Extended pasture forage sward responses to Canada thistle (Cirsium arvense) control using herbicides and fertilization

Combining two or more treatments within an integrated weed control strategy can provide benefits to crops. While several studies have examined Cirsium arvense L. Scop control in pastures, few have assessed corresponding aggregate forage responses. These responses include the tradeoff of improved grass yield due to weed reduction combined with the potential loss of desirable forbs. We evaluated the 3-year response in pasture forage yield and quality in central Alberta, Canada, to one-time mowing or herbicides (2,4-D ester, 2,4-D+mecoprop+dicamba, clopyralid, or picloram+2,4-D) applied at the early bud stage of C. arvense growth, both with and without annual spring fertilization, at 4 locations. Forage (grass and non-thistle forb) dry matter yield and quality (crude protein concentration and yield, phosphorus, and acid detergent fiber) were determined. Herbicide treatment increased grass yields during the second year in response to reduced weed competition, with these increases consistent in relative magnitude regardless of fertilization. In contrast, herbicides reduced non-thistle forbs, including legumes. Where herbicides were combined with fertilization, grass yield and quality increased, but was partly offset by a decline in forb yield and associated quality. When combined with fertilization, picloram+2,4-D at 0.24+0.89 kg a.i. ha−1 nearly eliminated legumes by the second year. While the loss of desirable forbs is likely to be offset by improved grass yields following weed control in forage swards containing only a minor forb component, this was less likely in swards containing abundant forbs. In this latter situation, alternative weed control strategies may be necessary to maintain forage yield and quality.