Environmental and Chemical Influences on Dicamba Volatility and Soybean Response
Creators: name: Osterholt, Matthew Joseph
Publisher: name: Purdue University Graduate School
Year: 2023
Subjects: Agrochemicals and biocides (incl. application)

description: Since the commercialization of dicamba-resistant soybean and cotton, numerous instances of suspected off-target dicamba movement onto sensitive plant species have been reported. Further investigation into chemical and environmental factors that influence dicamba volatilization is warranted to better understand the mechanisms that lead to increased dicamba off-target movement via volatilization and plant response to dicamba vapor. The environmental impacts of dicamba must be minimized in order to ensure the sustainability and continued use of dicamba, which is an important herbicide for controlling broadleaf weeds in key cropping systems and non-crop sites. Controlled environment experiments were conducted to characterize the chemical interactions with dicamba volatility for three formulations of dicamba on glass slides. Dicamba volatility was similar for spray solution pH levels 4 to 8 for the diglycolamine (DGA) and the diglycolamine with VaporGrip® (DGA+VG) formulations. For the N,N-Bis-(3- aminpropyl) methylamine (BAPMA) formulation, dicamba volatility increased at a pH level of 5 with continued increases in volatility occurring as spray solution continued to decrease indicating that BAPMA formulation is more sensitive to pH fluctuations than the DGA and the DGA+VG formulations. While spray solution pH levels below 4 increased dicamba volatility for all three formulations compared to each formulation applied at a native pH level (5.53, 5.2, and 6.28 for the DGA, DGA+VG, and BAPMA formulations, respectively), the largest increase in dicamba volatility occurred when ammonium or iron was added to spray solution. Therefore, applicators should avoid mixing dicamba with other tank-mix partners that contain ammonium or iron to minimize the likelihood for dicamba volatilization. While extensive research exists documenting the process of dicamba volatilization, there has been little confirmation regarding how volatilized dicamba enters sensitive plants. Dicamba-sensitive (DS) soybean with different levels of canopy conductance, from different watering regimes and exposure time of day, were exposed to dicamba vapor. The DS soybean response was positively correlated with soybean canopy conductance during the dicamba vapor exposure suggesting that dicamba vapor route of entry into soybean is facilitated through the stomata. An additional experiment was conducted that exposed the single side of a hypostomatic leaf to dicamba vapor on different northern red oak trees. Northern red oak tree response was substantially greater when the abaxial leaf surface (high stomatal density) was exposed to dicamba vapor compared to when the adaxial leaf surface (no stomata) was exposed to dicamba vapor. Thus, dicamba vapor entry into plants is largely facilitated via stomata and secondly through re-deposition onto the leaf surface, where dicamba is absorbed through the plant cuticle, albeit minor. If dicamba vapor is redeposited onto leaf surfaces, dicamba acid absorption through the cuticle can be limited without the presence of a surfactant. Field and greenhouse experiments confirmed that the presence of surfactants from applications of other formulated herbicides can exacerbate soybean response to dicamba acid that was deposited on the leaf surface. In the midwestern United States, off-target dicamba movement to DS soybean has been problematic as DS soybean are extremely sensitive to very low concentrations of dicamba. Field and greenhouses studies confirmed that there are phenotypic differences amongst different soybean genotypes and their response to dicamba. Estimations of visual soybean injury was approximately 10% less for genotypes that were less sensitive to dicamba compared to genotypes with increased sensitivity. The future identification of the mechanisms that lead to decreased sensitivity to dicamba could be used to identify soybean cultivars that could mitigate the impacts of dicamba off-target movement to DS soybean. Lastly, a field experiment was conducted that investigated the influence of simulated dew on dicamba volatility from dicamba treated soybean leaves, in addition to soybean response in the presence of dicamba vapor. The results from a field experiment determined that consecutive simulated dew applications increase dicamba volatility from dicamba treated soybean. Furthermore, this is the only research demonstrating that DS-soybean response increases from dicamba vapor in the presence of dew. The results from this dissertation provide further insight into the chemical and environmental factors that influence dicamba volatility, the route of entry of dicamba vapor into plants, and soybean response to dicamba.