Applications Of Dicamba Research Articles

Effect of Irrigation Regime and Fertilization on Recovery of Dicamba Injured Soybean

With the release of the dicamba-resistant crop technology and subsequent increase in dicamba off-target movement to non-dicamba-resistant crops, discovering means of mitigating yield loss through studying dicamba injury to soybean and interactions with factors such as irrigation regime and fertilization would prove beneficial. Field experiments were conducted in 2019 in Fayetteville and Colt, Arkansas, to evaluate the effect of irrigation regime to non-dicamba-resistant soybean that was injured by dicamba at a low dose at multiple timings. Another experiment was conducted in Fayetteville in 2019 and 2020 evaluating the impact of nitrogen (N) and potassium (K) fertilization on soybean recovery following injury by dicamba at multiple reproductive stages. Visible injury in both experiments was affected by application timing. Soybean yield components were impacted by dicamba applications within the irrigation regime experiment, and yields were decreased by dicamba applications; however, soybean yield was higher from branches than from the mainstem in dicamba-treated compared to nontreated plants. In the fertilization experiment, soybean treated with a low dose of dicamba that received N fertilization tended to have reduced biomass compared to treatments receiving no fertilizer or K alone, with greatest biomass reduction tending to occur among treatments receiving both N and K. Total grain yield was not affected by either irrigation regime or fertilization. While an increase in yield due to neither irrigation nor fertilization was observed, these results may help improve understanding of the effect of low-dose dicamba on soybean and aid producers making management decisions.

Effect of cotton herbicide programs on weed population trajectories and frequency of glyphosate-resistant Palmer amaranth (Amaranthus palmeri)

AbstractThe adoption of dicamba-resistant cotton (Gossypium hirsutum L.) cultivars allows using dicamba to reduce weed populations across growing seasons. However, the overuse of this tool risks selecting new herbicide-resistant biotypes. The objectives of this research were to determine the population trajectories of several weed species and track the frequency of glyphosate-resistant (GR) Palmer amaranth (Amaranthus palmeri S. Watson) over 8 yr in dicamba-resistant cotton. An experiment was established in North Carolina in 2011, and during the first 4 yr, different herbicide programs were applied. These programs included postemergence applications of glyphosate, alone or with dicamba, with or without residual herbicides. During the last 4 yr, all programs received glyphosate plus dicamba. Biennial rotations of postemergence applications of glyphosate only and glyphosate plus dicamba postemergence with and without preemergence herbicides were also included. Sequential applications of glyphosate plus dicamba were applied to the entire test area for the final 4 yr of the study. No herbicide program was entirely successful in controlling the weed community. Weed population trajectories were different according to species and herbicide program, creating all possible outcomes; some increased, others decreased, and others remained stable. Density of resistant A. palmeri increased during the first 4 yr with glyphosate-only programs (up to 11,739 plants m−2) and decreased a 96% during the final 4 yr, when glyphosate plus dicamba was implemented. This species had a strong influence on population levels of other weed species in the community. Goosegrass [Eleusine indica (L.) Gaertn.] was not affected by A. palmeri population levels and even increased its density in some herbicide programs, indicating that not only herbicide resistance but also reproductive rates and competitive dynamics are critical for determining weed population trajectories under intensive herbicide-based control programs. Frequency of glyphosate resistance reached a maximum of 62% after 4 yr, and those levels were maintained until the end of the experiment.

Optimizing weed control using dicamba and glufosinate in eligible crop systems

AbstractA field experiment was conducted in 2019 and 2020 that included six site-years and four locations in Arkansas to determine the optimal sequence and timing of dicamba and glufosinate applications when applied alone, sequentially, or in combination to control Palmer amaranth by size: labeled (<10 cm height) and non-labeled (13 to 25 cm height). Single applications of dicamba, glufosinate, and dicamba plus glufosinate (not labeled) resulted in less than 80% Palmer amaranth control, regardless of weed size. The mixture of dicamba plus glufosinate was antagonistic for Palmer amaranth control and percent mortality. Sequential applications, averaged over all time intervals and herbicides, improved the percentage of Palmer amaranth control 11 to 17 percentage points over a single application, regardless of weed size at application 28 d after final application (DAFA). Palmer amaranth control with glufosinate followed by (fb) glufosinate and dicamba fb dicamba, pending weed size, were optimized at intervals of 7 d, and 14 to 21 d, respectively. Because single site of action (SOA) postemergence herbicide systems increase the likelihood of the development of resistant biotypes and are not a best management practice (BMP) in that regard; sequential applications involving both dicamba and glufosinate were more effective. Furthermore, the sequence of application mattered with a preference for applying dicamba first. Dicamba fb glufosinate at a 14-d interval was profit-maximizing and the only herbicide treatment that resulted in 100% weed control when size was <10 cm. For larger weed sizes, economic analysis revealed that dicamba fb dicamba performed better than dicamba fb glufosinate when no penalty was assigned for using a single SOA. This resulted in greater yield loss risk and soil weed seed bank in comparison to timelier weed control with the smaller weed size. Hence, timely weed control and two SOAs to control Palmer amaranth are recommended as BMPs that reduce producer risk.

