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Fall herbicides may help combat resistant marestail

“You can pay me now or pay me later.” This line from the old TV commercial may be on the minds of some farmers as they begin making fall herbicide applications to control glyphosate-resistant horseweed and other problem weeds.

What once was a time to rest up from the harvest or head for the duck blind has become another application window for growers spraying tank mixes of herbicides such as flumioxazin (Valor) and dicamba (Clarity) or 2,4-D or flumioxazin alone to control resistant horseweed or marestail.

“Marestail has become a year-round management problem for us,” says Eddie Cates, a cotton consultant from Marked Tree, Ark. “If you ask a farmer about glyphosate-resistant marestail, he will tell you there’s not a field or a ditch-bank around here that doesn’t have it.”

“Glyphosate-resistant marestail has been getting worse in our area,” says Brett Roper, a sales representative for Agriliance in the Missouri Bootheel. “Last year, it really seemed to spread. I know Tennessee’s been dealing with it for the last few years, and it’s getting here now.”

One of Roper’s customers, Allen Below of Dexter, Mo., tried a fall-applied herbicide program with Valor and Clarity last November.

“He sprayed a field and left off the point rows,” said Roper. “He called me last March and said, ‘You’ve got to come see this,’ after he saw the difference between the two areas of the field.”

Cates had run 10-acre test plots using Valor and/or Clarity applied in the fall for the past two or three seasons. Last fall, growers he works with treated 2,000 acres with fall burn-down applications.

“We’ve applied 2 ounces of Valor and up to 8 ounces of Clarity in November and December,” he said. “It left some grasses, which probably help prevent erosion, but the treatment took out all of the marestail.” (Cates and Roper add Clarity or 2,4-D to the treatment if winter vegetation is present.)

The fields stayed clean of marestail until the Valor started breaking down and allowing some vegetation to come through in mid-March, he said. At that point, Cates and his growers applied Roundup and Clarity to control the new vegetation.

“In the fall, you’re dealing with smaller marestail plants and not with plants that have big tap roots,” said Cates. “Those plants are much more difficult to control with Clarity or Ignite when they’ve gotten larger in the spring.

“We’re dealing with a numbers game in northeast Arkansas,” he says. “We’re just reducing the numbers of the plants. We may spend the same amount of money if we just come in and spray Roundup and Clarity or Ignite and Clarity in the spring, but the fields are a lot cleaner.”

Farmers can apply Roundup in the tank mix in the fall, but if they leave it out, they’re more likely to see poa anna and henbit provide winter cover than to have bare ground out there.

“The key to making Valor work in a fall application is to put Clarity in with it,” says Cates. “The Clarity will take out the marestail that has already emerged, and the Valor will prevent more marestail from emerging until mid-March.”

Roper and Cates said making fall herbicide applications may be more challenging in 2006. Cates said his farmers had just started picking when he was interviewed. Since then, intermittent rains have interfered with harvesting and herbicide spraying with forecasts calling for more of the same.

“Farmers can apply Valor and Clarity in November, December and on out through February and March, if needed,” says Frank Carey, field market development specialist with Valent U.S.A. Corp. “It would be better not to apply them when it’s freezing, but when the temperatures warm up into the 40s and 50s, you can put them out.

“You can even do it now,” said Carey, interviewed in late October. “You want the temperatures to cool down so you don’t have too much degradation of the herbicide.”

Resistant marestail, which appears to be spreading across much of the northern Mid-South, has changed many farmers’ weed control strategies in the last few years, according to Carey.

“The thing about it is that it takes less herbicide to kill small weeds when you go with a fall application,” he notes. “When marestail and these other problem weeds over-winter, they are more difficult to control. After they’ve bolted and put down a taproot, it may take two shots of a herbicide to kill them.”

“If you can’t control them with a herbicide, then get ready to work the ground,” says Cates. “That’s something many of our growers are not prepared to do any longer.”

Carey says some growers are also applying Valor in 100-yard strips to create buffer zones around glyphosate-sensitive crops such as wheat. Not having to spray the Valor-treated strips could reduce the likelihood of Roundup drifting on to the wheat.

“We’ve seen reports that the number of drift complaints doubled or tripled in states like Mississippi last spring,” said Carey. “Farmers are looking at buffer zones to try to reduce the potential for damage in their fields.”

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