Dicamba Losses to Air after Applications to Soybean under Stable and Nonstable Atmospheric Conditions

Challenges to control broadleaf weeds in broadleaf crops prompted development of soybean [Glycine max (L.) Merr.] and cotton (Gossypium hirsutum L.) with dicamba resistance. 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 used to determine concentrations of dicamba in air after treatment to soybean. 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 the highest amounts (22.6 to 25.8 ng m(-3)) were detected in the first 8 h after treatment (HAT). 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 HAT was nearly threefold higher in applications made under stable atmospheric conditions. All experiments resulted in detection of dicamba through the last time point 72 HAT, indicating that volatility occurred regardless of application timing or formulation. Applications that included glyphosate resulted in higher dicamba concentrations than applications lacking glyphosate. 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 to reduce off-target movement of dicamba.