Marestail, also called horseweed, was the first confirmed resistant broadleaf weed. Now waterhemp appears to be resistant, and giant ragweed is a strong suspect.
With the list of glyphosate-resistant weeds growing, many might wonder if this trend is inevitable, or if there's something that can be done to reduce the spread of glyphosate resistance to other areas and other weeds.
“There are four steps that can help,” says Jeff Stachler, Ohio State University weed scientist. “The first is to include 2,4-D and/or Gramoxone in a spring burndown. The 2,4-D is most effective if applied before marestail has produced a stem.
“Second, include a residual herbicide in the burndown, such as Valor, Canopy XL or Sencor.
“The third step is to apply the burndown as early as practical to nail weeds while they are small.
“Fourth,” says Stachler, “if you have a field with some marestail that glyphosate has missed, consider a fall burndown with something containing 2,4-D.”
Glyphosate resistance in marestail has been confirmed in Ohio, Indiana, Tennessee, Kentucky, Missouri, Maryland, Delaware and New Jersey, primarily on continuous soybeans. Other states are sure to follow.
“Marestail is especially dangerous because the seeds are light and can easily travel great distances,” says Stachler.
When using glyphosate, always apply the appropriate rate for the size of the marestail — or any other weed — at the time of spraying.
Stachler says the majority of marestail in Ohio and probably Indiana is ALS-resistant. For areas where it's not ALS-resistant he suggests a tankmix of FirstRate or Classic with glyphosate.
The best long-term strategy for avoiding glyphosate resistance is to use a pre-emergence product rather than rely on glyphosate alone, Stachler says. Another option is to do some spring tillage.
He adds that atrazine is the most effective herbicide for combating marestail in corn. “It appears that 1 lb/acre is the minimum to use, but we need more research on this.”
Bryan Young, weed scientist at Southern Illinois University, focuses his message on preventing glyphosate resistance.
“A key factor is to rotate herbicides with different modes of action,” Young says. “Ideally, you would make only two or fewer applications of glyphosate in two years. However, you could stretch that if the applications are in different seasons and there are different weed species present at the time of application. For example, there might be a burndown to get winter annuals, and then later an in-season application on a totally different weed spectrum consisting of summer annual weeds.”
Young sees solid reasons for using soil-applied herbicides in addition to a postemergence product, such as glyphosate, as a way to head off resistance. “Once resistance to the postemergence product occurs, effectiveness against a resistant weed is lost forever,” he points out. And there may be no suitable rescue treatment available.
“Also, once you have resistant weed biotypes to postemergence herbicides, you may be forced to live with incomplete weed control. That's because it's rare to achieve season-long control of some weeds with a soil-applied herbicide,” Young says.