In 2002, glyphosate-resistant marestail was found sparsely distributed in the Missouri Bootheel. The next year, Andy Kendig was finding it “almost everywhere, just driving up and down the road. And since then it’s only gotten worse.

“I’m operating under the premise that, at least for the Delta region of southeast Missouri, resistant marestail is well-established,” says the Missouri Extension weed scientist stationed at the Delta Center in Portageville, Mo. “Growers should just assume they’ve got it.”

Producers in Tennessee should assume the same.

“We aren’t checking horseweed for resistance much anymore,” says Bob Hayes, weed scientist and superintendent of the West Tennessee Research and Extension Center. “It’s already everywhere and we’re telling producers to manage as though they’ve already got it. Resistant horseweed is in all our row-cropping areas.”

Any Tennessee producer utilizing a no-till system “must assume they have problem marestail from the get-go. Doing so helps with an overall burndown program, anyway. If you go out there with just glyphosate, the field will likely have cutleaf evening primrose and other weeds. Glyphosate plus dicamba will result in a more effective burndown. That’s a good strategy.”

Hayes says Tennessee researchers are currently investigating other weeds suspected of resistance. Most are pigweed samples “and if they are proven glyphosate-resistant we want to know how resistant they really are.”

Across the Mississippi River in the Bootheel, Kendig has yet to confirm glyphosate-resistant pigweed.

“That doesn’t mean we haven’t gotten a few phone calls and tested a few samples, though.”

As for pigweeds, “most producers will have some regardless of resistance,” says Hayes. “A residual herbicide allows a pretty good hedge against inclement weather or other situations that can develop.”

Both states expect a spike in corn acreage. How might that play into the resistance issue?

“For corn, atrazine remains fairly inexpensive for what it brings to the table,” says Hayes. “And things like Direx are inexpensive for cotton, considering the insurance it affords. We’re just trying to at least delay resistance from developing for a few generations of weeds.

“We’re continually saying, ‘Please don’t use glyphosate as your sole herbicide. That is a very bad idea even if you’ve planted Flex cotton or Roundup Ready corn.’”

Producers need to put some residuals, or additional herbicide mode of action, into the mix.

“That’s the minimum that needs to be done to abate the development of resistance,” says Kendig. “And any weed escapes must be removed.”

With Flex cotton coming on line, “it will probably mean a bit more Roundup being used. I don’t know if additional corn — mostly Roundup Ready — will mean more glyphosate being applied. It’s tough to say because corn will likely be displacing cotton and soybeans. So the Roundup Ready crops will almost be a wash, they’ll cancel each other out.

“In fact, I’ll make a point of saying that atrazine works surprisingly well in a Roundup Ready corn program. Atrazine is certainly an option for getting some alternative chemistry into the field.”

Roundup Ready corn seed remains a hot commodity.

“One of the chief concerns is producers want it for drift protection,” says Hayes. “It’s also a concern that more glyphosate could be going out due to planting more Roundup Ready corn or Flex cotton. Many producers claim they’ll plant Roundup Ready mainly as a defense mechanism against drift. They’re too afraid not to plant it — we’ve got many more corn fields mingled in with cotton and soybean fields.

“You know, it is one thing to drift on your own crop. It’s quite another to drift onto your neighbor.”

Weed resistance is “for real,” says Hayes. “Producers should use every strategy available to keep it off their farm because there’s nothing new to combat it. In a few years, Monsanto is supposed to have a dicamba gene in crops. But other than that there’s nothing that will help with resistance. There are certainly no world-beater herbicides in the pipeline.”

The fundamentals of dealing with resistant weeds haven’t changed. “The most important thing is to keep your eyes on your fields and watch all developments, look for the unusual,” says Kendig. “Second, do what you can to include alternative chemistry. Third, talk to someone who is familiar with resistance — a consultant, an Extension agent, someone who knows.”