Grain crops are being gobbled up faster than farmers can grow them, and that could portend trouble down the road if production doesn't catch up, said a Purdue University agricultural economist.

There have been two major demand surges in the past five years, including the rising use of corn to produce ethanol and China's purchases of soybeans, Chris Hurt said. The former has been driven by government biofuels mandates and high oil prices, while the latter is the product of China's growing food demand brought on by rapidly increasing incomes that have enabled the Chinese people to buy more food.

"These greater levels of usage have placed a strain on the agricultural production system, resulting in low inventories that leave little room for any production shortfalls," Hurt said. "Producers certainly have responded to try to meet those demands, but what we've seen is that demand has really outpaced the ability of the world to supply."

Inventories of corn and soybeans are near "bare minimums" in the United States, Hurt said, with wheat stocks in better shape.

Many years going into harvest there would be enough corn and soybeans in storage to last 45-60 days, or more, Hurt said. That likely won't be the case this fall.

"With corn, it looks like we could be down to about a 24-day supply at the end of this marketing year," he said. "That, of course, means any further threats to the 2011 crop yields would send markets into deeper shortages and higher prices."

Most of the corn grown in the world becomes livestock feed. But in recent years ethanol production has claimed a larger percentage of the crop, leading to increased corn acreage and higher prices, Hurt said.

"Fifty-nine percent of all the growth in corn use in the entire world over the last five years has been in a category where ethanol would be placed: industrial use," he said. "Here in the United States over the last five years 100 percent of the increase in corn usage is for ethanol, representing 2.5 billion bushels of corn."

About 27 percent of the U.S. corn crop is used for ethanol, compared to 10 percent in 2005, Hurt estimated. All told, 16 million additional acres of corn from the 2010 crop was required to produce ethanol versus 2005.