Onarm research was conducted in 2011 and 2012 to determine the postemergence and residual control by dicamba of glyphosate-resistant (GR) Palmer amaranth (Amaranthus palmeri S. Wats.). Preemergence dicamba at 0, 0.28, 0.56, and 1.1 kg ae ha-1 and 0.07 kg ae ha-1 flumioxazin was applied at 30, 15 and 0 days prior to planting. Postemergence dicamba at 0.28, 0.56, and 1.1 kg ae ha-1 with and without 0.84 kg ae ha-1 glyphosate was applied to 5, 10 and 15 cm Palmer amaranth. In addition, a greenhouse experiment was conducted in 2012 to evaluate and confirm the optimum rate for control of Palmer amaranth with a new formulation of dicamba (BAS 18322H). In the greenhouse, dicamba at 0.14, 0.28, 0.56, 1.1, and 2.2 kg ae ha-1 was applied to 5, 10, and 15 cm Palmer amaranth.

Auxin resistant cropping systems will provide producers with an alternative option for weed management, but with this new technology also comes the concern of off target movement of dicamba and/or 2,4-D to susceptible crops. Research was conducted over multiple site years in order to determine the effect of soybean response to different application timings and rates of 2,4-D and dicamba. 2,4-D was applied at 1X (0.56 kg ae/ha), 1/4X, 1/16X, 1/64X, and 1/256X rates, and dicamba was applied in a separate study at 1X (0.56 kg ae/ha), 1/4X, 1/16X, 1/64X, 1/256 and 1/1024X. All rates were applied at the V3 and R1 growth stages. Greatest yield losses occurred from dicamba applied at the R1 growth stage. Additional studies were conducted to determine at which growth stage soybeans are most sensitive to 2,4-D and dicamba. Greatest yield losses occurred at the late vegetative and early reproductive growth stages for both herbicides.

With the adoption of the Xtend® technology growers began to report reduced control levels of weedy grasses when applying tank-mixes of glyphosate and dicamba. Therefore, research was conducted at the Delta Research and Extension Center in Stoneville, MS from 2019 to 2021 to examine influence of herbicide mixtures with dicamba, as well as carrier volume and nozzle type. Results indicated barnyardgrass control 14 and 28 DAT was greatest with glyphosate alone. Control was not different when DGA or BAPMA dicamba or 2,4-D choline were mixed with glyphosate. At 28 DAT, barnyardgrass control was 15 to 18% lower with herbicide mixtures compared with glyphosate alone. Barnyardgrass control with glyphosate plus dicamba was not decreased by drift-reducing nozzles. Carrier volume of > 94 L ha-1 provided greater control of barnyardgrass compared to 47 L ha-1. These results indicate potential of reduced barnyardgrass control with glyphosate plus dicamba.

