"Over a long period commodities contribute right around zero to export growth," L. Josh Bivens of the Economic Policy Institute told The New York Times.

"Don't get too used to commodity export booms; as any third world country will tell you, they tend to go away pretty quickly," he said.

Grain exports have benefited from rising prices and the shrinking value of the dollar, which makes U.S. goods cheaper overseas. But, prices have begun to fall lately and the dollar is lately gaining in strength.

Corn exports rose 20 percent this year as farmers have added corn acreage in part to meet demand for ethanol production. But, buyers can quickly turn to corn grown in other countries as price and currency values shift.

"It is my fondest hope that exports will stay strong," Indiana grain farmer John Hardin Jr. said, "although I don't think it is realistic to expect a percentage increase equal to what we are seeing this year."