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Controlling pasture thistles
by Jim Morrison, Extension Educator, Crop Systems


How to control those ‘darn’ thistles in pastures was a topic at the recent Upper Midwest Grazing Conference in Dubuque, Iowa. The spiny nature of thistles prevents livestock from grazing near them and heavy thistle infestations may cause large areas of pastures to be left ungrazed. The overall result is that thistles lower the productivity of pastures.

Thistle management consists of proper identification, and then combining various cultural, mechanical, chemical, and perhaps biological measures. Perennial thistles, Canada thistle being the major one, can exist for many years and they reproduce from both seed and underground parent rootstocks. The creeping root system enables this weed to spread yearly.

Biennials, like musk, plumeless, and bull thistle, live for two years and reproduce only by seed. After germination, they form a prostrate rosette ranging from 4 to 18 inches in diameter before becoming dormant in the late fall. Exposure to cold winter temperatures is necessary to trigger these thistles to flower the second year after sending up a flower stalk (called bolting). Each plant can send up several stalks and produce numerous flower heads, each with viable seeds. After flowering or with the first frost, biennial thistles die in the second year.

Using good cultural practices (including rotational grazing, maintaining optimal soil fertility, and periodic mowing) that result in vigorous, dense, and uniform stands will help keep pastures competitive with weeds. Selective grazing, resulting from overgrazing, often leads to invasion by perennial weeds.

Crop rotation is a valuable weed management strategy for temporary pastures, especially since biennial thistles cannot tolerate tillage or crop competition.

Once biennials bolt or produce a seed stalk, they are less sensitive to herbicides. Mowing at this stage (before flowers open) will help reduce seed production. Some regrowth will occur, so a second or third mowing may be necessary. Even close mowing does little to control biennials the first year or during the rosette stage. Repeated cutting of the crown of biennial thistles 1 or 2 inches below the soil surface will eventually reduce the stand by preventing seed production. Mowing is generally less successful on deep-rooted perennial weeds and brush.

Weeds vary in their susceptibility to herbicides. Be sure to read and follow the label when selecting and using pesticides. A new broad-spectrum herbicide, Aminopyralid, manufactured by Dow AgroSciences, may be registered by the end of 2005. Once registered, it will provide another option for thistle control. Depending upon the livestock species, some herbicides have a legal interval between application and reentry for grazing/harvest. Certain herbicides may also have long-term residual effects on the site. Many herbicides will kill forage legumes that are present in the pasture.

Biennials are most easily controlled by herbicides while in the seedling or rosette stage in the year of germination and up to bolting stage in the second year. A fall or spring herbicide application will be effective. After bolting, they are much less susceptible to herbicides and mowing becomes the best option.

Perennials should be treated by a translocated herbicide while in the bud-to-early flower stage or in the fall regrowth stage. At this stage, the herbicide can move downward with food reserves to the roots, thus killing the entire plant.

No single practice will result in thistle-free pastures. A coordinated approach that combines various measures is needed. Details on pasture weed control are found in the 2005 Illinois Agricultural Pest Management Handbook, available at Extension offices.

Column adapted from material prepared by Dr. Jerry Doll, former Extension weed scientist, University of Wisconsin, Madison.

Jim Morrison, (815) 397-7714, morrison@uiuc.edu
Date: August 7, 2005


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