Residues of 2,4-D in wheat following application after heading

Abstract

When drought conditions persist throughout the early part of the cereal growing season, vigorous weed growth may not occur until the crop is maturing and thus may cause seed harvesting and storage problems. Weed desiccation by application of 2,4-D could alleviate such problems but would also raise concerns regarding the magnitude of 2,4-D residues in the seed. This concern has been addressed in a 2-yr study in which residues of 2,4-D were determined in wheat following application of 526 and 1052 g acid equivalent ha−1 after the crop had headed and was in the soft-dough stage. In both years, residues in the crop (whole plants) immediately after spraying were of the order of 20 and 30 mg kg−1 for the 526 and 1052 g ha−1 application rates, respectively. At maturity, the corresponding maximum residues detected in the straw were 4.63 ± 0.77 and 8.76 ± 0.70 mg kg−1, whereas those in the seed were at or exceeded the maximum residue level of 0.1 mg kg−1 permissible in Canada, being 0.10 ± 0.04 and 0.26 ± 0.03 mg kg−1, respectively. Key words: 2,4-D, wheat, residues, preharvest application