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Mid-South researchers prepare for glyphosate-resistant pigweed

As if Mid-South farmers did not have enough to worry about, they may be dealing with a new species of pigweed: Amaranthus inbetweenus.

Before you rush off to your weed control guide, let me hasten to add that this “new” strain is an attempt at humor from Andy Kendig, weed scientist with the University of Missouri's Delta Center at Portageville, Mo.

Kendig used the term he coined as part of the lead-in to a discussion of a very serious subject — the specter of glyphosate-resistant pigweed — at the Regional Cotton Pest Management Workshop at the Memphis, Tenn., Agricenter, Nov. 2.

Glyphosate-resistant pigweed has not been found in the Mid-South, although weed specialists have reported escapes in Roundup Ready cotton in Arkansas, according to Ken Smith, Extension weed scientist with the University of Arkansas. It was confirmed in Georgia last summer.

But those reports have gotten the attention of weed specialists and of crop consultants like Chuck Farr of Crawfordsville, Ark. “If we get glyphosate-resistant pigweed, we're out of business in northeast Arkansas,” he said during a panel discussion at the workshop.

Whether resistance is just around the corner or not, Kendig says Mid-South growers have been fighting an uphill battle in their efforts to control pigweed since the 1990s.

“When I was in graduate school in the 1980s, the answer to questions about controlling pigweed was to ‘spray something,’” he said. “But, in the 1990s, pigweed became one of the most troublesome weeds in the Mid-South.

“We were having problems controlling pigweed before Roundup Ready came along, and, with the advent of the Roundup Ready system, pigweed hasn't disappeared and continues to be troublesome.

One reason is that pigweeds are genetically diverse, says Kendig. “The Southern Weed Guide shows 10 species, the botany textbooks list 18 and some people even grow amaranth species for grain.”

Because of that diversity, pigweeds can often crossbreed with each other, hence Kendig's term, Amaranthus inbetweenus. “Spend a little time looking, and you will see species that are clearly half-breeds,” he said.

“Pigweeds have very low morals,” said Smith, picking up on Kendig's line. “That phenomenon gives us diversity that can lead to problems with resistance.”

The efforts to control waterhemp, a problem pigweed species in the Midwest, and Palmer amaranth, one of the more troublesome pigweed species in the Mid-South, have some interesting parallels, according to Kendig.

Waterhemp was one of the first pigweed species to develop resistance to ALS inhibitors such as Pursuit herbicide, which was used on about 85 percent of the soybean acres in the Midwest. You could replace waterhemp with Palmer amaranth and Pursuit with Staple or Scepter and soybeans with cotton and have basically the same situation, he says.

“Palmer amaranth and resistance to herbicides like Staple helped drive Roundup Ready cotton in the Mid-South,” says Kendig. “Ten years ago, morning-glory was one of our weed control problems. Since then we have changed to a weed control program that is generally said to be stronger on pigweeds and weaker on morning-glory. So why are we now complaining of pigweed problems with Roundup?”

Some of the reasons for increased control problems with pigweed: (1) significant reductions in residual herbicides, including Treflan or trifluralin preplant incorporated; (2) significant reductions in tillage; (3) reductions in competition from other weeds and pigweeds; and (4) weather.

Palmer amaranth can be a particularly bad weed in Roundup Ready cotton because it grows rapidly — up to 12 inches a week. “If it's in the drill, you can lose your height differential very quickly,” Kendig notes. “A pigweed plant produces millions of seed. Ninety-nine percent control of pigweed may come up a little bit short.”

Kendig — along with other Mid-South weed scientists like Smith and University of Tennessee Extension specialist Larry Steckel — has been conducting studies on the use of resistance prevention strategies, including the addition of residual herbicides to the Roundup Ready program.

“I don't think I have an ironclad pigweed recommendation from these studies, but when you want to start talking about resistance prevention, suddenly it's a lot more important to bring these alternative chemistries into your weed control program,” he notes.

Roundup Ready Flex cotton, which Monsanto expects to introduce in 2006, could mean an increase in glyphosate use because growers can apply it over the top of cotton much later in the season than with conventional Roundup Ready. That could also translate into some decreases in layby herbicides.

The new technology literally cries out for the use of more residual chemistry, and Monsanto has been talking about using those herbicides either pre-emergence or at layby.

Adding a residual herbicide may not be the total answer, either. “If you use a non-post residual tank mix partner with Roundup and the weed comes up, then the glyphosate application has to kill it,” he says. “So there's no reduction in selection pressure.”

Whether it's resistant or not, controlling Palmer amaranth requires a “systems” approach, says Kendig. That might include applying Prowl and/or Cotoran pre-emergence; Sequence (a tank mixture of Touchdown and Dual) postemergence; Roundup early post-directed and Valor plus Roundup at layby.

“Don't ask me to run the costs on that because I know it might be expensive,” he said. “But treatments like this generally look good, even if you swap out one herbicide for another.”

The weed scientist said he believes Cotoran still has a place in cotton weed control although it has almost gone the way of the buggy whip in the shift to Roundup Ready. “I like to see a half rate of Cotoran go out pre-emergence,” he says. “It might eliminate the one-leaf Roundup spray. I can't say it will eliminate resistance.”

Kendig has also been looking at some “unusual” residual herbicides but the future of the labeling is uncertain.

Regarding layby treatments, Kendig says his research shows that when you spray might be just as important as what you spray. “In locations where we had to spray late due to weather, one herbicide looked about as bad as another. I hope growers will keep their post-directed and hooded spray equipment,” he said, “because they may need it again.”

“Our job is to try to stay one step ahead of the problem,” said the University of Arkansas' Ken Smith. “This is the kind of research we need to help us figure out what will work when we do encounter resistance.”

Farr says small increases in herbicide expense to combat resistance may not be a problem for many growers. “It may cost another $1 or $2 on the front end, but look at what can happen down the road if we don't address this problem.”

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