixed in barrels for weed control in C. oleifera forest was the mixture of glyphosate and quizalofop-p-ethyl, and no lateral buds of weeds were observed at 30 day after spraying the mixture. These tests could provide an efficient, economical, and practical way to control the weeds in C. oleifera forest. *Corresponding author, Y. Hu, e-mail: huyhongwangyi@163.com

In contrast to high commercialization cost for GM herbicide resistant (HR) traits, non-GM HR traits bears great promise for immediate application to address weed problems in essentially all crops, especially minor crops. However, efficiently creating HR mutations on plant endogenous genes remains a great challenge. Recently, a modified CRISPR/Cas9 system performed base-editing and efficiently generated point mutations rather than indels in mammal cells and T0 rice plants, offering an immediate promise to address this issue. We established a plant base-editing system referring to those worked in mammal cells, and successfully generated precise point mutations to the codon of Pro197 on acetolactate synthase (ALS) gene in Arabidopsis. After a simple transformation, 1.7% (4/240) transgenic T1 plants harbored point mutations at desired positions, and these point mutations are inheritable to T2 generation, conferring T2 plants high resistance to tribenuron herbicide. Moreover, we recovered six non-transgenic (T-DNA free) HR plants out of 64 examined T2 plants, three harboring known HR mutations (P197S) and the other three bearing novel HR mutations (P197F). In addition, using this new technology, we successfully created HR mutations (P197S and P197L) on watermelon ALS gene, offering a non-GM tool to address broadleaf weeds in watermelon production. In conclusion, this base-editing system is a powerful tool to create point mutations conferring agronomically important traits, such as herbicide resistance, in virtually all crops.

A great deal of work continues to address the issue of herbicide resistance.  However, resistance is still a concern for many farmers, punctuated by the fact that resistance is continuing to increase and changes in weed management have not been realized on a broad scale.  We do not yet understand the grassroots concerns, challenges, and proposed solutions specific to different regions and cropping systems.  In 2016, WSSA provided funds to help support a series of regional listening sessions to obtain local input to help us understand how managing herbicide resistant weeds, both the problem and solutions, varies by region. The goals of the listening sessions were developed by the Herbicide Resistance Education Committee after reviewing results from past efforts and considering options for future efforts.  The committee considered how WSSA has worked with many groups on the issue of resistance, has learned a great deal and has been very productive, but felt that we need to take this effort further to be more successful dealing with resistance proactively rather than reactively.  The committee also recognized that our efforts had primarily focused on the national audience, whereas a more localized approach through regional and state efforts was needed to more completely understand the variety of complex issues that may preclude a practitioner’s ability to adopt best management practices for herbicide resistance management and the opportunities to launch more effective strategies. This symposium will provide WSSA members a report on the regional herbicide listening sessions that were held in seven locations from December 2016 through March 2017.  The introduction will provide background and context for the sessions along with over-arching findings. 

This symposium will provide WSSA members a report on the listening sessions.  The introduction will provide background and context for the sessions along with over-arching findings. The panel discussion will include a question/answer session with stakeholders who represent each of the listening sessions as well as a discussion about next steps within each region and nationally.  The panel members will be asked: What were your impressions of the listening session?; What did you hear at your listening session that caused you to think, or that you hadn’t heard before?; What have you done since you attended the listening session to address herbicide resistance? What do you recommend that we do about this problem?  How do we do a better job of addressing resistance at the farm level?  

This symposium will provide WSSA members a report on the listening sessions.  The introduction will provide background and context for the sessions along with over-arching findings. This panel discussion will include a question/answer session with Regional Coordinators who represent each of the listening sessions as well as a discussion about next steps within each region and nationally.  The panel members will be asked: What were your impressions of the listening session?; What did you hear at your listening session that caused you to think, or that you hadn’t heard before?; What have you done since you attended the listening session to address herbicide resistance? What do you recommend that we do about this problem?  How do we do a better job of addressing resistance at the farm level?  The Regional Coordinators included Annie Klodd, Bill Curran, Mark VanGessel (NE); Ian Burke and Don Morishita (NW); Brad Hanson and Brian Schutte (SW); Phil Stahlman (Great Plains); Jeff Gunsolus and Christy Sprague (MidWest); Darrin Dodds and Larry Steckel (MidSouth); Ramon Leon and Stanley Culpepper (SE). 

Paraquat products are valuable components in integrated weed management programs associated with conventional and genetically modified crops.  Furthermore, paraquat is extremely important where glyphosate resistance in weeds has been identified and is also an essential component in delaying the development of resistance to glufosinate in crops that are designed to tolerate that herbicide.  Additionally, but no less important, paraquat is a critical tool in reduced and no-till farming which leads to reduced soil erosion and a significantly reduced carbon footprint when compared to conventional cultivation.  On December 14, 2016, the United States Environmental Protection Agency issued the Paraquat Dichloride Human Health Mitigation Decision that specified required changes for who and how paraquat containing products may be used.   These changes include label changes, creation and distributions of supplemental warning materials, new training requirements for paraquat users, requirementd for closed system packaging and restrictions on who may use paraquat products.  These changes will be implemented in a three phase process with the final requirements having to be in place by October 1, 2020.

Data,  more data…  As the world gets smaller and trade of US commodities increases with increases in the interdependence among countries and greater liberalization of trade.  This increase in data requirements has intensified the minor use issue. The IR-4 program generates regulatory residue data to support pesticide use on specialty crops and for minor uses on major crops. While it is true that more data are needed to support trade of the valuable US commodities, there are a number of initiatives that are expected to help ease the load of data requirements.  The US EPA and Canadian Pest Management Regulatory Agency are supporting a global proposal for exchangeability of residue field trials. This proposal would lend to a further sharing, globally, of residue data that are used to set Maximum Residue Level (MRLs) standards used for trade.  Another initiative is the global implementation of using residue data from representative crops to serve as surrogate crops, within a grouping of similar crops, for setting MRLs.  The expectation of this work is to allow for greater cooperation and sharing of data to reduce the influence of pesticides on trade.  Progress in these areas will be discussed in detail.

The significance of both managed pollinators such as honey bees and mason bees as well as native pollinators such bumble bees, butterflies and beetles in the agricultural landscape has been widely reported.  One area that needs to be addressed is pollinator habitat and forage establishment in agricultural or rural landscapes.  Challenges in establishment and maintenance range from proper seed mixture selection, site selection, planting methods and long-term management including weeds.  Help is needed from the Weed Science Society of America on best practices for the weed challenges.

Triazine herbicides have been important in the development of conservation tillage and other best management practices.  Even with the advent of new technologies, these products are continually used because of the many benefits associated with no-till, yields, sustainability, soil savings, costs, herbicide resistant weed management, and weed control. Information will be presented to demonstrate the benefits of atrazine in today’s modern agricultural environments.

Successful atrazine stewardship at the watershed level is dependent upon an active feedback mechanism by which farmers and applicators can evaluate their progress. A robust water monitoring program can be used to inform a stewardship program based on adaptive management. A case study from Syngenta’s Atrazine Ecological Monitoring Program will be presented to illustrate how monitoring data at the watershed level is used to encourage adoption of effective best management practices.

The ongoing difficulty in harmonizing pesticide regulatory approvals with the Endangered Species Act’s consultation requirements is a major challenge for registrants, end users and government agencies. Syngenta is piloting innovative optional conservation approaches that may help resolve this issue. Syngenta is partnering with Delta F.A.R.M./Delta Wildlife in Mississippi  to implement conservation practices to benefit several endangered species.  The conservation activities being conducted in Mississippi as part of this pilot will be presented by Trey Cooke, Executive Director of DELTA F.A.R.M and Delta Wildlife.

The ongoing difficulty in harmonizing pesticide regulatory approvals with the Endangered Species Act’s consultation requirements is a major challenge for registrants, end users and government agencies. Syngenta is piloting innovative optional conservation approaches that may help resolve this issue. Syngenta is partnering with Iowa Soybean Association in Iowa  to implement conservation practices to benefit several endangered species.  The conservation activities being conducted in Iowa as part of this pilot will be presented by Keegan Kult, Environmental Scientist, Iowa Soybean Association.  

