COLUMBUS – Populations of marestail (horseweed) in Ohio have been found to be resistant to glyphosate, the first indications that herbicides like Roundup are losing their effectiveness in weed control.
Ohio State University research found that 10 fields from four counties (Clermont, Brown, Clinton and Highland) contained glyphosate-resistant marestail (horseweed) at three to five times the normal application rate.
The postemergence herbicide, carrying such trade names as Roundup WeatherMax, Touchdown, Glyphomax, Cornerstone and Mirage, is commonly used in no-till corn and soybean fields and some tilled fields to control a variety of weeds, including marestail.
Growing trend. This is the first time a weed in Ohio has been identified as glyphosate-resistant. Missouri and Indiana have also recently joined the list of states with glyphosate-resistant marestail.
Other states include Kentucky, Tennessee, Maryland, Delaware and New Jersey.
Beans after beans. Jeff Stachler, an Ohio State Extension weed scientist, said the majority of the resistant weeds are being found in fields that have been growing continuous soybeans in no-till situations for several years, and where only glyphosate products have been used at burndown and during the growing season.
Not good news. The find, he added, paints a frightening picture of where herbicide programs are headed: If management practices don’t change and the discovery of herbicide-resistant weeds continues, growers will eventually run out of control options.
“What this discovery is telling us is that growers must know how their herbicides work and change herbicides frequently enough to delay the onset of resistance,” said Stachler.
“You can’t stop resistance from happening, but depending on how often the selection pressure comes depends on how quickly the weed population changes from sensitive to resistant.
“The more you use the same product, the higher the chances are Mother Nature will find a way to get around it. It’s just a matter of when it will happen.”
Stachler said that growers are relying too heavily on postemergence herbicides to control weeds, mainly because it’s cheaper to apply only one application in the hopes of getting that one-two knock out punch.
But with weeds like marestail it is becoming evident that other control options are warranted.
Marestail (horseweed) is typically a winter annual that can also emerge in early spring, and doesn’t mature until late summer, competing with corn and soybean growth during the growing season.
The weed also produces a large number of seeds (1/4 million seeds per plant) that have little or no dormancy, allowing for plants to germinate any time during the year and increasing the likelihood that a resistant population will develop and quickly spread.
One researcher found that marestail can travel by wind up to 400 feet away from the mother plant, but Stachler believes it can easily travel farther than that.
ALS-resistant, too. Additionally, marestail is one of nine weed species in Ohio that is resistant to ALS-inhibitors (acetolactate synthase), a group of herbicides that kill weeds by preventing the plants from producing essential amino acids that are needed for proper growth and development.
Many ALS-inhibitor herbicides are post-emergence, and marestail resistance to such chemicals further narrows control options available to growers.
“If we have a weed that is both glyphosate and ALS-resistant, there will be fewer and, in some cases, no postemergence herbicides available in soybeans that would be able to control that weed. We’d have a lot of people backed up against the wall because of it,” said Stachler.
“Marestail and common lambsquarters are two of those species in which there would be no postemergence herbicide options in soybeans for their control.”
Mix it up. Such a scenario is a reminder to growers the importance of mixing up their weed management practices.
Stachler recommends, among other things, that growers incorporate preemergence herbicides into their control program.
“For some species such as common lambsquarters, one of the easiest ways to clean a no-till field at the time of planting is to use a combination of preemergence herbicides and an application of 2,4-D before planting,” he said.
Stachler also recommends that growers scout their fields for possible weed resistance to herbicides.
“If you start to see dead plants next to severely injured plants next to slightly injured or healthy plants within the same patch or within the same field, then you ought to be concerned that those weeds are developing resistance. Then it’s time to change the name of the game.”
That new game, said Stachler, would be to try a herbicide that has a different site of action.
Stachler also encourages growers to more wisely manage their fields, from tillage or no-till practices to improving crop performance and rotation. Such techniques are important, he said, to help stem the development of weeds with multiple herbicide resistance.
“We theorize that weeds like giant ragweed or common lambsquarters (both ALS-resistant) may be next in glyphosate-resistance,” said Stachler. “Based on greenhouse research, we found it very difficult to consistently control these weeds with glyphosate.”
Other glyphosate-resistant weeds identified in the United States include waterhemp, rigid ryegrass, goose grass and Italian ryegrass.
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