Influence of Broadcast Nozzle Design and Weed Density on Dicamba Plus Glyphosate Deposition, Coverage, and Efficacy in Dicamba-Resistant Soybean

Dicamba injury to sensitive soybean and other broadleaf crops due to drift is a major issue. Dicamba label restrictions have been created to mitigate the off-target movement of dicamba. One restriction is the mandated use of low-drift nozzles to spray dicamba; these nozzles produce large droplet spectrums and minimize the production of driftable fines. Experiments were conducted to evaluate herbicide coverage, deposition, and efficacy as influenced by spray nozzle design and density of waterhemp, goosegrass, and large crabgrass in dicamba-resistant soybean. Dicamba plus glyphosate was applied to 5- to 10-cm-tall weeds with a Turbo TeeJet (TT11005) nozzle and two drift reduction nozzles approved for dicamba applications: Turbo TeeJet Induction (TTI11005) and Pentair Ultra Lo-Drift (ULD12005). Weed densities were categorized into different levels and established in a 0.25-m2 quadrat prior to postemergence application. Deposition of herbicide spray solution onto targeted weeds was not different despite coverage differences observed on Kromekote spray cards. Coverage of herbicide solution was consistently lower with the low-drift TTI11005 nozzle as compared to the TT11005 nozzle. Herbicide efficacy on waterhemp plants was the lowest at the highest waterhemp densities of 54 plants per m2 with the drift-reducing TTI11005 nozzle, although weed control was not lowered at any density when applications were made with the ULD nozzle as compared to the TT11005 nozzle. Additionally, herbicide efficacy was reduced as large crabgrass density increased. Overall, the use of a drift-reducing nozzle can be successful for waterhemp control and Poaceae control postemergence in soybean when weed densities are suppressed or reduced through methods such as the use of a residual preemergence herbicide or cereal rye cover crop.

Potassium carbonate effects on spray mixture pressure changes and final pH

AbstractThis research examined a potential nuisance aspect of the use of the volatility-reducing agent (VRA) potassium carbonate when combined with glyphosate in spray-tank mixtures. A VRA is now required to be added to dicamba applications to reduce off-target movement from volatility. When no VRA potassium carbonate was added to the spray mixture, there was no pressure buildup. The addition of VRA potassium carbonate plus glyphosate (which lowers the pH) resulted in an observed pressure buildup. Although the gas produced was not identified, it would be expected to be carbon dioxide formed by the dissolution of the carbonate anion from the VRA. Source water pH range from 3.2 to 8.2 had no effect on pressure buildup. Pressure buildup was directly related to water temperature, with a linear response to temperature when the VRA was added last; in contrast, a less direct relationship of temperature to pressure buildup existed at temperatures >30 C when the VRA potassium carbonate was added first. There was no effect on the pressure increase from adding a defoamer or a drift control agent.

