Asian soybean rust has made the ag headlines for a year now. But don't forget another devastating fungus that breeds dreaded aflatoxin. Aflatoxin has infected many U.S. corn and cotton fields for years.
Central Texas grower Scott Averhoff has battled aflatoxin invasions more than once since 1990. He'd just as soon not face another one. His work as chairman of the national Mycotoxin taskforce is helping in the national drive to control aflatoxin or get rid of it completely.
The task force was formed by the National Corn Growers Association in 2002. The seven-member team is helping coordinate aflatoxin research among USDA and university scientists in affected states along the Gulf Coast and the eastern seaboard.
Aspergillus flavus is the fungus that causes aflatoxin. It thrives in drought-stricken areas with high humidity. “The more stress on corn, the more chance for the toxin,” says Averhoff, who farms a corn and wheat rotation at Waxahachie, south of Dallas.
Depending on where it is to be used, aflatoxin-infested corn faces everything from blending with clean corn or ammoniation to detoxify and degrade the toxin, to the need for complete crop destruction.
Corn for human consumption or that used for dairy cows can have no more than 20 parts per billion (ppb). Corn fed to finishing beef cattle can be 300 ppb. Blended corn can also be discounted by up to $1/bu., says Averhoff. If the rate of aflatoxin is more than 500 ppb, it must be destroyed.
Averhoff has seen all phases of contamination. He got a wake up call in 1990 when some of his corn tested positive. Then, 1996 was another bad year. Corn at 300-500 ppb had to be blended with clean corn to bring it below 300 ppb. But, 1998 was the worst.
“We had some fields that were 1,400 ppb and we averaged about 1,000,” he says. “All our corn had to be destroyed.”
It's difficult to tame the toxin. To help prevent aflatoxin, Averhoff uses several production and handling methods. First, he plants in 36-in. rows at a population of only 20,000 seeds/acre. “There is less stress on the plants,” he says. “We also plant Roundup Ready corn to hold down weed pressure, which also can cause stress to the corn plant.”
His yields for dryland production can top 150 bu./acre, but average about 90 bu./acre. “We try to harvest (earlier) at 20% moisture, then dry down the corn to 15%,” he says. “We dry the grain with ambient air. No heat is added.”
Averhoff's goal is to have aflatoxin-resistant corn hybrids to plant. The task force is behind research for resistance and other methods of aflatoxin control. “There is about $800,000 going toward aflatoxin research nationwide,” he says.
Much of the research is through Texas A&M University and North Carolina State University, where scientists are looking at poultry and dairy tests using binding agents. Other southern universities are seeing research, as is the University of Arizona, where aflatoxin in cotton has been a major problem, and USDA research stations such as the Food & Feed Safety Research Unit at the USDA Southern Regional Research Center in New Orleans.
Ed Cleveland, a leader in the center's aflatoxin programs, is working directly with a colleague in Phoenix who continues to see strong results in controlling aflatoxin through a counter-fungus that can turn back the toxic strain in cotton. Peter Cotty, an ARS plant pathologist, says the atoxigenic strain of aspergillus flavus does not damage corn or cotton.
Working with growers on some 20,000 cotton acres, he's found the atoxigenic strain competes head on with the toxic strain. “The non-aflatoxin producing strains are natural,” he says. “When they infect a field at the same time as the toxic strain, it can limit contamination a great deal.
“These influences of the atoxigenic have long-term effects. It can survive more than a year. We treat one year and get displacement. It overwinters to the next year. When we treat the second year and rotate to different crop, that crop benefits,” Cotty says. “If we keep treating a field, the non-aflatoxin ‘producers’ may eventually dominate throughout the field.”
The atoxigenic fungus is actually grown in wheat seed. About 10 lbs./acre is applied at a cost of about $5/acre for material. In south Texas, where growers regularly plow up aflatoxin-hendered corn, Cotty has helped obtain a Section 3 registration for the atoxigenic material in cotton, as well as for Arizona and southern California.
This should help growers who have corn rotated with cotton, he says.
Meanwhile, Cleveland and other researchers are looking to determine the genome of the atoxigenic strain as well as the toxic strain. “Knowing the fungal genome (of the toxic strain) will tell us more about its vulnerabilities,” says Cleveland.
Averhoff and the mycotoxin task force are also looking into alternative uses for contaminated corn. For example, he points out that there are problems associated with corn run through an ethanol plant.
About one-third of corn processed in an ethanol plant becomes dry distillers' grain (DDG). The problem with 200 ppb aflatoxin-infected corn is that all of the contaminated corn winds up in the DDG. “That means it can be 600 ppg,” says Averhoff. He says it's then unfit for beef cattle consumption, much less dairy cows.
He reminds Corn Belt growers that in hot, dry years, they can also see aflatoxin problems. In the late '80s, hundreds of millions of dollars in corn damage was experienced through aflatoxin contamination.
Aflatoxin is not the only toxin that worries the task force. “We named it a mycotoxin task force for a reason,” says Averhoff. “Fumonisin (a toxin caused by fungus commonly found in most of the world's corn and a toxin produced by the soil-borne disease fusarium) is also prevalent in Midwestern corn. If national standards on acceptable fumonisin consumption levels are set by the Food and Drug Administration, it will be an increasing problem for growers to deliver corn for food or feed.”