Nearly 20 years ago I summarized research from the University of Minnesota investigating factors that influence volatilization of dicamba (Behrens and Lueschen. 1979. Dicamba volatility. Weed Sci. 27:486-493). That article was prompted by an increase in off-target injury to soybean. Well, once again Iowa is suffering widespread damage to soybean from off-target dicamba movement. Several extension field agronomists have reported that injury is more widespread in 2020 than in the previous three years following registration of over-the-top applications of dicamba. In response to these problems, I decided to summarize some of the recent research investigating factors that influence off-target movement of dicamba. Not surprising, the recent research mirrors the earlier research.
Mueller, T.C. and L.E. Steckel. 2019. Dicamba volatility in humidones as affected by temperature and herbicide treatment. Weed. Technol. 33:541-546.
Mueller and Steckel at the University of Tennessee evaluated dicamba losses due to volatilization as affected by temperature and tank-mixing with Roundup Powermax. Xtendimax with VGT was applied to soil contained in trays, and then placed inside humidors maintained at different temperatures. As would be expected, dicamba volatilization increased as temperatures increased (Figure 1). Tank-mixing dicamba with glyphosate increased dicamba concentrations in the air by 2.9 to 9.3 times across the temperature ranges evaluated compared to dicamba alone. The addition of Roundup Powermax (or other glyphosate formulations) to dicamba reduces the pH of the spray solution, this change in pH has been shown in other research to increase volatilization losses.
Bish, M.D., S.T. Farrell, R.N. Lerch, and K.W. Bradley. 2019. Dicamba losses to air after applications to soybean under stable and nonstable atmospheric conditions. J. Env. Q. 48:1675-1682.
Bish et al. measured dicamba concentrations in the air above a soybean canopy sprayed with dicamba. High volume air samplers were positioned 6 inches above the canopy 30 minutes following the application of dicamba (Engenia plus Xtendimax w VGT). Applications were made during the daytime and evening, the evening applications occurred during stable environmental conditions (low wind speeds). Applications were made according to label restrictions, a drift retardant was included but not glyphosate.
No differences were determined between the two dicamba formulations (Figure 2). Dicamba concentrations in the air above the soybean canopy during the first 8 hours after application was approximately 5X greater than at later sampling dates. However, dicamba was still detected three days following the application.
Time of application influenced dicamba presence in the air above the soybean canopy (Figure 3). Evenings were characterized by low wind speeds; under these conditions higher dicamba concentrations were detected in the air than when dicamba was applied during periods with higher wind speeds. The authors stated the low wind speeds during evening applications could prevent dispersion of dicamba in the atmosphere, resulting in the higher concentrations in the first 8 hours after application.
Oseland, E., M. Bish, L. Steckel, and K. Bradley. 2020. Identification of environmental factors that influence the likelihood of off-target movement of dicamba. Pest. Manage. Sci. 76.
Weed scientists at University of Missouri evaluated factors that influenced whether commercial applications of dicamba on soybean were successful at preventing off-target movement and injury to adjacent crops. They evaluated 135 applications, 45% of the applications were classified as ‘successful’. Applications that had problems with off-target injury had a mean application temperature 3 degrees warmer than successful ones. Impact of wind speed on application success was less clear (at least to me). Maximum wind speed on the day of application was inversely related with the chance of success. With most pesticides the primary concern with drift is the movement of spray droplets with wind. They suggested that higher winds could disperse dicamba, reducing the amount of dicamba contacting sensitive plants in the area. Bradley’s group documented higher concentrations of dicamba in the atmosphere when the product was applied during calm conditions (Bish et al. 2019; Figure 3). The likelihood of a successful application decreased with increasing winds the day following application.
The researchers found that the likelihood of an unsuccessful application (off-target injury) increased as the soil pH decreased. They conducted trials with pH-adjusted soil to evaluate volatilization of dicamba off the soil surface. Plastic hoop structures were erected over susceptible soybean. Dicamba was applied to soil contained in a 20” by 11” flats, following application flats were placed within the hoops for 72 hours to allow volatilization from the soil surface. Dicamba volatilization increased as pH decreased (Figure 4). It is important to note that it is the pH of the soil surface that will determine vapor loss; pH values from routine soil tests may not be valid for evaluating soil pH influence on volatility. Fields under no-till production or that have surface applications of N likely have a surface pH more acidic than pH provided by a soil test. Other research has shown greater volatilization losses when the pH of the carrier solution is decreased.
Several dicamba formulations were evaluated, and while there were significant differences in vapor loss among formulations, all resulted in measurable soybean injury (Figure 5). All formulations had greatest losses when applied to a soil with a pH of 4.3 (data not presented).
Soltani, N. et al. 2020. Off-target movement assessment of dicamba in North America. Weed Technol. 34:318-330.
A recent paper in Weed Technology evaluated off-target movement of dicamba applied to dicamba-resistant (DR) soybean to adjacent susceptible soybean. A combination of dicamba (Xtendimax plus VGT) + glyphosate was applied to a block of DR planted within a field of susceptible soybean. All applications were made according to label restrictions (sprayer set up, environmental conditions). To distinguish particle drift from vapor drift (secondary movement) tarps were placed over plants at regular intervals downwind during application. Dicamba symptoms on plants under tarps was attributed to vapor drift since the tarps would intercept spray droplets leaving the treated area during application. Soybean were rated 21 to 28 day after application, and a model was developed to estimate the distance vapor drift would cause 1 and 10% injury.
As experience has shown, the risk of off-target movement is hard to predict. The distance where 10% injury was observed was more than 10 times greater in Arkansas than the other locations (Figure 6). This is likely due to higher temperatures at and following application. At the other locations, secondary movement causing 10% injury ranged from 0 ft in Ontario to 16 ft in WI.
Secondary movement resulted in soybean injury occurred at all but one location, and the authors concluded that high temperatures associated with low air movement increases the likelihood and magnitude of secondary movement resulting in crop injury.
Summary All of the aforementioned research supports that secondary movement (volatilization) is a significant contributor to dicamba movement from treated areas. Combining this volatility with the extreme sensitivity of non-resistant soybean makes it essentially impossible to use current formulations of dicamba in a landscape where both resistant and susceptible soybean are grown without significant crop injury.