<p>  </p> <p>Since the commercialization of dicamba-resistant soybean and cotton, numerous instances of suspected off-target dicamba movement onto sensitive plant species have been reported. Further investigation into chemical and environmental factors that influence dicamba volatilization is warranted to better understand the mechanisms that lead to increased dicamba off-target movement via volatilization and plant response to dicamba vapor. The environmental impacts of dicamba must be minimized in order to ensure the sustainability and continued use of dicamba, which is an important herbicide for controlling broadleaf weeds in key cropping systems and non-crop sites. </p> <p>Controlled environment experiments were conducted to characterize the chemical interactions with dicamba volatility for three formulations of dicamba on glass slides. Dicamba volatility was similar for spray solution pH levels 4 to 8 for the diglycolamine (DGA) and the diglycolamine with VaporGrip® (DGA+VG) formulations. For the N,N-Bis-(3- aminpropyl) methylamine (BAPMA) formulation, dicamba volatility increased at a pH level of 5 with continued increases in volatility occurring as spray solution continued to decrease indicating that BAPMA formulation is more sensitive to pH fluctuations than the DGA and the DGA+VG formulations. While spray solution pH levels below 4 increased dicamba volatility for all three formulations compared to each formulation applied at a native pH level (5.53, 5.2, and 6.28 for the DGA, DGA+VG, and BAPMA formulations, respectively), the largest increase in dicamba volatility occurred when ammonium or iron was added to spray solution. Therefore, applicators should avoid mixing dicamba with other tank-mix partners that contain ammonium or iron to minimize the likelihood for dicamba volatilization. </p> <p>While extensive research exists documenting the process of dicamba volatilization, there has been little confirmation regarding how volatilized dicamba enters sensitive plants. Dicamba-sensitive (DS) soybean with different levels of canopy conductance, from different watering regimes and exposure time of day, were exposed to dicamba vapor. The DS soybean response was positively correlated with soybean canopy conductance during the dicamba vapor exposure suggesting that dicamba vapor route of entry into soybean is facilitated through the stomata. An additional experiment was conducted that exposed the single side of a hypostomatic leaf to dicamba vapor on different northern red oak trees. Northern red oak tree response was substantially greater when the abaxial leaf surface (high stomatal density) was exposed to dicamba vapor compared to when the adaxial leaf surface (no stomata) was exposed to dicamba vapor. Thus, dicamba vapor entry into plants is largely facilitated via stomata and secondly through re-deposition onto the leaf surface, where dicamba is absorbed through the plant cuticle, albeit minor. If dicamba vapor is redeposited onto leaf surfaces, dicamba acid absorption through the cuticle can be limited without the presence of a surfactant. Field and greenhouse experiments confirmed that the presence of surfactants from applications of other formulated herbicides can exacerbate soybean response to dicamba acid that was deposited on the leaf surface. </p> <p>In the midwestern United States, off-target dicamba movement to DS soybean has been problematic as DS soybean are extremely sensitive to very low concentrations of dicamba. Field and greenhouses studies confirmed that there are phenotypic differences amongst different soybean genotypes and their response to dicamba. Estimations of visual soybean injury was approximately 10% less for genotypes that were less sensitive to dicamba compared to genotypes with increased sensitivity. The future identification of the mechanisms that lead to decreased sensitivity to dicamba could be used to identify soybean cultivars that could mitigate the impacts of dicamba off-target movement to DS soybean. </p> <p>Lastly, a field experiment was conducted that investigated the influence of simulated dew on dicamba volatility from dicamba treated soybean leaves, in addition to soybean response in the presence of dicamba vapor. The results from a field experiment determined that consecutive simulated dew applications increase dicamba volatility from dicamba treated soybean. Furthermore, this is the only research demonstrating that DS-soybean response increases from dicamba vapor in the presence of dew. The results from this dissertation provide further insight into the chemical and environmental factors that influence dicamba volatility, the route of entry of dicamba vapor into plants, and soybean response to dicamba.</p>

Peas (Pisum Sativum L. Var Alaska) were allowed to absorb calcium and magnesium chloride for 8 hours and then were germinated in a potassium phosphate buffer pH 6.5 for 40 hours. Peas were then treated with 0, 0.1, 0.3, or 0.5 ppm dicamba (3,6-dichloro-o-anisic acid) and harvested at 24, 48, and 72 hour intervals. The following determinations were recorded: Root elongation, nuclear volume, and anatomical modifications. Root elongation recordings showed that dicamba prevented normal root elongation. Treatments induced very short thick primary roots. Measurements of nuclear volume indicated that all herbicide concentrations were able to reduce the total volume. Several other compounds, (chloramphenicol, actinomycin D and colchicine), were introduced to better characterize the actions of dicamba. Chloramphenicol and colchicine were responsible for nuclear volume reductions. Dicamba induced major anatomical alterations of treated pea roots. In the region 1 millimeter from the root tip, cortical cells were induced to divide more profusely. The diameter of treated roots exceeded that of untreated root tips. In the region 3 millimeters from the root tip, excessive cellular division and swelling resulted in cortical damage.

Dicamba is often tank mixed with glyphosate to increase herbicidal efficacy but may contribute to off-target movement (OTM). In recent years, volatilization has become problematic for dicamba-containing herbicides, resulting in increased regulatory requirements necessitating the use of volatility-reducing agents (VRA) for application. Research was conducted in 2021 and 2022 using low tunnels in a field environment and humidomes in a greenhouse environment to further assess how glyphosate salts and VRAs affect dicamba volatility. Our data indicate that the inclusion of glyphosate to dicamba can increase dicamba volatility, depending on the glyphosate salt used. The inclusion of the evaluated VRAs will decrease dicamba volatility when applied to a tank mixture of dicamba plus potassium salt of glyphosate.

