There’s No Hiding Thistles in Pastures

By Jane Fyksen Crops Editor
Thursday, November 27, 2008 8:37 PM CST

If there are weeds in their pastures producers really don’t want their neighbors to see they’re thistles. In addition to being unsightly, heavy infestations can cause large areas of pastures to be left ungrazed.

Wisconsin is “home” to four thistles. Canada thistle and full thistle occur throughout the state. Musk thistle is common in southeastern counties plus other “hot spots” around the state. Plumeless thistle n often incorrectly called Russian thistle, which is the name of another thistle - is principally in southwestern and southcentral Wisconsin.

Mark Renz, UW-Madison forage weeds specialist, says the most common thistles in pastures are biennials n bull, musk and plumeless thistles. They only grow vegetatively the first year, forming a rosette before going dormant in late fall. Cold temperatures (vernalization) triggers them to bolt and flower the second year. Biennial thistles reproduce only to be seed. For successful control, they must be prevented from going to seed.

Renz, who’s zeroed in on thistles in pastures with emeritus weed scientist Jerry Doll, says Canada thistle is a perennial. Infestations can start from seed, but plants primarily regrow and spread each year via a creeping root system. Canada thistle is one of the first pasture weeds to resume growth after winter. Buds at the crown and on below-ground vertical stems produce shoots after mowing. And Canada thistles are very good at spreading, too.

Researchers planted either a single root segment 12 inches long or a six-inch diameter plug of Canada thistle plants. Within 12 to 16 months, buds on those roots produced, on average, 174 shoots and 930 feet of new roots! What’s more, viable seeds are formed 8 to 10 days after flowering and single plants can release over 5,000 seeds.

Renz and Doll say one of the most important cultural methods of pasture weed control is rotational and controlled grazing. Overgrazing weakens pastures and makes them competitive with weeds, including thistles. Don’t graze thistle-infected pastures and then immediately move livestock to thistle-free areas without allowing them time to “clean themselves of weed seeds in their digestive systems,” they advise. Likewise, clean mowers or choppers after leaving thistle-infected pastures so you don’t transport seeds to other pastures and hayfields.

The rapid spread of musk thistle is due in part to lack of natural enemies. Scientists found a seed-eating weevil in Italy and released it in southeastern Wisconsin in the mid ‘70s, but there’s been little evidence of reduced musk thistle populations. That’s because the weevils attack primarily the “first generation” of flower heads; additional buds that show up on secondary branches flower 10 to 20 days later, when weevils are no longer laying their eggs. Further, there’s been no attempt to spread this introduced weevil to new sites in Wisconsin because of concern it might attack the dune thistle, a threatened species found on the shores of Lake Michigan.

A natural organism infects Canada thistle that produces a toxin that inhibits chloroplast formation; Canada thistle plants appear cream-colored. It weakens them and minimizes flowering and seed formation. Infected populations often diminish and on occasion completely disappear over a period of years.

Persistent and timely mowing, clipping and even pulling by hand can greatly reduce biennial thistle infestations, but won’t have much impact on Canada thistle. Biennial thistles should be cut as close to the ground as practical each time they begin to flower. To prevent seed production, plants must be cut at the soil surface as higher cutting allows crown buds to resprout and flower.

Despite producers’ best efforts, thistles might become established in pastures. Herbicides that work on the biennial species are: Curtail, Cimarron/Escort, Crossbow, Dicamba, Forefront, glyphosate, Milestone, Stinger/Transline, Weedmaster and 2,4-D. Generally, this same line-up doesn’t control Canada thistle quite as well. The best options, according to Renz and Doll, are glyphosate, Milestone and Stinger/Transline.

Biennial thistles must be treated when plants are in the rosette stage. After bolting, they become much less susceptible to most herbicides. These experts say that 2,4-D amine or ester applied early spring or mid to late fall controls biennial thistles because at those times all the plants are in the rosette stage n when they’re easily controlled with herbicides with a growth regulator mode of action. The ester formulation of 2,4-D should be used unless crops sensitive to vapor drift are close by.

Once thistles start to bolt in late May or flower mid-June, aminopyralid is more effective than dicamba alone or the combination of 2,4-D and dicamba, and 2,4-D.

All herbicides that control thistles also kill desired pasture legumes. Rope wicks or rollers can be used to target the taller biennial thistles. The UW has shown that glyphosate applied with two passes with either a roller or rope wick applicator gave partial kill of bull thistles. However, more importantly, flowering (i.e. seed production) was completely stopped by the two passes.

Treatment timing is much different for the perennial Canada thistle. Effective herbicides are systemic (i.e. move from foliage into the roots). Such movement is greater when plants are in bud to early flower, than at earlier growth stages. Recommended herbicides include: Aminopyralid (Milestone, Forefront), dicamba (Clarity, Banvel, Overdrive), clopyralid (Stinger, Curtail), triclopyr plus 2,4-D (Crossbow) and glyphosate (Roundup and many other brands). A single application will reduce but not eliminate Canada thistle. Renz and Doll say Milestone and Stinger provide the best long-term suppression, but even they won’t eradicate populations. Again, all herbicides for Canada thistle also kill forage legumes, and glyphosate kills all pasture vegetation.

Fall applications to any thistle specie are better than a spring treatment. That’s because thistles are building up root reserves in the fall, so herbicide moves readily from foliate to roots. In the spring, there’s less downward herbicide movement. Fall applications also ensure that biennial thistles will be controlled because any shoots present in the fall are in rosette and very susceptible to herbicide. What’s more, winter weather is apt to finish what herbicide starts. And there’s less risk of drift to sensitive crops like soybeans.

Fall applications should be made while daytime highs are still in the 60s and 70s and plants are actively growing. That’s usually early to mid September in southern Wisconsin or a week or two earlier in the North.

Producers can view Renz’s new online weed ID tool to see the differences between the various types of thistles, and determine for certain what they’re dealing with. Go to http://weedid.wisc.edu.

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