The influence of carrier volume was evaluated in field experiments for glyphosate applied to wheat at rates representing 12.5 and 6.3% of the usage rate of 1,120 g ai/ha (140 and 70 g/ha, respectively). Wheat at first node and at heading was exposed to glyphosate applied in a constant carrier volume of 234 L/ha, where herbicide concentration declined with reduction in dosage, and in proportional carrier volumes of 30 L/ha for the 12.5% rate and 15 L/ha for the 6.3% rate, where herbicide concentration remained constant. At 28 d after treatment, glyphosate applied at first node in proportional carrier volume (an average for 30 and 15 L/ha adjusted proportionally to glyphosate rate) reduced wheat height 42% compared with 15% when glyphosate was applied in 234 L/ha. Height reduction was no more than 15% when glyphosate was applied at heading in 234 L/ha or in the proportional carrier volumes and at first node in 234 L/ha. Wheat yield was reduced 42% when glyphosate at 140 g/ha was applied in 234 L/ha but was reduced 54% for the same rate applied in proportional carrier volume. For 70 g/ha glyphosate, wheat yield was reduced 11% when applied in 234 L/ha, but was reduced 42% when the same rate was applied in proportional carrier volume. Wheat yield reduction was equivalent when glyphosate was applied in 234 L/ha at first node and at heading (29 and 24%, respectively), but yield reductions of 60% for first node application and 36% for heading application were observed when glyphosate was applied in a proportional carrier volume. When averaged across carrier volumes and glyphosate rates, the greater yield loss from application at first node was attributed to decreased number of spikelets per spike and seed weight per spike.

Nomenclature: Glyphosate; wheat, Triticum aestivum L. ‘USG 3209’