While Monsanto and Georgia agronomy and weed specialists look for further answers to this summer's apparent finding of glyphosate resistant pigweed (Palmer amaranth) in central Georgia, the National Corn Growers Association and others are reemphasizing the need for growers to diversify their herbicide programs for field crops.
Nathan Danielson, NCGA's director of biotechnology, says growers are urged to follow recommendations listed on NCGA's Weed Resistance Management Learning Module (WRMLM) Web site www.ncga.com/biotechnology/main/reference_guide.html.
The site, set up earlier this year, guides users through a step-by-step herbicide management and weed control program. “It might seem remedial to some producers,” he says, “but it is a good reference to consider as we rely more and more on herbicides for weed control.”
Randy Boman, Texas A&M University cotton agronomist, adds that the best resistance management approaches “include using multiple weed control methods in the field, including diverse herbicide chemistries and modes of action.” He stresses that is essential to make sure all applications are made according to herbicide labels, including rates.
Meanwhile, Steve Brown, University of Georgia agronomist, tells The Corn and Soybean Digest that the university and Monsanto hope to have a clearer view of the exact depth of the pigweed resistance problem in early or mid-September.
“Seeds are being collected and tested to determine how (widespread) this thing is,” says Brown. “Apparently there has been some ‘rogueing’ on a farm. There is a chance that this is a limited situation. That would be nice.”
He says there are “rumors of other sites with pigweed resistance. We don't know if it is more widespread. We are trying to determine that.”
Few would argue that the reliability of Roundup as an over-the-top herbicide has been one of agriculture's major success stories in recent years. The large majority of cotton and soybeans produced in the U.S. are Roundup Ready varieties. Roundup Ready corn is also popular among growers. However, additional herbicide types are needed to help enhance an overall weed control program, experts say.
“There are places where there is extreme reliance on Roundup alone,” says Brown, noting that Stanley Culpepper, Georgia weed scientist, is working directly with Monsanto in conducting heritability studies to confirm resistance.
“But we have recommended for a long time the use of residual herbicides and different layby and conventional products in a Roundup Ready cotton program.”
A good yellow herbicide program is important for early weed control in cotton, Brown and Boman say, before a glyphosate is applied over the top. Additional chemistries should be considered to help control weeds and hopefully prevent resistance.
“Weed resistance is an issue that growers need to pay attention to,” says Danielson, stressing that corn growers “have the tools, through NCGA, to manage the technology really well.
“Any time they have questions, they can look at our Web site,” he says. “We also encourage them to talk with the agricultural chemical providers and county extension officials.”
Danielson says some growers may see a potential economic incentive to use lower chemical rates than crops are labeled for. But in the long run, this plan can hurt. “Growers control their destiny on this,” he says.