Changes in Dicamba Use are Ahead

The first steps in the development of dicamba tolerant transgenic plants were conducted by Sandoz Crop Protection in Palo Alto, CA and then contractually followed up at the University of Nebraska. The subsequent development by Monsanto of soybean and cotton varieties that would tolerate post-emergent application of dicamba substantially changed the use patterns of dicamba in the United States. Transgenic dicamba-tolerant (DT) seeds were first approved in 2016 in the United States, although the post-emergent use of dicamba was not legal that year. In 2017 and 2018, there was substantial market penetration of DT soybean and cotton seeds into the market and the occurrence of dicamba off-target movement (OTM) was highly variable across the United States. The driving force behind these new seed traits was the widespread failure of glyphosate to control broadleaf weeds effectively, especially those from the Conyza and Amaranthus genera. Herbicide research and development in the United States has historically involved both industry and academic weed scientists usually operating in a symbiotic and mutually respectful relationship, although there may have been disagreements at times about some aspects of herbicides and their development. The relatively recent introduction of DT varieties and legal post-emergent dicamba in the United States was a dynamic time for weed control, and adoption of DT crops and subsequent OTM of dicamba greatly changed the working relationships between academic scientists and representatives from Monsanto and some other private companies. Perhaps the greatest change was the lack of access of research materials that would be available to academic scientists for evaluation prior to the retail sale of those materials. Historically, academic scientists would have the ability to evaluate various new technologies and provide objective, independent comments on their potential utility prior to commercialisation. Monsanto largely restricted access to the DT seeds or new herbicide formulations. There are some states that have long-established policies that they will not recommend a new herbicide technology unless they have examined it under their specific field conditions. For example, University of Arkansas researchers would not recommend the use of post-emergent dicamba on DT crops when it first became legal to use. Monsanto responded by filing legal challenges of various types against the University of Arkansas faculty, including 64 exhibits of various legal aspects. Many other states had varying degrees of restrictions placed upon their research efforts, and most scientists had to sign various forms of confidentiality agreements to obtain access to the research material from Monsanto. There were two major differences in the dicamba labels when first introduced in 2017 and then again two years later. The first major difference was an entire section on herbicide resistance confirmation validation and management that was clearly stated on the label. The authors of this paper remind the readers that the reason we have dicamba being used is because of the wholesale failure of glyphosate resistance management systems. The golden era of weed control, essentially 100% weed control with no crop injury, was lost due to the complete lack of any resistance management strategies. The US EPA added label language hopefully to avoid evolution of resistance in weeds from happening again with essentially any new herbicide. Many herbicide users expect all new labels to include resistance management language. The second obvious change to the label was the duration of validity, which was normally many years in previous pesticide registrations. The first dicamba label, which came out in 2017, was valid for only two years. The next label which came out in 2019, was only valid for five years, although there is widespread speculation on the future of that label.

Utility of roller wiper applications of dicamba for Palmer amaranth control in soybean.

BACKGROUNDThe commercialization of dicamba‐resistant soybean has resulted in increased concern for off‐target movement of dicamba onto sensitive vegetation. To mitigate the off‐target movement through physical drift, one might consider use of rope wicks and other wiper applicators. Although wiper‐type application methods have been efficacious in pasture settings, the utility of dicamba using wiper applicators in agronomic crops is not available in scientific literature. To determine the utility of roller wipers for dicamba applications in dicamba‐resistant soybean, two separate experiments were conducted in the summer of 2020 and replicated in both Keiser and Fayetteville, AR, USA.RESULTSUtilizing opposing application directions and a 2:1:1 ratio of water: formulated glyphosate: formulated dicamba were the most efficacious practices for controlling Palmer amaranth. The high herbicide concentrations and wiping in opposing directions increased dicamba‐resistant soybean injury when the wiper contacted the crop, but no yield loss was observed because of this injury. Broadcast applications resulted in greater Palmer amaranth mortality than roller wiper applications, and the most effective roller wiper treatments were when two sequential applications were made inside the crop canopy.CONCLUSIONSDicamba applications require adequate coverage for optimum weed control. While efforts can be made to increase roller wiper efficacy by optimizing coverage and timing of applications, broadcast applications are superior to roller wiper applicators for weed control. Roller wiper applications did not reduce soybean yield, thus wiper‐type applications may be safely used in dicamba‐resistant soybean, albeit the likelihood for off‐target damage caused by volatilization of these treatments would need to be investigated. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Characterization and inheritance of dicamba resistance in a multiple-resistant waterhemp (Amaranthus tuberculatus) population from Illinois

