Dicamba Translocation and Metabolism in Susceptible Soybean and Evaluation of Factors Contributing to Volatilization

The expansion of dicamba-resistant technology increased the use of dicamba in season and relocated the off-target movement towards the growing season, negatively affecting susceptible vegetation, such as soybean. This research aimed to understand different aspects of the off-target movement of dicamba and impact on susceptible soybean. A study determined the accumulation of dicamba in soybean seed (grain) when parental plants at the pod-filling stage were treated with 1/200 of a labeled rate of dicamba at 560 g ae ha-1 and radiolabeled herbicide used as a tracer. This research found that 44% of the total herbicide absorbed by the parent plants was transported toward seeds, and at least 99% of the total remained active, potentially impacting seed quality. Off-target movement investigations conducted by state regulatory officials include collecting and analyzing plant tissue for pesticide content and photos of the alleged damage. The second experiment assessed the persistence of dicamba and 2,4-D in Palmer amaranth and 2,4-D- and dicamba-resistant soybean. The findings revealed that the likelihood of detecting either herbicide in plant samples decreased rapidly following exposure; therefore, early sampling is critical to recovering potential herbicides in plant tissue. A third experiment evaluated the influence of volatilization and suspension of physical particles on the off-target movement of dicamba plus glyphosate with imazethapyr, a non-volatile herbicide added as a tracer. Results showed that the ratio of dicamba to imazethapyr detected in air samples was at least 50:1, several orders of magnitude more than the ratio of herbicide applied to the field (5.3:1), indicating that volatility contributed to most of the off-target movement that occurred. The fourth study established relationships between soybean responses, including visible injury and height reduction and concentration of volatilized dicamba. A predicted dicamba concentration of 1.60 ng m-3 per day resulted in 10% visible soybean injury The fifth study investigated differences in the off-target movement of dicamba impacted by herbicide and adjuvant mixtures or types of surfaces treated, using low tunnel trials. These experiments focused on causes of enhanced dicamba volatility where it was found that both glufosinate and glyphosate increase detection of the dicamba in air. Dicamba volatility was reduced by 70% when a volatility reduction agent was added to dicamba treatments, an adjuvant currently required for any in-crop application with the herbicide

Dicamba Translocation and Metabolism in Susceptible Soybean and Evaluation of Factors Contributing to Volatilization

The expansion of dicamba-resistant technology increased the use of dicamba in season and relocated the off-target movement towards the growing season, negatively affecting susceptible vegetation, such as soybean. This research aimed to understand different aspects of the off-target movement of dicamba and impact on susceptible soybean. A study determined the accumulation of dicamba in soybean seed (grain) when parental plants at the pod-filling stage were treated with 1/200 of a labeled rate of dicamba at 560 g ae ha-1 and radiolabeled herbicide used as a tracer. This research found that 44% of the total herbicide absorbed by the parent plants was transported toward seeds, and at least 99% of the total remained active, potentially impacting seed quality. Off-target movement investigations conducted by state regulatory officials include collecting and analyzing plant tissue for pesticide content and photos of the alleged damage. The second experiment assessed the persistence of dicamba and 2,4-D in Palmer amaranth and 2,4-D- and dicamba-resistant soybean. The findings revealed that the likelihood of detecting either herbicide in plant samples decreased rapidly following exposure; therefore, early sampling is critical to recovering potential herbicides in plant tissue. A third experiment evaluated the influence of volatilization and suspension of physical particles on the off-target movement of dicamba plus glyphosate with imazethapyr, a non-volatile herbicide added as a tracer. Results showed that the ratio of dicamba to imazethapyr detected in air samples was at least 50:1, several orders of magnitude more than the ratio of herbicide applied to the field (5.3:1), indicating that volatility contributed to most of the off-target movement that occurred. The fourth study established relationships between soybean responses, including visible injury and height reduction and concentration of volatilized dicamba. A predicted dicamba concentration of 1.60 ng m-3 per day resulted in 10% visible soybean injury The fifth study investigated differences in the off-target movement of dicamba impacted by herbicide and adjuvant mixtures or types of surfaces treated, using low tunnel trials. These experiments focused on causes of enhanced dicamba volatility where it was found that both glufosinate and glyphosate increase detection of the dicamba in air. Dicamba volatility was reduced by 70% when a volatility reduction agent was added to dicamba treatments, an adjuvant currently required for any in-crop application with the herbicide

