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.

Weed data were collected over three tillage systems and two years in southwestern Ontario, in fields of corn, soybean, and winter wheat. Tillage systems involved soil inversion by moldboard plow, soil disturbance without inversion, and no tillage. The data were subjected to multivariate ordination techniques to determine if weed communities were associated with the different crops, tillage systems, or years. Crop, year, and tillage system all had significant effects on the weed community. Weed communities in corn and winter wheat were least similar; those in soybean were intermediate. Several grass weeds were more abundant in the drier of the two years. Some variation was related to tillage system but the effect of tillage was less than that of either crop or year.

Field studies were conducted from 1987 through 1990 on Sharkey clay to determine effect of weed control treatment (WTRT) on yield and net return from soybean grown with no irrigation (NI) or irrigation (I) during reproductive development. Weed control treatments were: 1) PRE glyphosate plus metribuzin plus metolachlor or pendimethalin, followed by POST bentazon plus acifluorfen or 2,4-DB plus linuron (high WTRT); 2) PRE glyphosate at a rate to kill existing weeds at planting (medium WTRT); or 3) PRE paraquat at a rate to desiccate but not necessarily kill existing weeds at planting (low WTRT). POST cultivation was used in all treatments. Weed ground cover in the high, medium, and low WTRT's was composed of annual grass and moderately competitive broadleaf species, and averaged 7, 25, and 49%, respectively, at harvest in 1990. Yield declined with decreasing weed control in the I treatment after the first year, but average net returns from the high and medium WTRT's were not different, and both exceeded average returns from the low WTRT. Yields from the high and medium WTRT's in the NI treatment were not different, and both exceeded yield from the low WTRT. Average net returns from the medium WTRT were greater than those from the high and low WTRT's in all years. Yield increase from irrigation and difference in average net returns between I and NI declined with decreasing weed control.