Photo of Conyza canadensis - Canadian horseweed

Photo of Conyza canadensis in flower. This is a native plant growing wild in Rock Creek Park, Washington, D.C., USA. This species is a member of the Asteraceae family. Fritzflohrreynolds, CC BY-SA 3.0, via Wikimedia Commons.

Biotypes of Conyza canadensis (marestail, horseweed, or Canadian horseweed) with up to a five-fold increase in resistance to glyphosate, the active ingredient of Roundup® and other herbicides, have recently been confirmed in Richland County, Montana.

Marestail is an annual plant belonging to the Asteraceae family, and it is native to North America. As a winter or summer annual species, marestail emerges in fall or early spring, but it also can germinate in midsummer if growing conditions are adequate. In general, marestail plants start to bolt in April/May, begin to flower in July, set and disperse seed from August to October, and then die. Marestail plants can produce up to 200,000 seeds that are transported by wind, providing for effective spread of herbicide-resistant populations. Reports indicate that marestail seeds can easily travel more than 100 miles in a single fight with moderate wind speeds.

As a native to temperate regions, marestail plants can be found throughout southern Canada, the United States, and tropical America. In recent years, due to the spread of resistant biotypes, marestail has become a challenging weed to manage in reduced-till and non-till cropping systems. In Montana, marestail has primarily been reported in Richland, Valley, and Phillips Counties where it colonizes croplands, disturbed meadows, grasslands, and roadsides.

In the U.S. glyphosate resistance in marestail was first confirmed in 2000 in Delaware. Since then glyphosate-resistant marestail has been documented in more than one-third of the continental U.S. In all cases, the evolution of glyphosate resistance in marestail occurred in row crop systems, including cases of multiple herbicide resistance. This is the first confirmation of glyphosate-resistant marestail in Montana.

Herbicide resistance is defined by the Weed Science Society of America as “the inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide normally lethal to the wild type. In a plant, resistance may be naturally occurring or induced by such techniques as genetic engineering or selection of variants produced by tissue culture or mutagenesis.” The full distribution of herbicide-resistant marestail biotypes in Montana and the mechanisms driving this resistance are still unknown. Whether glyphosate resistance was selected in Montana or it moved from other states, its selection occurred due to the over-reliance of glyphosate and the failure to develop effective integrated weed management programs.

MSU Extension offers the MontGuide, Preventing and Managing Herbicide-Resistant Weeds in Montana, which is available free at county and reservation Extension offices or the Extension Cropweeds website. The website, from the International Society of Weed Scientists, is also an excellent resource for more information on herbicide-resistant weeds.

If you have questions on preventing or managing herbicide-resistant weeds, please contact your local Extension office.

Originally published in the Spring 2016 issue of the Montana IPM Bulletin.  Authored by Timothy Fine, Richland County Extension Agent, and Fabian Menalled, Cropland Weed Extension Specialist.


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