Experts from the Montana State University Extension Service have identified glyphosate-resistant Conyza Canadensis, also known as marestail or horseweed, in northeast Montana. The discovery is concerning given how easily this species can disperse in the wind.
According to Timothy Fine, Extension agent in Richland County, and Fabian Menalled, MSU professor and Extension specialist in cropland weeds, marestail with up to a five-fold increase in resistance to glyphosate, the active ingredient of Roundup and other herbicides, has recently been confirmed in Richland County.
Marestail is an annual plant native to North America. As a winter or summer 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 and 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 flight 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 Montana, marestail have mostly been reported in Richland, Valley and Phillips counties, where it colonizes croplands, disturbed meadows grasslands and roadsides.
In recent years, due to the spread of herbicide resistance, marestail has become a challenging weed to manage in reduced-till and non-till cropping systems. 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, however, 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.” The full distribution of herbicide-resistant marestail genetics in Montana and the mechanisms driving this resistance are still unknown. Whether glyphosate resistance evolved in Montana or it moved from other states, its selection and spread has occurred due to an over-reliance on glyphosate and the failure to develop effective integrated weed management programs.