Influence of fenchlorazole-ethyl on the metabolism of fenoxaprop-ethyl in wheat, barley, and crabgrass

The safener, fenchlorazole-ethyl (ethyl-1-(2,4-dichlorophenyl)-5-trichloromethyl-1H-1,2,4-triazole- 3-carboxylate), eliminated and reduced, respectively, the phytotoxicity of the herbicide fenoxaprop-ethyl (ethyl-(+/-)2-[4-[(6-chloro-2-benzoxazolyl)-oxy]phenoxy] propanoate) to wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.). In barley the ED50 value for foliar-applied fenoxaprop-ethyl was 150 g ai/ha. In wheat an ED50 value could not be determined because of its tolerance to the herbicide but there was a 30% reduction in dry weight after application of 600 g ai/ha. In both wheat and barley no reduction in dry weight occurred when the same respective doses of the herbicide were applied in combination with the safener (4:1 w/w, herbicide:safener). Conversely, addition of fenchlorazole-ethyl increased the toxicity of fenoxaprop-ethyl to crabgrass (Digitaria ischaemum (Schreb, Muhl.)). Metabolism studies on foliar-applied [14C]fenoxaprop-ethyl indicated that the herbicide was deesterified to fenoxaprop in all three species. Regardless of whether the safener was present or not, three major metabolites, 6-chloro-2,3-dihydro-benzoxazol-2-one (HOE 054014), and two unidentified water-soluble metabolites, were found in wheat, barley, and crabgrass 48 hr after treatment. When [14C]fenoxaprop-ethyl was applied alone, approximately 29, 63, and 9% of the applied radioactivity was metabolized to products other than fenoxaprop 48 hr after treatment, whereas 48, 96, and 8% was metabolized when the safener also was applied to wheat, barley, and crabgrass, respectively. These results indicate that fenchlorazole-ethyl may protect barley and wheat from the phytotoxic action of fenoxaprop-ethyl by increasing both the rate of deesterification of fenoxaprop-ethyl to fenoxaprop and the rate of metabolism of fenoxaprop to other metabolites. In crabgrass, deesterification of fenoxaprop-ethyl was increased by the safener but the subsequent detoxification of fenoxaprop was not increased in a manner similar to that found in the other two species.