The discovery, development and use of herbicides has been the focus of weed science for the past 60 years.  The accomplishments of the agrochemical industry in development of scores of herbicide molecules as well as transgenic crops with engineered resistance to herbicides is remarkable reaching an estimated global value in 2016 of $27 billion. Despite increases in the value of herbicide sales, there has been a steady consolidation of the agrochemical industry throughout the decades; such that the industry is now is coalescing into four large companies. Motivation for this industry consolidation has primarily been to control costs and maintain profitability. The high cost of bringing new herbicides to the market and limited number of companies that can perform this task will likely continue to constrain the number of new products coming to the market. Countervailing factors working against the existing herbicide inventory are increasing number of herbicide resistant weeds that reduce product efficacy, and an increasing number of off-patent or “generic” herbicides that are marginally profitable for registrants. In an era with few new herbicides, where then must we look for new weed control tools? Many suggest that the key to preserving the efficacy of existing herbicides is to use less of them. However, if we must use less herbicide, then what technologies can be used to supplement weed management programs? Technology with potential to improve weed control may come from engineering companies, which are new to the weed control business. New weed control products include automated weeders that detect and spray or uproot weeds within the crop. The implications of these trends on the future of weed control business will be explored as follows:

1.      What are the trends in the world herbicide market and what are the drivers towards industry consolidation? Will include a speaker familiar with the operations of the agricultural chemical industry, and can explain the reasons why the industry continues to consolidate. What are the constraints of herbicide development and why is it so expensive? Speakers: Jonathon Shoham, Syngenta (retired), James MacDonald, USDA ERS.

2.      Evolution of the herbicide marketplace. Most products in the herbicide inventory were patented decades ago and consist of large numbers of “generic” products. As the herbicide inventory accumulates increasing numbers of off patent products, what are the implications for product service, label revisions, regulatory challenges and industry profitability? Speaker: Peter Porpiglia of Amvac.

3.      New entrants into the weed management business.  Small engineering companies such as are developing commercial weed control products like automated weeders. These are new tools and how they will affect the weed, control market will be discussed.  Speakers: William Patzoldt, Blue River Technology, Ryan Herbon Agmechtronix.

4.      Balancing a declining herbicide efficacy with new technology. The increase in herbicide resistant weeds means that the herbicide efficacy of many existing products is in decline. Where will the new technologies come from? Automated technology is feasible for specialty crops, but how can it be scaled up to deal with major crops? The symposium will close with an integrated summary by David Mortensen, Penn State University.

Panel and open discussion. Steve Fennimore University of California, Davis will moderate.

Herbicides remain the largest sector of the crop protection market with growth being driven by innovation, the introduction of herbicide tolerant traits, no-till and rising labor costs in developing countries.  Having said that, growth has been slower than for insecticides and fungicides in recent years.  This is partly due to the impact of herbicide tolerant crops in allowing the substitution of cheaper glyphosate products for more expensive selective herbicides.  As a result glyphosate remains far and away the leading herbicide product, despite the increasing prevalence of glyphosate resistant weeds.  The growth of herbicide tolerance traits has also had a dampening effect on the rate of introduction of new herbicide active ingredients.

Having said that innovation remains an important driver of the herbicide market.  New herbicide tolerant traits conferring resistance to dicamba, 2,4-D and HPPD herbicides promise to change the balance of herbicide use in the industry.  There are also around 10 new active ingredients in the industry pipeline due to be launched over the next 5 years or so.

As well as changes to the overall herbicide market, there are developments within the competitive landscape for herbicides, with the current round of industry consolidation resulting in a redistribution of herbicide product portfolios amongst the leading players, and an increasing volume of product originating from China.

Other drivers of potential future changes to the herbicide market are the growth in precision agriculture and the increasing regulatory pressure to which glyphosate is being subjected.

Proposed mergers will, if carried out, reduce the Big 6 global seed and chemical firms to a Big 4. In markets for certain specific seeds or chemicals, the number of competing firms will fall to 1, 2, or 3. Proponents of the mergers argue that scale economies in scientific discovery, and the capital requirements for discovery in the genomic era, argue for mergers as a way tio encourage greater innovation. At the same time, merger can reduce the incentives to innovate, and can lead to increased product prices, in certain circumstances. I discuss these tradeoffs in analyzing the mergers, and review the role of antitrust agencies in reviewing mergers.

Abstract.  Herbicide discovery efforts were successful for a long time. The herbicide industry grew into a vibrant business with the discovery of twenty-eight commercial modes-of-action from the 1940s to the 1980s, but amazingly none since. The industry has remained successful relying on the discovery of chemistry with improved features within known modes-of-action, the development of value-added mixtures with other herbicides and safeners, new salts and formulations, manufacturing process improvements, and opportunities created with herbicide-resistant crops. Weed control is still the largest segment of the over $60 billion pesticide business, but the lack of proprietary protection for some of the most important technologies has forced a consolidation among the suppliers of proprietary products and led to the growth of generic companies. Today, it is difficult for any company to survive just with proprietary chemistry and most companies do not want to simply be the lowest cost supplier of post-patent chemistry. The distinction between proprietary and generic companies has become blurred with few companies relying fully on proprietary chemistry, and most relying to some extent on post-patent products with claims of added value. The business of chemical weed control has evolved to a search for sustainable solutions with mostly post-patent herbicides used in mixtures and planned sequences with other herbicides and systems including non-chemical weed management technologies.

Blue River Technology is bringing the next generation of smart machines to address current challenges being faced in agriculture.  With the use of artificial intelligence and machine learning, sprayers are being taught to recognize crops and weeds in real-time, thus allowing the application of herbicides to only weeds with a high degree of accuracy and precision.  Smart machines for weed control bring several advantages to agricultural producers, including the ability to more easily implement best management practices (BMPs) to combat the evolution of herbicide-resistant weeds.  Several of the BMPs addressed by smart machines include: 1) reduction of chemical input costs that allow producers to economically use full labeled rates, 2) the ability to use cost effective herbicide mixtures containing multiple sites-of-action to combat herbicide resistance evolution, and 3) uncoupling weed size from weed density that allows applications to take place at the proper timing without waiting until sufficient weed densities exist to make broadcast applications feasible.  In addition, smart machines collect high resolution images from all parts of the field with every pass.  With these images, it becomes possible to create weed maps that can be used by the producer to make informed decisions about weed population dynamics and herbicide performance.  Prototype machines were deployed in 2017 to manage weeds in cotton production.  Research will continue in 2018 with expanded efforts to include both cotton and soybean weed management.

As labor and immigration laws make uncertain times for growers, automation is rapidly making its way to lettuce fields.  Agmechtronix released its first automated lettuce thinning machine in 2012 and now has units spread across the United States.  In 2018, a new weeding machine will also be available to further help growers manage their available labor effectively.  Insight will be presented to show the design and development of a machine to identify and remove weeds from commercial lettuce fields.

 When we ask farmers and those involved with crop production decision-making we hear that weeds represent one of the handful of top challenges in their production systems. Interestingly, 30 years ago, the answer to that question would have been the same, the cast of characters (species) may not have been the same but weed management would have been at or near the top of the list. While much has been learned during those intervening years, our response (scientists both applied and basic, input developers and providers) has been disappointing in that it all too often has failed to appreciate the extent to which the input industry shapes how the latest science and technology is implemented. For us to improve on the current state of the weed control industry it is imperative that we employ a systems approach to assessing prospective tactics and technologies, then as much as possible, objectively identify bottlenecks that currently or likely will constrain the success of new tactics and technologies.  The talk will highlight steps our discipline can take to better position itself to develop robust approaches to weed management highlighting promising tactics and technologies that will get us there.

Glufosinate-ammonium is a Group 10 post emergence herbicide used in non-crop situations and in crops with the LibertyLink Seed Trait.  It has greater, more consistent use in the Mid-Western United States than on the High Plains and Western United States perhaps due to differences in relative humidity and temperature. New adjuvants could potentially improve weed control efficacy and consistency of glufosinate in these areas. A series of trials were conducted to evaluate the impact of several adjuvants on glufosinate weed control efficacy. These trials consisted of three greenhouse studies conducted at Colorado State University, as well as three field studies conducted in Colorado, and one field trial conducted in eastern South Dakota. Across these trials, five candidate adjuvants were compared with the Bayer suggested adjuvant of three pounds per acre ammonium sulfate plus 0.5% V/V non-ionic surfactant.  All the candidate adjuvants performed as well as or better than the standard.  AQ 127 at 0.375% V/V was superior in both states.  Generally, the addition of 1.5 lb per acre ammonium sulfate per acre slightly improved control over the adjuvants applied alone with glufosinate.  Results suggest that the use of new candidate adjuvants over currently used adjuvants could help improve weed control with glufosinate-ammonium.

AccuDrop™ (AG13064) is a non-oil, surfactant based drift and deposition adjuvant formulated without nonylphenol ethoxylates from Winfield^® United. AccuDrop™ is designed to maximize pesticide performance by improving spray deposition onto the intended target. Also, being surfactant based, AccuDrop™ can be used with many herbicides, fungicides or insecticides with minimal expected crop injury. The use rate of AccuDrop™ is 0.223 l ha^-1.

