The FDA has set use guidelines for corn containing aflatoxin. In general, they are based on maintaining performance and avoiding disease related to aflatoxin, except for dairy cattle in which prevention of aflatoxin residues in milk is the main concern. For example, human foods and feed intended for dairy cattle must contain less than 20 ppb.

Some aflatoxin levels have been observed in several U.S. Corn Belt states, with almost all below the 20 ppb limit. For example, one private grain inspection service in Nebraska said most of the tests it has completed were zero or only one ppb. Those lots of corn with elevated aflatoxin levels will be diverted into feed for local beef cattle that can consume that grain without harmful health effects.

"So far this harvest season, aflatoxin does not appear to be a significant problem," O'Neil said.

"However, we will know more once more of the crop has been harvested, and certainly we will keep an eye on it."

About 10 percent of the crop has been harvested as of Sept. 2.

In regards to Iowa's corn crop, Alison Robertson, from the plant pathology department at Iowa State University said, "Thus far, the problem does not appear widespread; however, fields across the state are at risk for aflatoxin considering the hot, dry conditions we had during pollination and are having now as much of the crop reaches black layer."

Corn-based ethanol plants, which produce distiller's dried grains with solubles (DDGS), generally have a lower aflatoxin threshold — even zero — because aflatoxin can be concentrated in the DDGS. U.S. DDGS importers who are concerned can require aflatoxin testing and set limits in their purchase contracts.

Local grain elevators screen all incoming loads of corn for the aflatoxin-producing fungus. Grain elevators can refuse corn that is over 20 ppb aflatoxin unless they can segregate it from non-contaminated corn and they have a known, approved local use for it.