Residues of glyphosate and its metabolite AMPA in canola seed following preharvest applications

Abstract

Residue data for glyphosate and its major metabolite aminomethylphosphonic acid (AMPA) in canola seed following preharvest application of glyphosate were lacking when this 2-yr study was initiated. Residues of glyphosate and AMPA were measured in the seed of canola (Brassica rapa L.) at maturity following preharvest application at rates of 0.45, 0.9 or 1.7 kg acid equivalent ha−1 at four sites in western Canada. Herbicide treatments were applied in early August to mid-September at four stages of crop development, which encompassed seed/pod moisture contents ranging from 74 to 12%. Rate of glyphosate application and the physiological maturity of the crop at application played important roles in determining the magnitude of glyphosate and AMPA residues in the seed of preharvest-treated canola. In general, for a given site, mean residues of glyphosate and AMPA in the seed increased with increasing application rate, and decreased with application at later stages of crop development. In 1989, mean residues of glyphosate in the seed for all application rates at the latest stage of development at each site were <1 mg kg−1. In 1988, when seed/pod moisture content generally declined very slowly from stage 1 to stage 3 and then decreased rapidly, corresponding mean residues were higher and varied from <1 to 19.7 mg kg−1. Seed/pod moisture was not a reliable indicator of stage of canola development and, consequently, the magnitude of glyphosate and AMPA residues in the seed. Key words: Glyphosate residues, AMPA residues, canola, preharvest treatment