AbstractWaterhemp [Amaranthus tuberculatus (Moq.) Sauer] is one of the most troublesome agronomic weeds in the midwestern United States. The rapid evolution and selection of herbicide-resistance traits in A. tuberculatus is a major challenge in managing this species. An A. tuberculatus population, designated CHR, was identified in 2012 in Champaign County, IL, and previously characterized as resistant to herbicides from six site-of-action groups: 2,4-D (Group 4), acetolactate synthase inhibitors (Group 2), protoporphyrinogen oxidase inhibitors (Group 14), 4-hydroxyphenylpyruvate dioxygenase inhibitors (Group 27), photosystem II inhibitors (Group 5), and very-long-chain fatty-acid synthesis inhibitors (Group 15). Recently, ineffective control of CHR was observed in the field after dicamba application. Therefore, this research was initiated to confirm dicamba resistance, quantify the resistance level, and investigate its inheritance in CHR. Multiple field trials were conducted at the CHR location to confirm poor control with dicamba and compare dicamba treatments with other herbicides. Greenhouse trials were conducted to quantify the resistance level in CHR and confirm genetic inheritance of the resistance. In field trials, dicamba did not provide more than 65% control, while glyphosate and glufosinate provided at least 90% control. Multiple accessions were generated from controlled crosses and evaluated in greenhouse trials. Greenhouse dicamba dose–response experiments indicated a resistance level of 5- to 10-fold relative to a sensitive parental line. Dose–response experiments using F1 lines indicated that dicamba resistance was an incompletely dominant trait. Segregation analysis with F2 and backcross populations indicated that dicamba resistance had moderate heritability and was potentially a multigenic trait. Although dicamba resistance was predominantly inherited as a nuclear trait, minor maternal inheritance was not completely ruled out. To our knowledge, CHR is one of the first cases of dicamba resistance in A. tuberculatus. Further studies will focus on elucidating the genes involved in dicamba resistance.

Chemical control of wild radish and volunteer EnlistTM soybean and selectivity to wheat crop

The incidence of volunteer EnlistTM soybean in the post-emergence of crops in succession, such as wheat, requires changes in chemical control. Thus, the objective of the work is to evaluate the efficiency of different post-emergence herbicides in the control of volunteer EnlistTM soybean and wild radish and their selectivity to wheat. For this, four experiments were conducted in 2017 and 2018 in field and greenhouse. The treatments tested were pyroxsulam, saflufenacil, pyroxsulam + saflufenacil, pyroxsulam + bentazon, pyroxsulam + metribuzin, saflufenacil + bentazon, and saflufenacil + metribuzin in 2017, and triclopyr, saflufenacil, MCPA, quinclorac, dicamba, pyroxsulam + metribuzin, metribuzin + metsulfuron, pyroxsulam + bentazon and bentazon + metsulfuron in 2018. The variables were the control of wild radish and volunteer EnlistTM soybean phytotoxicity to wheat crop, yield components, and yield total. The association of the herbicides pyroxsulam and saflufenacil is efficient in the management of volunteer soybean EnlistTM, showing selectivity to wheat. The isolated application of dicamba and the associations of pyroxsulam with metribuzin and metribuzin with metsulfuron represent alternatives for selective management of volunteer EnlistTM soybean in wheat, in addition to efficiently controlling wild radish in post-emergence.

EFICIENCIA Y RENTABILIDAD DEL CONTROL QUÍMICO DE MALEZAS EN EL CULTIVO MAÍZ

<p>Background: Corn is one of the main crops in Mexico. The crop is affected by different pests such as weeds and high-cost inputs are required for its control. Objective: To determinate the efficiency of five post-emerging herbicides on the control of monocotyledonous and dicotyledonous weeds in a corn crop, as well as to identify the optimal economic herbicide. Methodology: The study was carried out in Cocula, Guerrero, Mexico under irrigation conditions. Treatments were: 1) Nicosulfurón + 2,4-D; 2) Nicosulfurón + atrazine; 3) Nicosulfurón + dicamba; 4) Rimsulfurón + 2,4-D; 5) Rimsulfurón + dicamba; 6) Rimsulfurón + atrazine and 7) Treatment without application. Sampling was performed 7, 15, 30 and 60 days after application. The variables evaluated were, weed number and control percentage. In addition, an economic analysis was carried out, depending on the grain yield obtained. Results: Application of nicosulfuron and rimsulfuron in mixture with 2,4-D, dicamba and atrazine archived control levels than or equal to 90% for 30 days on Sorghum halepense and Panicum reptans weeds. For Melampodium divaricatum and Argythamnia neomexicana nicosulfuron + dicamba and rimsulfuron + atrazina herbicides-controlled weeds for 30 days. Kallstroemia maxima weed, was controlled by 100% with the application of nicosulfuron + dicamba. As for the financial analysis, the greatest economic benefit was with the mixture of rimsulfuron with atrazine and 2,4-D. Implications: The mixture of rimsulfuron + 2,4-D is efficient on the weed control in corn crop, in addition to being the most economical. Conclusion: The application of nicosulfuron and rimsulfurn achieves the best control over monocotyledons and dicotyledons, the greater control was observed with the nicosulfuron +dicamba application, however, the rimsulfuron application with atrazine and 2,4-D generated the best profitability.</p>

