Residue, Chemical Placement, And Metolachlor Mobility

Monolithic soil cores were used to evaluate the impact of tillage and point-of-chemical placement on off-site movement of metolachlor. Nine 254-mm diameter soil cores were removed from an agriculturally productive field site and positioned under a rainfall simulator. Provisions were made for collecting surface runoff and deep seepage from the cores. A one-time application of metolachlor at a rate of 2.2 kg active ingredient per ha was made to three bare, tilled surfaces, three untilled surfaces beneath a covering of wheat straw, and three untilled surfaces over-the-top of a covering of wheat straw. The nine cores were subjected to simulated rainfall events of 26.5 mm per hr intensity at 4, 48, 168, 504, 1008, and 2016 hours after application of metolachlor. A small quantity of metolachlor exited the cores in both runoff and in deep seepage water. There were no significant differences in concentrations of metolachlor in deep seepage among cores with the three different surface treatments. There was, however, significantly more metolachlor in runoff from cores where the chemical was applied over-the-top of wheat straw. Results, thus, indicate that a contribution to improved water quality may be made by developing under-residue herbicide application practices.