Effect of initial quackgrass density on control with glyphosate prior to spring wheat

Abstract

Field studies were initiated in 1985 and 1986 at two sites near Quebec City to evaluate the population response of three quackgrass (Elytrigia repens (L.) Nevski.) infestation levels of 13, 72, and 180 shoots m−2 treated with glyphosate at 0, 0.45, 0.90, and 1.80 kg a.i. ha−1 prior to seeding wheat. During year 1, almost complete shoot and rhizome control was achieved by the 0.90 kg a.i. ha−1 + ethylan and the 1.80 kg a.i. ha−1 glyphosate treatments, regardless of the initial quackgrass infestation levels. Quackgrass control was less for the 0.45 kg a.i. ha−1 rate applied to the high infestation. During year 2, this trend was similar. Shoot density and biomass varied more than rhizome biomass in the glyphosate-treated plots. In the absence of wheat competition during year 3, glyphosate still had a significant impact on quackgrass infestations, although rhizomes had built up to a considerable level by mid-July. Reduction in rhizome biomass in the low, medium, and high quackgrass infestation by glyphosate averaged 56, 53, and 34%, respectively, but no further difference in control existed among glyphosate treatments. The data indicate that a reduced rate (0.45 kg a.i. ha−1) of glyphosate can be used effectively when the density of quackgrass shoots is below ca. 70 shoots m−2 at time of treatment. Key words: Glyphosate, quackgrass control, quackgrass density, rhizome, Elytrigia repens (L.) Nevski.