ATRAZINE AND METOLACHLOR LOSS IN SURFACE AND SUBSURFACE RUNOFF FROM 3 TILLAGE TREATMENTS IN CORN

Conservation tillage with residue management has been proposed as the most effective means to reduce soil deterioration in southwestern Ontario. Two conservation tillage treatments, ridge (RT) and zero (ZT) tillage, were compared with conventional moldboard plow (CT) tillage from 1987 to 1990 for atrazine [6-chloro-N-ethyt-N'-(1-methyethyl)-1,3,5-triazine-2,4-diamine], deethylatrazine [6-chloro-N'(1-methylethyl)-1,3,5-triazine-2,4-diamine] and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide] loss in surface runoff and tile discharge. Twenty-five to 44% of the annual rainfall resulted in total field runoff (surface runoff or tile discharge). Conservation tillage increased surface runoff 42% and decreased tile discharge 15% compared with conventional tillage, but total held runoff was the same from all treatments. Runoff events shortly after herbicide application produced the greatest herbicide concentrations and losses in both surface runoff and subsurface drainage. Averaged over years, herbicide concentration in surface runoff and tile discharge was, respectively, 1.3 and 1.8 times greater from conservation than conventional tillage. Conservation tillage altered the source of herbicide loss compared with conventional tillage, but environmental factors after herbicide application were more important in determining total herbicide loss than tillage. Deethylatrazine was found in both surface runoff and tile drainage and accounted for 10 to 27% of combined atrazine and deethylatrazine loss.