SURFACE AND SUBSURFACE TRANSPORT OF ATRAZINE AND ALACHLOR FROM A BROOKSTON CLAY LOAM UNDER CONTINUOUS CORN PRODUCTION

Three tillage practices, conventional, zero, and ridge tillage, were compared for water quality in the first three years of tillage establishment. The tillage treatments were on a poorly drained, Brookston clay loam soil of <l % slope, tiled to a depth of 95 cm. Tillage had no significant effect on runoff volume, distribution between surface and subsurface runoff, herbicide concentration or herbicide loss. The volume of tile discharge exceeded the volume of surface runoff in all years. Triazine losses varied from 1.2 to 7.7 % of atrazine applied (1.8 kg/ha) while alachlor losses were less than 0.5 % of application (2.5 kg/ha). Herbicide losses were greatest when runoff producing events occurred soon after herbicide application. The dealkylated metabolite, de-ethyl atrazine, was detected in the runoff water at low concentrations throughout the study period consistent with its occurrence as an intermediate metabolite. Herbicide concentrations were higher in surface runoff water than subsurface runoff water which resulted in a greater proportion of herbicide loss from the surface. In most cases more atrazine was lost during the growing season that at other times, however, in drier years losses after harvest may account for a significant proportion of total loss. In all years, alachlor losses occurred during the growing season, because of its shorter persistence in soil relative to atrazine.