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Resistant pigweeds zap cotton in Southern states

Without a doubt, weed resistance to herbicide was the hot topic at the 2012 Beltwide Cotton Conferences in Orlando, Fla.


Without a doubt, weed resistance to herbicide was the hot topic at the 2012 Beltwide Cotton Conferences in Orlando, Fla.

While this has been seen as a “Southern” problem in recent years, it appears to be a matter of time before the horror stories from Dixieland spread west to major cotton-growing states like Texas.

“I don’t believe we will ever hear again someone say we won’t have resistance in anything,” said Bob Nichols of Cotton Incorporated in Cary, N.C.

Nichols said cotton growers must deal with resistance to Palmer amaranth (aka pigweed or careless weed) before it takes over. He suggests adding preplant incorporated, or so-called “yellow” herbicides, to the weed control arsenal.

“If resistance is found two counties away, take action,” Nichols said. “Use PPI herbicides and residuals.”

Key Points

• Weed resistance in South is a hot topic at the Beltwide conferences.

• Experts advise using different modes of action to fight weeds.

• Growers cannot rely on over-the-top weed treatments alone.


And there is some help from over-the-top herbicides, too. Ignite (now called Liberty) herbicide has come on the scene as a savior to use with cottons having the LibertyLink trait, especially where pigweeds have been showing resistance to glyphosate .

Nevertheless, weed experts worry if growers totally rely on Liberty — the same way they had on glyphosate for so many years — that overuse of Liberty also could lead to resistance in years ahead. That’s why the phrase “use different modes of action” is advised again and again.

To keep Ignite, “we’ve got to use multiple modes of action,” said Larry Steckel of the University of Tennessee, Jackson. “We may not like it, but we may have to ‘go back’ to hooded sprayers, too.”

More than pigweed

Steckel said resistance is not just a pigweed issue; up to nine weeds in the U.S. are known to be resistant to herbicide now.

Some of those others include horseweed (aka “marestail”), which is common in Texas, giant ragweed, common ragweed, water hemp, Italian ryegrass, johnsongrass, goosegrass and annual bluegrass.

The annual bluegrass actually was found to be resistant when it was discovered on a Tennessee golf course. But cotton field or golf course, you don’t want it.

“Although pigweed is the big problem, goosegrass is coming on strong now,” Steckel said.

However, pigweed remains the titanic headache. Steckel said more than 300 counties in the Mid-South and the Southeast have pigweed resistance now — not just in cotton fields, but in soybeans, too.

No magic cure

Don’t expect a silver bullet. Steckel said the days are gone when growers could simply “control 2-foot-tall pigweed with Roundup like the old days.”

Steckel said farmers need to look at all their tools. For example, dicamba, used pre- or postemergence, has worked well to control pigweeds in soybeans. Glufosinate plus 2,4-D, or glufosinate plus dicamba also offers good control of pigweed.

Steckel said the strategy of growers should be to think for the long haul, and use different modes of action that will help extend the effective life of new products like Liberty herbicide.

Watch turn rows and ditches

One pigweed plant may produce a million seeds. If just a small percentage of the seed germinates into new weeds in your field, you have a major problem.

Jason Bond, Mississippi State Uni-versity, Stoneville, said if a producer is going to spend a lot of money trying to clean a field of pesky weeds, including pigweeds, he needs to be sure turn rows, ditches and roadsides are clean of weeds, too.

“If not, the weed seed will just wash back into the field,” he said.

Bond noted if weeds in places like turn rows and ditches can’t be sprayed because of their location, perhaps the weeds can be mowed or disked.

Andy Vangilder of the University of Arkansas, Piggot, said when you see an isolated, thick patch of pigweeds all to themselves, don’t be surprised if the weeds are mostly females, or that you have pigweeds resistant to glyphosate herbicides. Be on the lookout for that, and contact specialists that can identify whether or not you have resistant weeds when you encounter such a situation in a field.

Vangilder said he agrees with Bond on the importance of keeping turn rows, ditches, and roadsides clean of weeds. “But that means your neighbors must be cooperative,” he said.

Can’t wait for weeds

Ken Smith of the University of Arkansas, Monticello, cautioned cotton growers not to get behind on weeds.

“You can’t wait until you see the weeds,” Smith said. “You have to have a plan before the season to start clean.”

Smith warned not to hinge your hopes on some new technology alone saving you.

“You can’t afford to let Palmer [amaranth] get a head start,” he assured. “The numbers game will beat you.”The veteran weed expert said it’s a whole new ballgame now.

“Overall, the over-the-top herbicides are not our base staple anymore,” Smith said. “If we rely on over-the-top, we are in big trouble.”

Smith said he now sees over-the-top treatments as “just a cleanup now” and a way “to manage the escapes.”He also noted growers need to look at controls for individual fields.

“There are no 100% herbicide programs,” Smith said.

Close that canopy

Cotton row width and vigorous growth is important because a canopy that closes quickly can slow down pigweed growth and reproduction.

Don Parker of the National Cotton Council, Cordova, Tenn., said growers must change to survive.

“With weed resistance to herbicide issues, cotton producers are reverting back to tools in their arsenal that, in some cases, they haven’t used in years. Also, don’t expect technology to move as quickly as in the past because of regulations that slow [new] products.”

And it may sound like blasphemy in areas that have fine-tuned no-till farming, but Steckel said in some particular cases, the weed war may even mean hitching up the plow and returning to some cultivation.



This article published in the March, 2012 edition of THE FARMER-STOCKMAN.

All rights reserved. Copyright Farm Progress Cos. 2012.

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