Although the impact of Canada thistle (CT) on annual crop production is relatively well established, few investigations report on this weed's impact within perennial pastures. This field study assessed herbage yield losses within eight central Alberta pastures from 1999 to 2001. Each pasture was sampled in 1999 to quantify thistle and herbage biomass within 25 permanent plots. CT was controlled in 2000 and the response of vegetation measured in 2000 and 2001. Before removal, significant negative relationships (P < 0.05) between thistle abundance and herbage were noted at six sites. After thistle removal, herbage at several sites displayed positive responses. Both thistle density and biomass adequately predicted herbage yield loss. Yield losses due to CT can be substantial, peaking at 2 kg/ha for each kilogram of standing thistle biomass and 4.3 kg/ha with each additional thistle stem per square meter. Demonstrated yield losses were variable among sites however, likely due to factors such as heterogeneity in soils, available moisture, and variation in disturbance history or pasture vegetation composition. CT management in perennial pastures of western Canada may enhance pasture production, but further research is required to reliably predict the ability of pastures to respond.

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Weed-wipers may provide effective weed control while minimizing the application of herbicide to nontarget species in rangeland and pasture. To date, few herbicides are recommended for use in weed wiping systems. We assessed Canada thistle and non–Canada thistle herbage responses in two experiments in pastures, the first examining wiped glyphosate, the second comparing glyphosate with three broadleaf herbicides at cost-equivalent concentrations [on a volume to volume (v/v) dilution basis]. In both studies, wiping with a glyphosate solution (33% v/v, equivalent to a one to two dilution ratio of herbicide to water) resulted in lower Canada thistle density and biomass than check plots, with control lasting up to 2 yr. However, significant reductions in grass biomass also occurred and were associated with an increase in the abundance of weedy annual forbs. In contrast, wiping with a concentrated solution of clopyralid (2% v/v), picloram plus 2,4-D (20% v/ v), or 2,4-D plus mecoprop plus dicamba (24% v/v), resulted in similar levels of Canada thistle control but no reduction in grass biomass. Despite direct application of herbicides to tall weeds, clover species in mixed stands were injured. In grass-dominated pastures, wiping with broadleaf herbicides was superior to nonselective glyphosate because the former more effectively balanced Canada thistle control with the retention of grass production.

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