Ethametsulfuron methyl metabolism and crop selectivity in spring oilseed rape (Brassica napus L.)

Ethametsulfuron methyl (methyl 2-[[[[(4-ethoxy-6-methylamino-1,3,5-triazine-2-yl)amino] carbonyl]amino]sulfonyl]benzoate, formerly DPX-A7881) is a sulfonylurea herbicide for postemergence broadleaf weed control in spring oilseed rape (Brassica napus L.), including canola varieties. Plant response data are presented demonstrating the tolerance of spring oilseed rape to application of 64 g a.i./ha ethametsulfuron methyl while sensitive weeds including Chenopodium album, Sinapsis arvensis, and Amaranthus retroflexus are controlled at rates of 16 g a.i./ha or lower, under greenhouse conditions. The relative tolerance of different weed species and oilseed rape varieties to this sulfonylurea herbicide is related to the rate at which the compound is metabolized to inactive metabolites. Ethametsulfuron methyl is metabolized with a half-life of 1.2-2.5 hr in tolerant oilseed rape, while sensitive weed species or oilseed rape varieties metabolize this herbicide more slowly, with half-lives ranging from 5 to 14 hr. Metabolites of ethametsulfuron methyl were isolated from canola seedlings and analyzed using thermospray liquid chromatography/mass spectrometry (LC/MS), packed capillary LC/MS, and cochromatography with authentic standards. The major early metabolites are O-desethyl ethametsulfuron methyl, which was confirmed by cochromatography with an authentic reference standard, and N-desmethyl-O-desethyl ethametsulfuron methyl. O-Desethyl ethametsulfuron methyl is herbicidally inactive in whole plant studies and is a poor inhibitor of acetolactate synthase, the target site for sulfonylurea herbicides. Additional data are presented which show that the concentration in foliage of ethametsulfuron methyl applied postemergence to oilseed rape at 30 g a.i./ha declines from an initial value of 890 ppb to less than 0.5 ppb 60 days after treatment, and the total radiolabeled burden is only 5 ppb. Thus, the ability of oilseed rape to rapidly metabolize ethametsulfuron methyl accounts for its tolerance to this herbicide and, along with growth dilution, contributes to an extremely low residue level 60 days after treatment.