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 www.gaweed.com.
"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.
- December 2008