Issue No. 22, Article 3/September 2, 2005
Aflatoxin in the Corn Crop?
Inquiries about the possibility of aflatoxin being a problem for this harvest have been trickling in recently. Mike Roegge, Extension crops educator with the Adams/Brown Unit, reports that growers are very concerned in the western part of the state as well as other drought-stricken areas.
Aflatoxin is one member of a larger group of chemicals called mycotoxins, which are toxins produced by fungi. Aflatoxins are some of the most potent carcinogens on earth, and that fact gets everyone's attention. The allowable amount of this toxin in grain for food or feed is highly regulated by the FDA. Testing is aimed at detection of 20 ppb of the toxin in grain intended for human consumption and immature animals. Table 2 lists FDA action limits for aflatoxin.
Aflatoxin can be produced by the fungi Aspergillus flavus and A. parasiticus. These fungi are present everywhere in soils and organic matter. Stored grain can be infected by the fungi, or they can infect in the field. This typically occurs when the weather is dry and hot (more than 86°F) during silking to late kernel development. We saw this happen in 1988, and it may happen again this year, certainly for some of the drought-stricken areas of the state.
Aflatoxin produced by A. flavus is most likely to be produced in the field, and particularly in storage, after the kernels have been damaged by insects, birds, mites, hail, early frost, heat and drought stress, windstorms, and other unfavorable weather conditions. The presence of the fungus A. flavus in a feed sample does not imply that the feed is unwholesome or that it will contain high levels of aflatoxin. Testing is necessary to determine how much toxin may have been produced.
Sampling grain for aflatoxin at the elevator follows established federal protocols. A 10-pound sample is usually collected by pooling five or more probes collected from an auger discharge of one or more combine hopper loads. The sample is ground for analysis, and one of three methods is then used to determine the presence of aflatoxin. One method is a visual examination, typically a black light test. The second method is a quick qualitative laboratory immunoassay procedure; the quick test now also includes a quantitative test kit to determine the range of toxin levels. Table 3 lists six sources for USDA-approved quick tests. The third method is one of a number of laboratory tests for actual quantification of the toxin. Quantification will be necessary if aflatoxin is detected by one of the other methods.
If you think your corn may have been at risk, what can you do to minimize the chance of fungi and toxin development now or in storage? First, harvest drought-stressed and insect-infested grain at early maturity as soon as the moisture content allows minimum grain damage (23% to 25% moisture for shelled corn, 25% to 30% moisture for ear corn). Set the blower on the combine high. Adjust the combine header speed to minimize cracking, and reduce the content of trash, fines, and small broken or mold-infected kernels, especially those kernels near the tips where mold infestation is most likely to be present. Upward of 50% reduction in existing aflatoxin levels (or below 20 ppb) can be achieved in some fields by carefully monitoring combine cylinder, screen, and airflow levels.
Drying is then essential to prevent further growth. Dry all grain to at least 13% to 14% moisture as rapidly as possible, not to exceed a 24- to 48-hour period after harvest.
Safe, long-term storage can be accomplished at a uniform moisture level of 13% or somewhat less. See Table 4 for maximum storage times for shelled corn at various temperatures and moisture levels. Moisture may be 14% if grain is to be moved or sold in a shorter period of time. For slow drying, the grain should contain no more than 18% to 20% moisture in full-bin drying; this may not be the best method when toxins are a concern. Another possibility is high-temperature drying until the grain reaches 18% to 20% moisture, followed by low-heat drying to 13% moisture. Avoid air-drying mold-damaged corn without heat.
Research has shown aflatoxin increases of 100 to more than 2,000 ppb in 3 days when recently harvested field corn was stored at high moisture levels. Delays in transit to the storage bin or buying point need to be minimal. Aflatoxins have been shown to increase in truckloads of contaminated corn by as much as 6% per hour of delay.
Cool the grain after drying and maintain dry storage conditions. When possible, continue cooling until the grain temperature reaches 35°F to 40°F. Thoroughly screen and clean the grain and all bins before storage to remove dirt, dust, and other foreign matter, crop debris, chaff, and cracked or broken seeds and kernels. Most contaminated corn is in the small and broken kernels, which will drop through a screen. Store the grain in watertight, insect-tight, and rodent-tight structures. Continue periodic aeration and probing for hot spots at intervals of 1 to 4 weeks throughout the storage period.
And, finally, if you have crop insurance, you know that the crop is insured while it's in the field, but what about afterward? Contact your insurer before harvest to learn the company's policy if aflatoxin is detected in your grain. You may be covered until December, or it may be that once you harvest you are uninsured. Be sure to check first.
Some good online resources about mycotoxins and grain testing follow:
Federal Loss Adjustment Manual, pp. 220-228 (Adobe PDF)