U.S. corn futures are expected to open sharply lower Friday after a government report showed corn stockpiles much larger than traders were expecting.

Chicago Board of Trade futures could open 30 to 40 cents lower, analysts said. Corn for December delivery closed Thursday at $6.32 3/4 per bushel. The exchange limits one-day price moves in either direction to 40 cents.

Friday's quarterly grain stocks report from the U.S. Department of Agriculture showed total U.S. supplies of 1.128 billion bushels as of Sept. 1, above any of the analyst forecasts in a Dow Jones Newswires survey. The average analyst guess was 962 million bushels.

The Sept. 1 stockpile estimate represents the end of the marketing year. Although it is down sharply from 1.708 billion a year ago, traders have for months assumed the final supply number would dip below 1 billion.

With outside market pressure from crude oil, equities and a stronger dollar, "it's setting up to be an ugly day," said Jim Riley, analyst with Linn Group.

While supplies are still smaller than average, the gap between the USDA's estimate and the average trade guess essentially adds 1 million acres of harvested corn to the supply pipeline, Riley said.

The report puts to rest talk that farmers will plant significantly more acreage next year, he said. With a high cost of production, corn prices are unlikely to be high enough to compel farmers to plant more.

"Everybody talking about farmers planting 94 million to 96 million acres of corn next year? That's over," Riley said.

Farmers planted an estimated 92.3M this year, second highest total since World War II.

Traders said the market was trading around $6 per bushel in over-the-counter trade ahead of the market's open.

Some traders questioned the USDA's estimate and how usage could have slowed. Farm Futures analyst Arlan Suderman noted that cattle placed in feedlots were at record levels in the past quarter, and that ethanol output has been high.

The market's slide Friday could be limited by the fact that traders already appeared to be leaning toward a negative report. Prices have tumbled throughout September.

Some traders expect the break in prices will stimulate new demand, particularly from China.

Already, South Korean importers have purchased more than 1 million tons feed grains, including at least 15 cargoes of corn totaling 837,000 metric tons in the last 11 days, capitalizing on the latest slump in prices, trading executives said Friday.

--Sameer Mohindru contributed to this article.