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Canadian Thistle Woes in Alfalfa

by Wyoming Livestock Roundup

By Jeremiah Vardiman, University of Wyoming Extension Agriculture and Horticulture Educator

            Every year, without fail, the question, “What is the best control for Canadian thistle in an alfalfa field?” is bound to be asked. There is no fix or management technique that will miraculously rid an alfalfa field of established Canadian thistle patches.

             Since alfalfa and Canadian thistle are both broadleaf plants, there is no chemical control option that will eliminate the thistle without killing the alfalfa crop.  The only exception to this would be Roundup Ready alfalfa varieties. Diligent weed management and persistence is the best control. Of course, not all fields are created equal and control options vary depending on the thistle population and distribution.    

            Starting with the easiest situation, thistle control follows the old adage, “An ounce of prevention is worth a pound of cure.”

The best situation and control option is to never allow a thistle plant to establish in the field. Typically this is accomplished through establishing alfalfa into a clean and weed-free field. If that can be accomplished, then diligent monitoring and suppression of new thistle plants needs to occur for the life of the stand.

            For small, established patches of thistle, there are two main options for control –  suppression or dead spots. The best option for suppression of the thistle and maintaining the alfalfa crop is spraying Raptor mixed with Methylated Seed Oil (MSO) and Urea Ammonium Nitrate (UAN) on the patches after the field has been cut and prior to three inches of re-growth.

This option will only help keep the thistle in check and needs to be a reoccurring task. Mechanical suppression, such as string trimming the thistle patches, should be utilized as needed to prevent seed production and spread.

            Another control option would be to sacrifice those areas within the field by spot spraying a nonselective herbicide, then reseeding those dead spots once the thistle is eliminated. Expect to retreat these patches as needed, which in heavy infestations can take multiple years until full control is achieved.  Depending on the size and location of the patches, this might be a more cost effective method than the suppression option.

            If the thistle infestations cover large areas or there is thistle throughout the entire field, the field is probably at a point that the alfalfa crop needs to be abandoned until the thistle is under control. At this point, crop rotation is the best option. Rotating the field to a perennial grass or annual grass hay crop provides various effective control options with broadleaf herbicides, while still providing a hay crop for the operation. Another option would be rotating the field to a Roundup Ready alfalfa. 

            There are other control options for Canadian thistle, which include mechanical (mowing) and biological (insects) control, however these options are not effective or feasible in an alfalfa hay production system. Mowing alone has minimal control at best, unless it occurs at monthly intervals over several growing seasons. If conducted long enough, the constant mowing will deplete the root reserves of the thistle to a point of death.

This constant mowing is not feasible in a hay production system because it will impact the yield, quality and economics of the operation. Biological control agents are more suited for rangeland conditions and are not ideal for crop production areas where there are other control options that are more effective.

            Canadian thistle is a difficult weed to control, especially within an alfalfa crop that limits the control options even further. Only through diligent weed management and persistence can thistle be removed from the field. There is still time for herbicide treatments to Canadian thistle patches this fall.

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