Palmer amaranth (Amaranthus palmeri) control in postharvest wheat stubble in the Central Great Plains

AbstractLate-season control of Palmer amaranth in postharvest wheat stubble is important for reducing the seedbank. Our objectives were to evaluate the efficacy of late-season postemergence herbicides for Palmer amaranth control, shoot dry biomass, and seed production in postharvest wheat stubble. Field experiments were conducted at Kansas State University Agricultural Research Center near Hays, KS, during 2019 and 2020 growing seasons. The study site had a natural seedbank of Palmer amaranth. Herbicide treatments were applied 3 wk after wheat harvest when Palmer amaranth plants had reached the inflorescence initiation stage. Palmer amaranth was controlled by 96% to 98% 8 wk after treatment and shoot biomass as well as seed production was prevented when paraquat was applied alone or when mixed with atrazine, metribuzin, flumioxazin, 2,4-D, sulfentrazone, pyroxasulfone + sulfentrazone, or flumioxazin + metribuzin, and with glyphosate + dicamba, glyphosate + 2,4-D, saflufenacil + 2,4-D, glufosinate + dicamba + glyphosate, and glufosinate + 2,4-D + glyphosate. Palmer amaranth was controlled by 89% to 93% with application of glyphosate, glufosinate, dicamba + 2,4-D, saflufenacil + atrazine, and saflufenacil + metribuzin resulting in Palmer amaranth shoot biomass of 15 to 56 g m−2 and production of 1,080 to 7,040 seeds m−2. Palmer amaranth control was less than 86% with application of dicamba, 2,4-D, dicamba + atrazine, and saflufenacil resulting in Palmer amaranth shoot biomass of 38 to 47 g m−2 and production of 3,110 to 6,190 seeds m−2. Palmer amaranth was controlled 63% and 72%, shoot biomass was 178 and 161 g m−2, and seed production was 35,180 and 39,510 seeds m−2, respectively, with application of 2,4-D + bromoxynil + fluroxypyr, and bromoxynil + pyrasulfotole + atrazine. Growers should use these effective postemergence herbicide mixes for Palmer amaranth control to prevent seed prevention postharvest in wheat stubble.

Effectiveness of glufosinate, dicamba, and clethodim on glyphosate-resistant and -susceptible populations of five key weeds in Australian cotton systems

AbstractXtendFlexTM cotton with resistance to glyphosate, glufosinate, and dicamba may become available in Australia. Resistance to these herbicides enables two additional modes of action to be applied in crop. The double-knock strategy, typically glyphosate followed by paraquat, has been a successful tactic for control of glyphosate-resistant cotton in fallow situations in Australia. Glufosinate is a contact herbicide and may be useful as the second herbicide in a double knock for use in XtendFlexTM cotton crops. We tested the effectiveness of glufosinate applied at intervals of 1, 3, 7, and 10 d after initial applications of glyphosate, dicamba, clethodim, and glyphosate mixtures with dicamba or clethodim on glyphosate-resistant and glyphosate-susceptible populations of flaxleaf fleabane, common sowthistle, feather fingergrass, windmill grass, and junglerice. Effective treatments for flaxleaf fleabane with 100% control were dicamba and glyphosate+dicamba followed by glufosinate independent of the interval between applications. Common sowthistle was effectively controlled in Experiment 1 by all treatments. However, in Experiment 2, effective treatments were dicamba and glyphosate+dicamba followed by glufosinate (99.3% to 100% control). Timing of the follow-up glufosinate did not affect the control achieved. Consistent control of feather fingergrass was achieved with glyphosate, clethodim, or glyphosate+clethodim followed by glufosinate at 7-d and 10-d intervals (99.7% to 100% control). Control of feather fingergrass was inconsistent. The best treatment for windmill grass was glyphosate+clethodim followed by glufosinate 10 d later (99.8% to 100% control). Junglerice was effectively controlled with all treatments except for glyphosate on the glyphosate-resistant population. Additional in-crop use of glufosinate and dicamba should be beneficial for weed management in XtendFlexTM cotton crops, when using the double knock tactic with glufosinate. For effective herbicide resistance management, it is important that these herbicides be used in addition to, rather than substitution for, existing weed management tactics.

