Volatilization of three herbicides applied to corn

Abstract

Secondary drift of herbicides through volatilization can become a significant source of airborne concentrations of the active ingredients. The physicochemical properties, such as vapor pressure, of the active ingredients play a significant role in the rate of volatilization post application. This study utilized a laboratory-scale wind tunnel exposed to ambient conditions to evaluate the volatilization of three herbicides with a range of vapor pressures following application to corn at the V3 growth stage: S-metolachlor (SMOC), bicyclopyrone (BIR), and pendimethalin (PDM). A series of air samples were collected from the wind tunnel with sampling durations of 8-, 16- and 24-h to characterize the temporal profile of pesticide volatilization. About 19% of SMOC application volatilized during the first 8 h while 5% volatilized during next 40 h of sampling. Relative lower fluxes of about 0.5% application were measured for PDM and BIR. All measurements of BIR samples were below laboratory detection limits except for one 8-h sample. Experimental results from this study were also compared to results from three predictive models ranging from a simple linear regression to mechanistic models utilizing physio-chemical properties to predict volatilization rates. The two mechanistic models captured the magnitude and diurnal behavior of the SMOC volatilization rate. However, the volatilization rate of PDM, which has relatively high leaf adsorption characteristics, was significantly overpredicted by all the models, with the exception of the Scholtz's model (scenario II). This specific scenario simulates a wetleaf with no active ingredient on the surface of the leaf. Pesticide uptake by the cuticle is accounted for in this scenario, and it provided the best estimates for all three herbicides.