Each fall a number of horses die from eating corn containing the mycotoxin fumonisin. This is a specific toxin produced from a mold that infects and damages corn kernels while still on the corn plant. Severe stress during the stage of kernel formation plus infestation with kernel-damaging insects increase the risk of fumonisin development.
Fumonisin is frequently found in varying amounts in corn, peanuts, and other field crops. The dangers are dose-related, and horses are the most susceptible of the domestic species. A recommended safe level for horses is less than five parts per million (ppm). Three to five ppm is a typical level during normal years. Stressed corn may have much higher levels. Fifteen ppm is fatal in equine species such as horses, mules, and burros.
Most fumonisin is found in damaged kernels and, therefore, in the fines or screenings that separate out when corn is being handled. Twenty-five to forty-five ppm of fumonisin and higher concentrations are frequently found in screenings or fines.
In every case of equine fumonisin poisoning that has been investigated, screenings/fines have been involved. Fines and screenings are cheap, sometimes free. They do have some energy value, but the risks are too high to feed them to horses.
Recommendations for safe feeding of corn to horses are as follows:
Screen or clean all corn and remove screenings before feeding or processing the corn for horse feed. Keep stored corn and feeds containing corn dry and protected from moisture. Avoid the use of corn of unknown sources for feed or mixing in horse feed. Avoid mixing corn at above 50 percent of the mixture in horse feeds.
Corn is an excellent source of energy and a useful feed for horses, as long as some precautions are taken.
Original Article: http://www.cvm.uiuc.edu/ope/enotes/showarticle.cfm?id=20