There have been many changes in production agriculture over the last 20 years. The use of herbicide resistant crops has become common place in production agriculture. However, the release of crops resistant to auxin herbicides has brought more attention to the pesticide application process regardless of the type of pesticide applied. Moreover, controlling off-target movement of pesticides has become an integral part of the day to day farming tasks. The use of deposition aids when applied with water has been documented; however, the effect of deposition aids on pesticide application is not well understood. Based on these findings, selecting a deposition aid not only can be affected by the pesticide used but also the crop in question. Additionally, label restrictions on auxin herbicide application in auxin tolerant crops brings an additional problem to cotton growers. The use of insecticides in conjuction with herbicide applications has been commonplace for many growers across the cotton belt. However, smaller droplets have been utilized to increase coverage of these insecticides. Data presented in these findings suggest that larger droplets can still have good levels of efficacy and in some instances increase yield. The use of auxin tolerant crops allows for usage of addition POST herbicides to cotton; however, the effect of these tank mix applications on ctrop injury is not well understood. In both EnlistTM and Xtend® Flex cotton levels of injury were increased when glufosinate and S-metolachlor were applied in a tank mix. However, yield was not negatively impacted in either study.

Distinguishing 2,4-D and dicamba herbicide formulations in cotton and soybean tissue is challenging in regulation of crop injury from these herbicides. Additionally, stewardship of 2,4-D and dicamba technologies is important to maximize their longevity and efficacy. Research was conducted to (1) characterize cotton and soybean response to various formulations of 2,4-D or dicamba with or without glyphosate, (2) develop a method for classifying these formulations in crop tissue, and (3) optimize use of chloroacetamide herbicides in dicamba systems for mitigation of selection pressure on dicamba. Formulations evaluated include dicamba diglycolamine (DGA), dimethylamine (DMA), N,N-Bis-(3-aminopropyl) methylamine (BAPMA), and DGA plus potassium acetate (KAc); and 2,4-D DMA, acid, isooctyl ester (ESTER), and choline. Weed management by the chloroacetamides s-metolachlor and acetochlor was evaluated with applications preemergence (PRE), early postemergence (EP), late postemergence (LP), PRE followed by (fb) EP, PRE fb LP, and EP fb LP. Cotton and soybean response differed by 2,4-D and dicamba formulation, and glyphosate presence. Cotton yield was reduced by 200 to 500 kg ha-1 following exposure to 2,4-D choline or DMA relative to acid or ESTER. Glyphosate presence led to a reduction in cotton and soybean yield of 377 and 572 kg ha-1, respectively. Exposure to dicamba DMA resulted in a 263 kg ha-1 reduction in soybean yield relative to dicamba DGA, and glyphosate presence reduced yield by 439 and 246 kg ha-1 in cotton and soybeans, respectively. Chemometric analyses generated models capable of up to 85% accuracy in identifying dicamba formulation in cotton and soybean tissue, and up to 80% accuracy in identifying 2,4-D formulation. Split chloroacetamide applications improved cotton yield up to 60%, reduced weed densities up to 90%, and improved control up to 56% relative to single applications. Cotton height was reduced up to 23% if a single chloroacetamide application was made. Soybean yield was maximized following any chloroacetamide application timing except PRE alone, and weed control was reduced up to 31% following single chloroacetamide application relative to split applications. These results will aid regulatory bodies in managing use of new weed control technologies and will assist producers in stewarding these new technologies.

