Reducing stress in corn slows aflatoxin

If an ethanol plant rejects a load of corn, chances are it is due to aflatoxin contamination. Aflatoxin is a toxic by-product of the soil-borne Aspergillus flavus mold that threatens both human and animal health.

Aflatoxin is the most damaging of a group of fungi-caused mycotoxins that can show up in harvested corn.

Brad Kusterman is the director of grain marketing and feed ingredient sales at First United Ethanol in Camilla, Ga. He says aflatoxin levels in corn tend to increase during ethanol processing which produces dried distiller’s grain as the main feed product. Studies in Mississippi show that aflatoxin levels can increase three or four times the initial levels during ethanol processing. Kusterman says all corn coming into his plant is tested for aflatoxin. “We know our customers in the poultry industry check our end products,” he says. “We reject incoming loads of corn that test more than 20 parts per billion of aflatoxin.”

Though aflatoxin can occur wherever corn is grown, it typically occurs in Southern states, especially during years of severe moisture stress. Aflatoxin, and how to avoid it, was a featured topic at a recent well-attended corn short course held in Tifton, Ga., organized by Georgia Extension Agronomist Dewey Lee.

“Heat, humidity, drought, plant disease and insect feeding can all result in increased mycotoxins,” says Lee. “Hybrid selection, especially hybrids selected for drought tolerance, can be an important step in managing aflatoxin. Management to reduce stress can also help. These practices include irrigation, conservation-tillage and pest control.” He adds that insects such as corn earworms, fall armyworms and stink bugs can make plants susceptible for molds that cause aflatoxin and other mycotoxins.

“Early harvesting reduces the risk of aflatoxin contamination by reducing the time corn is exposed to environmental pressure,” adds Lee.

University of Georgia entomologist David Buntin says corn hybrids available for planting in 2010 will have new strains of Bacillus thuringiensis bacteria that should help reduce insect damage and the mycoxins that can result from such damage. This new genetically engineered technology includes the VT Triple PRO technology from Monsanto for DeKalb hybrids. Buntin says the insects targeted by this technology include corn borers, corn earworms along with improved fall armyworm control. “The Bt is a stomach poison for corn earworm,” says Buntin. “The larvae begins to feed and you will see some minor damage, but we see a reduction in corn earworm size. They are being killed and the Bt stunts their growth.”

In his tests, Buntin saw no yield advantage from VT Triple PRO in tests at Tifton, but saw yield increases at his tests in Griffin due to fall armyworm control. “Monsanto suggests farmers will see a 2.5 percent yield advantage for Triple PRO, and this seems reasonable to me,” he adds. “The best results will be seen in improved overall grain quality as a result of using Triple PRO.” One advantage of the VT Triple PRO is that the required non-Bt corn refuge acreage in southern cotton-growing states will be reduced from 50 percent to 20 percent.

Another new stacked gene Bt technology is Viptera from Syngenta that may be approved in time for use in the 2011 growing season.

Monsanto is also developing SmartStax, a stacked gene platform for corn hybrids featuring protection from corn borers and corn earworms along improved fall armyworm control and Roundup and Liberty Link herbicide tolerance. “SmartStax has been good for earworm and fall armyworm control in my tests,” adds Buntin.

Overall, use of Bt corn in Georgia has been low, according to Buntin, mainly because most Georgia corn growers rotate corn with other crops and because the insects targeted by single-gene Bt hybrids have not been problems in the state. He has seen positive results from Bt corn in north Georgia at Calhoun where southwestern corn borers have been a problem.

When Buntin received results from his 2009 tests, he was disappointed that the new stacked gene transgenic corn hybrids did not significantly reduce aflatoxin. “We need more testing,” he suggests. “This technology will need to be combined with other management steps to reduce aflatoxin.”

Lee says another practice that may reduce aflatoxin is the application of non-toxic strains of the Aspergillus flavus fungus. Now-retired USDA Microbiologist Joe Dorner developed this biological control. “He found a non-toxic strain of Aspergillus flavus,” says Lee. When Dorner’s work showed a 90-percent reduction in aflatoxin in peanuts, Lee asked him to try the product on corn.