Interaction of contact herbicides and timing of dicamba exposure on soybean

AbstractDicamba residues in sprayers are difficult to remove and may interact with subsequent herbicides, including contact herbicides labeled for use in soybean. Without proper tank cleanout, applicators treating dicamba-resistant and non–dicamba resistant crops are at risk of contaminating the spray solution with dicamba residue from previous applications. Experiments were conducted in Fayetteville, AR, in 2018 and 2019, with the first experiment evaluating consequences of dicamba tank contamination with contact herbicides and the second experiment addressing the impact of dicamba exposure on a glufosinate-resistant soybean cultivar relative to a contact herbicide application. Experiments for tank contamination and timing of dicamba exposure were designed as a three-factor and a two-factor randomized complete block with four replications, respectively, considering site-year as a fixed effect in each experiment. Dicamba at 0, 0.056, 0.56, and 5.6 g ae ha−1 was applied alone, with glufosinate, with acifluorfen, or with glufosinate plus acifluorfen to V3 soybean. Dicamba applied in combination with contact herbicides exacerbated visible auxin symptomology over dicamba alone at 21 and 28 d after treatment (DAT), while dicamba at 5.6 g ae ha−1 reduced soybean height. Injury and height reductions caused by dicamba mixtures with contact herbicides did not reduce grain yield. In the second experiment, dicamba was applied at 2.8 g ae ha−1 at VC, V1, V2, and V3 and at 3, 7, and 10 d after a glufosinate application to V3 soybean (DATV3). Greater soybean injury was observed when dicamba exposure followed a glufosinate application than when dicamba preceded glufosinate or was applied in a mixture with glufosinate, with yield reductions resulting from 7 and 10 DATV3 dicamba applications. Dicamba exposure in the presence of contact herbicides resulted in increased auxin symptomology and can be intensified if soybean are exposed to dicamba following a contact herbicide application.

Response of non-dicamba-resistant soybean (Glycine max) varieties to dicamba

AbstractIntroduction and rapid adoption of dicamba-resistant (DR) soybean led to an increase of postemergent applications of dicamba. This resulted in a widespread increase in nontarget dicamba injury to non-DR soybean in 2017. Field studies were conducted in Manhattan, KS, in 2018 and 2019 and in Ottawa, KS, in 2019 to investigate the injury and yield response of soybean varieties with varying herbicide-resistance traits and maturity groups when exposed to dicamba. Four varieties were tested: ‘Credenz 3841LL’ (glufosinate resistant), ‘Credenz 4748LL’ (glufosinate resistant), ‘Asgrow AG4135RR2Y’ (glyphosate resistant), and ‘Stine 40BA02’ (glyphosate and isoxaflutole resistant), abbreviated as CR3841, CR4748, AG4135, and ST40B, respectively. Soybeans were treated with 5.6 g ae ha−1 of dicamba at V3 and R1 stages. Percent soybean injury, soybean height, soybean yield and yield components, and injury to offspring were evaluated. Four weeks after treatment (WAT) at V3, the greatest injury was observed in AG4135 and ST40B. Dicamba application at R1 resulted in the greatest injury to ST40B both 4 WAT and at senescence. Minimal injury was observed in all varieties treated at V3 at senescence and yield loss was 5% or less. Dicamba application at R1 resulted in 19 to 34% yield loss, with the least yield loss in CR4748, and the greatest in ST40B. Varieties with greater injury at senescence generally yielded less than other varieties.

Simulated dew increases volatility of dicamba from soybean leaves

AbstractAdoption of dicamba tolerant soybeans [Glycine max (L.) Merr.] accompanied by postemergence applications of dicamba resulted in off‐target movement and damage to nearby crops. The underlying cause, in some cases, has been attributed to weather‐related phenomena such as temperature inversions, which are often accompanied by the formation of dew. The objective of this controlled study was to determine if the formation of simulated dew on soybean leaves treated with dicamba led to increased short‐term dicamba volatility. Over a 48‐h period after the application of dicamba, dicamba volatility concentrations quantified in air samples increased by 20% when soybeans were exposed to simulated dew for 3 h compared with soybeans that were not exposed to dew. Concentrations of volatilized dicamba were elevated in the first (0–24 h) and second (24–48 h) 24‐h period after initiating collection of air samples. Dew may alter the stability of dicamba, ultimately contributing to increased off‐target movement.

