Dicamba Volatilization and the Effect of Synthetic Auxin Herbicides on Sensitive Soybean
The broad adoption of soybean resistant to synthetic auxin herbicides has led to increased risk for off-target exposure of sensitive plants to these herbicides through both tank-contamination and drift. New dicamba and 2,4-D formulations have been commercialized in an attempt to reduce the potential for off-target movement. A great deal of research has been conducted on soybean response to auxin herbicides alone, but when other postemergence herbicides are introduced into the equation, the effects of 2,4-D and dicamba have not been sufficiently studied. Additionally, the volatilization of dicamba formulations available prior to the registration of new formulations for use in dicamba-resistant soybean has been characterized in order to determine factors that influence off-target movement. However, the volatilization of these new formulations has not been extensively investigated.
Field experiments were conducted to determine 1) the response of glyphosate-resistant soybean to dicamba and 2,4-D, and 2) the influence of a full rate of dicamba applied with tank-contamination doses of 2,4-D on the response of dicamba/glyphosate-resistant soybean. Glyphosate-resistant soybean response to 2,4-D and dicamba were similar to published literature. The ED10 values for injury 14 days after the V2 and R1 exposure timings were 0.03 and 0.18 g ae ha-1, respectively, for dicamba and 35 and 31 g ae ha-1, respectively for 2,4-D. For soybean grain yield, ED10 values for dicamba were 4.61 and 1.66 g ha-1 at the V2 and R1 timings, respectively, and 34 g ha-1 for 2,4-D, combined across exposure timings. Additionally, dicamba/glyphosate-resistant soybean yield response to 2,4-D (ED10 = 34 g ha-1) was similar to glyphosate-resistant soybean (ED10 = 34 g ha-1) and the addition of a full rate of dicamba to 2,4-D tank-contamination did not increase soybean yield response to 2,4-D (ED10 = 59 g ha-1). Thus, no interaction of 2,4-D plus dicamba was apparent on the dicamba/glyphosate-resistant soybean and practices to mitigate exposure to 2,4-D on these soybeans would be no different than other sensitive varieties.
An experiment was conducted to evaluate soybean response to dicamba in conjunction with other labeled postemergence herbicides that are known to cause soybean injury, such as lactofen, acetochlor, and 2,4-DB. Soybean injury at 28 days after the R1 application was influenced primarily by the timing of dicamba exposure rather than the labeled POST herbicide, with up to 14% injury from POST herbicides alone and up to 37% injury from dicamba exposure at a dose of 5.6 g ha-1 alone at R1. Soybean yield reduction in response to dicamba alone was greater during the 2017 growing season with a 37% reduction in yield from dicamba exposure at R1 in 2017 compared with a 17% reduction in 2018. Regardless of the difference in yield response between years, the primary factor that influenced yield was the timing of dicamba exposure. In general, glyphosate-resistant soybean response to a reduced rate of dicamba was not influenced by additional postemergence herbicides applied at either the V3 or R1 growth stage.
Controlled environment experiments were conducted to evaluate the relative volatilization of dicamba formulations applied with drift reduction agents, turbid water carrier, ions in spray solution, and a spray solution pH range. Drift reduction agents and turbid water carrier did not affect dicamba volatilization. Spray pH levels of 4 and above did not result in increased levels of volatilization, while a spray pH of 3 increased volatilization by 2.8X and 3.9X for the DGA + VG and BAPMA formulations, respectively, compared with each respective dicamba formulation applied alone at a native pH of 5.4 and 6.4, respectively. Of the ions tested, diammonium and ferrous sulfate increased dicamba volatilization by 5X and 9X for the DGA + VG formulation, respectively, and 11X for the BAPMA formulation compared with dicamba alone. Additionally, the sulfate and chloride anions present in other ions tested did not cause an increase in volatilization. These results indicate the importance of spray application parameters and the continual attention to details such as tank-mix pH and carrier water ion content that must be practiced prior to an application of a synthetic auxin herbicide to avoid off-target movement.