A field lysimeter study to evaluate herbicide transport in a wyoming irrigated pasture

Abstract

A field study was conducted to assess the fate and mobility of two commonly used herbicides, dicamba (3,6‐dichloro‐2‐methoxybenzoic acid), and picloram (4‐amino‐3,5,6, trichloro‐2 pyridinecarboxylic acid), in an irrigated pasture. The herbicides were applied at two application rates to four plots (two replicates) that contained soil‐water collectors. Herbicide concentrations as a function of depth and application rate were evaluated statistically to determine if replicate data could be combined. Results indicated dicamba concentration variations were low, suggesting the mean value of the replicate plots could be compared; however, picloram concentration variations were high, and the replicate plot data could not be combined. Half‐time values for herbicide disappearance (time required for herbicide concentration to diminish from its maximum to half maximum levels), calculated from the arithmetic mean of the logistic equations, were found to be useful in describing herbicide transport. Half‐time values were dependent upon application rate, collector depth, pesticide type, and plot location. Dicamba applied at the low application rate was detected at 15 and 30 cm depths only, and was not detected throughout the entire study period (467 days) at deeper depths. Significant differences occurred between plots receiving the high dicamba application rate, which was attributed to permeability differences that affected herbicide migration in the soil profile. Picloram half‐time values were also affected by soil organic carbon, which increased picloram adsorption. In general, differences in herbicide concentrations were observed within plots receiving the high picloram application rates, which was primarily attributed to spatial differences in hydraulic conductivity. Herbicide disappearance was most rapid at the shallow depth (e.g., 15 cm) within plots receiving lower herbicide application rates.