REDUCING HERBICIDE LOSSES TILE-OUTLET TERRACES

This study evaluated the effectiveness of various tillage/herbicide ma nagement practices for reducing atrazine and cyanazine runoff from the -outlet terrace systems. Of particular interest was the effectiveness of the 66 ft (20-meter), no-spray setback area around tile-outlet rise r pipes neeeded to meet label requirements compared to (1) tilled, 100 % sprayed area; (2) no-till, 100% sprayed area; and (3) tilled, 100% s prayed area with herbicide incorporation. Four practices were replicat ed three times on 12 individual tile-outlet terraces in Iowa, Missouri , and Nebraska. Atrazine and cyanazine were applied at rates ranging f rom 0.88 to 2.5 lb./arce (0.99 to 2.8 kg/ha). Three runoff events were sampled in Iowa and Missouri, and one in Nebraska. Concentrations of atrazine and cyanazine were greatest from the tilled and no-till plots where 100% of the contributing area was sprayed. Conclusions include (1) incorporation into no-till or disk-till reduced herbicide runoff c oncentrations by approximately 25% compared to surface application alo ne; (2) thc setback reduced herbicide losses in proportion to the area not sprayed; (3) no-till reduced total herbicide mass loss 94% and 91 % for cyanazine and atrazine, respectively, because of a 72% reduction in total runoff volume; and, (4) a comparison of tilled and no-till b ased an one runoff event suggests greater herbicide mass loss from no- till; however, based on three events no-till shows a reduction in mass loss.