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Why Has Horseweed (Marestail) Become a Concern?

May 8, 2003

In last week's newsletter (issue no. 6, May 2, 2003), we pointed out that horseweed (marestail) was added to the list of the six most common broadleaf weeds found in Illinois soybean fields. In this article, we attempt to explain why this weed has become more predominant throughout the state.

Horseweed Biology

Horseweed is an annual weed species that historically has been found in waste areas, fallow fields, and fencerows. However, with the increases in reduced tillage acres over the past 10 to 15 years, horseweed has become a major problem in no-till fields and more recently a problem in some tilled fields. One reason for its predominance in these situations is its life cycle. We mentioned previously that horseweed is an annual; it can follow either a winter or summer annual life cycle. Although the majority of plants emerge in the fall, horseweed also can emerge in the spring, perhaps even into early summer. Horseweed does not mature until late summer, so unlike many other winter annual weed species that complete their life cycle early in the spring, horseweed can compete with soybean during the growing season and also can interfere with harvest.

Horseweed emerges and forms a basal rosette in the fall or early spring.

Shortly into the spring, the plant bolts, and the stem elongates and produces numerous hairy leaves that alternate around the stem.

Once the stem elongates, the basal leaves deteriorate. The stem leaves are lanceolate and gradually become smaller up the stem.

An erect horseweed plant can grow to be 6 feet tall. Numerous tiny flowers are present from July to October; these flowers produce thousands of tiny seeds that are easily dispersed by wind due to the attached pappus (tiny hairlike bristles). This dispersal mechanism allows horseweed seeds to travel great distances and establish themselves in several fields.

Challenges with Horseweed Management

A number of factors can influence horseweed control. First, size and growth stage of plants at the time of herbicide application can certainly influence horseweed management. Horseweed is relatively easy to control when it is small and in the rosette stage. One of the more effective and economical preplant options is 2,4-D ester or a combination of 2,4-D ester and glyphosate. We also have seen good postemergence activity with Gramoxone Max and FirstRate. However, once horseweed starts to bolt, it becomes much more difficult to control.

Second, because of the plasticity in horseweed emergence, it is difficult to know when to apply burndown applications to control the highest percentage of emerged plants. In many cases, using a herbicide with soil residual activity can help control horseweed plants that haven't yet emerged. We have had good results with early preplant applications of Canopy XL, Canopy, Sencor, Valor, and Authority. Fall applications of some of these herbicides also have done well in reducing the overall horseweed population and reducing the variation in plant size in the spring.

A third challenge is that a limited number of herbicide options with foliar activity on horseweed exist, especially in soybean. Because of this constraint, it is important to use wisely the herbicide options that we have for controlling horseweed so that we don't limit our options with development of herbicide resistance.

Herbicide-Resistant Horseweed

In the United States, populations of horseweed are resistant to the ALS-inhibiting herbicides, paraquat, atrazine, diuron, simazine, and glyphosate. Currently in Illinois, there are no confirmed cases of horseweed resistant to these herbicides. However, populations of ALS-resistant horseweed have been prevalent in Ohio for a number of years, and the spread of glyphosate-resistant horse-weed is moving west. Glyphosate-resistant horseweed was first confirmed in Delaware in 2000. Since this time, there have been populations confirmed resistant in New Jersey, Maryland, Tennessee, and Kentucky. In 2002, glyphosate-resistant horseweed was reported in Ohio and Indiana, making us only a state away from the problem. In most cases, glyphosate-resistant horseweed does not act like other resistant weeds that we have seen in Illinois. Usually, with the ALS-resistant weeds that we have in the state, the herbicide does not have any effect on the weed. However, with the glyphosate-resistant horseweed populations, glyphosate usually causes some stunting, and many times the terminal bud may be killed, but the plant will branch out and survive the glyphosate. Generally these horseweed plants may exhibit only an 8- to 10-fold level of resistance; however, these plants are resistant enough to survive, produce seed, and become a problem.

Although glyphosate-resistant horseweed is a concern, it doesn't necessarily mean we should stop using glyphosate. Rather, we should keep our eyes open for glyphosate resistance in Illinois and use effective management strategies to help delay the development of resistance. Recommendations include use of fall herbicide applications to reduce horseweed density and size in the spring, apply herbicides when horseweed is less than 2 inches tall, consider using herbicides with residual activity on horseweed, and use preplant tillage whenever feasible. If you think you have a horseweed population resistant to glyphosate, please contact one of us.--Christy Sprague and Aaron Hager

Author: Aaron Hager Christy Sprague

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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