As part of the testing program, Winfield^® United conducted 126 field efficacy trials as well as screening though the Winfield^® United Spray Analysis System, a patented recirculating low speed wind tunnel. In numerous field trials, herbicide plus AccuDrop™ performance versus the herbicide alone showed significantly increased weed control. Field drift studies also showed significant drift reductions; 3.02 m with the addition of AccuDrop™ compared to 6.83 m with no drift control added. Wind tunnel testing was utilized to evaluate spray particle size with various pesticides and nozzle tips. AccuDrop™ added to glyphosate and sprayed through XR11003 nozzles reduced the percent of spray particle droplet fines from 16% to 6%. Likewise, with a AIXR 11004 nozzle, percent fines were reduced from 16% to 4% vs glyphosate alone. 

In 2017, ~70% of the cotton in Georgia was planted using auxin-tolerant varieties.  One of the application requirements for this new technology is the use of nozzles that produce extremely-coarse to ultra-coarse droplets (≥ 503 microns).  Since most cotton growers also plant peanut, there have been questions about the use of these nozzles for weed management in peanut.  Several peanut herbicides are contact in nature (acifluorfen, bentazon, lactofen, and paraquat) and medium droplets (236-340 microns) are recommended when these herbicides are used.  Using a commercial John Deere R4030 sprayer (110’ boom, 20” nozzle spacing, 36” boom height, 40 PSI, 9.8 MPH tractor speed, and 4-6 MPH wind speed), the coverage and droplet size of 2 nozzles (XRC-11005VP and TTI60-11005VP) were compared.  A computer analysis of 30 Kromekote water-sensitive cards indicated that the TTI60-11005 nozzle (8.4% coverage, 514 VMD₅₀) provided better coverage and produced larger droplets than the XRC-11005 nozzle (6.6% coverage, 302 VMD₅₀).  Using these same two nozzles, a commercial peanut field (105 acres) was treated with a standard herbicide program.  Field observations from this site would suggest that there was no difference in efficacy between the XRC and TTI-60 nozzles.  Additional results from a small-plot research trial comparing the efficacy of 3 nozzles (DG11002, TTI60-11002, and TADF02-D) indicated that nozzle type had no effect on the control of Palmer amaranth (Amaranthus palmeri).  However, the TADF02-D nozzle provided 4% less control of annual grasses than the DG11002 nozzle.      

New dicamba and 2,4-D herbicides have created a greater level of interest in ensuring that these products are applied effectively. To use these products, websites have been implemented which show the most up-to-date adjuvant and herbicide tank-mix partners as well as the nozzles and pressures labeled for use with these herbicides. Selecting a nozzle at a pressure within the range is not difficult, but offering sound advice to the best of these combinations is less well-known. A study was conducted at the Rodney Foil Plant Science Research Center in Miss. State, MS to measure coverage produced by nozzles labeled for use with new dicamba and 2,4-D herbicides. Seven nozzles (ID 11003, ID 11004, TDXL-D 11003, TDXL-D 11004, TTI 11003, TTI 11004, and ULD 12004) which are labeled for use with at least two of the herbicides were applied at two of three application volumes (140, 187, 281 L ha^-1) at pressures of 207 and 276 kPa. The 03 nozzles were applied at 140 and 187 L ha^-1, and the 04 nozzles were applied at 187 and 281 L  ha^-1. Treatments were applied using a two-nozzle research track sprayer (Generation III Sprayer, DeVries Manufacturing, Hollandale, MN). The 281 L ha^-1 application volume was included as the sprayer’s maximum travel speed is 10 km h^-1 which makes it unable to apply 04 nozzles at 140 L ha^-1, the recommended application volume for applying the new dicamba and 2,4-D products. The spray solution was comprised of water plus a 0.4 g L^-1 addition of Brilliant Blue dye (Flavors and Colors.com, Diamond Bar, CA). Applications were made to 5 by 7.5 cm Kromekote cards, a specialty type of photo paper that stains when droplets impact its surface. Percent coverage was measured using Image J for each card. Card coverage for each application volume was analyzed using PROC GLIMMIX in SAS 9.4 with means separations at α = 0.05. Nozzle and pressure were both significant (P < 0.001) for all 03 and 04 nozzles, where the   higher pressure increased coverage at each of the three application volumes. Volume was significant for the 04 nozzles, which demonstrates that application volumes above 187 L ha^-1 would improve coverage even with a TTI, the largest droplet size producing nozzle labeled for use with all of these herbicides. Nozzle selection plays a crucial role in improving coverage made with nozzles labeled for Xtendimax^™, Engenia^™, and Enlist One^™ applications. Selecting a pressure towards the upper end of the allowable pressures on these herbicide labels will improve coverage and should increase efficacy of these herbicides. 

Many national and international organizations (e.g. US EPA, USDA, FAO and EPPO) have stated that the efficacy of plant protection products, including herbicides, would be fully evaluated if the potential risks of the product have been also evaluated. This statement imposes the need for assessments in terms of agronomic sustainability, which is mainly focused on the elimination of the herbicide resistance risk. The amount of seed production that enters the soil seedbank over a number of seasons will affect the sustainability of any weed control strategy. Therefore, knowledge of weed seed production, which is an important element of weed population dynamics, is required for the development of improved weed management strategies and the minimization of herbicide resistance risk. To understand what data are collected in trials involving herbicide performance evaluations, the journals Weed Science and Weed Technology were surveyed for the last 40 years. Survey results exposed 3,500 relevant articles/trials which are mostly citing 9 weed species, 14 active ingredients within 11 herbicide action groups and 5 major crops. Most of these species, including pigweeds, foxtails, wild oat, and large crabgrass, have developed multiple herbicide resistance in many herbicide products, crops, and locations. The natural, spontaneous mutation rate for a single gene is estimated between 1:10⁵ and 1:10⁶ per generation. Given the large fecundity potential of most weed species particularly those that have developed multiple herbicide resistance, there is a probability that at least one seed could contain the mutation for herbicide resistance. Palmer amaranth, for example, has been reported to produce 600,000 seeds plant^-1, foxtails 28,000 seeds plant^-1, wild oat 200 to 1500 seeds plant^-1, and large crabgrass 120 to 7000 seeds plant^-1. A meta-data analysis including data filtering and multivariate analysis showed that seed production was evaluated in only 81 articles the association of which was found to be with biomass and to lesser extent with density but not with visual rating, which was associated most with weed density; a collinear association. Knowledge on herbicide efficacy through accurate and representative evaluation processes including seed production, which is crucial to understanding population dynamics and predicting the evolution of herbicide resistance, secures the effectiveness of weed management systems.

The recent introduction of new dicamba herbicide formulations has led to increase attention and focus on Drift Reduction Adjuvants (DRAs). DRAs are adjuvants that lower the volume fraction of driftable droplets (≤ 150 μm) compared to a pesticide alone and are designed to reduce the chance of off target movement of the pesticide being applied.  Most DRAs are polyacrylamide or polysaccharide based. New EPA regulations mandate that a DRA is required to be included into the spray application solution when a new dicamba herbicide formulation is tank-mixed with certain tank-mix partners. The new DRA requirement was initiated to reduce the chance of off target movement of new dicamba formulations.

Shear is defined as a strain in the structure of a substance produced by pressure, when its layers are laterally shifted in relation to each other. In the presence of shear, the bonds that hold polymers together can be broken, causing a change in the physical structure of the polymer. This change in the physical structure could change the drift reduction capabilities of a DRA.

To determine a DRAs susceptibility to shear, three DRAs were exposed to a pumping system and the volume fraction of the driftable droplets was measured after a specific number of passes through the pumping system. Samples were collected after 0, 10, 25, and 50 passes through a closed-loop pumping system. The samples were then sprayed and measured in a low speed wind tunnel. At 0 passes, all DRAs reduced the volume fraction of driftable droplets compared to the tank mx of dicamba and glyphosate alone. As passes through the closed-loop pumping system increased, the polyacrylamide products failed to reduce the driftable droplet reduction, however the polysaccharide DRA continued to reduce the volume fraction of driftable droplets after all passes through the pumping system. This data concludes that polyacrylamide DRAs could begin to lose the ability to reduce driftable droplets after passing thorough a pump in an agricultural sprayer. 

