Reformulation of dicamba herbicide: Impacts on offsite transport and soybean damage

Authors: HAMMER, CALEB - University Of Minnesota; GRIFFIS, TIMOTHY - University Of Minnesota; Baker, John; Rice, Pamela; Frankson, Lara; GUNSOLUS, JEFFREY - University Of Minnesota; ERICKSON, MATTHEW - University Of Minnesota; XIAO, KE - University Of Minnesota; MISTRY, AARTI - University Of Minnesota; SARANGI, D - University Of Minnesota

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/24/2024
Publication Date: 9/29/2024
Citation: Hammer, C.R., Griffis, T.J., Baker, J.M., Rice, P.J., Frankson, L.E., Gunsolus, J.L., Erickson, M.D., Xiao, K., Mistry, A.P., Sarangi, D. 2024. Reformulation of dicamba herbicide: Impacts on offsite transport and soybean damage. Agronomy Journal. 116(5):2200–2216. https://doi.org/10.1002/agj2.21630.
DOI: https://doi.org/10.1002/agj2.21630

Interpretive Summary: In Minnesota, soybeans are a crucial crop with over 2.8 million hectares harvested in the year 2022. Dicamba, an active ingredient in herbicide formulations, is commonly used to prevent the growth of broadleaf weeds in soybeans. However, dicamba is susceptible to volatilization and drift, thereby causing significant plant damage to non-target crops downwind. A commercially available herbicide formulation containing dicamba was reformulated to reduce volatility and off-target movement. We conducted field experiments and assessed the effectiveness of the dicamba reformulation by quantifying dicamba emissions following spray application and investigated how weather factors influenced off-target movement. The experiments were conducted at the University of Minnesota Agricultural Experiment Station during the growing season of 2018, 2019, 2021, and 2022. Air samplers were used to measure dicamba concentrations downwind from a 4-hectare soybean field that was sprayed with the reformulated herbicide. Dicamba emissions were modeled. The results indicate that dicamba emissions and downwind transport were significant for several days following application. Further, standard soybeans (not bred for resistance to dicamba injury) that were located within 15 to 45 m of the dicamba application showed substantial dicamba-related damage. During the hotter, drier growing seasons we observed large dicamba emissions and downwind damage to soybean plants was more extensive, presumably because the dicamba-injured soybean plants were unable to recover under drought-stress conditions. These results indicate that the dicamba reformulation has not adequately prevented significant post-spray volatilization losses and downwind transport, information important for soybean producers.

Technical Abstract: The herbicide dicamba (3,6-dichloro-2-methoxybenzoic acid) is commonly used to control broadleaf weeds in soybeans. Dicamba, however, is susceptible to volatilization and drift, thereby causing significant plant damage to nontarget crops downwind. Dicamba was reformulated to reduce volatility and off-target movement. The effectiveness of the dicamba reformulation was assessed by quantifying dicamba emissions following spray application and investigated how meteorological factors influenced the off-target movement. The experiments were conducted at the University of Minnesota Agricultural Experiment Station (UMORE Park) during the growing season of 2018, 2019, 2021, and 2022. Multiple high-flow polyurethane foam air samplers were used to measure dicamba concentrations downwind from a4-ha soybean field sprayed with dicamba. Dicamba emissions were estimated using backward Lagrangian modeling constrained by the air sample observations. The results indicate that dicamba emissions and downwind transport were significant for several days following application. Further, non-traited soybeans located within15–45 m showed substantial dicamba-related damage. In warmer, drier seasons, increased dicamba emissions caused more severe damage to downwind soybeans, likely worsened by drought stress preventing recovery. Favorable atmospheric conditions that reduced potential drift can be difficult to achieve in terms of the typical weather experienced over agricultural sites in the Upper Midwest. These results indicate that the dicamba reformulation has not adequately prevented significant post-spray volatilization losses and downwind transport.