With increased concern over glyphosate resistance, more Southern growers are trying to balance their rotations of Roundup Ready cotton, soybeans and corn without losing the benefits of the over-the-top herbicide program.
Stephen Logan, who farms with his father, Danny, in Gilliam, LA, depends on pivot-irrigated cotton with Roundup- and Bollgard-stacked genes. It's his No. 1 cash crop. The 2,600 acres of cotton, mostly no-till, are either continuous or rotated with 600 acres of mainly dryland conventional corn. About 450 acres of Roundup Ready dryland soybeans are also grown nearly on a continuous basis, but future rotation with cotton is anticipated.
Logan has seen no weed resistance to glyphosate in his northwestern Louisiana location. But he realizes that, since such resistance has been recorded in parts of the Midsouth, it could creep toward his region.
“Roundup works great for us,” he says, noting that even though tech fees for glyphosate-resistant varieties can be steep, they're worth it in the long run. Because of the threat of resistance, he would like to see some additional chemistry.
“We try to rotate corn with cotton as often as possible because the cotton usually does better due to the organic matter remaining after corn,” says Logan, adding that cotton yields are around 1,200 lbs./acre. “It has more early season growth, more drought tolerance and enables us to break the glyphosate cycle.”
Because his cotton acres out-number his corn acres about four to one, there is still a lot of continuous cotton. The only Roundup Ready corn is planted in an eight-row buffer zone adjacent to cotton ground sprayed with glyphosate.
While corn and cotton are in a 38-in.-row bedded pattern, Group IV beans are planted in a 20-in.-row flat system.
“We want to do more soybean rotation with cotton in the narrow-row flat-cropping system,” says Logan. “But for us, cotton still does better after corn in most of our soils, which range from sandy, low organic to heavy, sticky clay.”
For early burndown in cotton, Logan mixes glyphosate with 2, 4-D, then later with Dual in the first or second over-the-top application for additional weed protection.
Most Southern cotton farmers rotate with conventional corn and/or Roundup Ready soybeans. But don't expect a few rotations out of Roundup Ready crops to be a sure cure against resistance, says Robert Hayes, University of Tennessee plant science professor, Jackson, TN.
Delaware was the first state to report significant resistance to glyphosate, followed by Tennessee. Marestail, or horseweed, spread from a few acres in 2000 to more than 500,000 acres in Tennessee and Kentucky in 2002.
“Marestail is a windblown seed,” Hayes says. “Some growers hope greater rotations of Roundup Ready cotton or beans with conventional corn will control resistance. But no matter what you rotate with, if the Marestail seed blows in, you'll still see problems.”
Mixes of 2, 4-D and a Dicamba herbicide, such as Clarity or Banvel, are used by some for early preplant burndown. But with the still-heralded overall success of Roundup Ready cotton, Hayes sees little if any reduction in its use, either in rotation with Roundup Ready soybeans or convention corn, or on a continuous basis.
According to Syngenta, maker of Touchdown IQ, the rapid and widespread adoption of Roundup Ready technology for soybean, cotton and corn production will prompt weed resistance to glyphosate herbicides. The company recommends farmers make no more than two applications of glyphosate-based herbicide in a given field during any two-year period.
That cropping practice might not take place until major resistance invades farms and causes economic damage. There are still many Roundup Ready cotton/soybean/corn areas that have seen no such problems. In North Carolina, where at least 80% of the cotton and 80% of the soybeans are Roundup Ready, growers don't worry much about rotations with conventional varieties.
“We are obviously concerned with the potential for resistance, but so far haven't encountered a problem,” says Alan York, a weed scientist at North Carolina State University, Raleigh. “We discuss it with our growers and tell them to keep their eyes open. We're not likely to convince growers not to plant Roundup Ready, but we encourage integration of other chemistry into the system, such as conventional herbicides for lay-by application in cotton.”
He adds that his region's growers are primarily concerned with continuous cotton. “Most of the Roundup Ready soybeans are rotated with conventional corn, so that helps break the cycle,” he says. “However, that could change if we see more growers planting Roundup Ready corn.”