With aflatoxin contamination looming as a major issue in international peanut markets, it behooves Southeastern growers to do everything possible to prevent problems at the farm level.
"It's extremely important that growers reduce the risk of carrying Aspergillus flavus-contaminated peanuts to the buying point," says John Beasley, University of Georgia Extension peanut specialist. "Several things can be done at the grower level - during pre-harvest, harvest and post-harvest - to manage aflatoxin and reduce the risk of contamination.
During the growing season, irrigation drastically reduces the risk of aflatoxin contamination, says Beasley. "Southeastern research clearly documents that the critical stage for watering peanuts, as far as yield and quality are concerned, is that 40- to 100-day period - essentially, from initial bloom to about three to five weeks prior to harvest.
"Drought injury during that time frame directly affects yield and grade. When you get to within that last 30 days before harvest, you don't need to back off completely on watering. High soil temperatures mean a greater risk to aflatoxin, as kernel moisture declines," says the agronomist.
Growers should be careful, he adds, not to water too much in the final 30 days before harvest. "Use some type of scheduling system for irrigating peanuts during this time, something like IrrigatorPro from the National Peanut Lab. Do whatever is necessary to prevent peanuts from going into stress during those last 30 days prior to harvest," says Beasley.
Managing pests, including insects, weeds, disease and nematodes, also is important in preventing aflatoxin contamination, he continues. "When we talk about insects, we're talking primarily about soil insects. We're not so concerned about foliage-feeding insects when we're managing for A. flavus," he says.
Lesser cornstalk borers can be found under hot, dry conditions in sandy soils, says Beasley. Southern corn rootworms, meanwhile, are found mostly in heavier soils under wet conditions. Wireworms, he adds, are prevalent in most any conditions, feeding on mature pods.
"This past year, the hot, dry start of the season was very favorable for lesser cornstalk borers. We experienced tremendous problems in south Georgia, south Alabama and north Florida. We even saw fields under irrigation that had to be treated two or three times for lesser cornstalk borers.
"Research has shown that these insects don't have to make a complete puncture through the pod and into the kernel to set up aflatoxin contamination. Lesser cornstalk borers can cause a small amount of damage on immature kernels, and those kernels will be set up for aflatoxin contamination. "Lesser cornstalk borers can be found under hot, dry conditions in sandy soils, says Beasley. Southern corn rootworms, meanwhile, are found mostly in heavier soils under wet conditions.
Turning to weed control, Beasley says weed pests that are the most concern in managing aflatoxin are those with fleshy fruit, such as bur gherkin. When these fruit are crushed among the peanuts, they set up zones of excess moisture that are favorable for aflatoxin contamination, he says.
"Do your best to control these types of weeds. Often times, it's very difficult to find the right herbicide combination to control these weeds."
Aflatoxin contamination also has been linked to calcium nutrition problems in peanuts, explains Beasley. "If you have a weakened pod due to a lack of calcium, several organisms in the soil - including pythium, rhizoctonia and sclerotia - will attack the pods. And once the pod becomes damaged or weakened in any way, they are most susceptible to A. flavus development.
"Once your peanuts are planted and begin emerging, you need to get out there and take a pegging zone sample. It's just like a regular soil sample, but you go only three inches deep. If this sample shows less than 500 pounds per acre of calcium, or if you have a calcium ratio of less than three to one, then you need to apply gypsum based on the soil test result.
"Contact you county Extension agent about the proper way to take a pegging zone sample. This is important even if you have a fall sample that shows good calcium in the eight-inch zone. A lot of rainfall in the winter can move some of that calcium below the top three inches. The calcium that's needed for healthy pods must be in that top three inches where the peanuts are developing."
Fertilizing heavily with potash or magnesium can interfere with calcium uptake, says Beasley. This is a concern for those growers who plant cotton ahead of peanuts and use a high rate of fertilizer. Obviously, drought conditions make it difficult for calcium to be taken up by the pods.
Harvesting mature peanuts also is important in preventing aflatoxin, he says. "If you harvest a number of immature kernels, you'll be more susceptible to A. flavus. Use the hull-scrape chart to help determine when to begin digging."
And, says Beasley, if you have center pivot irrigation with dry field corners, it's very important that you harvest the dry corners separately from the irrigated crop and keep them in separate trailers.
Properly adjusting digger shakers and combines also are good ways to help manage aflatoxin, notes Beasley. "Make sure that you adjust your digger shakers and combine to do the most efficient job possible. We conducted a test where we intentionally dug too shallow and dug too far to the left or too far to the right. This test revealed that digging too shallow can make just enough of a cut to create an opening in the pod zone. Aflatoxin can develop in this opening."
Combines also should be adjusted properly, he adds. "These adjustments sometimes must be done several times during the day while you're combining. Make the proper adjustments throughout the day as conditions change."
After harvest, growers should level off the peanuts in the trailer to prevent uneven drying, says Beasley. "And if you're drying on the farm, make sure that you have the proper settings. Monitor the process closely so that you're not drying too fast or too long. Follow storage recommendations."