One of the most utilized herbicides in the world is 2,4-D which came to market as an herbicide in 1945.  Traditional 2,4-D formulations are dimethyl amine salt (DMA) of 2,4-D and low volatile (LV) ester of 2,4-D. New low loaded (1-3 lbs/gallon) 2,4-D acid formulations have recently come to market. A new higher loaded formulation, AQ 990 (4lbs/Gallon), has been developed. Two newer high loaded formulations with drift reduction technologies in the formulations are now being developed (AQ1175 DRT1 and AQ1181DRT2). Two drift reduction systems have been used (DRT1 and DRT2). Both drift reduction systems have also been used to evaluate a new 5lb/gallon dicamba acid formulations (AQ1177DRT1 and AQ1179DRT2).  Trials were conducted with the four formulations to study spray droplet size, physical drift, volatility, and efficacy compared to commercially available standards.  Results indicate that significant new advances in formulation can be made with some older herbicides to make them compatible with modern weed control needs.

Soluble phosphate availability is a major limiting factor for plant growth, development and yield. To assure a constant phosphorous supply, plant employ both high- and low-affinity phosphate acquisition mechanisms. Phosphonomethyl glycine (glyphosate) is an herbicide widely used throughout the world, and previous studies have suggested that glyphosate can be transported across the plasma membrane via phosphate transporters in herbaceous species. The effects of phosphate status on glyphosate uptake was investigated in the tree Eucalyptus grandis. Eucalyptus grandis putative phosphate transporters showed differential gene expression in leaves and roots in response to phosphate deficiency. Overall, the expression of high-affinity phosphate transporters increased in phosphate starvation conditions, particularly in roots. ¹⁴C-glyphosate was absorbed at a higher rate and translocated faster in phosphate-starved plants compared to control plants grown in phosphate replete conditions. In leaf mesophyll protoplast assays, rapid uptake of ¹⁴C-glyphosate into protoplasts was observed, and addition of NaH₂PO₄ to the assays competed with ¹⁴C-glyphosate uptake. This indicates one mechanism of glyphosate uptake is via phosphate transporters. These results have implications for best management practices for weed control, and glyphosate application under phosphate application regimes.  

Intensive use of pesticides contributes to watercourses contamination and runoff is an important process involved in movement of herbicide molecules and their metabolites from fields to surface water. In lowland areas - that are close to watercourses - and where soybean was introduced for crop rotation with rice, the use of pre-emergence herbicides like as S-metolachlor may facilitate surface water contamination by herbicides runoff. To mitigate possible environmental impacts the use of vegetative filter strip (VFS) is a promising alternative. Therefore, the objective of this work was to evaluate the efficiency of VFS composed by bermudagrass (Cynodon dactylon) and sudangrass (Sorghum sudanense) in S-metolachlor and metolachlor oxalic acid (MOA) retention. The experiment was conduced in the field during the 2015/2016 seasons. The field soil is classified as yellow Ultisol (sandy loam texture and with 6% of declivity). The plot measured 22m² (10x2 m cultivated with soybean, and 2x1m with the VSF). The S-metolachlor was applied at the rate of 1.65 kg a.i. ha^-1. At the end of each plot was installed a storage recipient to collect the water moving through the VFS. After each rainfall, water collection was performed, and the amount of runoff water was measured. An aliquot was separated, for detection and quantification of the herbicide and the metabolite, which was performed by LC-MS/MS. VFS reduced on average 80% of surface water runoff compared to treatment without VSF. The total amount of S-metolachlor runoff in the four rainfalls was 0.21% of the total applied (3.5 g a.i ha^-1) in the treatment without VFS. However, in plots with VFS, was observed S-metolachlor runoff reduction of 89% using sudangrass, and 96% using bermudagrass. For the metabolite (MOA) the reduction was similar, 77% on sudangrass and 94% in bermudagrass. The concentration of herbicide and metabolite was not significantly different among treatments. The main difference was the amount of water runoff that was reduced in the treatments with VSF. Therefore, reduction in water flow was the main mechanism that contributed for herbicide runoff reduction. Thereby, VSF use is an efficient alternative to reduce S-metolachlor and MOA transport to water courses.

Glyphosate is the most commonly used herbicide in the world due to its wide-spectrum of weed control and popularity of glyphosate-resistant (GR) crops. It is possible that concentrations of glyphosate and its metabolite aminomethylphosphonic acid (AMPA) might have increased in the soil due to the continuous use of glyphosate over last two decades in the Midwestern United States. Recently, more emphasis has been put on the use of cover crops for soil health benefits and as a part of integrated weed management approaches. Though glyphosate has no soil residual activity and has a short half-life, it is not known whether increased soil concentration of glyphosate or its primary metabolite AMPA may injure cover crops especially when the soil water content is at field capacity. The objective of this study was to evaluate the impact of increased concentration of glyphosate or AMPA in silty clay soil on commonly planted cover crop species in the Midwest including winter pea (Pisum sativum), crimson clover (Trifolium incarnatum), hairy vetch (Vicia villosa), cereal rye (Secale cereale), and winter wheat (Triticum aestivum). Greenhouse studies were conducted with glyphosate or AMPA thoroughly mixed with the soil at varying rates including 0, 3.5, 7, 14, 35, 70, and 105 mg per kg of soil (which is equivalent to 0, 2.7, 5.4, 10.8, 26.9, 53.8, and 80.7 kg per ha). Separate experiments were conducted for glyphosate and AMPA with four replications and repeated in time. The study was also conducted in a hydroponic setup using 100 ml test tubes to grow pre-germinated pea and cereal rye seedlings with glyphosate or AMPA applied at 0, 7, 35, 70 mg per 100 ml of water (equivalent to 0, 53.8, 269, 538 kg per ha). Additional, studies were conducted to test the germination of aforementioned cover crop species in glyphosate or AMPA solution (70 mg per 100 ml of water) and compared with control (treated with water). The data were subjected to linear regression and ANOVA in R. The results indicated that glyphosate or AMPA applied in soil or hydroponics setup did not reduce the aboveground or root-biomass production in any of the cover crop species tested. However, glyphosate reduced the germination of cereal rye, wheat, and hairy vetch by 48, 66, 75% compared to AMPA or water control, respectively. Overall, the results suggested that increased concentrations of glyphosate or its metabolite AMPA will not have an impact on the biomass production of tested cover crop species at the rates tested in this study.

Previous research has shown herbicides can move from the intended site laterally via subsurface soil pathways; however, this has been minimally investigated in turfgrass systems. Field research (Raleigh, NC) was conducted to elucidate the effect of soil volumetric water content (VWC) at application, on subsurface lateral herbicide movement. White clover (Trifolium repens L.) plots were established downslope of hybrid bermudagrass [Cynodon dactylon (L.) Pers. × Cynodon transvaalensis Burtt-Davy] plots treated with aminocyclopyrachlor (AMCP; 105 g ae ha^-1), fluroxypyr (210 g ai ha^-1), or metsulfuron-methyl (MET; 42 g ai ha^-1). Each bermudagrass-white clover plot represented a unique soil VWC condition at herbicide application including wilting point (15% v/v), field capacity (35% v/v), or saturation (48% v/v). Data suggest soil VWC at application differentially affected herbicide subsurface lateral movement. Fluroxypyr, the least water soluble compound, did not affect white clover growth across all soil VWC, while AMCP and MET movement increased with increasing soil VWC. Neither herbicide reduced white clover aboveground biomass when applied at wilting point, while applying AMCP at saturation resulted in 92 to 100% biomass reduction at all downslope intervals (0 to 1.5 m from treated surface), and MET resulted in 45 to 64% reduction to a 0.6 m downslope distance. Applying all herbicides at wilting point or field capacity resulted in ≤11% biomass reduction beyond 0.9 m, suggesting turfgrass managers can minimize off-target movement by monitoring soil moisture prior to application. 

