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Issue No. 1, Article 7/March 19, 2009

Horseweed: Best to Control Before Planting

Horseweed (Conyza canadensis) has become a challenging broadleaf weed in minimum and no-tillage cropping systems across much of the southern half of Illinois. Horseweed, often widely recognized by its other common name, marestail, is native to North America but historically has been a weed predominantly of waste areas and fallow ground. With the advent of limited tillage agronomic cropping practices and the substantial increase in their adoption during the past 20 years, horseweed has become very adapt at populating Illinois corn and soybean fields.

Horseweed, like other annual plant species, completes its life cycle in one year. Unlike many other annual species, however, horseweed may exist as a winter or summer annual. Populations of winter annual horseweed typically emerge during the fall months, within a few days or weeks after seed is dispersed from the parent plant. Summer annual populations can emerge in early or late spring, perhaps as late as early summer in some instances. In northern areas of Illinois, most horseweed demonstrates a winter annual life cycle, whereas a substantially higher proportion of spring emergence occurs in areas in Illinois south of (approximately) Interstate 70. Both winter and summer annual life cycles can be found across central Illinois.

Fall-emerging plants form a basal rosette that represents the plant's overwintering stage. Rosettes are merely a cluster of small leaves that appear to be attached together at a common point. In the spring, plants bolt by rapidly elongating the main stem. Leaves, very numerous and hairy, alternate around the stem and become progressively smaller in size toward the top of the plant. The leaves on mature plants lack petioles and have entire or slightly toothed margins. As the plant matures, leaves toward the base of the plant deteriorate and fall off the stem.

Mature horseweed plants may reach heights in excess of 6 feet, but plants ranging from 3 to 5 feet are perhaps most common. The stems are erect and unbranched, except toward the top of the plant. Flowers are produced in a panicle-type inflorescence at the top of the plant. The seeds, known as achenes, are produced with an attached "parachute" (known as a pappus) to aid in windborne dispersal. Horseweed seed can travel long distances with this type of dispersal mechanism, which becomes especially important when considering the spread of herbicide-resistant biotypes.

Herbicide-resistant biotypes of horseweed are common in many areas of the United States. Resistance to several herbicide classes, including bipyridiliums (e.g., paraquat), glycines (e.g., glyphosate), ALS inhibitors (e.g., cloransulam), and triazines (e.g., atrazine), has been documented in U.S. populations. Illinois has not been immune to this phenomenon, with glyphosate-resistant horseweed biotypes identified as early as 2005.

Glyphosate resistance in marestail, especially during the initial stages of selection, may not always demonstrate the "black or white" response many are accustomed to observing with other types of herbicide resistance (such as resistance to ALS inhibitors). For example, it's altogether likely that glyphosate-resistant and glyphosate-susceptible plants will be found in close proximity. Following a foliar application of glyphosate, it's common to observe plants showing very few signs of injury interspersed among plants that were completely controlled. Additionally, glyphosate-resistant horseweed may or may not demonstrate symptoms of herbicide injury. Resistant plants might be stunted and display some yellowing in the meristem area after treatment. In some instances the top of the plant dies back, but this is usually followed by profuse branching along the lower stem of the resistant plant. Contrary to this, most ALS-resistant plants rarely show signs of injury following treatment with an ALS-inhibiting herbicide.

Ideally, horseweed should be controlled prior to corn or soybean planting. However, because horseweed can emerge in the spring as well as during the fall it is likely that some horseweed will emerge following planting, especially in southern Illinois. Thus, farmers will need to consider how to manage horseweed, and particularly glyphosate-resistant biotypes, both before planting and after crop emergence.

Control of existing horseweed before planting: Tillage can be a very effective tool to control existing horseweed prior to planting. Tillage continues to be widely utilized in Illinois corn production, which, in combination with a broad spectrum of effective soil-residual and foliar-applied herbicides, has helped alleviate the severity of herbicide-resistant horseweed populations in corn. With a higher percentage of Illinois soybean grown without tillage (including interrow cultivation), managing horseweed in soybean becomes very reliant on herbicides.

