RESIDUES OF GLYPHOSATE AND ITS METABOLITE AMPA IN WHEAT SEED AND FOLIAGE FOLLOWING PREHARVEST APPLICATIONS

In a 2-yr study at four locations in western Canada, residues of glyph osate and its major metabolite aminomethylphosphonic acid (AMPA) were measured in the seed and foliage of wheal (Triticum aestivum L.) follo wing preharvest applications at rates of 0.45, 0.9 or 1.7 kg acid equi valent ha(-1). Herbicide treatments were applied in early August to mi d-September at seed moisture contents ranging from 52 to 12%. Glyphosa te and AMPA residues in the seed increased as the rate of application increased, and decreased as the seed moisture content at the time of a pplication decreased. However, when the maximum application rate of 1. 7 kg ha(-1) was sprayed at seed moisture contents of 40% or less, glyp hosate residues in the seed were < 5 mg kg(-1) the Maximum Residue Lev el recently established by Health Canada. Glyphosate and AMPA residues in the straw also increased with increasing application rate, but the re was no consistent pattern in residues of either chemical with seed moisture content at the time of application. Physiological maturity of the crop, rainfall washoff, and application rate appeared to play imp ortant roles in determining the magnitude of glyphosate and AMPA resid ues in the seed and straw of wheat.