Research HighlightsInvestigations of Off-Target Movement of Dicamba
By Bryan Young, Purdue University
The commercial launch of Xtend (dicamba-resistant) soybean in 2017 provided growers with an alternative site of action for the control of herbicide-resistant weeds. However, applications of dicamba in Xtend soybean resulted in significant off-target movement of dicamba to sensitive plants, primarily non-Xtend soybeans. The amount of soybean acres affected, and the level of soybean damage, was beyond acceptable levels in some areas. In many cases, traditional models or assumptions of spray particle drift could not explain how adjacent soybean fields were exposed to injurious levels. Secondary movement of dicamba in the air or volatility was often suspected. This $446,004 project aims to characterize the factors and practices that contribute to off-target movement of dicamba.
Field research is being conducted across multiple states to advance knowledge of how and why dicamba movement occurs. This information will be shared with applicators and growers involved with dicamba stewardship and Xtend soybean. Specific parameters being investigated include air concentrations of dicamba following application across different landscapes and weather conditions, the influence of rainfall within 24 hours of application and spray solution characteristics.
Initially, label restrictions prevented the application of dicamba formulations approved for use in Xtend soybean if rain was predicted to occur within 24 hours of application. It was demonstrated that half an inch of overhead irrigation (simulated rainfall) at six hours after dicamba application reduced dicamba air concentrations by at least 75% in the 18 hours immediately following irrigation. Furthermore, no dicamba volatility was detected after the first 24 hours where irrigation was applied, compared with as long as three days without irrigation.
Off-target herbicide movement was defined as “primary” drift, which occurs during the herbicide application, and “secondary” drift, which occurs after the first 30 minutes. The 30-minute window allows any spray droplets (particles) to settle in the target area or move with the wind. Both primary and secondary drifts were evident, with the primary movement explained by prevailing wind direction during application. Secondary off-target movement has not been fully explained, but could be due to temperature inversions and/or dicamba volatility.
Controlled environment experiments demonstrated that drift reduction agents (DRAs) and spray water contaminated with organic matter or sediment did not increase the volatility of Clarity (not approved for Xtend crops), Xtendimax or Engenia. Furthermore, a spray solution with pH from four to six did not increase the volatility when applied to Xtend soybeans. The only spray pH that increased volatility was a pH of three, which would seldom be reached. The addition of ammonium sulfate (AMS) increased dicamba volatility by as much as 16 times. Inclusion of AMS in dicamba solutions had a minor influence on spray pH (0.1 pH units); thus, some other factor is responsible for the increased volatility of dicamba with AMS, and it should be avoided in combination with dicamba for commercial applications.
Results were presented at scientific meetings and extension meetings, and discussed with the U.S. EPA and state regulatory agencies. It may have informed the U.S. EPA when the 24-hour restriction related to rainfall was modified to provide greater flexibility in dicamba applications.
A greater understanding of how herbicide off-target movement occurs supports the overall goal of improving herbicide and environmental stewardship.
Published: May 15, 2020
The materials on SRIN were funded with checkoff dollars from United Soybean Board and the North Central Soybean Research Program. To find checkoff funded research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.
