Corn is the primary feedstock of ethanol, and 5.5 billion bushels of U.S. corn were used for that purpose in 2011-12.

"Roughly 40 percent of the corn that's produced in this country is used in ethanol, although some of it is later used as distillers grain for livestock feed," Abbott said. "That's up from about 10-12 percent five years ago. The amount of corn that makes up the increase is more than we export."

Because the law requires that ethanol be produced, there is less corn available for other non-ethanol users, including foreign buyers and U.S. livestock producers. The high demand for corn, coupled with the partially regulated market, has pushed corn prices higher.

Secondly, the U.S. has not kept up with many other nations that have significantly increased their corn acreage. Although U.S. farmers have shifted acreage away from other crops and into corn, competing nations and customers have significantly increased their area planted.

"We haven't expanded overall planted area like the rest of the world. Our acreage is basically flat," Abbott said.

Since the late 1990s, South America has boosted crop acreage 53 percent. The nations that make up the former Soviet Union are growing crops on 24 percent more acres, with acreage up 13.4 percent in sub-Saharan Africa and Oceania. By contrast, crop acreage in the European Union is off 4 percent.

U.S. corn exports were further hurt by the summer drought. According to the USDA, domestic corn production is expected to be down 13 percent from 2011, at 10.7 billion bushels. That would represent the lowest corn production volume since 2006.

For those reasons, the outlook for U.S. corn exports going forward is less positive than a few years ago, Abbott said.

"We've tried to accommodate the export markets by working to increase production, but we haven't managed to do that," he said. "We've had to keep feed use flat and watch exports shrink.

"All the hard work we did to build export markets has been hurt by the high commodity prices of the last four or five years. As a result, the world has figured out ways to meet their own needs. And with a couple of exceptions like China buying more soybeans, we're probably going to see weaker export demand in the future."