Aflatoxin contamination in corn has been a real problem for Texas growers for two years in a row. In 2008, the insurance payments to Texas growers for aflatoxin exceeded $14 million and economic damage was far greater than can be measured.
A little background: Aflatoxin is a human and animal health hazard regulated by the federal government beginning at 20 parts per billion of grain; this level is equivalent to one pint of water in an Olympic-size swimming pool! Aflatoxin is caused by a fungus, Aspergillus flavus that infects the grain, the cob, and the crop residue where it is believed to overwinter. The fungus prefers hot, dry and humid conditions. 2009 broke records for high temperatures and drought in many areas of Texas, so high levels of aflatoxin were not surprising.
Aflatoxin is a major concern not only for growers, but for anyone purchasing or feeding grain. The fungus is a severe threat to a safe and sustainable food supply here and abroad. Unfortunately, researchers have not yet found a silver bullet and there is unlikely to be one. Reducing the threat requires an integrated set of solutions. Texas AgriLife researchers, with the support of the Texas Corn Producers Board, the USDA, and industry, are investigating many different methods including breeding and genetic improvement of corn, preventive inoculations with A. flavus that does not produce the toxin (atoxigenic strains), crop and pest management to reduce stress, and natural clay binders that can be added to absorb aflatoxin after harvest but before consumption.
For growers, the most important and least expensive component of protection is choosing the right hybrid. A number of observable characteristics can reduce the hybrids susceptibility. Maturity, husk coverage, ear nod (ear droop), grain hardness and earworm resistance all appear to play a role.
If a proper maturity is chosen, drought and heat stress can be minimized resulting in less infection. Tight husk with complete coverage of the ear tip is a major factor for preventing fungal spores from contacting the ear and causing infection. Ear nod, when the tip of the ear points to the ground at maturity, reduces the possibility of spores being washed into the ear with rain. Harder grain endosperm types also appear to be much more difficult for the fungus to penetrate than softer grain types.
Earworm resistance is also a component because the earworm can move spores from outside of the husks to inside the ear. However, earworm resistance alone is not a complete solution; we have observed many ears that have had no earworms, but still have high fungus and aflatoxin levels.
As of yet, researchers have not been able to clearly identify any completely resistant corn inbred or hybrid, such has been found for a foliar disease like rust. Varying quantitative levels of resistance, however, have been noted. Texas AgriLife and USDA breeding programs are working together to find ways of pyramiding these sources of resistance and incorporating them with better agronomics and yield.
Currently, the highest level of genetic resistance to the fungus and to aflatoxin accumulation is most often found in tropical corn. Tropical corn has always been grown where it is continuously exposed to and evolved with the fungus. Unfortunately, this corn often has other less desirable characteristics, such as lower yields and maturity too late for the United States. To develop improved Southwestern inbreds plant breeders cross elite lines, those adapted to the temperate United States with high yield but low resistance, to tropical resistant lines which have low yield but high resistance.
Breeders then try to select new lines with a high yield potential and a high level of resistance from the resulting populations. In many cases it is tough to separate resistance from plant characteristics (maturity, husk coverage, etc.) mentioned earlier. It is also difficult to make sure that plants that look resistant are not simply escaping fungal spores. When researchers evaluate a varieties resistance, they often perform controlled inoculations with the fungus. This makes disease pressure as uniform as possible across all cultivars being evaluated, increasing the chances of identifying and selecting an inbred line that is truly resistant.
An additional issue for growers, regulators and researchers alike is the cost and difficulty to measure aflatoxin levels in a cultivar. Currently multiple tests are available for both home and official measurement of aflatoxin contamination which your county Extension agent can help direct you towards.
Unfortunately, since aflatoxin comprises such a tiny proportion of the sample, and is not found uniformly across plants in a field, large errors and widely varying estimates in contamination levels are often seen. Many growers have observed that when they go to sell their grain they can get varying estimates of its contamination – sampling is a real problem. Research on detection technologies will hopefully improve this issue in the future.
Texas AgriLife researchers are exploring a new method using near infrared spectroscopy which uses a samples absorbance pattern of infrared light to predict the contamination level. This method could make detection possible in less than two minutes with minimal labor costs. By increasing the number and decreasing the cost of screening samples, more samples can be taken leading to a better estimate of the true contamination level.
What about other aflatoxin control measures? Initial results of the commercial application of atoxigenic fungus (naturally occurring strains of Aspergillums flavus that does not produce the toxin and outcompetes toxigenic strains) by Dr. Tom Isakeit with Texas AgriLife Research appear promising. Although aflatoxin was not completely eliminated, toxin levels were decreased and other experimental strains seem to have promising results as well. The jury is still out on value for growers, since there is an application cost and the grain will still have some fungus contamination, but the toxin can be greatly reduced.
Management strategies are still being researched in cooperation with pathologists; your county Extension agent can recommend best management practices to reduce contamination. Sadly, grain may still be contaminated even after all preventive measures have been taken. However, promising remediation technologies are being evaluated. A group of soil researchers with Texas AgriLife has identified special mineral clay soils with unique properties that bind aflatoxins so the toxins pass through the digestive system without being absorbed, limiting toxin problems. Pending deregulation in the United States, this approach may decrease concerns about aflatoxin contamination of grain for human or animal consumption in the future.
Aflatoxin is a problem that has been with us for a very long time and is here to stay, but many promising and emerging technologies exist. These include new hybrids with greater resistance, atoxigenic strain preventative inoculation, better management approaches, and binders to ultimately minimize the risk of aflatoxin consumption to livestock and consumers.
Dr. Murray is an Assistant Professor of Quantitative Genetics and Corn Breeding at Texas A&M University in College Station, Texas. He is conducting research on the genetics of corn aflatoxin resistance and working with other researchers to identify integrated solutions. He may be contacted at [email protected].