“For marestail, glyphosate resistance is again the big issue,” Sandell says. “However, even non-glyphosate-resistant marestail can really reduce corn yields very quickly when in high density.”

Horseweed can be controlled with either a fall or spring burndown, Sandell says. “However, it’s unrealistic to expect residual control to last from fall through the first postemergence application, especially when planting soybeans,” he adds. “Controlling marestail prior to planting is critical, as postemergence control can be challenging if the plants have any size to them.”

Marestail tends to be more of a problem in no-till fields, Owen notes. “For marestail, if you include a little tillage, then that will take care of it, as long as you control it while it is still a rosette and before it bolts and sends up a flower stalk,” he says. “At that stage, it’s much more difficult to control.”

In Iowa, there are fewer issues related to herbicide resistance with marestail than with waterhemp or ragweed, Owen says, “but resistant marestail populations do indeed exist here.”

Sandell agrees that “tillage is another good tool to consider for Nebraska” but advises caution so that tillage doesn’t expose fields to erosion or cause farmers to jeopardize their benefits from government conservation programs.

Glyphosate-resistant marestail populations are also taking root in Illinois. “These resistant populations are more common across the southern third of the state,” Hager says. However, he adds, anecdotal observations during 2011 indicate that glyphosate-resistant horseweed populations have become a greater concern among farmers in central Illinois as well.