The corn tests also showed dramatic reductions in aflatoxin on corn, especially in the Southwest where alfatoxin has been a nagging problem on drought-stressed corn. The product was earlier registered for use on peanuts and it received a clearance for corn in 2008.

USDA licensed this technology to Circle One Global, a Georgia-based company. Circle One marketed the beneficial fungal strain as Afla-Guard. And this past year, Syngenta acquired the technology from Circle One Global.

Dave Ross with Syngenta Special Products explains that Afla-Guard works by competitive displacement. In other words, it out-competes the bad Aspergillus flavus. He likens its action to biological musical chairs. “You fill the chairs with the good Afla-Guard and there is no place for the bad Aspergillus flavus to grow,” he explains. “On average, we expect to see an 85-percent reduction of aflatoxin in corn and peanuts if Afla-Guard is used properly. We don’t say that Afla-Guard offers a complete control, but it should make crops more marketable.”

Ross hopes label changes will allow a wider window of application in corn and a reduction in the recommended rate. Currently, Afla-Guard is labeled for application at 20 pounds per acre. “We’d like for research to show we could lower this rate to 10 pounds per acre,” he adds.

Afla-Guard cost 97 cents per pound in 2009, and its price to growers has not yet been set for 2010, according to Ross. He adds there may be some carryover effect from the product. For example, if Afla-Guard was applied in 2009, some of it may remain active in subsequent years. But more research is needed to determine if an application one year will be enough to control aflatoxin during the next year or two. “Our best guess is that it carries over, but not at effective levels,” says Ross.

Lee says the beneficial fungus has been well received in the Southwest by both growers and grain elevators. Lee says, “In Georgia, the future of Afla-Guard will be good, but we need to figure out the timing of applications and how to best use it.”

Brian Scully, research leader with the USDA-Agricultural Research Service in Tifton, Ga., is heading a team to evaluate aflatoxin resistance and tolerance in corn hybrids. “Our goal is to develop corn with resistance to Aspergillus flavus, insects and aflatoxin, and to work on agronomic traits such as drought and heat tolerance along with higher yields,” he says.

Scully cited surveys taken in the Southeast since 1977 showing a gradual decline in overall aflatoxin contamination in corn. “Over time, we expect this trend to further decline,” he adds. Two thirds of Georgia corn is now irrigated, and Lee believes that irrigation is largely responsible for the increasing state average yields and for much of the decline in aflatoxin contamination.

“Our breeding program has aflatoxin resistance, insect resistance, lodging resistance and drought tolerance, but not in high yielding plants,” says Scully. He and his group are also working to develop resistance to emerging corn pests, the chocolate milk worm and the corn silk fly.

He also reported on Georgia field tests evaluating damage from maize weevil and stinkbugs. In these tests, insect infestation sites coincided with sites of increased aflatoxin. “This damage and the aflatoxin typically occurs along field edges,” adds Scully. He hopes to use this information to spray field edges or harvest corn grown along the edges of fields separately to see if these practices will reduce overall aflatoxin contamination.

Scully also works with molecular biologists looking for genes that confer aflatoxin resistance. “We know that drought increases aflatoxin,” says Scully. The team members have found more than 800 drought-related genes, and are working to select four or five of the best ones for drought tolerance to add to inbred corn lines. So far, the team has selected five inbred lines to add to regional tests prior to possible release.

Lee is excited about a National Corn Growers initiative to fund an Aflatoxin Mitigation Center with $5 million requested from Congress. Corn growers in states such as Texas, Alabama, Georgia, North Carolina, Mississippi and Louisiana are cooperating on the project. They’ve received strong support from the land grant universities in their states. Corn farmer Danny Willingham of Irwinville, Ga., chairs the Georgia Corn Commission and has been working on initiative. “I’ve been to a lot of meetings on this topic, and I believe it is now time to approach Congress to ask for funding for this project,” he says.

“When you couple biological controls such as Afla-Guard with good corn genetics and stress tolerance in hybrids, we will have effective multiple barriers to aflatoxin in Georgia-grown corn,” adds Lee.

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