Feed and Forage--Harvesting

AFLATOXIN IN PREHARVEST CORN

Drought and high air temperatures raise concerns over the potential of aflatoxin contamination of the corn crop that may impact the usability of much of the remaining drought-stricken crop.

The role of the weather, primarily moisture and temperature stress, in the preharvest contamination of corn with aflatoxin is well documented. The toxin, produced by the fungus Aspergillus flavus, can have a detrimental effect when fed to most livestock including swine, poultry and cattle. In the midwest, Aspergillus flavus and aflatoxin are considered primarily a storage problem. Corn harvested at a high moisture content and improperly dried will mold producing large quantities of aflatoxin as a byproduct. In South Carolina aspergillus flavus and the resulting aflatoxin can be found in preharvest corn.

Moisture and Temperature

Periods of drought result in dramatic increases in preharvest aflatoxin in field corn. Many of the cultural practices that reduce plant stress such as subsoiling and proper nitrogen fertilization in turn also reduce aflatoxin. The drought prone sandy soils of the coastal plain are more likely to develop aflatoxin than heavier soils with greater water holding capacity. Generally, irrigated corn will have substantially lower levels of aflatoxin than non-irrigated corn, although several studies have shown that irrigation does not always reduce aflatoxin. In addition, aflatoxin levels tend to be higher in irrigated corn during drought years. One characteristic of drought is the substantially higher air temperatures. Most fungi flourish between 68 and 86 degrees but A. flavus has a much broader temperature range and an optimum growth temperature in the range of 96 degrees. The combined effects of temperature and moisture result in a corn predisposed to aflatoxin formation.

Mode of Entry

No visible spore production can be seen on undamaged kernels infected with Aspergillus flavus, making visual identification of aflatoxin laden corn difficult. Testing by the minicolumn technique or the more qualitative methods (HPLC) will detect the aflatoxin in the intact kernels. In these methods the toxin is extracted chemically.

Insects can play a substantial role in moving the fungus into close proximity to the developing corn kernels (infection court). Insects, such as the corn earworm, feed on the developing kernels and can carry the fungus on their body, placing the spores near freshly damaged kernels. Typically Aspergillus flavus will colonize the damaged kernels, resulting in aflatoxin concentrations ranging in the parts per million range. This type of damage is easily identified by the abundant sporulation of the fungus on the damaged kernels. Although the number of affected kernels is small, the enormous amount of toxin in these damaged kernels makes insect damage a significant component in the aflatoxin problem. In addition, the infrequent kernel with extremely high levels of toxin complicate the analysis of corn for aflatoxin. This is the major reason why a 10 pound sample of corn is required for analysis for aflatoxin contamination in corn. Insect damage is frequently more severe in drought years adding to the overall problem.

Solution

The most effective strategy for the control of aflatoxin in corn is to prevent or reduce its development in the field by using good production practices such as subsoiling, weed control and adequate nitrogen fertilization.

Efforts should be made to minimize additional development of aflatoxin in the harvesting process and storage of the grain. Harvest corn as soon as it is mature and make certain your combine is properly adjusted to prevent excess damage to the kernels. After harvest bring the grain down to 20% moisture within 24 hours. Do not allow the corn to stand in wagons for extended periods of time without air movement through the grain.

For storage, grain moisture should be further reduced to 13% or less and maintained at this moisture or lower to prevent the buildup of aflatoxin in storage. Drying the corn to 13% or less will not kill the fungus or reduce the amount of aflatoxin in the (the) commodity but will retard the further development of the fungus.

Once the corn is contaminated, very few practical options exist to remove it. Utilization of anhydrous gaseous ammonia can be used as a decontamination agent but several distinct disadvantages exist. Ammonia treatment will discolor the grain, (turning yellow corn dark tan to mahogany in color) and results in the release of noxious vapors. One additional option for consumers of the grain is the use of NovaSil (processed hydrated aluminosilicate). Research has shown that aluminosilicates applied at the recommended rate to increase flow properties and reduce caking will detoxify aflatoxin to some extent when ingested by livestock. At the present time NovaSil is not labeled for this use. However, it is labeled as an anticaking material (0.5% or 10 lb/ton) and could be used for that purpose with the added benefit of some aflatoxin detoxification. Although this is not the entire solution to aflatoxin contamination of corn it may aid in the use of corn containing aflatoxin. NovaSil will not detoxify many other toxins produced by other fungi. Samples suspected of containing aflatoxin can be analyzed by the South Carolina Department of Agriculture.