By Madison Dapcevich

Special to Forum News Service



SIDNEY, Mont. -- Marestail, with up to a fivefold increase in resistance to glyphosate, the active ingredient in Roundup and other herbicides, has been identified in northeast Montana by experts from the Montana State University Extension Service.

Fabian Menalled, MSU professor and Extension specialist in cropland weeds, says the number of herbicide-resistant weeds in Montana has nearly doubled since 2004, with some cases of multiple herbicide resistance, and the problem is getting more difficult and expensive to manage.

"We need to be aware that the more we rely on that really great herbicide Roundup, the higher the chances are that we're going to be running into resistance," Menalled says. "Farmers need to start thinking outside the box and try to go back and think how they can manage weeds with a decreased reliance on herbicides through crop rotation, increasing competitiveness of the crops and increasing the mortality of the weeds."

The weed was discovered after a farmer sprayed his Chemfallow field with a glyphosate product and it failed to perish. Samples were pulled from several farm fields and an overgrown lot in Sidney, and were sent to the International Survey of Herbicide Resistant Weeds in Oregon, where they were tested and confirmed.

"We have a pretty good understanding of why certain plants are better at selection pressure," says Richland County Extension Agent Timothy Fine. "If you have a resistant plant and you spray all the other plants around it that aren't resistant, then the one resistant is going to produce seed and increase the population."



A mutation

Herbicide resistance is the evolution of plant mutation at random. When a producer sprays a field, weeds without that genetic mutation are killed off, while the resistant plant is able to go to seed and reproduce. Over time, the DNA of the mutated gene creates an increase in population.

"It is not the herbicide that changes the genetics of the plant, that's very important," Menalled says. "The herbicides select for the resistant plants. The mutants are there when we use the same management practice over and over again. It is pure evolution occurring in your backyard, literally."

While Marestail is not a common weed in Montana, it is a typical weed in no-till soybean systems across the nation.

Perhaps the bigger question is whether these resistant plants were accidentally imported or if it is a natural selection process from the use of glyphosate herbicides.

Marestail is an annual winter or summer plant species native to North America. Generally, Marestail plants start to bolt in April and May, begin to flower in July, set and disperse seed from August to October, and die.

The plants can produce up to 200,000 seeds that are transported by wind reportedly more than 100 miles in a single flight. In Montana, Marestail is mostly found in Richland, Valley and Phillips counties, where it colonizes croplands, disturbed meadows, grasslands and roadsides.

Ongoing study

Fine hopes to collect more samples this summer to verify if there are multiple weeds resistant to herbicides other than glyphosate. If that is the case, Montana Extension Services will begin to formulate a strategy for control that will examine which herbicides are still effective and what alternative cropping systems might be available.

"The method of control with Marestail is to spray in the fall when the plants are still young, and that's certainly a recommendation we're going to give if that works into an operator's weed control program and cropping system," says Fine, who adds the best recommended practices also include rotating classes of herbicides from year to year.

"Our hope is, by finding out early, we can work with producers to prevent any further instances from happening," Fine says. The MSU Extension Service encourages producers to familiarize themselves with the different strains of herbicide-resistant weeds and contact their local Extension Agent Office with questions, comments or concerns.