Sequential Applications of Synthetic Auxins and Glufosinate for Escaped Palmer Amaranth Control

Field and greenhouse studies were conducted to investigate the influence of sequence and timing of synthetic auxins and glufosinate on large Palmer amaranth (Amaranthus palmeri) control. Field studies were performed in Henry County, AL where treatments were applied to Palmer amaranth with average heights of 37 and 59 cm in 2018 and 2019, respectively. Sequential applications of 2,4-D/dicamba + glyphosate followed by (fb) glufosinate at labeled rates 3 or 7 days after initial treatment (DAIT) were used in addition to the reverse sequence with a 7-day interval. Time intervals of 3 or 7 days between applications did not influence Palmer amaranth control. Palmer amaranth was controlled 100% by dicamba + glyphosate fb glufosinate and 2,4-D + glufosinate fb glufosinate 7 DAIT in 2018. However, herbicide performance was reduced due to delayed application and taller plants in 2019 with up to 23% less visual injury. To further investigate Palmer amaranth response to dicamba and glufosinate applied sequentially, a greenhouse study was conducted in 2019 where physiological measurements were recorded over a 35-day period. Treatments were applied to Palmer amaranth averaging 38 cm tall and included dicamba + glyphosate fb glufosinate 7 DAIT, the reverse sequence, and a single application of dicamba + glufosinate + glyphosate. Glufosinate severely inhibited mid-day photosynthesis compared to dicamba with up to 90% reductions in CO2 assimilation 1 DAIT. In general, Palmer amaranth respiration and stomatal conductance were not affected by herbicides in this study. Applications of dicamba + glyphosate fb glufosinate 7 DAIT was the only treatment hindered Palmer amaranth regrowth with 52% reduction in leaf biomass compared to nontreated control. These data suggest Palmer amaranth infested fields are more likely to be rescued with sequential applications of synthetic auxins and glufosinate, but consistent control of large Palmer is not probable.

Dicamba emissions under field conditions as affected by surface condition

AbstractThe evolution and widespread distribution of glyphosate-resistant broadleaf weed species catalyzed the introduction of dicamba-resistant crops that allow this herbicide to be applied POST to soybean and cotton. Applications of dicamba that are most cited for off-target movement have occurred in June and July in many states when weeds are often in high densities and at least 10 cm or taller at the time of application. For registration purposes, most field studies examining pesticide emissions are conducted using bare ground or very small plants. Research was conducted in Knoxville, TN, in the summer of 2017, 2018, and 2019 to examine the effect of application surface (tilled soil, dead plants, green plants) on dicamba emissions under field conditions. Dicamba emissions after application were affected by the treated surface in all years, with the order from least to most emissions being dead plants < tilled soil < green plant material. In fact, dicamba emissions were >300% when applied to green plants compared to other surfaces. These findings suggest that dicamba applications made to bare ground will likely underestimate what may occur under normal field use conditions when POST applications are made and the crop canopy or weed groundcover is nearly 100% green material. A potential change to enhance the accuracy of current environmental simulation models would be to increase the theoretical findings to allow for the effect of green plant material on dicamba emissions under field conditions.

Efficacy of imazapic/imazapyr and other herbicides in mixtures for the control of Digitaria insularis prior to soybean sowing

Herbicide mixtures, use of multiple sites of action, and other weed management practices are necessary to avoid cases of biotype resistance. The aim of this study was to evaluate the efficiency of imazapic/imazapyr and other herbicides in mixtures to control Digitaria insularis at burndown before soybean sowing. This field research was conducted in Umuarama, State of Parana (PR), Brazil, in the 2018/19 soybean season. The experiment was conducted in a randomized block experimental design with four replications and 11 treatments composed of the application of glyphosate, clethodim, haloxyfop, imazapic/imazapyr, glufosinate, 2,4-dichlorophenoxyacetic acid (2,4-D), dicamba, triclopyr, and saflufenacil, in mixtures. Weed control was evaluated as well as soybean injury and yield. An analysis of variance and F-test were performed, and the treatment means were compared by the Scott-Knott test. All treatments showed great control over the weed and low crop injury rate while maintaining soybean yield. The application of imazapic/imazapyr in mixtures with other herbicides was effective in controlling glyphosate-resistant D. insularis in burndown before soybean sowing and with sequential application of haloxyfop + glyphosate at V3 stage of soybean. This chemical management was also selective for soybean.

