An established alfalfa stand infested with quackgrass received fluazifop-P, haloxyfop, or quizalofop at single applications of 0.28 ae kg ha-1 and sequential applications of 0.14 plus 0.14 ae kg ha-1 with the second treatment being made 16 d after the first. All herbicide treatments increased diffusive resistance and decreased transpiration of quackgrass compared to control plants. These effects became apparent 16 d or more after spraying. Visual injury symptoms were noted 8 to 10 d after treatment, several days prior to changes in diffusive resistance or transpiration. The alfalfa content of the first cutting was increased from herbicide treatment; however, total forage yield was reduced. Yields and composition of later cuttings were unaffected by the various herbicide treatments.

The effects of quackgrass shoot populations shortly after emergence, and dry weight at harvest on yield of canola were determined in four field experiments conducted near Vegreville, Alberta in 1986 and 1988. A hyperbolic model described the data well in most cases and indicated little intraspecific quackgrass interference among quackgrass shoots until populations exceeded 200 shoots m−2. Magnitudes of predicted canola yield losses among experiments were close, even though quackgrass-free yields varied. A pooled hyperbolic equation, based on shoot density, predicted that an intermediate quackgrass infestation of 50 to 100 shoots m−2 would reduce canola yield by 18 to 32%. An economic threshold model based on the hyperbolic model provides a means of estimating when control of quackgrass with herbicides is economical.