Herbicide Residues from Winter Wheat Plots: Effect of Tillage and Crop Management

Data on the magnitude and persistence of residues of metribuzin (4-amino-6-t-butyl-3-(methylthio-1,2,4-triazine-5-(4H)-one and bromoxynil octanoate (2,6-dibromo-4-cyanophenyl octanoate) in soils and runoff water from winter wheat (Triticum aestivum L.) fields were collected on plots managed with different tillage and crop rotation systems during three seasons from 1978 to 1982. On steeply-sloped runoff plots, the greatest runoff, erosion, and losses of herbicides occurred on plots that were planted in winter wheat after summer fallow and were tilled conventionally. The lowest erosion and herbicide losses were observed on plots directly seeded in winter wheat stubble without tillage, whereas the relative effectiveness of the other systems depended on the year, the volume of runoff, and the type of runoff events. Wheat residue from the previous crop was more effective than pea (Pisum sativum L.) residue in reducing erosion and herbicide losses, mainly because of the greater quantities of wheat residues produced. Compared to runoff of herbicides under summer conditions in the southeastern USA, runoff of herbicides under winter conditions in Eastern Washington was greatly extended, and there was very little movement or degradation of the herbicides while the ground was frozen. The half-life of metribuzin in soil was 102 to 112 d (k = 0.0069 × 0.0062 d⁻¹), but insufficient data were obtained to calculate the half-life of bromoxynil octanoate. The disappearance of bromoxynil octanoate and bromoxynil (2,6-dibromo-4-cyanophenol) from soil plots was rapid; only traces of bromoxynil octanoate or bromoxynil were present after 72 d under winter conditions. Small amounts of metribuzin and desaminodiketo metribuzin (DADK) moved down as far as 21 cm in the soil. Less than 0.060 µmol kg⁻¹ of metribuzin and less than 0.015 µmol kg⁻¹ DADK remained in the top 7 cm of soil 192 d after application of 0.43 to 0.56 kg ha⁻¹ of metribuzin.