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LSU researchers combat aflatoxin in corn

ST. JOSEPH, La. — LSU AgCenter scientists are looking at a way to “fight fire with fire,” so to speak, to help prevent the development of dangerous levels of aflatoxin in corn during periods of drought stress.

Trace amounts of aflatoxin, which is produced by a fungus called Aspergillus flavus, can force grain elevators to reject shipments of corn because they pose a danger to cattle and horses and to humans. But not all strains of Aspergillus flavus produce aflatoxin.

“Researchers working on cotton in Arizona have found that some atoxigenic strains of A. flavus do not produce the toxin under stress conditions and are very competitive with the toxigenic strains,” said Rick Mascagni, an agronomist with the AgCenter’s Northeast Research Station in St. Joseph, La.

“If the atoxigenic strains are out there, they can actually out-compete the toxigenic strains and possibly shut them down.”

Speaking at Northeast Station’s annual field day, Mascagni said LSU researchers have put out the atoxigenic strains on sterilized wheat or barley seed to have a nutrient source so they can develop and compete with the toxigenic strains.

LSU researchers have applied an A. flavus strain called K-49 that was isolated by scientists working at the USDA Agricultural Research Service/Mississippi Agricultural and Forestry Experiment Station complex at Stoneville, Miss., and a product called Afligard, which was developed in Georgia.

The latter was applied on barley seed at a rate of about 20 pounds per acre.

“I looked at it yesterday (June 9) and you could see the olive green fungus starting to grow on the barley and wheat seed kernels that are on the ground,” said Mascagni.

Louisiana corn growers have experienced the opposite of drought conditions with more than 20 inches of rain falling in portions of the state during May and early June. The Northeast Research Station had received 14 inches in 17 days in the month prior to the field day on June 10.

“This year, you would not expect aflatoxin to be a problem because of our wet conditions, but they say it’s caused by stress conditions, and too much water can be stressful,” he noted.

While most of the Mid-South has received ample rainfall in 2004, corn growers remember other years when they weren’t as fortunate. In 1998, Louisiana producers lost millions of dollars when more than 70 percent of their corn crop was infected with aflatoxin.

“It’s the No. 1 corn production problem in the South,” says Steve Moore, an agronomist with the Dean Lee Research Station in Alexandria, La. Like Mascagni, Moore is conducting tests with the atoxigenic strains of Aspergillus flavus.

Both researchers and their counterparts at other LSU AgCenter stations are also screening public breeding lines of corn for resistance to the aflatoxin-producing strains of A. flavus.

“Aflatoxin is considered the most potent natural carcinogen in existence,” says Moore, adding that the U.S. Food and Drug Administration prohibits interstate shipment of infected corn when the aflatoxin level exceeds 20 parts per trillion.

“Heat affects the activity of the fungus, and drought stresses the plant,” says Moore. “Any stress on a corn plant generally makes it more susceptible to aflatoxin contamination.”

Although corn has been an off-and-on crop for the Mid-South because of problems like aflatoxin contamination, some observers believe higher corn prices and falling cotton futures have led to a significant increase in corn in 2004. Some estimates put Louisiana corn plantings as high as 700,000 acres this year, compared to 520,000 acres in 2003.

“You drive around this area, and it almost looks like the Midwest with all the corn and Group 4 soybeans,” said David Lanclos, Extension corn and soybean specialist with the LSU AgCenter.

Mascagni said recommendations currently call for the atoxigenic strains to be applied between the “knee-high” and tasseling stages of corn development. “I put our test out about a week before tasseling,” said Mascagni, “and the atoxigenic strains seem to be doing quite well.”

Besides the work on aflatoxin, Mascagni and Ernie Clawson, an assistant professor at the Northeast Station, are conducting research on dual-row planting of corn, grain sorghum and soybeans.

Using a planter provided by Monosem, the AgCenter researchers have planted corn and grain sorghum in twin rows 9 inches apart on the Northeast Station and at the Macon Ridge Research Station near Winnsboro, La. Similar tests are being conducted on soybeans at both locations.

The twin-row or dual-row system may represent a compromise that could help Louisiana soybeans growers with several problems, says Clawson.

Soybeans planted in narrow rows (20 inches or less) canopy faster and make more efficient use of sunlight and moisture. But they are also more susceptible to waterlogging because those soybeans are typically planted flat.

“What this test shows is a compromise where you put two rows 9 inches apart on top of a 40-inch bed,” said Clawson. “It allows you to keep the plants up out of the water and still narrow the rows a little bit.”

Clawson planted the St. Joseph test on May 5. He used Maturity Group 4 varieties “that have performed well in our variety tests at least last year and preferably in the last three years.” The varieties used represented a range of maturities and plant heights.

Clawson used a planting rate of 131,000 seed per acre in the twin rows, 105,000 seed per acre in the single rows and 188,000 in the drilled beans or five seeds per row foot on the twin rows, eight seeds per row foot on the single and 2.7 on the drill-seeded beans in the study.

“I think we’re already seeing some beneficial effects,” he said. “The plants on the beds, whether in twin rows or single rows, are a lot bigger and greener than the ones planted in the flat. The latter had more problems with waterlogging and are still yellow.

“It’s interesting that the water moved off the flat areas before it did the row middles, but having the beans on beds still made a difference.”

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