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Horseweed ( Conyza canadensis ) is more commonly known as marestail to most Indiana farmers. Horseweed is native to the United States, but it has recently become much more problematic to control in Roundup Ready soybean production. Horseweed is well adapted to no-till crop production. It has evolved resistance to several different herbicides including glyphosate products (www.weedscience.com). Horseweed was the first broadleaf weed documented to evolve resistance to glyphosate (Vangessel 2001) in the United States. Since this initial report, glyphosate resistance has been reported in several other states including Indiana.

In Indiana, the first case of glyphosate resistance was confirmed in Jackson County in 2002. With 90% of the soybean acres planted to Roundup Ready varieties and 60% of soybeans grown utilizing no-till practices in Indiana, it is a high priority for Purdue Weed Science Extension to help Indiana soybean producers find the best alternatives to keep horseweed from becoming an even bigger threat to agriculture production and environmental quality.

The objective of this website is to provide information on the distribution, biology, and management of glyphosate-resistant horseweed in Indiana and selected other states. This site contains maps county showing locations of glyphosate-resistant horseweed, and our latest research results on the biology and management of this weed. If you have any questions about this site or if you have information and links you would like added to this site, please contact Vince Davis, Valerie Mock, Bill Johnson, or Glenn Nice.

Distribution of Glyphosate-Resistant Horseweed in Indiana and Ohio

An in-field survey to locate glyphosate-resistant horseweed populations in Indiana was conducted in September and October of 2003, 2004, and 2005. The survey was conducted utilizing GPS and GIS technologies to randomly locate sample fields. Additional information can be accessed through links provided below. To learn even more about this survey please see our recent manuscript in Weed Technology: Davis, V. M., K. D. Gibson, and W. G. Johnson*. 2008. A Field Survey to Determine Distribution and Frequency of Glyphosate-Resistant Horseweed (Conyza canadensis) in Indiana. Weed Technol. 22:331-338.

Sites Surveyed - this page displays the intensity and location of Indiana counties that have been surveyed for horseweed escapes in 2003 through 2005.

IN Screening Results - this page is an interactive map of IN that allows users to find detailed information regarding the location of glyphosate resistant samples collected in the fall of 2003 through 2005.

IN and OH Screening Results - this page has a two state map showing counties with confirmed glyphosate resistant horseweed in both IN and OH.

Biology and Ecology of Indiana Populations of Glyphosate-Resistant Horseweed

Horseweed is traditionally considered a winter annual weed. However, in southeastern Indiana problematic glyphosate-resistant horseweed routinely completes it's lifecycle as a summer annual. Research investigating these lifecycle characteristics was conducted in no-till soybean for two years at the Southeastern Purdue Agriculture Center near Butlerville, IN in Jennings county. To learn more please see our recent manuscript in Weed Science: Davis , V. M. , and W. G. Johnson*. 2008. Glyphosate-Resistant Horseweed (Conyza canadensis) Emergence, Survival, and Fecundity in No-Till Soybean. Weed Sci. 56:231-236.

Horseweed response following glyphosate rate titration experiments were also conducted on Indiana populations to investigate the response following a range of glyphosate rates. To learn more about the variable levels of resistance in Indiana populations visit this poster presentation titled "Response of Selected Indiana Horseweed Populations to Glyphosate Rates. Proc. North Cent. Weed Sci. Soc. 59:49. Columbus, OH. The abstract is available online.

Response of Glyphosate-Resistant Horseweed to Other Herbicides

Resistance to ALS inhibitors has been identified in several IN horseweed populations. To learn more about these experiments, please visit our recent presentation titled "Distribution and Characterization of ALS Resistance in Indiana Horseweed (Conyza canadensis) Populations". Proc. North Cent. Weed Sci. Soc. 62:39. St. Louis, MO. The abstract is available online.

Horseweed control failures following exposure to 2,4-D in early spring burndown applications were also reported to Purdue Weed Science Extension specialists in the spring of 2006 and 2007. Screening experiments have also been conducted to investigate the variable levels of response following exposure to 2,4-D in collected Indiana populations. To learn more please visit our recent presentation titled "Response of Indiana Horseweed (Conyza canadensis) Populations to 2,4-D." Proc. Weed Sci. Soc. of America 48:97. Chicago, IL. The abstract is available online.

Management of Glyphosate-Resistant Horseweed

A field experiment investigating the population dynamics of glyphosate-resistant horseweed and the effect on crop yields was initiated at the Southeastern Purdue Agriculture Center in the fall of 2003. This experiment examined the influence of crop rotation including cover crops, herbicide application timing, and herbicide selection. Cropping systems in areas with glyphosate-resistant horseweed should not rely solely on glyphosate for weed control. To learn more about this experiment please visit our recent manuscript in Weed Science: Davis , V. M. , K. D. Gibson, T. T. Bauman, S. C. Weller, and W. G. Johnson*. 2007. Influence of Weed Management Practices and Crop Rotation on Glyphosate-Resistant Horseweed Population Dynamics and Crop Yield. Weed Sci. 55:508-516.

Purdue University Home Page

For more information regarding this web page please contact:

Vince Davis, Valerie Mock, Bill Johnson, or Glenn Nice.

This web site was posted on Sept. 15, 2004 and last updated on Aug. 12, 2008.