Mycotoxins are natural toxins produced by fungi.
There are two classes mycotoxins of concern that can
occasionally develop in corn produced in Kentucky. Fumonisins,
the more common in Kentucky, are produced principally by the
fungus Fusarium verticillioides (formerly Fusarium moniliforme).
Fumonisins can cause fatal diseases of horses and swine, and
pose a known risk to other livestock. Fumonisins also have been
shown to cause cancer in laboratory rats, and in humans
fumonisins have been associated with esophageal cancer and
serious birth defects called neural tube defects. Aflatoxins are
produced by the fungus Aspergillus flavus. The aflatoxins are
highly potent liver carcinogens in animals and may be
carcinogenic to humans, as well. Aflatoxin contamination is
uncommon in Kentucky corn but can occur when sustained hot, dry
weather occurs during grain fill. Both mycotoxins are regulated
by the U.S. Food and Drug Administration. More information on
aflatoxins and fumonisins is available in the following
Extension publications:
Fumonisin, Vomitoxin, and Other Mycotoxins in
Corn Produced by Fusarium Fungi. ID-121.
http://www.ca.uky.edu/agc/pubs/id/id121/id121.pdf
Aflatoxins in Corn, ID-59.
http://www.ca.uky.edu/agc/pubs/id/id59/id59.pdf
Corn hybrids containing the Bt trait have been
commercially available for almost a decade. This trait results
in the produced of a delta-endotoxin which poisons the European
corn borer (ECB) as it feeds. The currently available Bt traits
are very effective against ECB but are less effective against
corn earworm and fall armyworm. All of these insects can play a
role in enhancing contamination of corn with fumonisins or
aflatoxins, by creating wounds in kernels that allow fungal
infection and by carrying spores on their bodies.
Because the feeding activity of these
lepidopterous insects enhances the risk of mycotoxin
contamination, a number of studies in the U.S. and elsewhere
have tested whether the Bt trait reduces the risk of mycotoxin
contamination.
To put it simply, there is strong evidence
from several well-conducted studies showing that many Bt hybrids
have a substantially reduced risk of fumonisin contamination as
compared to non-Bt counterparts. If the field has heavy pressure
from ECB and conditions favor fumonisin contamination,
reductions in fumonisin contamination of 80-90% are common in
these studies.
The most substantial and consistent reductions
in fumonisin contamination have been observed with hybrids in
which the delta-endotoxin is produced in all plant tissues,
including kernels. This makes sense, because insect feeding in
the kernels provides A. flavus and F. verticillioides with an
easy access into the kernel where it can produce mycotoxins.
Occasionally, fumonisin reductions are observed in hybrids where
the delta-endotoxin is produced in green tissue and pollen (but
not kernels). Although direct protection of kernels more
effective in reducing fumonisin contamination, these hybrids
probably lower overall ECB populations, resulting in fewer
kernels wounds and therefore fewer infection sites.
There have also been similar studies testing
the effect of the Bt trait on aflatoxin contamination.
Reductions in contamination levels have sometimes been observed,
especially in situations with high ECB populations and high
overall aflatoxin levels. In these circumstances, reductions
have commonly been in the range of 50-75%, substantial but not
as large as for fumonisins. It is also significant that in
several of these instances, levels of aflatoxin were above the
FDA limit for human consumption even in Bt hybrids.
Conclusion
Bt hybrids are not a magic bullet against fumonisins and
aflatoxins. However, Bt hybrids expressing the delta-endotoxin
in kernels have clearly been shown to reduce the risk of
fumonisin contamination in sites with high ECB pressure. They
also may reduce aflatoxin risk somewhat should environmental
conditions favor aflatoxin contamination.
