Dicamba emissions under field conditions as affected by surface condition

Abstract

The evolution and widespread distribution of glyphosate-resistant broadleaf weed species catalyzed the introduction of dicamba-resistant crops that allow this herbicide to be applied POST to soybean and cotton. Applications of dicamba that are most cited for off-target movement have occurred in June and July in many states when weeds are often in high densities and at least 10-cm or greater at the time of application. For registration purposes, most field studies examining pesticide emissions are conducted using bare ground or very small plants. Research was conducted in Knoxville, TN in the summer of 2017, 2018, and 2019 to examine the effect of application surface (tilled soil, dead plants, green plants) on dicamba emissions under field conditions. Dicamba emissions after application were affected by the treated surface in all years, with the order from least to most emissions being dead plants < tilled soil < green plant material. In fact, dicamba emissions were > 300% when applied to green plants compared to other surfaces. These findings suggest that dicamba applications made to bare ground will likely underestimate what may occur under normal field use conditions when POST applications are made and the crop canopy or weed groundcocver is nearly 100% green material. A potential change to enhance the accuracy of current environmental simulation models would be to increase the theoretical findings to allow for the effect of green plant material on dicamba emissions under field conditions.

Effect of 2,4-D formulation on volatility under field conditions

“…The 28.9 ng m -3 of detected dicamba was similar to what was detected for both 2,4-D formulations across both surfaces. These results would be consistent with Mueller and Steckel (2021), who reported that more dicamba emissions were detected from green plants than dead plants or tilled soil.…”

“…The following 2 yr added the variable of having green vegetation also present in some of the field experiments. Previous research has indicated that the surface condition affects dicamba volatility (Behrens and Lueschen 1979;Mueller and Steckel 2021). Because of the limited number of air samplers available, the authors wish to clearly state that each field experiment was independent and separate from all the other experiments.…”

Large-scale evaluation of 2,4-D choline off-target movement and injury in 2,4-D-susceptible soybean

“…Herbicides can also move to a neighboring area in the form of vapor within a few hours and up to ≥48 h after application (Matthews et al 2014;Soltani et al 2020). Herbicide physiochemical properties (i.e., vapor pressure) are key in predicting vapor drift potential; however, numerous environmental conditions such as temperature, humidity, wind, and the treated surface conditions (e.g., bare soil, dry vegetation, actively growing vegetation) can also influence herbicide volatility (Mueller and Steckel 2021;Strachan et al 2010;Striegel et al 2021;Sosnoskie et al 2015). The 2,4-D choline formulation is reported to be 88% and 96% less volatile than the amine and ester formulations, respectively, and application with an approved nozzle could reduce spray drift up to 90% (Havens et al 2018;Hillger et al 2012).…”

Conversations about the Future of Dicamba: The Science Behind Off-Target Movement

“…Dicamba vapor transport has also been associated with the density of foliage, soil pH, air temperature, relative humidity, wind speeds, and on-site meteorological conditions . However, a meta-analysis of 23 trials concluded that many of these parameters by themselves do not explain the variability in in-field exposures .…”