LISTED AS a noxious weed in 43 states and affecting some 12.7 million acres nationwide, Canada thistle has gobbled up land that could better be used as rangeland, pasture, wildlife habitat, or grounds for hunting or hiking. Herbicides like Tordon 22K and Stinger (Transline) have helped fight large patches of this perennial weed. But because they cannot be used near surface waters, they have had limited long-term results.

That's why ranchers, farmers, weed specialists and others are welcoming the introduction of a new active ingredient called aminopyralid. In addition to rangeland, pastures, habitat areas and other non-cropland areas, the compound can be safely applied right up to water's edge or in areas where there is a high water table. Under normal use rates and conditions, it also can be applied near and under most mature trees.

Reduced risk pesticide

Aminopyralid is the active ingredient in Dow AgroSciences' new Milestone liquid herbicide, which received full registration in August 2005. The EPA accepted aminopyralid under its Reduced Risk Pesticide Initiative, which includes compounds posing less risk to humans and the environment than many products currently on the market.

Application rates as low as 5 to 7 fluid oz. of Milestone per acre can be used to control Canada thistle. This is a much lower application rate than current market standards, Dow AgroSciences reports. The new herbicide provides control of several other broadleaf weeds that invade rangeland and pasture acres, including musk, bull and plumeless thistles; spotted, diffuse and Russian knapweeds; yellow starthistle; and tropical soda apple.

Robert Wilson, weed specialist, University of Nebraska Panhandle Research and Extension Center, has tested the herbicide for the past three years and believes it will be very beneficial. Some of the country's more productive rangeland sites are in wet meadows and subirrigated areas that ranchers have been unable to treat with older compounds because of water restrictions. Use of Milestone against Canada thistle and other invasive broadleaf weeds in these areas will allow grass to grow back, providing food for cattle and wildlife as well as clearer pathways for hunters and hikers, Wilson says.

Application

Wilson has experimented with Milestone over the last three years. Spring treatment with the herbicide (ranging from 5 to 7 oz./acre) has averaged 82% control of Canada thistle up to 12 months after application, while fall treatment averaged 93% control of the weed for the same period. This compares to 68% and 81% control with spring and fall applications, respectively, of Stinger (Transline), Wilson says.

In preparation for cold winter weather, perennial weed roots undergo a change in their carbohydrate composition. Fall-applied herbicides disrupt this cycle, preventing the plant from getting ready for winter. Fall application is generally considered better than spring treatment when one might get initial control of the tops of weeds, but not be able to prevent new shoots from recovering, Wilson explains.

Perennial thistles can reproduce from root fragments and their seeds can remain viable for as long as 20 years. Canada thistle develops a massive root system that can spread as much 20 ft. in one season.

Milestone may be applied when the ambient air temperature is 50°F or higher. It may be tank mixed with other herbicides, such as Remedy, Surmount or PastureGard herbicide or 2,4-D. If Milestone is applied while weeds are mature, Dow AgroSciences recommends that a nonionic adjuvant be used with the herbicide.

The time required for rainfastness depends on environmental conditions and the species targeted for control, Dow AgroSciences reports. It adds, however, that Milestone will be rainfast within 2 hrs. after application at labeled rates.

Dow AgroSciences says that there are no grazing or haying restrictions for any class of livestock, including beef cattle and lactating dairy animals, sheep, goats and horses, with the new herbicide.