Cotton producers battle late-emerging resistant horseweed

West Tennessee cotton producers are now battling a June emergence of glyphosate-resistant horseweed.

According to University of Tennessee weed scientist Bob Hayes, the late emerging horseweed, which is also known as marestail, is in many cases coming through in fields where there had been excellent control of the weed early in the season.

Corn and soybeans producers have been able to control it in-crop, but cotton producers have had a more difficult time, according to Hayes.

“I was in a field last week and there was cotyledon material (horseweed) pushing through the ground,” Hayes said. “There was a lot of horseweed less than 2-3 inches tall with about 20 leaves. Spots in the field were very thick. There have been at least three fields with that scenario. It's not going away.”

In addition, resistant horseweed has now been confirmed in conventional-till fields, according to Hayes. It was thought that resistance was primarily a problem in no-till.

For in-season control in both conventional and no-till cotton, “On real small horseweed, we recommend a post-directed application of MSMA and Karmex where they're sure they have resistance,” Hayes said. “If they have weeds out there that they need to kill with Roundup, we recommend a directed application of Roundup and Karmex and surfactant and dropping in some MSMA if they need to.”

Results have ranged from good to poor, according to Hayes. “Basically, you're into salvage mode, especially if the horseweed has any size on it. In most cases, it's not going to give complete control, but it's the best we can do.”

Hayes noted that earlier in the season, MSMA did not perform well on resistant horseweed. “But MSMA is just not the same herbicide under cool conditions as it was last week when temperatures were in the 90s.”

In most cases of June-emerged horseweed, growers are working with a good height differential for direct sprays, according to Hayes. However, in other instances (late replanted cotton for example) horseweed will have to be rogued.

Prior to the 2002 season, west Tennessee Extension specialists established a protocol for control of suspected infestations of the resistant weed, including the use of Valor in the fall, or a spring burndown of 2,4-D or a tank mix of Clarity and Roundup. But this year, resistant weeds often appeared in fields that had no previous history of an infestation.

Weed scientists suspect that seed from resistant plants may have been carried by the wind and deposited in those fields. To make matters worse, growers have had trouble controlling even susceptible horseweed due to cold, wet weather. “It's been a marestail year,” one grower noted.

There's little doubt that west Tennessee farmers will be advised to include a material for control of resistant horseweed when they burn down prior to planting in 2003.

“I thought at one time that conventional tillage might be the answer,” Hayes said. “But we've got it in conventional-till fields.”

Late emergence of horseweed is also being found in fields where producers went with Roundup and Clarity at burndown. On the other hand, growers, “are in a good situation to come in and post-direct and do a good job,” Hayes said.”

Another program Hayes is evaluating is to spray Roundup early and then come back with a full dose of Gramoxone and Cotoran at planting. “It's helped. We're not getting re-infestations.”

As for recommendations for 2003, Hayes said, “We're going to try and get our data together with the companies involved and come up with the same message. We don't want to confuse growers.”

Resistant horseweed has been on Bob Montgomery's radar screen for almost three years now.

The agronomic research manager for Monsanto first saw a farm field with suspected Roundup-resistant horseweed during the 2000 crop year. The following year, he set up a plot on the farmer's field to test potential control treatments at burndown.

The most effective products and application timing in Montgomery's tests was a tank mix of Roundup and Clarity applied from late March to early April in 2001 and 2002.

According to Montgomery, the timing of the application was important for controlling re-infestations prior to planting. The study indicated that Nov. 15 treatments which killed all existing vegetation allowed resistant horseweed to re-infest the field before planting.

Applications of the same materials on Nov. 30, a colder day, resulted in fewer horseweed at planting than with the Nov. 15 time frame, however both fall application timings resulted in less than acceptable horseweed control due to spring emergence. Roundup and Clarity completely controlled horseweed that had emerged prior to application in the fall and early spring. The key is to time the application so as not to encourage re-infestation by late emerging horseweed..

Growers might also consider a two-step approach with Clarity and Roundup, according to Montgomery.

“When Clarity is applied alone, it will allow poa annua (annual bluegrass) and other winter annual grasses to remain in the field and compete with emerging horseweed. The poa annua can then be burned down with Roundup ten to fourteen days before planting. The problem with removing all vegetation from the soil surface too early is that horseweed emergence is induced due to the lack of competition from other weeds.”

Montgomery says the development of resistance is not something “we are totally unprepared to deal with. There is a lot of glyphosate used all over the world. To have no more than three resistant weed species worldwide demonstrates how rare it is for plants to express an altered form of the enzyme that binds glyphosate or other resistance mechanisms to this herbicide. Glyphosate resistant horseweed is a manageable weed control issue.

“We're not the type of company to stick our heads in the sand,” Montgomery added. “We want to go after any problem early and use the principles of integrated pest management. We believe that it is more cost effective for growers adjust weed control practices and herbicides to deal with these types of issues if they happen rather than invest in solutions to problems that may or may not occur.”

That will mean changing the burndown program for west Tennessee no-till producers. “I don't think many producers in this area will consider using Roundup alone as a burndown. I hope they don't. It will remain the burndown of choice for many that do not have this weed problem.” he said.

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