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Glyphosate-resistant pigweed in a class all alone among cotton pests

• Growing at rates as fast as 2 inches a day, the weed has been known to reach a height of 9 feet and a weight of 40 pounds, far exceeding the dimensions of any of its competitors.• From 2005 until now, this resistant weed has continued to spread throughout the Eastern Seaboard and down into south Alabama.

Paul L. Hollis

April 3, 2012

5 Min Read
<p> <em><strong>AUBURN UNIVERSITY EXTENSION weed scientist Mike Patterson explains the continuing battle against glyphosate-resistant pigweed during the recent East-Central Alabama Row Crops Workshop held in Shorter.</strong></em></p>

Anyone who is a college football fan is familiar with the five-star rating system for high school recruits — the biggest, fastest and meanest receive the top rating of five stars.

If such a system was applied to cotton weed pests, glyphosate-resistant Palmer amaranth pigweed undoubtedly would be in a class all alone, at six stars, says Mike Patterson, Auburn University Cooperative Extension System weed scientist.

Growing at rates as fast as 2 inches a day, the weed has been known to reach a height of 9 feet and a weight of 40 pounds, far exceeding the dimensions of any of its competitors, he says.

Speaking at the recent East-Central Alabama Row Crops Workshop in Shorter, Patterson recalled the history of Palmer amaranth, and when glyphosate was first sprayed on the weed species.

“In past years, we would spray glyphosate at the standard rate on Palmer amaranth that was knee-high and burn it down to the ground,” he says.

“In 2004-2005, there was one cotton field in south-central Georgia where a farmer noted he couldn’t kill Palmer amaranth with glyphosate. From 2005 until now, this resistant weed has continued to spread throughout the Eastern Seaboard and down into south Alabama.

“In 2009, it was discovered in a soybean field in north Madison County in Alabama’s Tennessee Valley. To the best of my knowledge, there are probably 26 counties in Alabama that have at least one field of glyphosate-resistant Palmer-amaranth pigweed. Farmers in central Alabama also have reported resistant pigweed,” says Patterson.

Some refer to resistant Palmer amaranth as “Frankenweed,” the monster weed, he says.

“One of the reasons it’s so hard to manage is that the seed production is very high. More than 1.7 million seeds have been documented on one plant. Even if you get 99.9 percent control of female plants, and you leave one or two per acre, with each plant producing at least a half-million seeds, you still have a lot of plants out there,” he says.

USDA researchers have been looking at resistant pigweed seed mortality, says Patterson.

(For another look at the numbers game, click here).

“Their research has shown that in the first year after those seed hit the ground, during the winter before the next season, as much as 50 percent of the seed will rot and won’t come up. But when you’re dealing with populations of hundreds of millions, you’ve still got a bad problem.”

Most weed scientists in the Southeast agree that farmers need to maintain residual control of this weed, from burndown all the way through layby, he says.

“If it’s as tall as your cotton in a Roundup Ready system, it’s extremely difficult to control, and it’ll put you out of business. With most residual herbicides in cotton, you get two to three weeks of activity.”

Soil residual herbicides once again have become a standby of cotton production, and one of the critical factors in controlling this weed, says Patterson.

Must stay ahead of the weed

“However, since residual herbicides generally don't last more than three weeks even with sufficient rainfall, proactivity — staying ahead of the weed from burn down to layby — is another critical factor in controlling pigweed.”

 As an example of what farmer should be doing to control resistant pigweed, Patterson cites Walt Corcoran, who farms north of Eufaula in the southeastern corner of Alabama. He follows a proactive strategy that has set the standard for growers in the rest of the state, he says.

Corcoran puts Gramoxone and Diuron down two weeks ahead of planting, killing emergent pigweed. Then, he plants Roundup Ready cotton.

At planting, he applies Reflex alone or with Prowl and hopes for rain. “Both of these herbicides have soil residual effects against pigweed, but the key is to never let the pigweeds emerge,” says Patterson.

Two to three weeks later, this is followed by an over-the-top application of either Dual or Warrant plus glyphosate (Roundup).

Three to four weeks later, Corcoran runs a hooded sprayer in the middles with Gramoxone, followed with a layby application containing Valor plus MSMA or glyphosate, which provides additional residual effect.

“Layby is critical, and Valor is one of the best possible materials for a resistant pigweed that you haven’t yet killed. But everything depends on activation by rainfall or irrigation,” he says.

Patterson says his goal is to try and scare farmers into being proactive against resistant Palmer amaranth.

“Roundup is a good herbicide, but resistance can put you out of business. Unless you use residuals, you’ll go broke, no matter if you’re using glyphosate or Ignite. Liberty will kill a Palmer at 4 inches tall, but you won’t be able to keep up with it.”

While new products are on the horizon that’ll help with resistant pigweed, some won’t be available for awhile, says Patterson.

“Two of the most promising new technologies include Monsanto's dicamba-tolerant cotton and Dow’s new line of cotton resistant to 2, 4-D applications,” he says.

While these products won’t insure that resistant pigweed is “graveyard dead,” Patterson says they will provide at least 80-percent control when used alone and even better control when used with glyphosate or Liberty (gluphosinate).

“These are not magic bullets, and neither of these varieties will be available until the next three to four years.” 

 A more immediate option might be Bayer’s GlyTol-LibertyLink varieties that allow the application of glyphosate and Liberty. These should be available this season, he says.

Rotation always is a good plan, says Patterson, but consistent, proactive soil-residual herbicide applications to reduce seed populations will remain the best strategy for most producers.

[email protected]


About the Author(s)

Paul L. Hollis

Auburn University College of Agriculture

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