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 <70 d. However, 10 yr average soil temperatures to 1 m depth from a site in Saskatoon, SK, were considerably <20 °C for much of the growing season. Assuming soil temperatures to be a proxy for expected temperatures of on-farm biobeds, biobed temperatures would not be high enough for a long enough period of time to achieve complete dissipation of some herbicides. Consequently, biobeds in the Canadian prairie provinces may require supplemental heating, especially in spring and late fall, to maintain incubation temperatures of approximately 20 °C to optimize the degradation of herbicides used in prairie crop production.
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