Despite being hemmed in by reports of glyphosate-resistant pigweed in both Tennessee and Georgia, weed scientists in Alabama are saying there's not a similar problem in their state's cotton fields — not yet.
“They've found resistance to Palmer amaranth, but we don't have very much of that to my knowledge in Alabama,” says Mike Patterson, Auburn University Extension weed scientist. “We have a plot of Palmer amaranth at the Wiregrass Experiment Station, but I'm not aware of a grower in Alabama who has it.”
A pigweed species was tested this past year for pigweed resistance, but the test proved negative, says Patterson. “An east Alabama grower had trouble last year killing pigweed with glyphosate. We dug up some plants that survived his application and we grew them in the greenhouse. They made seed last winter, and we planted the seed in the spring and let them grow to about 4 inches. We then sprayed them with a range of glyphosate applications, even at low rates, and we killed them all. That was a rough indication that we weren't dealing with a resistant population.
“Monsanto officials feel that they have true resistance with the field in Georgia because they can spray high rates of glyphosate and still not kill the weed,” he says.
The weed science community always has felt that if a summer annual weed were going to develop resistance to glyphosate, then a pigweed species would be a good candidate, says Patterson.
“Pigweed makes so many seeds. It wouldn't surprise me to see more pigweed cropping up. But for now, we don't have that problem in Alabama,” he says.
The yellow or dinitroaniline herbicides are a good rotation chemistry for weed control in cotton, says Patterson, but they might not be as effective in conservation-tillage fields. “Many growers who are using reduced-tillage no longer incorporate, and everyone knows that yellow herbicides work best when you incorporate them. If you're in minimum-tillage, you generally give up that option.
“Prowl has activity as a pre-emergence treatment on pigweed, but it's not as effective if it's not disked into the soil. Growers might also use Dual Magnum for resistant pigweed or annual grasses. But on our soils, we can't use it as a pre-emergence application, so they have to put it over-the-top with that first glyphosate application. Georgia growers are using Dual Magnum as a pre-emergence treatment for tropical spiderwort, and it also has pigweed activity,” he says.
Alabama growers also haven't seen the problems with tropical spiderwort that Georgia growers have seen, says Patterson. “I've seen it in pecan orchards in Baldwin County, and one or two row-crop farmers have reported having it, but it hasn't exploded in Alabama as it has in south Georgia. Of course, we may be in a different position this time next year.”
Alabama growers should be cautious, he says, when bringing in equipment from south Georgia. “Some of our growers in the southern part of the state travel to Georgia to buy used equipment that has been in those invested fields. If you don't do a good job of cleaning the equipment, you could bring the weed with you. It's like a nematode — it doesn't take but one soil clod to start an infestation in the field.”
Patterson estimates that from 90 to 95 percent of Alabama's cotton producers are planting Roundup Ready varieties, with only a handful of growers sticking strictly to conventional cotton.
“That's the current generation of Roundup Ready, and they'll probably continue planting with the Roundup Ready Flex varieties. It's a Roundup Ready world, at least in cotton. We've always recommended that growers not depend too heavily on glyphosate in Roundup Ready varieties. When Roundup Ready cotton first came onto the market, we weed scientists starting talking about resistance issues. Then, we went for six or seven years when we didn't see anything. We've been crying “wolf,” but the wolf didn't show up until this year. Now, we'll go back to stressing resistance management in county meetings.”
There are growers who use only glyphosate on Roundup Ready cotton, says Patterson, and they try and make it with a couple of applications of the herbicide. “Some of them have made it work, but resistance will force you to change your mode of operation. Of course, you don't stop using glyphosate — it's an excellent herbicide, and it controls a lot of weeds.”
Everyone addressing the resistance issue agrees that the key is to have a “ground zero” policy, he says. “In other words, nothing should live to make seed. If you allow resistant pigweed to go to seed, it'll be difficult to control.”
Managing resistance will require vigilance on the part of growers, says Patterson. “We'll have to slow down, and it'll cost money. We have to go back to some of the older materials at post-direct and then a layby, and we'll have to do a good job with them. Glyphosate has made things too easy and too convenient for some.”
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