Influence of Application Factors on Dicamba Volatility

The commercialization of dicamba-resistant soybean will lead to increased use of dicamba herbicide and importance of proper stewardship in performing these applications. Off-target injury to broadleaf plants from dicamba may occur through physical spray particle drift during the application, volatility of dicamba from the soil or leaf surfaces following the dicamba application, or spray equipment contamination with dicamba in subsequent applications to sensitive crops. In order to help combat the concern for off-target movement by vapor drift in dicamba tolerant soybean, product formulations of dicamba that reduce the potential for volatility have been developed. Other spray application factors that can influence the volatility of dicamba include adjuvant, carrier volume, droplet size, and plant sensitivity to dicamba. A field experiment was conducted to determine dicamba vapor movement as influenced by target surface of the sprayed area and the addition of adjuvant. Volatility was reduced when applied to bare soil compared to vegetation based on soybean plants as the bio-indicator. At 21 days after treatment, an oil emulsifier adjuvant reduced soybean injury from dicamba volatility compared to no adjuvant. As has been previously published, this research, confirms that volatility from a soil surface was less than that from a vegetative surface. In a field experiment, efficacy of dicamba was not different across adjuvants on any broadleaf weed except common lambsquarters, where all adjuvants classified as activators increased weed control compared to no adjuvant used with dicamba. In order to quantify dicamba volatility under controlled environmental conditions, a series of experiments were conducted in a growth chamber with conditions set to an average day in June (i.e. 35 C, 40% R.H., 14-h day). The addition of adjuvant, increasing carrier volume, and ultra coarse droplets decreased volatility of dicamba from soybean vegetation. New formulations of dicamba that were developed specifically for use in dicamba-resistant soybean and have not previously been tested by a third party showed reduced dicamba volatility compared to an older formulation of dicamba. There was no difference in volatility of dicamba following applications to soybean plants tolerant to dicamba and plants that were sensitive to dicamba during the onset of leaf death. In order to optimize dicamba applications for improving weed management and reduce off-site injury, applicators must understand variable under their control than can contribute to dicamba volatility