Atmospheric deposition of dicamba herbicide can cause injury to sensitive soybean

The herbicide dicamba has injured millions of hectares of sensitive plant species in the United States since 2017. This injury has coincided with the commercialization of dicamba-resistant soybean [Glycine max (L.) Merr.] and cotton (Gossypium hirsutum L.). We quantified atmospheric deposition and mass flux of dicamba in 12 soybean production regions of Missouri. Dicamba was routinely detected in weekly deposition samples collected during agriculturally-intensive spray periods. Observed concentrations were indicative of both local (<1 km) and long-distance transport (>1 km) of air-borne dicamba. High deposition events (>100 µg m-2) occurred annually in southeast Missouri, and peak dicamba deposited at these sites (12.5-84.0 µg m-2) was sufficient to injure non-dicamba resistant soybean. Adoption rate of dicamba-resistant crops and atmospheric stability explained much of the variance, and it is difficult for a herbicide product label to address these variables. Overall, these results demonstrated that dicamba was commonly deposited from the atmosphere during the growing season and observed concentrations and fluxes were strongly related to the timing and magnitude of rainfall events, and the amount of dicamba usage near collection sites.