Since its introduction into the United States in the late 1700s, Canada thistle (Cirsium arvense) has spread dramatically, causing greater crop losses than any other perennial broadleaf weed in the north central region of the United States. In Wisconsin, it continues to be a major pest identified by growers, land managers, and consultants. In pastures studies indicate that, while highly variable, forage loss from Canada thistle can result in an average of 22% yield loss. While forage quality remains high for Canada thistle, its palatability can be extremely low due to the spiny nature of the leaves. This can result in partial-use (40%) or complete rejection by animals. In addition to these costs animals do not utilize the forage nearby effectively when Canada thistle is present. This can result in <50% utilization of desirable forage. Finally, spines present on the leaves can aggravate animals often resulting in reduced performance. Clearly Canada thistle is not a desirable plant.
Soil Science Extension
University of Wisconsin Madison