Dicamba-tolerant soybeans have been used in recent years to improve weed control of glyphosate resistant weeds by the POST application of dicamba.  Much interest has been generated by the potential for off-site movement of dicamba, which has resulted in discussions related to the causes of this phenomenon.  This report details two field studies, including optimizing field and laboratory conditions to enhance sensitivity and accuracy of dicamba sampling and analysis. The first study showed that diglycolamine salt of dicamba was more likely to volatilize from green plant surfaces compared to either tilled bare ground or dead plant material.  The second study showed that newer formulations of dicamba had slightly lower volatility under field conditions compared to the diglycolamine salt formulation.  All samples showed dicamba above detectable levels, and the typical pattern of dicamba arising from treated plots was correlated to temperature.  Humidomes have been proposed by other researchers as a test system to compare various attributes of herbicide behavior under more controlled conditions.  This report will detail our efforts to conduct “modified” humidome studies. Fixed experimental parameters include herbicide formulation (DGA, DGA + vapor grip, or BAPMA) and surface condition (dry soil, wet soil or soybean foliage).  Random variables focused on differences in temperature on herbicide behavior under greenhouse conditions, with studies conducted over a sufficient time interval to encompass temperatures expected to be encountered under field conditions.  A diurnal cycle of temperature was also evident in the experiments, which ranged in duration from 1.5 to ~ 4 days in length.  Dicamba formulation showed significant differences in detected amounts.  Temperature appeared to be a major factor driving the amounts of herbicide detected.  A surprising finding was the apparent detection of DGA dicamba, as well as the acid moiety of dicamba in the samples.  All treatments based on the DGA salt (Clarity and Xtendimax) showed consistent detection of a chromatographic peak co-eluting with the DGA salt of dicamba, while none of the BAPMA treatments ever had the corresponding peak. Possible explanations for this are direct volatilization of DGA salt of dicamba, or possibly the dicamba is moving co-incident with soil or plant particles.

The volatility of dicamba from soil surfaces is known to be influenced by a range of atmospheric factors such as temperature, relative humidity (RH), and wind speed. The impacts of soil properties on dicamba volatility, however, are not well characterized. The objective of this study is to investigate potential impacts of soil salt content, measured as electrical conductivity (EC), on dicamba volatility from soil. A field soil was collected from the Arkansas Agricultural Research and Extension Center in Fayetteville, Arkansas and native EC, pH, texture, and organic matter content were characterized. In addition to native EC (470 µS cm^-1), soil EC was adjusted to high (7000 µS cm^-1) and medium (3200 µS cm^-1) levels by adding CaCl₂. Soil with known moisture content was added to plastic trays and sprayed with Engenia herbicide at 1.12 kg ae/ha using a track sprayer. Immediately following application, two potted early vegetative growth stage soybean and two potted early vegetative growth stage tomato were placed on the trays, and enclosed with a plastic dome lid. A humidome containing low EC soil with no dicamba application was maintained as a control. Humidome units were placed in a growth chamber with RH and temperature control. Air was pulled through the humidomes under vacuum (2 L/min) for 24 hours and passed through a filter apparatus containing polyurethane foam (PUF) and XAD-2 resin to capture volatilized dicamba. After 24 hours, indicator plants were transferred to a greenhouse and visually assessed 2 and 4 weeks after exposure for dicamba symptoms. Air filter materials were collected for dicamba extraction and analysis. Identifying soil properties that influence dicamba volatility will help scientists, farmers, and policy-makers steward the use of much-needed weed control tools.

Herbicide movement in the atmosphere after application has become an important consideration as new herbicide uses are conducted in agronomic crops. While much interest has been on the herbicide dicamba, this project focused on the similar herbicide 2,4-D. Two field studies were conducted in Knoxville Tennessee in 2017 to compare Enlist Duo (choline salt of 2-4-D formulated containing glyphosate) to Weedar 64 (amine salt of 2,4-D, treatment applied tank mixed with glyphosate). The presentation will detail various field and laboratory procedures that were used to quantify 2,4-D concentrations at two sampler heights, 0.3 m and 1.3 m. Sampling interval was for 0 to 24 and a separate 24 to 36 hr after a normal field application of a full labeled rate of each chemical. 2.4-D concentrations were similar between the two formulations and there were positive detections at all sampling intervals, including the 24 to 36 hr interval. Concentrations were greater at the lower 0.3 m sampling height compared to the 1.3 m height. Concentrations expressed as nanograms per cubic meter range from 1 to 2.6 ng/m;. Of the two field studies the study with higher temperatures tended to have higher levels of 2,4-D detections

Benzobicyclon is a new pro-herbicide from Gowan for use in U.S. rice (Oryza sativa). In the presence of water, benzobicyclon is converted into the potent 4-hydroxyphenylpyruvate dioxygenase inhibitor benzobicyclon hydrolysate and this will be a novel site of action for weed control in U.S rice. Benzobicyclon tolerance in rice was recently discovered to be conferred by the HPPD INHIBITOR SENSITIVE 1 gene (HIS1). The expression profile of the HIS1 gene is not ubiquitous and further research is needed to characterize its expression pattern spatially and temporally. An experiment was conducted to determine the relative gene expression of HIS1 in different tissue types of the tolerant inbred rice cultivars RoyJ, LaKast, and Diamond, and in the tolerant hybrids RT CL XL745 and RT XL753. Results will be presented.       

Abstract：To instruct precise application of pesticide and reduce the amount of pesticide to control the barnyard grass Echinochloa crusgalli in direct seeding rice field, the sensitivity of barnyard grass to bispyribac-sodium and cyhalofop-butyl and the virulence of the two herbicides to barnyard grass at four different leaf stages were determined by the whole plant bioassay method. The result suggested that fresh weight of barnyard grass at 1 to 4-leaf stage decreased more than 70% at the recommended bispyribac-sodium dose, while decreasing about 37.25% at 5 to 6-leaf stage. Fresh weight of barnyard grass at 1 to 6-leaf stage all reduced more than 80% at recommended cyhalofop-butyl dose. The ED₅₀ of bispyribac-sodium on barnyard grass at 1~1.5, 2~2.5, 3~4, and 5~6 leaf age was 11.39, 17.03, 21.15 and 56.07 g•hm^-2  respectively; The ED₅₀ of cyhalofop-butyl at the four leaf stage was 11.93, 13.72, 31.70 and 34.00 g•hm^-2  respectively. The sensitivity of barnyard grass at different leaf stages to bispyribac-sodium and cyhalofop-butyl was different. Recommended bispyribac-sodium dose and 1/4 to1/2 recommended cyhalofop-butyl dose was recommended to control barnyard grass at 1~4 leaf stage.

Glyphosate-resistant Italian ryegrass (Lolium multiflorum) is widespread in annual and perennial crops in California. Two glyphosate-resistant populations from an almond orchard (MR1) and an alfalfa field (MR2) in Glenn County were also not controlled with a labeled field rate of sethoxydim (515 g ai h^-1), glyphosate (867 g ae h^-1), imazamox (45 g ai h^-1) and paraquat (560 g ai h^-1) in a greenhouse study. The objectives of this study were: (1) to determine the magnitude of herbicide resistance in these Italian ryegrass populations, (2) to evaluate alternative herbicide options, and (3) to characterize the physiological and molecular bases of resistance. Whole plant bioassays were conducted to determine the response of the resistant populations to glyphosate, paraquat and ACCase inhibitors over a range of rates and to evaluate cross resistance to the labeled field rates of four ACCase-inhibiting herbicides from two different chemical families [aryloxyphenoxypropionate (fops) (e.g. fenoxaprop-P-ethyl, cyhalofop-butyl and fluazifop-P-butyl) and cyclohexanedione (dims) (e.g. clethodim and sethoxydim)]. All herbicide treatments were applied to 3- to 4-leaf stage plants. A research track sprayer fitted with a boom equipped with a flat fan E8001 nozzle was calibrated to deliver 20 GPA of herbicide at 40 PSI. Dose-response experiments revealed that the levels of resistance to glyphosate, sethoxydim and paraquat were ˃ 45, ˃ 122, and ˃ 20-fold, respectively, for the MR1 population and ˃ 24, ˃ 93, and ˃ 4-fold, respectively for the MR2 population. Both resistant populations were cross-resistant to aryloxyphenoxypropionate (fops) herbicides. MR1 plants were also cross-resistant to clethodim (146 g ai h^-1) whereas, interestingly, all MR2 plants were controlled (93 to 100%) with the labeled field rate of this herbicide. Based on leaf disc assays, the shikimic acid level in Sus plants was significantly greater than in MR plants 32 h after light pretreatments. Sanger sequencing of a region of the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene revealed Pro₁₀₆-to-Ala and Pro₁₀₆-to-Thr substitutions in MR plants. This amino acid substitution is known to cause target-site resistance to glyphosate in ryegrass species. EPSPS gene copy number and expression level were similar in plants from the Sus and MR populations. Alignment of ACCase gene sequences in plants from Sus and MR populations revealed a missense mutation, Ile₁₇₈₁-to-Leu, in both MR populations, which has been previously found to endow resistance to ACCase-inhibiting herbicides in several weed species, including ryegrass. Sus plants had less ¹⁴C-paraquat radioactivity in tissues above the treated leaf and greater radioactivity in tissues below the treated leaf than MR plants, which was also visualized via phosphorimaging. This research reveals that multiple herbicide resistance in Italian ryegrass populations of California is due to both target-site and non-target-site resistance mechanisms.

