Runoff of herbicides can promote adverse impacts in receiving waters. The objective of this study was to assess the effects of rainfall delay, herbicide application rate, rainfall intensity, and pre-application rainfall on runoff of TRIMEC (a combination of 2,4-D, dicamba, and mecoprop), a herbicide that is commonly used in central Kentucky. The levels of rainfall delay were 0, 2, and 4 d following application; and the levels of herbicide application rate were 0, 0.5, 1 and 2 times the recommended rate. Simulated rainfall was applied at intensities of 64, 102, and 140 mm h-1; and the depths of water applied prior to TRIMEC application were 0, 13, and 25 mm. Flow-weighted composite runoff samples were analyzed by gas chromatography. Maximum concentrations in runoff for treatment combinations studied were: 2,4-D, 45.5 µg L-1; dicamba, 1.59 µg L-1; and mecoprop, 212 µg L-1. The rainfall delay affected both 2,4-D and dicamba concentrations but not mecoprop concentration, suggesting that its foliar half-life might be longer than suggested. As anticipated, runoff concentrations of all TRIMEC constituents were significantly (p < 0.05) affected by herbicide application rate. Rainfall intensity affected only the concentration of mecoprop, with concentrations at the highest intensity being significantly (p < 0.05) greater than those at the two lower concentrations. Pre-application rainfall had no significant effects on runoff concentrations. Mass transport averaged 1.51, 0.38, and 14.8% of amounts applied for 2,4-D, dicamba, and mecoprop, respectively, reflecting differences in degradation rates, wash-off characteristics and other factors. Mass transport was in no case significantly affected by the treatments. The findings of this study suggest that when TRIMEC is applied at the recommended rate under comparable soil, vegetation and weather conditions, the potential for 2,4-D to exceed the maximum contaminant level of 70 µg L-1 in runoff is low.

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Published in Transactions of the ASAE, v. 42, issue 6, p. 1631-1636.

© 1999 American Society of Agricultural Engineers

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