Manage Weed Resistance

December 13, 2008 07:27 AM

Across big swaths of the Southeast and mid-South regions, cotton farmers already miss the simpler days when spraying Roundup handled their weed problems. Now, with widespread Palmer amaranth and horseweed resistance to Roundup, old-style multi-herbicide programs are necessary.
If your fields don't yet have resistant weeds but other farmers in your area are fighting them, you better take action to prevent them. If you do have resistant weeds, then you have to find ways to stop their spread—and fast.

"Early detection is critical. When you see a spot that looks resistant, don't ignore it. That one isolated weed escape out there: Can you really afford to let it go to seed? We've had a lot of people pulling weeds this year to stop just that," says Alan York, North Carolina Extension weed specialist, who spends much of his time working on the resistant weed problem.

Palmer amaranth, better known as pigweed, presents special challenges. Pigweed is pugnacious, designed by Mother Nature to survive tough conditions. It can produce more than half a million seeds per plant and it's promiscuous as well, mutating easily. When sprayed with a herbicide such as Roundup (glyphosate), which gives almost perfect control, it's a genetic recipe for developing resistance. Plenty of farmers found exactly that, to their dismay, after pigweed resistance to Roundup was discovered in 2005.

Best advice. Catch it early, and hit it hard. That's the only approach to Roundup-resistant pigweed that works, says Stanley Culpepper, Georgia Extension weed scientist.

"Growers who have resistant Palmer amaranth must adopt aggressive management. Growers who do not have resistance must delay its arrival, as there are no economical programs to manage this pest in cotton," he says.

Culpepper and his coworkers have developed a three-pronged approach for pigweed using preplant incorporated or pre-emergence herbicides; postemergence materials; and lay-by directed sprays. The system varies depending on whether the field has Roundup Ready or LibertyLink cotton and the extent of glyphosate-resistant pigweed infestation. For details, go to

"Anytime we rely heavily on a product or group of products with a single mode of action, we're putting pressure on for resistance. We backed off other herbicides. We backed off cultivating. It was inevitable that we were going to run into resistance," York says.

Complicating the problem, pigweed also developed resistance to ALS (acetolactate synthase) inhibitor herbicides, further narrowing the range of chemistry available to fight it, says Don Parker, National Cotton Council manager of integrated pest management.

"This is resistance to one chemistry we've heavily relied on, glyphosate, plus another one that won't control pigweed, either. Two different modes of action," Parker says.

The answer, he says, is to keep Roundup and other glyphosate materials in the weed control arsenal for their wide-ranging control but also add other still-viable chemicals to prevent resistance problems.

"We're going back where we started from, basically. We need to use some of the old chemistry and weed control techniques to bring the yellow herbicides back," Parker says.

Make a plan now. York agrees with Culpepper that a multilevel herbicide plan now works best in areas with resistant weed problems.

"We've got to reduce selection pressure and need to reduce reliance on herbicides. That's easy to say, but what can we do? Will a cover crop help? Cultivation? Crop rotation with rotation of chemicals? We're 99% Roundup Ready cotton and soybeans and 75% Roundup Ready corn. We've got to get some diversity in the chemistry we're using," York says.

Diversity starts with preplant and pre-emergence herbicides. "In cotton, you get pigweed pre-emergence or you don't get it at all," York says.

He suggests applying Valor herbicide preplant followed by a pre-emergence material or going with a two-way or three-way tank mix of pre-emergence herbicides. The most effective two-way mixes in his work are Reflex plus Staple, Reflex plus Direx, and Direx plus Staple. Prowl can be added to any of these tank mixes for an added punch.

To help fight resistant weeds, Monsanto Company is offering a rebate program in 2009 called Roundup Ready Cotton Performance Plus. Participating farmers must plant cotton varieties that contain Roundup Ready or Roundup Ready Flex technology and choose from the three preplant herbicide options, two early post herbicide options and the lay-by herbicide option. Any product can be used individually to qualify for the rebate, or one herbicide from each category can be combined for a rebate of up to $12 per acre.

If you spot even one weed that survived herbicide application, don't let it go to seed, advises weed specialist Alan YorkBe proactive. Whatever your plan, pay close attention to what's happening in your fields, advises Bill Robertson, National Cotton Council manager of cotton agronomy, soils and physiology.

"You may go with that burndown herbicide, then at your first over-the-top spray it's the first time you realize you have weed resistance. A lot of farmers get to that point thinking they have no problem, but then they wind up with resistant pigweed scattered across their fields. There are a lot of tools to combat resistance, but by that time, it's beyond the point to employ those, so prevention is a big part of dealing with resistant weeds," Robertson explains.

"Prevention is the most critical thing. If you're in an area where there have been problems, even if you didn't have it last year, count on having it this year. If you don't have it, your neighbors do. You need to be prepared," Robertson says.

New herbicide technologies and traits for cotton, such as dicamba tolerance, that provide additional methods of controlling resistant weeds are still a few years away from being commercially available to growers. Meanwhile, properly managing resistant weeds could extend the effectiveness of Roundup and the glyphosate materials, Robertson says.

"Glyphosate has been so good, so easy to use and so convenient. It's a tool we can't afford to lose," he says.

You can e-mail Charles Johnson at

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