Extensive herbicide usage has led to the evolution of resistant weed populations that cause substantial crop yield losses and increase production costs. The multiple herbicide resistant (MHR) Avena fatua L. populations utilized in this study are resistant to members of all selective herbicide families, across five modes of action, available for A. fatua control in U.S. small grain production, and thus pose significant agronomic and economic threats. Resistance to ALS and ACCase inhibitors is not conferred by target site mutations, indicating that non-target site resistance (NTSR) mechanisms are involved. Understanding the inheritance of NTS MHR is of upmost importance for continued agricultural productivity in the face of the rapid increase in resistant weed populations worldwide. Since few studies have examined the inheritance of NTSR in autogamous weeds, we investigated the inheritance and genetic control of NTSR in the highly autogamous, allohexaploid species A. fatua. We found that NTSR in MHR A. fatua is controlled by three separate, closely-linked nuclear genes for flucarbazone-sodium, imazamethabenz-methyl, and pinoxaden. The single-gene NTSR inheritance patterns reported here contrast with other examples in allogamous species, and illustrate the diversity of evolutionary responses to strong selection. 

Synthetic auxin herbicides have been shown to bind to F-box proteins in the Skp-Cullin-F-box (SCF) ubiquitin ligase complex causing degradation of AUX/IAA repressor proteins. Transport inhibitor response 1 (TIR 1) from Arabidopsis was the first F-box protein to be shown to bind the natural auxin indole acetic acid (IAA) and the synthetic auxin 2,4-D. A homolog of TIR1, auxin signaling F-box protein 5 (AFB5), has been shown to be the preferred F-box protein for binding of the pyridinecarboxylic acid synthetic auxin herbicide picloram. Binding of synthetic auxin herbicides to TIR1 and AFB5 utilizing surface plasma resonance with the DII degron repressor peptide revealed differential binding affinities of auxin chemistry classes between these two F-box proteins.

The degradation of AUX/IAA repressor proteins via the SCF ubiquitin ligase complex leads to auxin specific gene expression. A separate study employed microarray analysis to identify gene expression that is regulated in Arabidopsis by treatment with synthetic auxin herbicides. A comparison was made of gene expression changes in Arabidopsis plants treated with 2,4-D or Rinskor™ active (florpyrauxifen-benzyl). Differential gene expression analysis found 215 genes differentially expressed at 4 HAT, 1457 genes differentially expressed at 24 HAT, and 58 genes differentially expressed at both time points between 2,4-D and Rinskor™. Gene ontology analysis of significantly differentially expressed genes between 2,4-D and Rinskor™ identified differences in various metabolic processes.

The results of the research could have implications on resistance management strategies as well as new herbicidal target site identification.

Compensatory evolution of abiotic stress tolerance via adaptive selection to weed management is a considerable threat to agricultural systems. While most selective processes tend to reduce the standing genetic variation within weedy populations, it is also possible to select inadvertently for background increases in fitness. The Echinochloa genus has a history of co-evolution within rice due to substantial biological similarity between this weed and rice. The crop and weed have similar morphology and both thrive in wetlands. In Arkansas, quinclorac is a primary herbicide used to manage Echinochloa despite the presence of resistance in approximately 25% of the populations. Using a transcriptomic approach through RNA-sequencing, two Arkansas populations of Echinochloa colona identified as quinclorac-resistant (ECO-R) and susceptible (ECO-S) were investigated for potential differences in abiotic stress tolerance based on their underlying genomic differences. Previously, our research group used this analysis for identifying the underlying mechanism of resistance to quinclorac in ECO-R via enzymatic detoxification with the UGT75D1 glycosyltransferase. Basal level gene expression without quinclorac and comparative differences following treatment were examined. Without quinclorac, ECO-R had elevated expression of several key components of the photosynthesis, carbon assimilation, and fatty acid biosynthetic processes. Trehalose biosynthesis, implicated by the elevation in TPP and TPS enzyme transcripts, was highly induced. Trehalose has been reported to play a role in abiotic stress mitigation. The elevation in TPP and TPS in ECO-R compared with ECO-S was validated via qPCR. The expression of these genes remained high, but similar to that of the non-treated check, after quinclorac treatment of ECO-R. Phenotypic differences in drought stress have been observed in ECO-R. Another gene, ALPL1, was also expressed higher in ECO-R than in ECO-S without quinclorac treatment but was expressed even more in response to the herbicide. ALPL1 is a gene suppressor antagonist protein induced by abiotic stress. This protein has the potential to activate downstream gene products that may include the trehalose enzymes or xenobiotic detoxification genes. We have evidence for increased detoxification of quinclorac in ECO-R compared to ECO-S. Collectively, the elevation and presence of these proteins indicate that ECO-R may be predisposed to better tolerate adversity from abiotic stress due to the selection imposed by herbicides used for management. Further investigation into the biochemical components of these mechanisms is required. 

Concerted enrichment of gene networks in multiple-resistant Echinochloa colona

Nilda Roma-Burgos^*,1, Christopher E. Rouse¹, Christopher A. Saski², Rooksana E. Noorai², Vijay Shankar², and Amy L. Lawton-Rauh²

¹ Department of Crop, Soil, and Environmental Sciences, University of Arkansas, Fayetteville, AR 72701, USA 

²Department of Genetics and Biochemistry, Clemson University, Clemson, SC, USA

Echinochloa species are among the most problematic grass weeds globally in rice and various upland crops. resistance to herbicides occurs at a higher frequency in grasses versus other monocots and dicots. Across the globe, E. crus-galli is second only to Lolium rigidum in the number of herbicide modes of action (10) to which it had evolved resistance. Of greater concern is the increasing incidence of multiple resistance within individual plants. One-third of E. colona populations in Arkansas are multiple-resistant. We analyzed the transcriptome of a multiple-resistant E. colona (ECO-R) in response to cyhalofop, glufosinate, propanil, and quinclorac herbicides to identify candidate genes involved in multiple resistance to herbicides and begin to understand biochemical pathway interactions in multiple-resistant plants. ECO-R has high resistance to propanil and quinclorac and elevated tolerance to cyhalofop and glufosinate. ECO-R and ECO-S (susceptible) seedlings with one fully extended leaf were treated with field-recommended doses of these four herbicides. Leaf tissues were harvested 24 h after treatment for RNA sequencing and transcriptome analysis. High resistance to propanil can be attributed to enhanced detoxification by CYP709B2 and CYP72A15, complemented by enhanced production of GSTU17 and other glycosyltransferases. Propanil also induced hypersensitive-response genes more in ECO-R, which could seal off affected organs and tissues, allowing survival and recovery. The application of quinclorac to ECO-R induced the expression of CytP450 genes including CYP709B2 and CYP71A(4,8) as well as transferases UGT75D1 and three others. Interestingly, ECO-R had only about 10% more metabolites of quinclorac than ECO-S, indicating the presence of other coping mechanisms. The high induction of ALPL1-like protein in treated ECO-R indicates supporting mechanism for increased DNA transcription to effect increased production of key functional and regulatory proteins. One notable aspect is the enrichment of the trehalose metabolism pathway in non-treated ECO-R compared to non-treated ECO-S. Nitrate metabolism (assimilation and transport) was also enriched. This was accompanied by increased transcripts for photosynthesis-associated proteins, carbon assimilation genes, fatty acid synthesis genes, and enzymes for trehalose biosynthesis. Thus, ECO-R is intrinsically more resilient to these stresses than ECO-S. The role of trehalose in stress adaptation (and even signaling) in plants is well characterized. With the components of nitrogen assimilation, photosynthesis, carbon assimilation, carbohydrate metabolism, lipid synthesis, and xenobiotic detoxification enriched in concert, ECO-R is able to survive extremely high doses of propanil and quinclorac. These processes in turn can impart elevated tolerance to an ACCase-inhibitor like cyhalop or a nitrogen-assimilation inhibitor like glufosinate. ‘Priming’ for resistance to herbicides with different modes of action can emanate from this.

Glyphosate resistant giant ragweed across north America presents two very distinct phenotypes in their response to glyphosate.  The most unusual of these two responses is a very rapid necrosis of mature leaves (within 24 hours) caused by a rapid release of H₂O₂ which is well recognized for its very destructive effects on plant tissue.  How this very unusual response to glyphosate is triggered is still unknown, although deep sequencing over the time course of a treatment has resulted in several candidate genes that may be involved.  Interestingly, this necrosis response is not observed in young tissues or meristematic points in the plants; it appears that mature leaves loaded with glyphosate are “amputated” from the plant to protect new meristematic tissue.  The second phenotype is called a “slow response” which takes up to a week or two to recover from a glyphosate treatment, although the plants never die.  Careful studies of classical mechanisms of glyphosate resistance do not account for this phenotype.  On-going research will be directed at more complex potential aspects of this source of resistance.

