Impact of rainfall and tillage systems on off‐site herbicide movement

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Abstract
Ground cover associated with conservation‐ and no‐till cropping systems can alter runoff and herbicide loss from the soil surface. This study was conducted in 1987 and 1988 to evaluate runoff losses of soil‐applied alachlor and imazaquin in five different soybean cropping systems. Cropping systems used were: 1) conventionally tilled monocrop soybean planted in May; 2) conventionally tilled monocrop soybean planted in June; 3) conventionally tilled doublecrop soybean with incorporated wheat stubble; 4) no‐till doublecrop soybean with burned wheat stubble; and 5) no‐till doublecrop soybean planted into standing wheat stubble. Runoff was lower in 1988 due to a lack of precipitation and dry soil conditions. Herbicide loss in both years was greatest in doublecrop soybean with incorporated wheat stubble, which corresponded to higher runoff. Lower herbicide loss in June‐planted monocrop soybean was attributed to a rougher seedbed that retarded runoff. May‐planted monocrop soybean also lost very little herbicide in 1987, although this treatment had the highest runoff. Lower concentrations of alachlor and imazaquin in runoff from this treatment may be attributed to degradation or soil sorption of herbicides between planting and the first runoff event 23 days later. Highest runoff losses of alachlor and imazaquin, as compared to their initial concentration, were approximately 10% in 1987, and less than 0.5% in 1988. Nomenclature: alachlor, 2‐chloro‐N‐(2, 6‐diethylphenyl) N‐(methoxymethyl) acetamide; imazaquin, 2‐[4, 5‐dihydro‐4‐methyl‐4‐(l‐methyl‐ethyl)‐5‐oxo‐1H‐imadazol‐2‐yl]‐3‐quinolinecarboxylic acid; soybean [Glycine max (L.)] Merr.; wheat (Triticum aestivum L.).