Canada thistle (Cirsium arvense) has been identified as a weed of concern in Wisconsin pastures. It can reduce forage yield and utilization, both of which can have a negative impact on animal performance (Undersander et al., 2002). Control typically involves the use of herbicides, an effective control that has been well-researched and documented. Though effective in controlling Canada thistle, herbicides also kill clovers, which are highly desired in Wisconsin pastures. Thus graziers are often left wondering if they should manage Canada thistle infestations in pastures with an herbicide, knowing it will remove the clovers, or if they should allow this problem weed to persist. To answer this question it is important to understand how much forage is being lost due to direct competition with Canada thistle and how much forage utilization is reduced by this spiny weed.