Chloris truncata and Chloris virgata are warm-season C4 grass that are problematic to control in chemical fallows and along roadsides in Australia as they are poorly controlled by normal rates of glyphosate used. Despite this, both species have evolved resistance to glyphosate. Populations of both species were investigated to determine the mechanisms of resistance to glyphosate. In C. virgata, all 4 resistant populations examined had a mutation at Pro 106 in EPSPS. A Pro 106 Leu substitution was identified in the most resistant population and Pro 106 Ser mutations in the other three populations. In C. truncata, resistance was the result of gene amplification of EPSPS, with resistant populations containing 32 to 48 times the number of EPSPS genes as susceptible populations. C. virgata is a diploid species, whereas C. truncata is polyploid. This has likely influenced the mechanisms of glyphosate resistance selected.

Amplification of EPSPS gene, the molecular target of glyphosate conferring glyphosate resistance in Amaranthus tuberculatus has been documented. We recently found that the EPSPS copies amplified at the native locus and on an extra chromosome in glyphosate-resistant A. tuberculatus. In eukaryotic (e.g., yeast) tumors, an increase in gene copy number is often associated with changes in the number and structure of chromosomes. However, stress-induced genome alterations including chromosomal changes is not completely understood in plants. In this research we identified the extra chromosome which was found in GR A. tuberculatus is a ring chromosome termed Extra Circular Chromosome carrying Amplified EPSPS (ECCAE). Furthermore, the ECCAE is heterochromatic, harbors four major EPSPS amplified foci and is sexually transmitted to 35% of the progeny. Two highly glyphosate-resistant (HGR) A. tuberculatus carrying the ECCAE with a chromosome constitution of 2n=32+1 showed somatic cell heterogeneity. We hypothesize that the ECCAE in the somatic cells of HGR A. tuberculatus plants underwent breakage-fusion-bridge (BFB) cycles to generate the variation found in somatic cells, including de novo EPSPS gene integration into chromosomes. Plants with such chromosomal alterations survive glyphosate selection during the sporophytic phase and are plausibly transmitted to germ cells leading to durable glyphosate resistance in A. tuberculatus. This is the first report of early events in aneuploidy triggered de novo chromosome integration by a yet unknown mechanism, which may drive the rapid adaptive evolution of herbicide resistance in crop plant weeds.

In the last decade, waterhemp (Amaranthus tuberculatus) has evolved resistance to 2,4-D and HPPD inhibitors in multiple states across the midwestern US. Two populations resistant to both chemistries, one from Nebraska (NEB) and one from Illinois (CHR), were studied using an RNA-seq approach to identify the genes responsible for resistance. In this study, cDNA libraries were generated and sequenced for eight replicates of herbicide-resistant (HR) and herbicide-sensitive (HS) plants from the two populations (32 total plants sequenced). Using both a waterhemp transcriptome assembly and a high-quality grain amaranth (A. hypochondriacus) genome as references, differential gene expression analysis was conducted to identify genes that were significantly over- or under-expressed in HR compared to HS. When these differentially expressed genes (DEGs) were mapped back to the grain amaranth genome, physical clustering of the DEGs was apparent at gene expression “hotspots” along several of the 16 grain amaranth scaffolds. SNP calling was performed across all 32 samples to look for condition-specific (CS) variants and all statistically significant CS variants were also mapped to the A. hypochondriacus genome. In almost every one of the expression hotspots, allele-specific expression was also observed, allowing for the development of allele-specific assays to diagnose resistance problems in fields. Further work has begun to identify any potential cis-acting regulators or epigenetic variation leading to this localized difference in expression between R and S plants. These allele-specific expression hotspots are a potentially useful tool in future RNA-seq studies to narrow down the regions of true regulatory control leading to resistance, and they may also provide insights into the evolution of herbicide resistance in weeds.

With several new auxin-herbicide-tolerant crops being developed and commercialized, a new emphasis must be placed on the understanding and control of auxin-resistant weed species as part of ongoing stewardship efforts to prevent or mitigate intensified selection pressure for resistant biotypes. Although our understanding of auxin perception, signaling, and metabolism in model plant species has advanced greatly in recent decades, to date there have been no reports of the identification of resistance mechanism(s) for any field-derived auxin-resistant weed species.  To address this issue, we have undertaken the study of one economically-important dicamba-resistant biotype of Kochia scoparia initially isolated from a field in Western Nebraska, and have successfully identified the mechanism of resistance. Our work indicates that the auxin resistance mechanisms identified to date in model plants such Arabidopsis thaliana can translate to weed species, offering insight into the future of auxin-herbicide resistance management. 

Cost effective Integrated Weed Management (IWM) involves combinations of non-chemical and chemical technologies adapted to the crops, the weed diversity, and environmental factors. When low diversity is present in the cropping system, including herbicide use, weed-herbicide-resistance can evolve with the results of poor weed control and significant crop losses. Herbicide-resistance has spread worldwide and concern a high number of weed species. It is of particular concern in certain locations in cereal, soybean, maize, cotton, and rice cropping systems. Among several resistance mechanisms, herbicide detoxification (EMR, enhanced metabolic resistance) can confer resistance to a broad spectrum of chemical classes representing one or several modes of action.  Molecular elements involved in herbicide detoxification are still poorly characterized and understood.  Our recent data using RNA-Seq transcriptome analyses to identify genes conferring EMR resistant populations in rye-grass and Amaranthus populations resistant to different herbicides will be summarized and compared. Among genes overexpressed in the herbicide-resistant plants compared to the sensitive plants, several, including CytP450s, GSTs, and GTs, were validated by genetics (co-segregation with the resistant phenotype), and functionally, by biochemical activity on the herbicide compounds. For some of the characterized genes, structure activity was performed on a range of herbicides representing several chemical classes. In addition data on protein modelling and herbicide docking in the active site of detoxification enzymes will be presented.  Not all overexpressed genes co-segregating with the resistance phenotype were found to be able to detoxify the herbicide(s) showing poor activity on a given resistant population. Possible resistance evolution mechanisms will be discussed in comparison to resistance evolution in other organisms, in particular in insects resistant to insecticides.

Amplification of the EPSPS gene and overexpression of EPSP synthase have been implicated as the causal mechanism driving resistance to glyphosate in Amaranthus palmeri. To determine the genetic drivers of glyphosate detoxification, we treated Amaranthus palmeri plants from glyphosate sensitive and resistant populations with water, water + Tween 20 (0.5%), or water + Tween 20 (0.5%), + glyphosate potassium salt (4.4 mg/ml), and sampled RNA at 0, 4, and 24 hours, respectively.   The de novo assembly of the first Amaranthus palmeri transcriptome presented a total of 68,181 putative genes, of which 38,229 produced a significant match to the SwissProt protein database.   Resistant and sensitive plants displayed drastically different diverse arrays of both up and down regulated gene expression profiles. Glyphosate was the main ingredient influencing gene expression. At the 4h time point, the resistant biotype had only 311 gene upregulated and 151 downregulated after exposure to surfactant + glyphosate; whereas the sensitive biotype had 2,296 genes upregulated and 858 downregulated.  At 24 hours, the resistant biotype had 4,654 upregulated genes, and 5,045 downregulated; and the sensitive had 7,885 upregulated and 8790 downregulated.  The predominant genes and biochemical pathways being upregulated as early and late responses to glyphosate exposure involved metabolism, defense, transport, detoxification, and histone modification.  These results indicate that glyphosate resistance may involve a sophisticated concert of genes and biochemical pathways to sequester and mitigate the toxic effects in the plant in addition to EPSPS amplification. 

Protoporphyrinogen oxidase (PPO)-inhibitor resistant Palmer amaranth (Amaranthus palmeri S. Wats.) is widespread in Arkansas. A previously conducted genotypic screen on Arkansas accessions found the dominant target-site resistance mechanisms to be the ΔG210 deletion or the R128G mutation in the PPX2 gene, with several accessions segregating for both mutations. One accession from Crittenden County was confirmed using TaqMan quantitative PCR assay to contain both the ∆G210 and R128G mutations. Out of the two known mutations in the PPX2 gene, R128G was recently reported and information regarding the level of PPO-inhibitor resistance it provides relative to ΔG210 is lacking. Therefore, the objective of the current study was to compare the level of fomesafen resistance conferred by the ΔG210 deletion and R128G mutation either alone or in combination (R128G/ R128G, R128G/∆G210, or ∆G210/∆G210). We hypothesize, based on ancillary observations that the single point mutation such as R128G is weaker than the codon deletion ΔG210. The plants that contain both resistant alleles from the Crittenden County accession were allowed to cross-pollinate and produce seed in the greenhouse. To compare the level of resistance conferred by each mutation alone or in combination, the genotyped progeny from the cross were subjected to increased rates of fomesafen and results will be presented. 

