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

Residue data for glyphosate and its major metabolite aminomethylphosphonic acid (AMPA) in canola seed following preharvest application of glyphosate w ere lacking when this 2-yr study was initiated. Residues of glyphosate and AMPA were measured in the seed of crtnola (Brassica rapa L,.) at maturity f ollowing preharvest application at rates of 0.45, 0.9 or 1.7 kg acid equiva lent ha(-1) at four sites in western Canada. Herbicide treatments were appl ied in early August to mid-September at four stages of crop development, wh ich encompassed seed/pod moisture contents ranging from 74 to 12%. Rate of glyphosate application and the physiological maturity of the crop at applic ation 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 stage s 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 v ery slowly from stage 1 to stage 3 and then decreased rapidly, correspondin g 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, c onsequently, the magnitude of glyphosate and AMPA residues in the seed.