CHICAGO -- The biggest slump in wheat prices in three years is expected to fall deeper as expanding harvests from Russia to Canada bolster inventories to the most in a decade.

The Department of Agriculture is expected to announce today a 3.4 percent gain in stockpiles to 202.89 million metric tons by June, according to analyst estimates. Prices had already fallen 35 percent from a 29-month high of $9.17 a bushel in February and are expected to drop another 11 percent to $5.30 in the next 12 months.

Global food costs declined 9.6 percent since February, according to United Nations reports. Corn and soybeans prices are tumbling after farmers throughout the world responded to record-high prices by planting more crops.

Combined output of wheat, corn and soy will jump 3.4 percent to a record 1.8 billion tons this season, 32 percent more than a decade ago, the USDA said last month.Wheat fell 25 percent to $5.95 on the Chicago Board of Trade this year, heading for the biggest annual drop since 2008.

"Every country that can plant more corn and wheat has done so," said Alex Bos, manager of agricultural commodities at Macquarie. "It's an incredible shift, and there's still spare farmland capacity that can be put into production."

Global wheat production is expected to jump 5.3 percent in the 12 months ending May 31, according to the USDA. Production will rise to 685 million tons, topping the record of 684.3 million in 2010. Demand will expand 3.3 percent to 673.3 million tons. The USDA will probably increase its inventory estimate by 290,000 tons, the Bloomberg survey showed.

Investors may be skeptical of the USDA's figures. It reduced the corn-crop estimates in 2010 and 2011 by the most for any two-year period since 1984. The estimates changed as rain delayed planting and hot, dry weather disrupted the growing season in the Midwest. The USDA cut its forecast four times in as many months this year, and critics such as Darrel Good, an agricultural economist at the University of Illinois, are questioning their reliability.

In June, at the start of the hottest Midwest summer in 50 years, corn futures for July delivery touched a record $7.99 in Chicago.

The grain then tumbled 17 percent that month as the government said U.S. farmers planted more than analysts were expecting and June 1 stockpiles were 12 percent larger than anticipated.

Even after this year's declines, grain prices remain costly historically. Wheat has averaged $7.31 this year, about 45 percent more than the 10-year average. Corn averaged $6.83, the most in records going back to 1958. While the U.N.'s food index has declined for four consecutive months, the gauge is still 46 percent above its average over the past decade.

Surging global grain output is compensating for a decline in the U.S., the world's biggest agricultural exporter. Farmers will reap a smaller wheat crop for a third consecutive year in 2012, and less corn for a second year, the USDA estimates. The nation's wheat exports are down 22 percent from a year earlier at 18.1 million tons, a sign that this year's record farm income of $100.9 billion predicted by the USDA may not be repeated next year.

"In the past, basically the U.S. has totally dominated the world export market, but we're starting to see increased competition," said Shawn McCambridge, a senior grain analyst for Jefferies Bache.