Dicamba losses to air after applications to soybean under stable and nonstable atmospheric conditions

Authors: BISH, MANDY - University Of Missouri; FARRELL, SHEA - University Of Missouri; Lerch, Robert; BRADLEY, KEVIN - University Of Missouri

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/20/2019
Publication Date: 11/1/2019
Citation: Bish, M.D., Farrell, S.T., Lerch, R.N., Bradley, K.W. 2019. Dicamba losses to air after applications to soybean under stable and nonstable atmospheric conditions. Journal of Environmental Quality. 48(6):1675-1682. https://doi.org/10.2134/jeq2019.05.0197.
DOI: https://doi.org/10.2134/jeq2019.05.0197

Interpretive Summary: Weed resistance to the commonly used herbicide, glyphosate (e.g., Roundup®), has prompted development of soybean and cotton with combined glyphosate and dicamba resistance to control broadleaf weeds in these crops. Since 2017, use of commercial products containing glyphosate and dicamba has resulted in an unprecedented number of dicamba drift-related injury cases, mainly to soybean, in the United States. The objective of this research was to study dicamba drift of commercially available products specifically formulated to minimize drift and to relate dicamba air concentrations to weather conditions. The movement of dicamba was studied under field conditions in central Missouri using air samplers to determine air concentrations following application to soybean. In the first set of experiments, new commercial dicamba formulations were applied to soybean. Similar levels of dicamba in air were detected for both formulations, and the highest amounts were detected in the first 8 hours after application. A second set of experiments involved comparisons of mid-day applications, when the atmosphere was unstable (e.g., no temperature inversion and windy), to early evening applications under stable atmospheric conditions (i.e., temperature inversion and low wind). Dicamba detected in the first 8 hours after application was nearly three-fold higher in applications made under stable atmospheric conditions. In all the experiments, dicamba was detected in air up to the last sample time at 72 hours after application, indicating drift occurred regardless of application timing or formulation. Applications that included glyphosate resulted in higher dicamba concentrations than applications lacking glyphosate. Dicamba concentrations were also higher when the atmosphere was more stable. These results provide field-level data that new commercial dicamba formulations can volatilize over time and that atmospheric conditions at application strongly affect dicamba concentrations in air. In addition to following label requirements, growers and pesticide applicators need to be familiar with these factors in order to reduce off-target movement of dicamba.

Technical Abstract: Challenges to control broadleaf weeds in broadleaf crops prompted development of soybean and cotton with dicamba resistance (DR). As a result of an unprecedented number of dicamba-related injury cases in the United States, the movement of dicamba was studied in an applied research setting. High volume air samplers were utilized to determine concentrations of dicamba in air following treatment to soybean (Glycine max. (L.) Merr.). In the first set of experiments, new commercial dicamba formulations were applied to soybean. Applications were made at the same time with treated areas at least 480 m apart to avoid cross-contamination. Similar levels of dicamba were detected for both formulations, and highest amounts (22.6 to 25.8 ng/m3) were detected in the first 8 hours after application. A second set of experiments involved comparisons of mid-day applications, when the atmosphere was unstable, to later applications under stable atmospheric conditions. Dicamba detected in the first 8 hours after application was nearly three-fold higher in applications made under stable atmospheric conditions. All experiments resulted in detection of dicamba through the last timepoint 72 hours after application, indicating volatility occurred regardless of application timing or formulation. Applications that included glyphosate resulted in higher dicamba concentrations than applications lacking glyphosate. Dicamba concentrations were also higher when the atmosphere was more stable. These results provide field-level data that new commercial dicamba formulations can volatilize over time and that atmospheric conditions at application affect dicamba concentrations. Pesticide applicators need to be familiar with these factors in order to reduce off-target movement of dicamba.