Updated February 3, 2020:
The commercial launch of Xtend (dicamba-resistant) soybean in 2017 provided farmers with an alternative for control of herbicide-resistant weeds. However, applications of dicamba in Xtend soybean from 2017 to present continue to result in off-target movement (OTM) and injury to sensitive soybean, and other plants, beyond acceptable levels. The long-term goal of this project is to reduce the threat OTM of dicamba represents to soybean farmers in terms of their freedom to operate, profitability, and sustainability. By characterizing the factors influencing the greatest movement of dicamba movement and evaluating practices to help mitigate movement we can move closer to this goal.
Large-scale field research was conducted across several states in 2018 and 2019 to determine the extent of dicamba OTM from applications of Xtendimax plus Roundup Powermax. Off-target herbicide movement in our research was defined as “primary” drift that occurs during the herbicide application, and “secondary” drift, which occurs after the first 30 minutes following the application. The 30-minute period after application was intended to allow for any spray droplets (particles) from the application to settle in the target area or move with the wind off-site. The major emphasis of our research was to document the secondary drift of dicamba following application. In the 12 experiments conducted, evidence of secondary movement of dicamba outside of the treated area was evident in 11 sites. Furthermore, secondary movement of dicamba upwind from the direction of the wind during the application was evident in the same 11 sites. The extent of soybean injury from secondary movement of dicamba was primarily less than 24%, but soybean injury beyond 50% was noted. The duration of dicamba OTM was 96 hours following application for three of the research sites, but was most commonly observed out to 48 hours from application. The peak amount of dicamba secondary movement was in the first 24 hours after dicamba application. The cause or factors responsible for the extent of secondary OTM of dicamba in our research have not been fully explained, but could be due to temperature inversions and/or dicamba volatility.
Overhead irrigation (simulated rainfall) applied at 6 hours after dicamba application to Xtend soybean markedly reduced the amount of dicamba found in air samples taken within the first 24 hours after the irrigation event. The influence of rainfall was even more dramatic out to 48 hours from the irrigation event with 38 times less dicamba found in air samples, down to near normal background levels of dicamba. These results suggest rainfall can reduce the amount of dicamba remaining in the air following an application and can effectively reduce dicamba OTM.
The potential for dicamba herbicide volatility as influenced by different spray solution aspects was investigated in controlled environment chambers using three dicamba herbicide formulations: Clarity, Xtendimax, and Engenia. The addition of drift reduction agents (DRAs) or using water carrier with high turbidity (high sediment from soil or organic matter) did not increase dicamba volatility when sprayed on Xtend soybean. Dicamba spray solutions with a pH range from 3 to 8 were evaluated and a solution pH of 3 doubled the amount of dicamba volatility compared with higher pH solutions. However, spray solutions ranging from a pH of 4 to 6 had no influence on dicamba volatility, regardless of the specific dicamba formulation. Why didn’t these slightly acidic solutions increase dicamba volatility when applied to Xtend soybean? Subsequent research showed that the leaf surface chemistry can buffer the pH of the spray solution droplets within minutes of application to a more neutral pH. Even though spray pH may not be as important as previously thought, the addition of ammonium sulfate (AMS) increased dicamba volatility by as much as 16X, while only reducing the spray pH by an average of 0.1 pH units. Thus, some other factor than acidification of the spray solution pH is responsible for the increased volatility of dicamba with AMS and this should still be avoided in combination with dicamba for commercial applications.
The investigator team has shared our research findings with interest groups as the results have become available. More specifically, our research activity was discussed directly with representatives from the U.S. EPA and the Association of American Pesticide Control Officials (AAPCO) on September 23 and 24, 2019 in Lafayette, IN. In addition, more complete results were shared with the U.S. EPA at a research meeting held in Alexandria, VA on December 16, 2020. Multiple webinar recordings to discuss this research are scheduled with the Plant Management Network on March 12 and 26, 2020 to be included in the Take Action initiative.