Effective two-pass herbicide programs to control glyphosate-resistant Palmer amaranth (Amaranthus palmeri) in glyphosate/dicamba-resistant soybean

AbstractField experiments were conducted in 2018 and 2019 at Kansas State University Ashland Bottoms (KSU-AB) research farm near Manhattan, KS, and Kansas State University Agricultural Research Center (KSU-ARC) near Hays, KS, to determine the effectiveness of various PRE-applied herbicide premixes and tank mixtures alone or followed by (fb) an early POST (EPOST) treatment of glyphosate + dicamba for controlling glyphosate-resistant (GR) Palmer amaranth in glyphosate/dicamba-resistant (GDR) soybean. In experiment 1, PRE-applied sulfentrazone + S-metolachlor, saflufenacil + imazethapyr + pyroxasulfone, chlorimuron + flumioxazin + pyroxasulfone, and metribuzin + flumioxazin + imazethapyr provided 85% to 94% end-of-season control of GR Palmer amaranth across both sites. In comparison, Palmer amaranth control ranged from 63% to 87% at final evaluation with PRE-applied pyroxasulfone + sulfentrazone, pyroxasulfone + sulfentrazone plus metribuzin, pyroxasulfone + sulfentrazone plus carfentrazone + sulfentrazone, and sulfentrazone + metribuzin at the KSU-ARC site in experiment 2. All PRE fb EPOST (i.e., two-pass) programs provided near-complete (98% to 100%) control of GR Palmer amaranth at both sites. PRE-alone programs reduced Palmer amaranth shoot biomass by 35% to 76% in experiment 1 at both sites, whereas all two-pass programs prevented Palmer amaranth biomass production. No differences in soybean yields were observed among tested programs in experiment 1 at KSU-ARC site; however, PRE-alone sulfentrazone + S-metolachlor, saflufenacil + imazethapyr + pyroxasulfone, and chlorimuron + flumioxazin + pyroxasulfone had lower grain yield (average, 4,342 kg ha−1) compared with the top yielding (4,832 kg ha−1) treatment at the KSU-AB site. PRE-applied sulfentrazone + metribuzin had a lower soybean yield (1,776 kg ha−1) compared with all other programs in experiment 2 at the KSU-ARC site. These results suggest growers should proactively adopt effective PRE-applied premixes fb EPOST programs evaluated in this study to reduce selection pressure from multiple POST dicamba applications for GR Palmer amaranth control in GDR soybean.

Long-Term Control of Hedge Bindweed (Calystegia sepium L.) with Single, Tank Mixture, and Sequential Applications of Glyphosate, 2,4-D, and Dicamba

Hedge bindweed (Calystegia sepium L.) is a widespread troublesome perennial weed species that has strong rhizome regenerative capacity. Four pot trials with randomised, complete block designs were conducted in 2015 to evaluate long-term control of hedge bindweed using individual, tank mixture, and sequential applications of selected herbicides. Two different formulations of N-(phosphonomethyl) glycine (glyphosate; isopropylamine, trimesium salts) were applied at 2000 g active ingredient (a.i.) ha−1. Additionally, two synthetic auxins were applied as 3,6-dichloro-2-methoxybenzoic acid (dicamba) at 500 g a.i. ha−1 and the dimethylamine salt of (2,4 dichlorophenoxy)acetic acid (2,4-D) at 1000 g a.i. ha−1. Tank mixtures and sequential applications (12/24 h separation) of these different herbicides were also included. Long-term control of hedge bindweed, Calystegia sepium L., growth was evaluated 8 months after treatments, as comparisons of shoot and rhizome growth (biomass) between untreated and treated plants. There were no differences between the two formulations of glyphosate alone, with shoot and rhizome biomass reductions of 83% and 42%, respectively. Dicamba alone inhibited shoot and rhizome biomass by 86% and 67%, respectively. By itself, 2,4-D provided the greatest reductions in shoot and rhizome biomasses, 93% and 79%, respectively. Antagonism was seen in the tank mixtures of glyphosate and dicamba or 2,4-D. Tank mixtures were generally comparable to treatments of glyphosate alone, and were less effective compared to dicamba or 2,4-D alone. The greatest reduction of bindweed rhizome biomass was for sequential glyphosate trimesium salt followed by 2,4-D 12 h later, thus showing significantly greater efficacy over glyphosate isopropylamine salt (94% vs. 84%; p ≤ 0.05). These data for reductions of the growth of the rhizome biomass show that the sequential application of glyphosate followed by 2,4-D significantly improves long-term control of hedge bindweed.