Canada thistle management in pastures
Mark J Renz
Extension Weed Scientist
Agronomy Dept., University of Wisconsin-Madison
Canada thistle has been identified as enemy #1 in pastures in the Midwestern U.S. It is important to remember that Canada thistle is a perennial, unlike other thistles which are biennial (e.g. plumeless, musk, bull thistle). Intensive management is therefore required to eliminate infestations. Management for this species focuses on depleting reserves from the creeping perennial root system. Thus multiple years are needed to see a substantial change in an infestation, unless herbicides are used. Below I highlight a range of management options. Remember these options all have costs and benefits, so please read each section carefully and then evaluate which technique is most appropriate for your field. Also remember that no single practice will produce or maintain thistle-free pastures. Effective plans typically combine the appropriate method(s) described below with proper pasture management.
Grazing: The optimal time for grazing is when plants have produced flower buds, however animals may avoid Canada thistle at this stage due to its spiny leaves. Rotational grazing can reduce avoidance and increase impact, especially with higher stocking densities. If animals eat most of the Canada thistle shoots for 2-3 years populations can be eliminated, but if utilization is low populations can persist.
Mowing: Mow when plants have produced flower buds. Then repeat mowing when plants again produce flower buds or 7–10 leaves. This method can result in 3-5 mowings per year depending on regrowth and will need to be implemented for many years to eliminate populations. If eradication is desired, other techniques should be selected or integrated with mowing.
Biological control: Several natural and introduced pests suppress Canada thistle. These control agents never eradicate infestations, but can reduce the density and size of the population. Suppression from insects that have been released as biological control agents has been variable, depending upon environmental conditions. Please check with your state department of agriculture if interested in these insects as some states require permits before releasing. A bacteria that turns the tips of Canada thistle white (Pseudomonas syringae pv. tagetis) however is unregulated and common on the landscape. While Canada thistle can be heavily suppressed in wet years, rarely does this bacteria result in plant mortality. If symptoms are visible on Canada thistle, this bacteria can be spread by mowing plants when moisture is present on foliage. Expect good suppression in wet years and poor suppression in dry years. Additional management will be required to eradicate populations.
Herbicides: Effective and economical herbicides are available to suppress Canada thistle in pastures for multiple years. The key to effective control with herbicides lies in making the application at the correct stage of development of Canada thistle. Applications should be applied to plants that have flower buds present, but few fully developed flowers (<50% flowering). Applications before or after this stage of growth in the summer will have reduced control. Alternatively, applications can be made to Canada thistle rosettes in the fall (plants should be mowed or grazed previously). Research has shown that fall applications through October can provide equivalent control to applications at the flower bud stage. While several herbicides have an impact on Canada thistle, applications with products that contain aminopyralid (Milestone, Forefront) or clopyralid (Stinger, Transline) are the most effective. If one of these active ingredients is applied at the labeled rate and at recommended timing, one can expect >90% control for up to two years. While these herbicides are safe to established pasture grasses they will kill/injure legumes in your pasture potentially reducing pasture productivity. Additionally it is not recommended to replant legumes into your pasture for 1 year after application unless reduced stands can be tolerated. Due to this, avoid broadcasting these herbicides if absence of legumes for 1- 2 years is not acceptable. Applying the herbicide to individual plants (spot treatment) can allow for legumes to persist, but if the infestation of Canada thistle is extensive this is not recommended. When making these applications make sure to apply at label rates, as higher rates can injure grasses. Another option that avoids injury to desirable legumes is to use a weed wiper and wipe glyphosate (e.g. Roundup) onto the leaves and stems of Canada thistle that is growing above the desirable forage. This method is effective if thistles are at least one foot taller than desirable forage. If this isn’t the case, injury or death of desirable forage may occur.
Mention of specific herbicides is for your convenience and is not an endorsement or criticism of one product over other similar products. You are responsible for using herbicides in full compliance with the current product label.