EFFECT OF INITIAL QUACKGRASS DENSITY ON CONTROL WITH GLYPHOSATE PRIORTO SPRING WHEAT

Field studies were initiated in 1985 and 1986 at two sites near Quebec City to evaluate the population response of three quackgrass (Elytrig ia 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 pri or 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 k g a.i. ha-1 glyphosate treatments, regardless of the initial quackgras s, 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 bioma ss in the glyphosate-treated plots. In the absence of wheat competitio n during year 3, glyphosate still had a significant impact on quackgra ss infestations, although rhizomes had built up to a considerable leve l by mid-July. Reduction in rhizome biomass in the low, medium, and hi gh quackgrass infestation by glyphosate averaged 56, 53, and 34%, resp ectively, but no further difference in control existed among glyphosat e 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.