Herbicide resistant Palmer amaranth becoming known as super weed

For growers in the Southeast with a history of continuous use of glyphosate-based and ALS-inhibiting herbicides, the question is not if, the question is when will resistant pigweed become a problem.

Since University of Georgia researchers first documented true glyphosate resistance in south central Georgia, the explosion in reporting of weeds resistant to several families of herbicides has rocked farming communities from North Florida to Virginia. The biggest newsmaker continues to be Palmer amaranth, commonly known as big-seeded pigweed, and rapidly becoming known as ‘super weed’.

Since first documented in 2005, counties in Arkansas, Kentucky, North Carolina, South Carolina and Tennessee have also confirmed resistant Palmer pigweed. Eight more counties in Georgia and two counties in Alabama are expected to confirm resistant varieties during the 2008 season.

In less than three years since his initial findings in 2005, University of Georgia Weed Scientist Stanley Culpepper contends there are 15-20 times more glyphosate and ALS-resistant weed acres in his state.

In North Carolina, venerable Weed Scientist Alan York has documented an alarming trend in weeds resistant to several popular herbicide families. The bottom line, he says, is that farmers have a problem that can be fixed, but first they have to admit they have a problem.

A recent international survey of weed resistance by the Weed Science Association confirms herbicide resistant weeds is a growing problem all over the world. Group G/9 herbicides are known as glycines, which cause the inhibition of EPSP synthase. Research has shown these particular biotypes are resistant to glyphosate and they may be cross-resistant to other Group G/9 herbicides and/or other herbicide families.

In North Carolina, York has found pigweed with resistance to both glyphosate and ALS-inhibiting herbicides. Because the ALS inhibitors are commonly used in other crops grown in rotation with cotton and soybeans, York says farmers need to pay special attention to what families of herbicides they are using on all crops.

Herbicides that target the enzyme acetolactate synthase (ALS) are among the most widely used in the world. Unfortunately, these herbicides are also notorious for their ability to select resistant weed populations. Now, there are more weed species that are resistant to ALS-inhibiting herbicides than to any other herbicide group.

South Carolina grower William McElveen says glyphosate resistant pigweed first started to become a problem on his farm in 2006 in soybeans and cotton. “We would see a weed escape here and another in another part of the field. We suspected we had a problem, but it wasn't widespread,” he says.

“Last year we really had a problem. In one field we brought in people to pull weeds, and the weeds came right back,” he laments. There are many fields in Lee County that have been called ‘suspect’, but we feel we have resistance, the South Carolina grower says.

Resistance created a big enough problem for McElveen that he revamped his herbicide program for cotton.

“This year, I am planning to broadcast the label rate of Valor in the tank-mix with Roundup two to three weeks prior to planting. After planting, I am going to apply Reflex right behind the planter wheel. Even though we may use the same family of herbicides, we feel like we are spreading them out far enough to help reduce the risk of causing resistance to any one herbicide, he says.

McElveen grows 600 acres of cotton, 600 acres of peanuts, 500 acres of corn and 500 acres of soybeans double-cropped with wheat.

Like so many growers, McElveen says he cannot quit using glyphosate-containing herbicides, even in fields in which he knows he has resistance problems. He explains that glyphosate has been the most economical way to manage cocklebur, sicklepod, grasses and a host of other weed and grass problems.

The recent rise in price of glyphosate-containing herbicides may change that theory for some growers in 2008. Farmers who did not buy their herbicides early in the year are finding prices doubled from 2007.

Reasons for the dramatic increase in price for all glyphosate-based herbicides vary, but include:

  • Price of fuel and general recession of the economy.

  • Production of generic glyphosate in China is decreasing.

  • Demand for glyphosate in Brazil and other growing farm economies around the world is increasing.

  • Increases in corn, wheat and soybean production in the U.S. is driving supply down.

The price increase may make it more viable for farmers in the Southeast to move away from glyphosate-based herbicide programs to a more diverse system that utilizes several different chemical families.

Fremont, N.C., grower Jerry West says his problems with glyphosate resistant pigweed came from using poultry litter that clearly included resistant pigweed seed in with the corn used in the poultry feed formulation.

“We didn't have any problems with resistant pigweed until three years ago. We like poultry litter for fertilizer — it fits our operation well. In one particular field, we used litter and the next year we had pigweed in the field that we could not kill with glyphosate,” he says.

For West it was one more reason to get out of the cotton business for a while. Not a primary reason, he notes, but one of many reasons that he and his sons tired of growing cotton. In addition to leaving off cotton the past two years, they now sell litter from their turkey production facility to a local energy company, rather than have it spread on farmer's fields.

Though glyphosate resistant pigweeds have probably been around for several years, the first documented cases came in 2005. Since that time the problem has skyrocketed, partly because of better detection and reporting. The big question is why?

York says we have an ideal recipe for weed resistance in the Southeast. A number of crops may be rotated with cotton. However, from 1996 to 2000, the vast majority of cotton, perhaps as high as 75 percent, was grown as a continuous monoculture.

Prior to the dramatic drop in cotton acreage in the Southeast in 2007, Texas, Georgia, Mississippi, Arkansas and North Carolina represented 10-12 million acres of cotton annually. With the exception of Texas, all these states planted 97-99 percent glyphosate-resistant varieties as late as 2006.

The end result, York contends is a perfect recipe for disaster, when you factor in growth potential of Palmer amaranth, compared to any of these crops.

Dubbed ‘the super weed’, Palmer amaranth, or pigweed, can produce well over 500,000 seed from a single mature plant. Mature Palmer pigweed can easily top six feet tall and 6-8 inches wide at the base of the plant. Harvesting cotton fields containing even a small percentage of mature pigweed is no bargain at best and impossible in some severe situations.

There is no single remedy to fight glyphosate resistant pigweed in cotton or soybeans, but with acreage in both crops expected to increase in the Southeast in 2008, farmers likely will be looking for some alternatives that fit into their production system.

York says some of rules of thumb to follow include:

  • Use both pre-mergence and postemergence herbicides that have different modes of action.

  • Use a herbicide at layby that has a different mode of action from previous treatments.

  • Reduce soil seed banks by continuously maintaining good weed control.

  • Survey weed populations each season and record observations, especially weeds that escape known herbicide applications.

  • Hand remove, if necessary target weeds not controlled by herbicides known to control these weeds.

  • In fields where herbicide resistance is known or highly suspected, clean vehicles and equipment prior to moving to fields where resistance is not suspected

  • Do not continue to treat weeds with herbicides that continue to show an inabilitly to control the target weed.

To help manage glyphosate resistant pigweed, several companies have brought back, or in the process of bringing back old, proven herbicides.

Most of these companies have detailed herbicide resistance avoidance programs on their respective Web sites. Perhaps the best, and most unbiased of these is the National Cotton Council.

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