No. 6 Article 5/April 29, 2005

Horseweed Control Issues with Glyphosate

Many Illinois farmers are aware that populations of horseweed (Conyza canadensis, also referred to as marestail) in several states have been confirmed resistant to glyphosate. We have speculated for several years that it was only a matter of time before we encountered similar issues in Illinois. Recent evidence indicates that we may soon move beyond the exercise of mere speculation. While we have not yet conducted all appropriate experiments to attach the moniker of "resistant," we have identified three horseweed populations from Illinois that are not effectively controlled by glyphosate.

The populations we have examined in greenhouse experiments originated from three counties in Illinois: Douglas, McLean, and Schuyler. Only one of these populations was sent to us by a concerned farmer, who noted poor control of horseweed following a burndown application of glyphosate in 2004. The other two populations were collected during weed survey and collection campaigns in 2004. The geographic dispersion of these populations suggests there may not be a "concentrated" area of concern in the state, but rather that farmers across Illinois should remain vigilant.

So why are we not using the term resistant to describe the response of these horseweed populations to glyphosate? The declaration that a particular plant biotype is resistant should satisfy the criteria established by the Weed Science Society of America (WSSA). In part, the WSSA defines resistance as "the inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide normally lethal to the wild type." The three horseweed populations we examined contained plants that survived a foliar application of 0.75 lb acid equivalent glyphosate, whereas all plants from a susceptible control population were completely controlled at this rate. However, we have not had sufficient time to determine if this lack of response to glyphosate is passed on to (i.e., inherited by) the progeny. If, in fact, the progeny of the surviving plants demonstrate a similar lack of response to glyphosate, it will then be appropriate to describe these populations as resistant. Regardless of the nomenclature, we felt it appropriate to inform our clientele of our preliminary findings.

Glyphosate-resistant horseweed was first identified in Delaware in 2000. Since that initial discovery, glyphosate-resistant horseweed populations have been identified in nine other states, including our neighboring states of Indiana and Missouri. Weed scientists in the 10 states where these populations are known to exist have conducted extensive field, greenhouse, and laboratory experiments to further our understanding of the mechanism these plants use to survive glyphosate, as well as how best to manage these resistant populations under field conditions. Much of the following text is based on their research findings.

Glyphosate resistance in marestail, especially during the initial stages of selection, may not always demonstrate the "black or white" response that we 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 susceptible plants will be found in close proximity. Following a foliar application of glyphosate, you might 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 may die back, but this is usually followed by profuse branching along the lower stem of the resistant plant. Contrary to this, most ALS-resistant plants generally show no signs of injury following treatment.

Because glyphosate use is pervasive both prior to planting (i.e., burndown) and following crop emergence (i.e., postemergence in glyphosate-resistant varieties/hybrids), identification of horseweed populations from Illinois that are not adequately controlled by glyphosate necessitates that farmers consider what options are available to control these populations. Since glyphosate use in Illinois is more extensive in soybean than in corn, the following discussion will be limited to soybean.

Ideally, all horseweed should be controlled prior to 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 soybean planting. Thus, farmers will need to consider how to manage glyphosate-resistant horseweed both before planting and after soybean emergence.

Control of existing horseweed before planting: 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 are needed to provide adequate control of glyphosate-resistant horseweed. Suggested tank-mix partners include 2,4-D ester, FirstRate, Gangster, and Canopy EX. Control of horseweed with Gramoxone is often improved when tank-mixed with Sencor and/or 2,4-D. Valor and Python alone are not very effective at controlling emerged horseweed but can provide soil residual control. Other herbicides that provide soil residual control include Sencor, Classic, and FirstRate. Utilizing tank mixtures and soil residual products can be considered as a proactive approach to slowing the spread of resistant populations.

Control of horseweed after soybean emergence: Tank-mix partners with glyphosate include FirstRate or Classic. Few other options, if any, are available for postemergence control of glyphosate-resistant horseweed in soybean.

The following Web sites provide much more information related to horseweed, including additional research on glyphosate-resistant populations: (Adobe PDF) (Adobe PDF) (Adobe PDF)

We appreciate the efforts of our colleagues at the University of Illinois, Dr. Patrick Tranel and Dr. Dean Volenberg, who have been intimately involved with the research to date on these horseweed populations.--Aaron Hager and Dawn Nordby

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