Response of wheat (Tritium aestivum) to low rates of glyphosate and glufosinate

Field studies were conducted in 2007 and 2008 at Hamilton Farms near Lonoke, Arkansas, USA to evaluate the potential impact of sub-lethal doses of glyphosate and glufosinate on winter wheat (Tritium aestivum). Three wheat cultivars were seeded, and glyphosate and glufosinate were applied at 1/10 and 1/20 times the labeled dose at the 3- to 4-leaf, panicle initiation, and boot stages. Both herbicides caused significant injury to wheat; however, wheat response to both herbicides was different. Glufosinate injury occurred rapidly and consisted of mainly leaf necrosis. Glyphosate injury to wheat was minimal and manifested in the form of leaf yellowing. Glyphosate caused shortened flag leaf when applied at PI and reduced canopy height when applied at boot. Injury was minimal to wheat across all treatments, with the highest level of injury being 34% following glufosinate applied at boot. Conversely, wheat was most sensitive to glyphosate at the 3- to 4-leaf stage. Flag leaf length was reduced the greatest by glyphosate at PI, whereas glufosinate reduced flag leaf length minimally. Even though glyphosate injury was lower than that of glufosinate, yield was reduced more by glyphosate. Wheat seed weight and germination were not affected by either herbicide. Wheat appears to be slightly more sensitive to glyphosate with respect to yield; however, it is slightly more sensitive to glufosinate with respect to foliar injury.