Field experiments were conducted at Vegreville, Alberta in 1984, 1985, 1986, and 1988 to determine the effects of green foxtail and pale smartweed on yield of wheat, barley, and canola. There was considerable variation among years in the response of crop yield to both weeds and in the relationship between weed dry weight and weed density. Mostly relationships between crop yield and either weed density or dry weight were poor, suggesting that the weeds competed weakly with the crops. Thus density or dry weight may be poor predictors of crop yield losses due to green foxtail or pale smartweed. Where the crops emerged ahead of these weeds, and where soil moisture was not a limiting factor, crop yield losses were minimal and control with herbicides probably uneconomical. In some instances, growth and development of the weeds was suppressed by the crops to the extent that little or no weed dry matter was present at crop maturity. This was most evident with barley, and where the crops emerged ahead of the weeds.

In continuous wheat or barley or in a canola/barley rotation, wild oat control every year over 4 yr maintained wild oat seedling populations at 3 plants/m2 or less. Failure to control wild oats annually increased wild oat populations (>200 plants/m2 by the fourth year) in continuous wheat dramatically, while in the other two cropping systems, populations increased to only 40 plants/m2 or less by the fourth year. In the continuous wheat and in the canola/barley rotation, wild oat control every year generally provided the best economic returns when prices and costs were averaged over 4 yr; in continuous barley, the average return was better when wild oats was controlled only in the second or third years rather than every year.

A long-term study was conducted at Lethbridge, Alberta, to determine the response of weed populations to various crop rotations and tillage treatments. Weed density and species composition differed with rotation, tillage, and date of sampling within years. Fewer weeds were found in winter wheat-fallow than continuous winter wheat, winter wheat-lentil, or winter wheat-canola rotations. A dense infestation of downy brome developed in the continuous winter wheat rotation. In all rotations, more weeds were present in zero tillage plots than in either minimum or conventional tillage plots. Dandelion and perennial sowthistle densities increased slightly over years in the minimum and zero tillage treatments. Flixweed, field pennycress, wild buckwheat, and common lambsquarters densities decreased in zero till but densities of downy brome, redroot pigweed, and Russian thistle increased. Russian thistle was not well controlled with trifluralin and its density increased in the winter wheat-canola rotation.