Environmental Behavior of the Chiral Herbicide Haloxyfop. 2. Unchanged Enantiomer Composition in Blackgrass (Alopecurus myosuroides) and Garden Cress (Lepidium sativum)

Haloxyfop-methyl is a chiral herbicide against grasses in dicotyledonous crops. In plants and soil, haloxyfop-methyl is rapidly hydrolyzed to haloxyfop-acid, whose R-enantiomer carries the actual herbicidal activity. In soil, S-haloxyfop-acid is converted within less than 1 day and almost completely into R-haloxyfop-acid. In this study, we investigated the possible interconversion of the enantiomers of haloxyfop-methyl and haloxyfop-acid in blackgrass and garden cress. Racemic or enantiopure haloxyfop-methyl was applied to the leaves of plants grown in agar. The metabolism was followed during 4 days using enantioselective GC-MS. In contrast to soils, no interconversion was observed in plants, and metabolism was nonenantioselective. These findings are consistent with the fact that after pre-emergence application to soil and uptake by roots, the observed herbicidal effect is basically independent of the enantiomer composition of the applied substance, whereas after postemergence application, the efficacy clearly is different for the two enantiomers.