| The purpose of this research was to compare the effects of tilled and no till systems, corn and soybean crop type, and four time periods (Apr–May, Jun, Jul–Aug and Sep–Nov) within the growing season, on soil and water quality as indicated by the herbicide, metolachlor. The amount of metolachlor in surface and subsurface (tile) water and the soil profile, from the surface to 30 cm depth, was monitored. Six pairs of fields (12 sites) with medium (silt loam) textured soils in southwestern Ontario were included in the study.; In this study, the concentration of metolachlor ranged from not detected in soil and water, to maxima of 1930 and 200 μg/kg in soil at the 0 to 10 and 10 to 30 cm depths, respectively, and 225 and 400 μg/kg in water from surface runoff and tile flow, respectively. Tilled fields contributed three times more water, by volume, from surface runoff, per time period, than did no till fields. The difference in the loss of metolachlor, expressed as a percentage of applied metolachlor, between till (59%) and no till (22%) systems during the June time period was large and unexpected. The initiation of the spring flush phenomenon (Thurman et al., 1991) was clearly demonstrated since water containing metolachlor exited fields as surface runoff and tile flow.; This study showed that regardless of tillage system or crop type, water quality was often impaired during the one-to-three-month period following the application of metolachlor to control weeds. However, during the growing season, the amount of metolachlor in soil or water was minimised when no till or soybean production systems were used. These findings support the continued promotion of no till crop production systems as a best management practice for medium textured soils in southwestern Ontario. |