<p dir="ltr">Waterhemp (<i>Amaranthus tuberculatus</i>)<i> </i>is one of the most problematic weeds in soybean production in the United States. The ability of waterhemp to quickly evolve resistance threatens the utility of many herbicides. The introduction of Xtend^®and Xtendflex^®soybeans allow for the in-season application of dicamba and glufosinate. With an increase in dicamba use in soybeans plus its continued use in corn, there have been many reports of dicamba failure on waterhemp.</p><p dir="ltr">Greenhouse dose response experiments were conducted to screen six populations of waterhemp for resistance to dicamba. Each population was compared to a known sensitive and known resistant population, with 50% growth reduction (GR₅₀) values of 39 g ae ha^-1and 226 g ae ha^-1, respectively. Low-level dicamba resistance was identified in all populations, as they had GR₅₀values that were different from the known sensitive and R:S ratios that varied from 1.7 to 4.4. Additionally, all six populations exhibited at least 50% survival at a 1/2X rate of dicamba where the sensitive only had 30% survival. Therefore, we can conclude that dicamba resistance in waterhemp is present in multiple counties in Indiana.</p><p dir="ltr">In addition to characterizing populations from Indiana, a growth monitoring study was conducted to determine how emergence timing impacted waterhemp growth. In 2021, plants that emerged in the earliest cohort were taller, had more branches, and accumulated more biomass in comparison to later emerging plants at six weeks after flagging. In 2022, drought conditions throughout the month of June impacted growth of earlier emerging plants, and waterhemp that emerged in the latest cohort were taller, had more branches, and accumulated more biomass than earlier emerging cohorts at six weeks after flagging. Seed yields per plant were low in both years, but all cohorts were able to produce seed. This research concludes that in years when soil sufficient moisture is available, earlier emerging waterhemp plants are competitive with crops and later emerging plants can still produce seed.</p><p dir="ltr">Additionally, field trials were conducted to determine herbicide programs in the Enlist^® and Xtendflex^® soybean systems that would best control a waterhemp population with multiple herbicide resistance. At 21 days after the second postemergence application, waterhemp control was highest in two pass systems. The addition of pyroxasulfone to the second postemergence application increased control of waterhemp in the Xtendflex^® system. Waterhemp densities were the lowest and soybean yield was the highest in two pass herbicide programs for both systems. The results indicate that waterhemp resistant to chemistries in HRAC Groups #2, #4, #5, #9, #14, and #27 was most effectively controlled by programs with two herbicides applications, either a preemergence followed by postemergence or two pass postemergence, and included 2,4-D and glufosinate in the postemergence application(s).</p><p dir="ltr">Finally, a waterhemp population from Francesville, IN was characterized for herbicide resistance via a series of field, greenhouse, and laboratory experiments. Preliminary laboratory analysis confirmed resistance to herbicide actives in the HRAC Groups #2 and #14 via target site mutations and to Group #9 via gene amplification. Field research trials indicated inadequate waterhemp control with preemergence applications of pendimethalin and atrazine and postemergence applications of herbicide actives from Groups #2, #9, #14, and #27 as well as glufosinate and dicamba. Greenhouse dose response experiments revealed GR₅₀values for the Francesville population that were significantly higher for dicamba, mesotrione, and topramezone than the known sensitive. R:S ratios of 4.4, 3.3, and 1.8, were documented for dicamba, mesotrione, and topramezone, respectively. Data from all experiments demonstrated that the Francesville population is resistant to herbicide actives in Groups #2, #4, #5, #9, #14, and #27.</p>

Horseweed (Conyza canadensis (L.) Cronq.) is prevalent in Kentucky and can be difficult to control. Research has shown multiple weed control methods to be more sustainable than relying on chemical control alone, so the use of multiple methods for horseweed management was examined in this study. The main objective was to determine best practice(s) to reduce horseweed prior to soybean [Glycine max (L.) Merr.]. Treatments included: fall-planted cover crop [CC; cereal rye (Secale cereale L.) or none], fall-applied herbicide (saflufenacil or none), and spring-applied herbicides (dicamba, 2,4-D ester, or none). We hypothesized horseweed densities would be reduced the most where all factors were combined. Saflufenacil suppressed horseweed densities from application through March, when densities increased due to a lack of competition from other winter weeds. Spring herbicides decreased horseweed densities until soybeans reached V1 in 2017, but in 2018 lost efficacy after CC termination. CC alone resulted in the longest horseweed suppression. The combination of spring herbicides and CC usually reduced horseweed densities to near zero between the CC termination and soybean planting. However, some low densities seen soon after soybean planting could be problematic. Further research must be conducted to determine the best integrated horseweed management system until soybean canopy closure.