If we grow it, can we sell it?” This appears to be one of the biggest questions facing farmers who might be considering growing herbicide-resistant corn hybrids, says Eric Prostko, University of Georgia Extension weed scientist.
“There's a perception out there that these transgenic crops are not safe for human consumption,” says Prostko. “Hopefully, with technology and good science, we can prove to the world that these hybrids are safe.”
In reviewing the total acres of herbicide-resistant crops grown in the United States in 2000, soybeans clearly was the leader with 54 percent of the U.S. crop planted in herbicide-resistant hybrids, says Prostko. Twenty-six percent of the U.S. cotton crop was planted in herbicide-resistant hybrids compared to only six percent of the nation's corn crop, he adds.
Three types of herbicide-resistant corn hybrids currently are available, he says. They include Roundup Ready, Liberty Link and Clearfield Corn — IR and IT hybrids.
IR hybrids, explains Prostko, are resistant to the imidazolinone or IMI family of herbicides, including Scepter, Cadre, Pursuit, Arsenal and Lightning. “Both parents of the IR hybrids carry resistance to IMI's and cross resistance to sulfur urea materials such as Accent and Classic. The problem with IR hybrids is that they take a longer time to develop. Most hybrids available in the future will be IT,” he says.
Tolerant to IMI
IT hybrids are tolerant of IMI herbicides, with only one parent showing resistance, he says. IT hybrids are easier and faster to develop than IR hybrids, he adds.
Growers should consider several factors before selecting a herbicide-resistant hybrid, says Prostko. “We need to make sure this technology controls the weeds, and that the herbicides work adequately on our problem weeds in southwest Georgia, such as bristly starbur, Florida pusley and others.
“We also need to know the cost-per-acre of this technology, and how it compares to our conventional weed control systems in corn production. We're also interested in the yield potential of these hybrids. Will they perform adequately, or will they at least be equal to what we're already growing?”
This past year, Prostko and other Georgia researchers conducted trials in three locations, comparing herbicide-resistant hybrids and conventional hybrids.
“All herbicides were applied when corn was about eight inches tall at all locations. There were no statistical differences in yield between the conventional varieties and the herbicide-resistant varieties. We also did not detect any yield loss associated with the herbicide applications made to the herbicide-resistant hybrids.”
In the 2000 tests, herbicide-resistant hybrids that performed equal to or better than the test averages included Pioneer 32K63LL (Liberty), Garst 8222IT (Clearfield) and Dekalb 626RR (Roundup), reports Prostko.
“Numerous varieties are out there and we intend to look at all of them. Our data thus far tells us that these varieties performed well at these locations in this year. Many more hybrids need to be evaluated, and we need more data. A grower who's interested in using this technology should look at the entire variety package, and not just the yield performance.”
It's very important, he notes, that growers know the cost associated with using herbicide-resistant technology. “We need to keep in mind that Roundup and Liberty have no residual activity, and we may have to go back in with another application or with atrazine to finish the job.”
In looking at the cost-per acre of the herbicide-resistant technologies, Prostko says the Roundup Ready will have a seed cost of $6 and a herbicide cost of $6 to $8, resulting in a total per-acre cost of $12 to $14.
Liberty Link will have no seed technology fee but the herbicide cost will be $13 to $19 per acre, resulting in a total per-acre cost of $13 to $19. Clearfield also will have no seed fee but the herbicide cost will be $12.61 per acre, he says.
“This is good technology, but we need to consider the marketability of the transgenic varieties. Also, will these products provide better control of our problem weeds than what we're already doing? In addition, are control costs going to be less or at least equal to what we're already doing?”
University of Georgia researchers also are looking at different weed management systems for herbicide-resistant corn, says Bill Vencill, research weed scientist.
“We're looking at a systems approach with these transgenic crops, to see if they're more effective than conventional systems,” he says. “And, we're looking at our most troublesome weeds in Georgia, including pigweed, Texas panicum Palmer amaranth, cocklebur, morningglory, sicklepod and coffee weed.”
Summarizing the research, Vencill says that greater than 90 percent weed control was achieved in a Roundup or glyphosate-only system, with Roundup Ready corn hybrids. “In working with other crops, we've seen that morningglories eventually can creep in where we're using only glyphosate. It might be best to look at potential herbicide partners for glyphosate. And with corn, it's fairly easy to find something that'll control morningglories.”
A different approach
In Liberty Link, a Liberty-only weed management system provided less than 70 percent weed control, he says. But with tank mixes of various postemergence herbicides, control was greater than 90 percent, he adds.
“We didn't see herbicide injury problems with either one of these systems,” says Vencill.
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