Cogongrass [Imperata cylindrica (L.) Beauv.] Control using Chemical Treatment with Cover Cropping Systems

Cogongrass management generally requires multiple herbicide applications, however, success is limited if not integrated with other methods. Experiments were conducted to evaluate the use of cover cropping systems with herbicides on cogongrass control. Field studies determined that sequential glyphosate applications in the summer were necessary to achieve 80% or greater control, but a single application could be effective if weather conditions allowed early planting and good cover crop establishment of Roundup Ready soybeans. Studies also indicated that the use of ALS-resistant Italian ryegrass and white clover crop combinations showed no effect, but imazapyr applications made in May or June provided 80% or higher control by October. Greenhouse experiments showed that delayed planting at least 1 month after imazapyr preemergence applications from 70 to 280 g ae ha-1, significantly reduced emergence failure, height and biomass reductions of legumes used for revegetation

Chlorophyll fluorescence as a marker for herbicide mechanisms of action

Photosynthesis is the single most important source of 02 and organic chemical energy necessary to support all non-autotrophic life forms. Plants compartmentalize this elaborate biochemical process within chloroplasts in order to safely harness the power of solar energy and convert it into usable chemical units. Stresses (biotic or abiotic) that challenge the integrity of the plant cell are likely to affect photosynthesis and alter chlorophyll fluorescence. A simple three-step assay was developed to test selected herbicides representative of the known herbicide mechanisms of action and a number of natural phytotoxins to determine their effect on photosynthesis as measured by chlorophyll fluorescence. The most active compounds were those interacting directly with photosynthesis (inhibitors of photosystem I and II), those inhibiting carotenoid synthesis, and those with mechanisms of action generating reactive oxygen species and lipid peroxidation (uncouplers and inhibitors of protoporphyrinogen oxidase). Other active compounds targeted lipids (very-long-chain fatty acid synthase and removal of cuticular waxes). Therefore, induced chlorophyll fluorescence is a good biomarker to help identify certain herbicide modes of action and their dependence on light for bioactivity. Published by Elsevier B.V

Dicamba air concentrations in eastern Arkansas and impact on soybean

Damage to non–dicamba resistant (non-DR) soybean [Glycine max (L.) Merr.] has been frequent in geographies where dicamba-resistant (DR) soybean and cotton (Gossypium hirsutum L.) have been grown and sprayed with the herbicide in recent years. Off-target movement field trials were conducted in northwest Arkansas to determine the relationship between dicamba concentration in the air and the extent of symptomology on non-DR soybean. Additionally, the frequency and concentration of dicamba in air samples at two locations in eastern Arkansas and environmental conditions that impacted the detection of the herbicide in air samples were evaluated. Treatment applications included dicamba at 560 g ae ha−1 (1X rate), glyphosate at 860 g ae ha−1, and particle drift retardant at 1% v/v applied to 0.37-ha fields with varying degrees of vegetation. The relationship between dicamba concentration in air samples and non-DR soybean response to the herbicide was more predictive with visible injury (generalized R 2 = 0.82) than height reduction (generalized R 2 = 0.43). The predicted dicamba air concentration resulting in 10% injury to soybean was 1.60 ng m−3 d−1 for a single exposure. The predicted concentration from a single exposure to dicamba resulting in a 10% height reduction was 3.78 ng m−3 d−1. Dicamba was frequently detected in eastern Arkansas, and daily detections above 1.60 ng m−3 occurred 17 times in the period sampled. The maximum concentration of dicamba recorded was 7.96 ng m−3 d−1, while dicamba concentrations at Marianna and Keiser, AR, were ≥1 ng m−3 d−1 in six samples collected in 2020 and 22 samples in 2021. Dicamba was detected consistently in air samples collected, indicating high usage in the region and the potential for soybean damage over an extended period. More research is needed to quantify the plant absorption rate of volatile dicamba and to evaluate the impact of multiple exposures of gaseous dicamba on non-targeted plant species