Effect of temperature on the dissipation of seven herbicides in a biobed matrix

Cold winters and short, warm summers in the Canadian prairies pose a challenge for the effectiveness of on-farm biobeds for degrading agricultural pesticides. A thermo-gradient plate was used to evaluate the effect of temperature on the dissipation kinetics of seven commonly used herbicides applied to a biobed matrix composed of materials typically available on a farm. The dissipation of all seven herbicides increased with increasing incubation temperature and duration. 2,4-D, bromoxynil, and thifensulfuron-methyl dissipated completely during the 35 d incubation at 13 and (or) 20 °C. Tribenuron-methyl, pyrasulfotole, thiencarbazone-methyl, and metsulfuron-methyl dissipated 93%, 70%, 64%, and 34%, respectively, at 20 °C. The order of decreasing dissipation in the biobed matrix reflected the relative soil half-lives and soil sorption coefficients of the herbicides. Metsulfuron-methyl and thiencarbazone-methyl had the lowest activation energies and temperature quotients and were the least sensitive to increases in incubation temperature. At 20 °C, the half-lives of all herbicides were