A Novel Mutation in Protoporphyrinogen Oxidase IX (PPO) Confers Broad Resistance to PPO inhibitors

Gulab Rangani,¹ Reiofeli A. Salas-Perez,¹ Raphael A. Aponte,² Michael Knapp,² Ian R. Craig,² Thomas Mietzner,² Andreas Landes,² Ana Claudia Langaro,³ and Nilda Roma-Burgos^*,1

¹ Department of Crop, Soil, and Environmental Sciences, University of Arkansas, Fayetteville, AR 72701, USA 

² BASF SE, Carl-Bosch-Strasse 38, 67056 Ludwigshafen, Germany

³ Department of Plant Science, Federal University of Viçosa, Viçosa, MG 36570-000, Brazil

Protoporphyrinogen oxidase (PPO)-inhibiting herbicides are used to control weeds in a variety of crops. These herbicides are particularly useful in managing broadleaf weeds (i.e. Amaranthus spp.) that are resistant to other herbicide modes of action such as acetolactate synthase (ALS)-, enol-pyruvyl-shikimate-phosphate synthase (EPSPS)-, and photosystem II (PSII) inhibitors. PPO herbicides inhibit heme biosynthesis and photosynthesis in plants. Amaranthus palmeri evolved resistance to PPO inhibitors lately, almost ten years from the first detection of PPO-inhibitor-resistant A. tuberculatus.  Deletion of PPO-G₂₁₀ was the predominant mechanism of resistance in both species. Two other resistance-conferring PPO mutations have also been reported lately in A. palmeri. While investigating the basis of high resistance to fomesafen in A. palmeri, we identified yet another amino acid substitution of G₃₉₉A in the catalytic domain of PPO2 (numbered according to the protein sequence of A. palmeri). Analysis of GenBank PPO sequence data revealed that G₃₉₉ is highly conserved in the PPO protein family across eukaryotes. Through molecular, computational, and biochemical approaches, we established that the G₃₉₉A mutation in PPO2 causes steric hindrance for binding of several PPO-inhibiting herbicides. This reduces herbicide affinity to the binding site, causing high-level in vitro resistance to different PPO inhibitors of the mutant PPO2 relative to the wild type. This is the first report of an amino acid substitution at this position of a PPO2 gene isolated from a field-selected weed population of any species. The G₃₉₉A mutation is very likely to confer resistance to other weed species under selection imposed by the extensive agricultural use of current PPO-inhibiting herbicides. We also identified two resistance-conferring weak mutations G₁₁₄E and S₁₄₉I in one allele. Single mutants were generated to determine the contribution of each mutation to resistance. Biochemical analysis showed that G₁₁₄E confers low-level resistance to lactofen and oxadiazon while S₁₄₉I confers low-level resistance to these two herbicides as well as to acifluorfen, carfentrazone, fomesafen and fluthiacet. On the other hand, the double mutant confers higher resistance to carfentrazone, fluthiacet, lactofen, and oxadiazon compared to the single mutants, but increased the susceptibility to acifluorfen and fomesafen. These weak mutations may not produce a resistant population in the short term, but may contribute to occasional escapes from PPO-inhibitor-herbicide application when conditions are suboptimal for herbicide activity.

Palmer amaranth (Amaranthus palmeri) is one of the most economically damaging glyphosate-resistant weed, mainly in row production systems in US. Our previous studies have shown that the glyphosate-resistant biotype (R-biotype) do sustain metabolic perturbation immediately (8hr) after the herbicide treatment, but recovers by 36-72 hrs after treatment, possibly due to the abundance of antioxidant machinery that complements EPSPS amplification. We predicted that the observed higher abundance of antioxidants could potentially help the R-biotypes to endure environmental stress. We tested this prediction by documenting the physiological perturbations sustained by resistant and susceptible biotypes (S-biotypes) of A. palmeri when subjected to drought stress following mutiple controlled dry-down experiments. Perturbations in both the primary and secondary metabolite pools were mapped by in-depth global metabolomic approaches. The cellular-level stress manifestation including the ROS production and lipid peroxidation, and the whole plant physiology including carbon assimilation were measured. Experiment was conducted both in illuminated growth-rooms and under natural sunlight in greenhouse. 

Spectral quality of light had a dominant influence on the cellular physiology of A. palmeri, with artificial lighting, irrespective of the overall light intensity, resulting in poor metabolic response. More than 32,000 unique mass features, which were curated to 1503 metabolites, showed significant response across the treatments. In the absence of drought stress, the cellular physiology, and the metabolite pools of A. palmeri were biotype dependent. In general, compared to R-biotypes, the S-biotype had marginally richer secondary metabolite profiles. Although, not in accordance with their EPSPS copy numbers, R-biotypes exposed to drought had significantly higher activities of key regulatory enzymes, which reflected in the metabolite pools. Overall, the metabolite profiles of the three R-biotypes were ~2 times more induced by drought, compared to that of the two S-biotypes. Also, compared to the R-biotypes, both the S-biotypes had approximately four times more compounds downregulated in drought treatment. Irrespective of the treatments, the physiological parameters including carbon assimilation, and stomatal conductance did not differ significantly between the R- and S-biotypes. Compared to the S-biotypes, the R-biotypes had a higher ROS pool, which did not vary with drought treatment. The R-biotypes had a lower overall cellular damage under the drought treatment, resulting in a marginally higher drought recovery.

There is currently no online repository for genomic information dedicated to the field of weed science. Sequencing information is customarily uploaded to the National Center of Biotechnology Information (NCBI) as a means of providing public access to sequence data referenced in peer-reviewed manuscripts. While a new online repository would not be meant to supplant NCBI, it could serve the greater weed science community more specifically. Such online repositories serve to unite scientific communities around a specific biological topic which in turn aids research advance of that area or species. Closely related to the field of weed genomics is the International Survey of Herbicide Resistant Weeds (http://www.weedscience.com) which has developed into a world-wide source for herbicide resistance information and has help to build a community of researchers focused on herbicide resistant weeds. To this end, we have developed an online repository (WeedGenomics) currently available at http://www.weedgenomics.com. WeedGenomics currently has 22 available annoated weed transcriptomes available for download or searchable by keyword or DNA search string. A database of >1600 herbicide related genes from common weed genera are available on WeedGenomics. Sequences are can also be searched for target-site resistance mutations using an online search tool or a separate command line tool can be downloaded from GitHub. WeedGenomics was deployed on on 25 June 2017. This seminar will discuss the future growth and development of WeedGenomics with the goal of developing the site into a hub for the study of weed genetics, genomics, and transcriptomics.

Microbiome organisms (including endophytic as well as phyloplane, rhizoplane and rhizosphere organisms) can degrade environmental xenobiotics including pesticides, conferring resistance to most types of pests.  Cases of herbicide resistance in weeds have been partially documented to be due to microbiome detoxification.  These include ‘problem soils’ and rhizosphere bacteria that protect rice from diclofop methyl. Fungal endophytes were correlated with diclofop-methyl resistance in Lolium multiflorum, and specific endophytic viruses were reported in non-target resistant grasses and one  strain, AMVV1, was not found in any of the herbicide-sensitive biotype plants tested.  No direct causative links were established in either of the cases, as the agents have not yet been used to infect herbicide susceptible biotypes to see whether they confer resistance, nor whether bacterial antibiotic or anti-viral  treatments abolish resistance.  Some cases of metabolic resistance may have been misattributed to weed metabolism, and not to organisms in the microbiome, as few researchers use axenic weeds in studying herbicide metabolism.  Instances of microbiomes evolving herbicide resistance contributing to resistance of their hosts may become more common due the erratic nature of climate change, as microbiome populations typically increase and evolve faster in stressful conditions.  One might predict that this evolution is most likely to occur first with herbicides that also affect the microorganisms’ own metabolisms such as those that inhibit pathways leading to amino acid biosyntheses.  Is it a stretch to assume that they will evolve the capacity to detoxify xenobiotics that do not directly effect them that will save their hosts?  Conversely, microbiome organisms could be engineered to provide crops with needed resistances to herbicides and insecticides, respectively, but there has not been sufficient efficacy to achieve commercial products useful at the field level, even with genetically engineered microbiome organisms.