Glyphosate-resistant horseweed may be appearing in west Tennessee fields with no previous history of an infestation.

University of Tennessee weed scientist Bob Hayes first documented horseweed resistance to glyphosate in west Tennessee in 2001, after it appeared in one field in 2000. Hayes used University of Tennessee equipment to put in plots on farmer fields which had a history of hard-to-control horseweed “to make sure that we were dealing with a resistant population. And in fact, that is the case.”

In research plots, Hayes applied glyphosate and other materials to the horseweed at 2X to 10X rates, “and all we were doing was burning the leaves with the surfactant. We were not getting the translocation to the growing point. We have stunted some of it, and in some cases, it turns a little yellow. But we didn't get complete control.”

The problem prompted the University of Tennessee to recommend certain protocols for control of the resistant weed this year, but only on fields where resistance was observed in 2001.

But on Willie German's farm, horseweed, also known as marestail, survived a burndown with Roundup and a follow-up application of MSMA and Direx in a field that had been previously clean.

“That's the case with a lot of people,” said Hayes, who suspects that an especially windy fall may have carried resistant horseweed seed from field to field. He added that resistance appears “to be much more widespread than we had thought.”

Documented resistance to horseweed has been reported in New Jersey and Maryland. Hayes has had “a couple of calls” from Mississippi farmers suspicious that they have fields with resistant horseweed.

German farms about 3,500 acres of no-till near Somerville, Tenn. On one of the problem fields, he burned down on April 10, going with 25.6 ounces of Roundup Ultra Max and 8 ounces of ammonium sulfate. He noticed the horseweed escapes about a week later. All other weeds had been controlled.

He briefly considered plowing the field before planting, but decided to go with a recommendation to spot spray a third of a gallon of MSMA, a pint of Direx and crop oil on April 26.

“We planted the farm on April 29. The MSMA/Direx wasn't killing the marestail. It stunted it, that's all. At that point, I knew I had one more chance to spray with Roundup over-the-top.”

On May 15, German made the over-the-top Roundup application and 13 days later “it looked like the Roundup stopped the marestail for a few days and yellowed them. But there's no brown on them, and they're not going to die. We're going to have them all year.”

German will clean the weeds out of the middles with cultivation, but “getting it out of the row is going to be tough. We don't have the height differential to direct spray.”

German says the worst of the problem, which is infesting about 15 percent of his cotton acreage, “has either been in no-till or where we haven't rotated. Some of the other farms that have been plowed up or have had beans or corn on it the last few years are not as bad. But this field has had glyphosate on it every year for burndown.”

“We had marestail last year here, but we cleaned them up,” said University of Tennessee Extension area specialist Craig Massey. “If we had known we were going to have this problem, we could have mixed some Clarity in with the Roundup (at burndown).”

German says another option would be to go to some tillage, “maybe every third year or something like that. I could also go to a different crop where I can use a different chemical.”

German, a Case IH dealer, added, “Case IH tillage has marestail on our label. I've never seen a weed resistant to cold steel.”

Hayes agrees that to control resistant weeds, “There may be situations where we can judiciously till. But just disk those spots, rather than treating the whole field.”

In addition, growers on highly erodible land “will likely have to get approval from NRCS before tilling.”

Several chemical options are available to control resistant horseweed prior to planting, according to Hayes. “Valor did a good job on the broadleaves — cutleaf evening primrose, geranium, horseweed — and left the poa annua, which provides some cover for the winter months. The Gramoxone/Karmex application killed all the vegetation.”

However, a spring application of Valor “did not perform as well.”

“We've had a few growers put out 2,4-D. That's about as cheap as you can go. It'll do a good job if you put it out in the middle of February. They bought an old sprayer and boom and designated it as their 2,4-D sprayer.

“Roundup and Clarity performed extremely well for the farmers who got it on,” Hayes added. “What we're running into now are that some growers didn't get much rain (to activate the Clarity).”

There were occasions when good control was achieved with Gramoxone and Direx applied early last fall or in very early spring, according to Hayes. “But then we went through a period when Gramoxone/Direx didn't work very well.”

The next few weeks could be crucial for growers like German, who are feeling some frustration.

“I thought things were getting kind of simple for us with the Bt cotton and the Roundup Ready cotton, sowing cover crops, building the land up. Now we get this,” German said.