Herbicide-resistant pigweed a real nightmare
Oct 28, 2008 9:37 AM,
By Jim Langcuster
Auburn University
Imagine a recurring nightmare in which an army of goblins slowly robs you of your money and eventually your livelihood and whose numbers multiply no matter what you do to stop them.
Day after day, Georgia row-crop farmers are dealing with something eerily similar — and so will Alabama growers unless some solution is found.
“For farmers, it's actually scarier than Halloween,” says Mike Patterson, an Alabama Cooperative Extension System weed scientist and Auburn University professor of agronomy, who has been closely monitoring the rapid progression of the weed.
The goblin, in this case, is herbicide-resistant Palmer amaranth, better known as pigweed. Desperate to contain its spread, farmers already have hand pulled resistant pigweed off some 15,000 acres of Georgia cropland.
“Workers are pulling up the entire plants — root and all — out of the ground and loading them on a wagon, then piling them up on the edge of the field,” says Patterson.
As farmers have discovered through experience, mere chopping isn't enough. Only root-and-shoot removal will work — otherwise weeds return with a vengeance as new plants sprout from the roots.
Patterson's Georgia counterparts estimate that more than a million acres of cropland are infested with the resistant weeds, which are spread not only through seed but also via pollen.
Why are farmers so frightened of this weed? By developing resistance to the standby herbicide glyphosate, commonly known as Roundup, pigweed has dethroned the system farmers have relied on for roughly a decade to control weeds and, equally important, to contain operating costs.
This system, a combination of chemistry and crop genetics, allowed farmers to spray glyphosate, directly over the cotton plants, killing a wide range of weeds while sparing the cotton plants and also relieving them of costly tillage and other herbicide applications.
For cotton producers, the only other option is using soil-applied herbicides to kill the pigweed before it emerges. But even this doesn’t guarantee success.
To work, the herbicides must be activated by rainfall; otherwise, they don't work. And if they don't work, the pigweed ultimately emerges and out-competes the cotton.
“By the time the cotton reaches 3 or 4 inches, you've got pigweed that is 7 or 8 inches.”
By then, it's too late, he says.
“Beyond that, there's nothing available in our cotton herbicide arsenal to kill the weed postemergence.”
In a classic case of survival of the fittest, the weeds outgrow cotton plants, robbing them of moisture and vital plant nutrients, slowly starving them.
One of the only remaining options is rotating cotton with corn. As a grass crop, corn will tolerate applications of herbicides that broad-leaf crops such as cotton won't, he says.
For example, pigweed still can be controlled using corn herbicides such as atrazine, which farmers have used for 40 years.
“Almost every acre of planted corn is treated with atrazine,” says Patterson, adding that a couple of other corn-related herbicides are also still effective in treating resistant pigweed.
University of Georgia researchers already are working with U.S. Department of Agriculture counterparts to identify other ways to attack the weed. They've discovered that pigweed seed may typically degrade after only three years in the soil.
“That may be one of the weak points of this weed, which is a good thing,” Patterson says, adding that other weed seed, such as sicklepod and morningglory, may survive for decades in the soil.
Researchers are searching for ways to keep these fields clean — managing them so that germination doesn't occur, leaving pigweed seed to decay in the soil.
“That's the only way we're going to beat this thing, unless chemical companies come up with another weed control system similar to the Roundup approach,” says Patterson.
“But don't hold your breath on that one, because Roundup represented a once-in-a-lifetime discovery that is not likely to be repeated.”
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