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Scientists watching resistant horseweed

“This certainly isn’t new to Monsanto or those of us watching trends outside the state,” says Ken Smith, Arkansas Extension weed scientist. “Tennessee has battled this weed for a while. But it is new in Arkansas: glyphosate-resistant marestail (or horseweed). This weed threatens our conservation tillage efforts.

“If we don’t control this weed early and do what needs to be done, the cotton crop will be behind the 8-ball and we’ll end up putting a plow back into the fields.”

Smith and others suspect this isn’t the first year the resistant weed has been in the state, but this is the first time it’s been found and carried to the greenhouse to confirm resistance.

Smith says the question always comes up: how do weeds become resistant?

Basically, in doing its job, a herbicide interrupts some vital process of a plant’s life – some chemical reaction is halted, some enzyme is affected.

“Some plants are able to develop an alternate pathway,” says Smith, who spoke at the July 24 Monsanto field day in Coy, Ark. “They’re just able to find some way to produce the needed chemical reaction. Other plants will restrict the uptake and absorption of a given herbicide. The morphology part of the plant is able to shut a product out. Then some resistant plants are able to metabolize herbicides quickly. If they’re able to do so well enough, a plant can escape injury.”

Likely candidates

In many weed species, there are a small percentage of plants with the genetic diversity to be resistant to one herbicide or another. The more specific a herbicide is to a chemical reaction inside a plant, the easier it is for a “special” plant to be found that can battle the herbicide.

Researchers’ best guess is that in any given horseweed population, there is a small percentage of plants resistant to glyphosate. When utilizing a con-till situation where glyphosate is used every year for burndown, resistant plants are selected out.

Horseweed plants produce about 50,000 seed each annually. So it takes only a year or two for an entire field to have resistant weeds.

“Horseweed is in bloom right now and its seed are wind-blown,” says Smith. “Usually the seed don’t move a huge distance (maybe a half mile), but under the right weather conditions they can.”

While the arrival of resistant horseweed is a “major obstacle, it is one we can manage,” says Smith.

“When talking about resistance, there are different levels. Sometimes you can kill a weed with a 2X or 3X rate. Other times you can’t kill it using any rate of a given herbicide.”

On the tour, Smith shows a photograph of a small horseweed plant.

Terminal illness

“This one was sprayed with a 2X rate when it was a small seedling. What happened was the terminal was burned out of it and three terminals took that one’s place. Typically, horseweed has only one stem unless something destroys the terminal early. But if something messes up the terminal early, it will branch. That’s what we’re finding with a 2X rate.”

Some of the resistant weeds were handling a 10X rate. “We were using 5.6 quarts of Roundup Weathermax and still didn’t kill them,” says Smith.

It will take a few more dollars up front, but resistance can be managed.

“We can come in early with 2,4-D and dicamba 30 days prior to planting,” says Smith. “That does a pretty fair job especially when followed by Roundup or Gramoxone to take out any weeds that have germinated.

“Or, in another situation, you may need to take out a wheat crop. If you’re planting a wheat crop for cover on a con-till field, you don’t want to take the wheat out too early. Remember, the 2,4-D and dicamba followed by Roundup. Leave the stubble standing.”

Alternative treatments

If you do get into a crisis situation, two quarts of MSMA just ahead of planting appears to be as successful as anything Smith has tried. “But,” he warns, “it’s not great.”

Resistant horseweed has been confirmed in the eastern part of the state in Mississippi, Poinsett, and Crittenden counties. Smith figured that’s where it would show up first.

“We think Lee and Phillips counties will have this problem next year. That doesn’t mean Chicot County – or some other county far away – can breathe easily for the next three years. If there’s a field there that, over several years, has used a lot of glyphosate, it could show up.

“We’ve looked at the expansion of this weed. If you look at where Tennessee’s resistant weeds were first found and the progression through the years, it was obvious we were going to have a problem this year. It was right on schedule. That means that for those without resistant horseweed in the state, your day is coming. We’re going to have resistant horseweed and will just have to learn to manage for it.”

Other weeds can become resistant as well.

“I hope it doesn’t happen,” says Smith, “but it can. The one we’re most concerned about it pigweed. If and when pigweed becomes resistant, our con-till efforts would be hit very hard. We’d be in a real storm.”

With the high number of Roundup resistant crops grown today, if pigweed became resistant to glyphosate, it would revolutionize agriculture in a negative manner.

But will it?

“Well, pigweed has become resistant to (a range of products) and one of pigweed’s first cousins are already resistant to glyphosate. Hopefully, it won’t happen, but we need to be very conscious of any escapes with this weed.”

Pigweed first?

Five or six years ago if you asked Smith if horseweed would be one that would become resistant to glyphosate, he’d have said no.

“I would have thought a species with a lot of genetic diversity – like pigweed – would have been first. But so far pigweeds remain extremely susceptible and easy to kill with glyphosate. I hope that continues.”

The timing of spraying along with using the correct rates are extremely important, says John Bradley, Monsanto’s tillage system expert. “We got into trouble in west Tennessee – and I’m not blaming everything on this, I agree with (Smith’s) comments about diversity – when farmers started using half rates. We’ve got to keep using the full label rate and pay attention to timing. Conservation tillage is all about timing, so this is no different.”

Smith agrees. “We’ve gotten a little lax waiting for weather to get right and if you miss a couple of days, no problem. Well, I think those days are coming to a close. Now, when it comes time to spray a con-till field, you’d be best served to get it done. It behooves everyone – farmers, Extension folks, consultants, everybody – to keep that in mind.”

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