As expected, glyphosate-resistant Palmer amaranth pigweed continued its march across Georgia in 2009, being confirmed in 14 more counties.
“This is not surprising,” says Stanley Culpepper, University of Georgia Extension weed scientist. “It’s now in more than 50 counties, and by the end of the year or early next year, we might find it in every agronomic county in the state.”
It will continue to spread across the state, primarily due to the pollens and the natural seed production, said Culpepper, speaking at the recent Bayer CropScience 2010 Southeast Technical Seminar held in Braselton, Ga.
Of the fields tested this past year, several fields in east Georgia had Palmer amaranth escapes that were not resistant, he says. “We still have areas where we can improve our typical cotton weed management programs  even though we don’t have resistance,” he says.
(For more see http://southeastfarmpress.com/cotton/herbicide-resistance-0226/index.html .)
An even greater concern than glyphosate resistance, says Culpepper, is multiple resistance. “I’m routinely pulling out samples and screening them for both glyphosate and ALS herbicide resistance. With the ALS herbicides, we’re talking about 20 to 22 herbicides that we commonly use on various crops in Georgia. The multiple resistance is of greater concern to me. Not that Staple is the greatest against emerged pigweed, but if there isn’t resistance, at least our growers can use it as a tool,” he says.
Multiple resistance is becoming very common, he adds, and it has been confirmed that ALS resistance will move through pollen. If growers continue to rely heavily on this chemistry, the resistance will continue to spread rapidly, he says.
“We all know our world has changed,” says Culpepper. “It began changing in 2007 and changed even more in 2008. In 2009, we saw a massive change, and I think it was a change for the better. Our growers are becoming more aggressive and more integrated in how they control Palmer amaranth.”
Grower surveys were conducted in some of the Georgia counties where pigweed resistance was more severe at the end of 2008 and this was compared to grower practices before resistance was detected, he says. “But we need to remember it’s not whether or not we have resistance — it’s population dynamics. The populations of Palmer amaranth you have will determine how you manage this problem,” says Culpepper.
In severely infested areas, change has been rapid, he says. “We’re incorporating more of our herbicides, and we did even more of this in 2009, especially the yellow herbicides. Ignite-based programs have increased significantly. In 2009, I’d estimate that 7 to 8 percent of our cotton was treated with Ignite-based programs. Cultivating and hand weeding also were very important, with hand weeding becoming more critical at the end of 2008.”
The hand weeding component was especially interesting in 2009, cays Culpepper.
“Everywhere I went, someone was hand weeding. We did a survey along with our county agents in 52 counties including in our training. These counties represent 799,000 acres of cotton, and these surveys showed that 437,000 or 54 percent of the cotton had been hand weeded. We talked to about 100 growers, and it was fascinating to learn their cost ranges for hand weeding, with expenses running anywhere from $3 per acre to $100. The $3 per acre included the more aggressive growers, and the average was about $26 per acre. That suggests we probably spent $11 million just in hand weeding this past year.
“You might think this is bad, but if you look at where we we’ve been since 2005, I would suggest this is really good, and the county agents and consultants are the reason our growers are doing more hand weeding. They have shared with them the information about how this resistance is moving through the pollen. They know how bad this problem can be and how aggressive it can be. If we’re aggressive for the next three or four years, I think we can turn the tide on this pest.”
Research during 2008 and 2009, says Culpepper, determined that Palmer amaranth has three potential weaknesses, including a shallow emergence depth, short seed life in soil, and a significant light requirement needed for germination. Each of these weaknesses can be manipulated to improve control by herbicide systems.
In conservation-tillage, heavy residue crops can be used to essentially block sunlight required for Palmer amaranth germination and can greatly improve control. It is critical to note that in strip-tillage production, Palmer amaranth will emerge in the strip if herbicides are not activated in a timely manner by irrigation or rainfall.
In conventional-tillage, Palmer amaranth control can be improved by deep turning or using a yellow herbicide preplant incorporated. The adoption of timely Ignite-based programs will improve control regardless of producing cotton in conventional or conservation-tillage systems.
For a complete listing of University of Georgia recommended programs for controlling glyphosate-resistant Palmer amaranth pigweed, see http://mulch.cropsoil.uga.edu/weedsci/HomepageFiles/Palmerhandout-2010.pdf .
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