Effect of Tillage Systems on Runoff Losses of Pesticides, A Rainfall Simulation Study

Abstract

ABSTRACT PESTICIDE runoff losses were measured from small plots subjected to simulated rainfall. The plots were established at three locations in Iowa, each with different soils and slopes, and were cropped to corn using six different tillage practices. Experimental con-ditions were conducive to pesticide runoff losses with tillage up and down hill, and large, intense rainstorms simulated shortly after pesticide application. One in-secticide (fonofos) and two herbicides (alachlor and cyanazine) were applied to the soil at recommended rates. A total of 21.6 cm of water was applied as three rains, two at 6.35 cm/h and a third at 12.7 cm/h. Con-centrations of all three pesticides in both sediment and water samples decreased significantly with time. Pesticide concentrations in water were correlated with percent residue cover; pesticide concentrations in sedi-ment were not correlated with percent residue cover except for cyanazine at one location and fonofos at two locations. Fonofos concentrations in water were so low (avg. 0.014 ppm) that even though much more water than sediment was lost, the major carrier was sediment (avg. of 1.8 percent of the fonofos applied was lost). Thus, tillage practices that decrease erosion should decrease runoff loss of fonofos. The ratio of herbicide concentra-tion in sediment to that in water (ranged from 5 to 10) was less than the ratio of water to sediment lost; thus, water was the major carrier of the two herbicides. De-creases in herbicide losses from decreased runoff and erosion from plots with conservation tillage practices were negated by higher herbicide concentrations. For alachlor the average loss from all plots was 7.9 percent of that applied (8.0 percent for conventionally tilled plots); the average for cyanazine was 11.0 percent (9.7 percent for conventionally tilled plots).