Glyphosate at simulated drift rates representing 12.5, 6.3, and 1.6% of the usage rate of 1,120 g ai/ha (140, 70, and 18 g/ha, respectively) was applied to wheat at first node, boot stage, or at early flowering. At 14 d after treatment (DAT) wheat injury, expressed as bleaching of leaf foliage and growth inhibition, was 40 to 55% for 70 g/ha applied at first node and for 140 g/ha applied at all growth stages. Wheat height 28 DAT was reduced 47% with glyphosate applied at 140 g/ha at first node and was reduced around 26% for 70 g/ha applied at first node and 140 g/ha applied at boot stage. Wheat height was not reduced with glyphosate at 18 g/ha applied at first node or boot stage and with all rates applied at early flowering. Wheat yield was reduced 72% when glyphosate was applied at 140 g/ha at first node, 45% when applied at boot stage, and 54% when applied at early flowering. For 70 g/ha, wheat yield was reduced 25 to 30% for the three application timings. Wheat yield was not reduced for 18 g/ha glyphosate. In another study, six wheat varieties responded the same to glyphosate applied at 140 and 70 g/ha. Wheat height 28 DAT was reduced an average of 34% for 140 g/ha glyphosate and 17% for 70 g/ha applied at first node, but height was not reduced when applied at early flowering. Yield was reduced an average of 58 and 43% for 140 and 70 g/ha applied at first node and 38 and 19% for 140 and 70 g/ha applied at early flowering. In both studies yield reductions in most cases were reflected in reduced spike density, spikelet number per spike, and seed weight.
Nomenclature: Glyphosate; wheat, Triticum aestivum L. ‘Coker 9663’, ‘Mason’, ‘LA 422’, ‘AGS 2000’, ‘Pioneer/26R61’, and ‘USG 3209’.