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 glyphosate and its major metabolite aminomethylphosphonic acid (AMPA) were measured in the seed and foliage of wheal (Triticum aestivum L.) following preharvest applications at rates of 0.45, 0.9 or 1.7 kg acid equivalent ha(-1). Herbicide treatments were applied in early August to mid-September at seed moisture contents ranging from 52 to 12%. Glyphosate and AMPA residues in the seed increased as the rate of application increased, and decreased as the seed moisture content at the time of application decreased. However, when the maximum application rate of 1.7 kg ha(-1) was sprayed at seed moisture contents of 40% or less, glyphosate residues in the seed were < 5 mg kg(-1) the Maximum Residue Level recently established by Health Canada. Glyphosate and AMPA residues in the straw also increased with increasing application rate, but there 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 important roles in determining the magnitude of glyphosate and AMPA residues in the seed and straw of wheat.