Stephen R. Koenning, Bruce A. Fortnum, and Pawel Wiatrak
Toxic metabolic by-products of fungi, known as mycotoxins, have received considerable attention during the past several years. Aflatoxin, produced by the fungus Aspergillus flavus, has been considered to be the most serious problem in North and South Carolina in recent years.
The detection of aflatoxin in corn can result in a reduced price for the grain or even rejection. The maximum concentration of aflatoxin in corn for interstate trade is regulated at 20 parts per billion (ppb) by the Food and Drug Administration (FDA). Although aflatoxin production is more severe in some years, low levels (less than 20 ppb) are frequently observed even in a good year. Low levels can increase dramatically if the corn is stored improperly.
Another class of mycotoxins are referred to as fumonisins. These toxins are produced by the fungus Fusarium moniliforme and are quite common in corn produced in North and South Carolina. Although there are no current restrictions on fumonisin in corn grain, new regulations restricting the levels of fumonisin in corn grain are in the process of being adopted by the FDA. These regulations state that for human or horse consumption corn cannot have more than 5 ppm of fumonisin in the grain. For swine the restricted level will be 10 ppm, and for cattle and poultry fumonisin levels must be below 50 ppm.
It is not uncommon for corn to have 1 to 3 ppm of fumonisin even in a good year. Corn shipped to Europe will probably be monitored for levels of fumonisin as well as aflatoxin.
Mycotoxins are known to cause serious health problems in animals including reduced weight gain, capillary fragility, reduced fertility, suppressed disease resistance, and even death. No animal is known to be resistant, but in general, older animals are more tolerant than younger animals.
Mycotoxins have been implicated in deaths from acute toxicoses in young animals, particularly poultry, as well as several animal health problems, including reduced fertility and growth rate. Aflatoxin is excreted in milk so special care should be exercised when feeding grain to lactating animals. Severe losses have been observed in commercial swine production in South Carolina following consumption of aflatoxin-contaminated grain. Inhalation of grain dust containing aflatoxin can be a serious health risk to producers.
Both Aspergillus flavus and Fusarium moniliforme are widely distributed in nature and are favored by high temperature. Temperatures ranging from 80 to 100 degrees F and a relative humidity of 85 percent (18 percent moisture in the grain) are optimum for fungal growth and toxin production in storage. Aspergillus flavus colonization of grain and subsequent toxin production can occur in the field. Air temperatures in the mid to high 90’s or higher allow silk colonization by A. flavus and eventual colonization of the kernel.
Aspergillus flavus can grow at temperatures high enough to inhibit growth of other fungi. Field contamination of corn can result in aflatoxin levels reaching 500 ppb or greater. Irrigation can dramatically reduce Aspergillus flavus infection of grain and aflatoxin production. Growth of these fungi does not occur below 12 to 13 percent moisture in the grain. Molding of grain can occur within moisture pockets within a grain bin making moisture control in storage essential to preventing losses. Grain sampling for aflatoxin contamination is difficult due to the localized nature of molding in storage. Extra care should be exercised to obtain a representative sample for analysis.
Aflatoxin contamination is higher in corn that has been produced under stress conditions. Thus, drought, heat, insect, and fertilizer stress are all conducive to high levels of aflatoxins. Factors that influence fumonisin production in corn are not well understood at this time. Certainly, insects provide an avenue of infection for both Aspergillus and Fusarium.
High rainfall and humidity at silking may increase infection of corn kernels by Fusarium spp. Hybrids genetically engineered to resist insects have been shown to have lower levels of fumonisin and aflatoxin. Therefore, use recommended production practices, plant early, irrigate to reduce drought stress, harvest early, avoid kernel damage during harvest, dry and store corn properly, and keep storage facilities clean in order to minimize the level of mycotoxins.