Impact of Initial Soil Water Content, Crop Residue Cover, and Post-Herbicide Irrigation on Herbicide Runoff

Herbicide loss in runoff is strongly influenced by rainfall immediately following herbicide application and by environmental conditions, such as crop residue cover and soil water content. A laboratory rainfall simulator was used to quantify the impact of initial soil water content (0.12 and 0.24 kg kg-1), crop residue cover (10% and 30% surface cover), post-herbicide irrigation (4 to 8 mm), and timing of first runoff event (1, 8, and 15 days) on atrazine and metolachlor runoff. Herbicides were applied (1.3 kg ha-1 a.i. atrazine; 1.6 kg ha-1 a.i. metolachlor) to the surface of self-contained soil trays (0.55 × 0.28 m), and simulated rainfall was applied at 55 mm hr -1. Herbicide concentration and mass loss in runoff were evaluated after 13, 25, 38, and 51 mm of rainfall, but treatment effects were independent of rainfall depth. Greater initial soil water content substantially increased herbicide concentration. When initial soil water content was 24% (versus 12%), 2 to 3 times more herbicide mass loss was observed when runoff occurred 1 and 8 days after herbicide application. After 51 mm of simulated rain, 30% crop residue cover resulted in 22% to 29% less water runoff and 35% to 50% less atrazine mass loss than 10% residue cover. Average herbicide concentrations were similar for both residue levels, indicating that differences in herbicide mass loss resulted from different water runoff volumes. The post-herbicide irrigation ("rain-in") reduced atrazine mass loss by 33% on day 1, largely from reduced concentration, but no mass loss reduction was seen on days 8 and 15, when soil crusting is believed to have increased runoff volume. These results demonstrate the importance of soil water content during the first runoff following herbicide application and quantify how low antecedent moisture, greater crop residue cover, and a light post-herbicide irrigation can reduce herbicide runoff.