Government reports about the U.S. corn crop have become increasingly unreliable of late, contributing to wild swings in corn prices, a Wall Street Journal analysis shows.

Over the past two years, the Department of Agriculture's monthly forecasts of how much farmers will harvest have been off the mark to a greater degree than any other two consecutive years in the last 15, according to a Journal analysis of government data. This year's early-season forecasts also appear to have been way off. The next monthly report is due on Friday.

At the same time, periodic stockpile reports -- government estimates of how much corn is stored in farm silos and other storage facilities -- have generated big surprises. The average monthly swings in stockpile estimates between May and October, the heart of the growing season, have been greater this year than in any year since 1996, according to the Journal analysis.

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The stockpile reports have had a big effect on markets. On Sept. 30, the USDA said a quarterly survey showed corn stockpiles were 23% higher than it had estimated earlier that month. Corn prices fell 6.3% in the futures market that day, shaving $5 billion off the value of corn in the fields.

"There is very much a lack of confidence right now among farmers" in the government data, says Bill Christ, who harvests roughly 100,000 bushels of corn a year in Metamora, Ill. "Can't they get it right?"

USDA officials blame unpredictable weather for recent errant production forecasts. They say the figures are snapshots that change based on fresh information, such as damage caused by heat waves or changes in consumption patterns.

"If somebody's going to be in these commodity markets, they better understand these things are subject to change," says Gerald Bange, chairman of the USDA's World Agricultural Outlook Board, which is involved in producing the data.

The U.S. grew 38% of the world's corn in 2010, when the domestic crop was worth $67 billion. Strong demand from foreign buyers, and from the growing ethanol industry, has added to market volatility. Jerry Norton, who tracks corn for the USDA, says because corn supplies are tight, "the market is much more sensitive" to changes in the department's reports.

The Chicago Board of Trade has long had limits on single-day price moves on corn and other commodities in the futures market. Corn prices have hit the basic limit 20 times since the start of 2009, with eight of those instances, or 40% of them, coming on the day of a USDA report, according to a Journal analysis of price data. Between 1996 and 2008, only 20% of such limit moves came on reports days.