Soil persistence studies with bromoxynil, propanil and [ 14 C]dicamba in herbicidal mixtures

Summary: The persistence of bromoxynil (3,5‐dibromo‐4‐hydroxybenzonitrile), [ 14 C]dicamba (3,6‐dichloro‐2‐methoxybenzoic‐7‐ 14 C acid) and propanil [ N ‐(3,4‐dichlorophenyl)propionamide] at rates equivalent to 1 kg ha −1 , were studied under laboratory conditions in a clay loam, a heavy clay and a sandy loam at 85% of field capacity and at 20±1°C, both singly and in the presence of herbicides normally applied with these chemicals as tank‐mix or split‐mix components. The degradation of bromoxynil was rapid with over 90% breakdown occurring within a week in the heavy clay and sandy‐loam soils, while in the clay‐loam approximately 80% of the bromoxynil had broken down after 7 days. In all three soils degradation was unaffected by the presence of asulam, diclofop‐methyl, flamprop‐methyl, MCPA, metribuzin or propanil. Propanil underwent rapid degradation in all soil treatments, with over 95% of the applied propanil being dissipated within 7 days. There were no noticeable effects on propanil degradation resulting from applications of asulam, barban, bromoxynil, dicamba, MCPA, MCPB, metribuzin or 2,4‐D. The breakdown of [ 14 C]dicamba in a particular soil was unaffected by being applied alone or in the presence of diclofop‐methyl, flampropmethyl, MCPA, metribuzin, propanil or 2,4‐D. The times for 50% of the applied dicamba to be degraded were approximately 16 days in both the clay loam and sandy loam, and about 50 days in the heavy clay.