In situations where tillage is not an option, existing horseweed plants should be controlled before they exceed 6 inches in height. While glyphosate remains effective on many other weed species, tank-mix partners or alternative herbicides are needed to provide adequate burndown control of glyphosate-resistant horseweed. Dicamba, paraquat, and 2,4-D are other herbicides that can be used to control horseweed prior to corn or soybean planting. Research has repeatedly demonstrated improved control of emerged horseweed from applying two- or three-way herbicide tank mixtures as compared with single-herbicide burndown treatments.

Both amine and ester formulations of 2,4-D are labeled for burndown applications prior to soybean planting, but the ester formulation is usually preferred over the amine formulation. The low water solubility of an ester reduces the potential for it to be moved into the soil by precipitation, where it could cause severe injury to germinating soybean seed. Also, the ability of esters to better penetrate the waxy leaf surfaces of weeds often results in improved control of large weeds and during periods of cool air temperatures. The labels of many 2,4-D ester formulations (3.8 lb acid equivalent per gallon) allow applications of up to 1 pint per acre 7 days prior to soybean planting; increasing the rate to more than 1 pint increases the waiting interval to 30 days.

Distinct and Clarity are dicamba-containing products labeled for preplant applications in corn (Clarity only) or soybean (both products). Corn is much more tolerant of dicamba than soybean, so additional use precautions must be followed when using dicamba prior to soybean planting. For example, both labels indicate that a waiting interval and precipitation event must occur between dicamba application and soybean planting. An application of Clarity, at 4 to 16 fluid ounces per acre, must be followed by at least 1 inch precipitation and a waiting interval of 14 days (for applications of up to 8 fluid ounces) or 28 days (for applications between 8 and 16 fluid ounces) before planting soybean. The Distinct label allows 2 to 4 ounces per acre to be applied, followed by a minimum of 1 inch precipitation and a waiting interval of 30 days before soybean planting. Applications of dicamba, either alone or tank-mixed with glyphosate for burndown of existing vegetation, may be better suited to areas of southern Illinois where soils tend to warm sooner in the spring, which helps increase the speed of dicamba dissipation in the soil. If you plan to use dicamba as part of your burndown herbicide program in 2009, keep these intervals and precipitation requirements in mind so as to make timely applications that preclude planting delays.

Unlike glyphosate, 2,4-D, and dicamba, which are extensively translocated in plants following absorption, paraquat shows very limited movement following absorption. This fact becomes important when considering application parameters that influence paraquat's activity. Thorough coverage of target vegetation with contact herbicides is essential for optimal performance. Coverage with contact herbicides is often enhanced through higher application volumes (around 20 gallons per acre) and flat-fan-type nozzles. The performance of paraquat on emerged horseweed is often improved with tank-mix partners, such as atrazine or metribuzin and 2,4-D, and when the application is followed by periods of bright sunshine and warm air temperatures.

Glufosinate is another option for burndown of exsiting horseweed prior to planting. Prior research has indicated that glufosinate has good activity on emerged horseweed, and control is often improved by tank-mixing with atrazine or metribuzin. The current Ignite 280 label recommends using 29 fluid ounces per acre for burndown applications of horseweed 6 to12 inches tall.

Residual control of spring-emerging horseweed: Several soil-residual herbicides can control horseweed after crop planting. Products containing the active ingredients atrazine, metribuzin, flumetsulam, cloransulam, flumioxazin, sulfentrazone, fomesafen, or chlorimuron can provide 3 to several weeks of residual control. Length of residual control can vary by active ingredient, application rate, and soil type/organic matter content.

Control of horseweed after crop emergence: In corn, growth regulator herbicides, such as dicamba, can control horseweed that emerges following planting. Some ALS-inhibiting herbicides also have activity against horseweed but would not provide control of biotypes that are resistant to this herbicide family (while no ALS-resistant horseweed biotypes have been confirmed in Illinois, these have been confirmed in states close to Illinois). Glufosinate, used in conjunction with glufosinate-resistant corn hybrids or soybean varieties, is another option for control. Keep in mind that if glufosinate is applied as a burndown treatment prior to crop planting, no in-crop applications of glufosinate can be made.

In soybean, cloransulam- or chlorimuron-containing products can be applied postemergence to suppress or control emerged horseweed. These products can be tank-mixed with glyphosate for postemergence applications in glyphosate-resistant soybean. Of course, glyphosate is an option for control of glyphosate-sensitive biotypes.--Aaron Hager

Aaron Hager

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