Corn and soybean prices opened lower Wednesday on the Chicago Board of Trade after the U.S. Department of Agriculture said Iowa’s average corn yield per bushel will be higher than earlier forecast and export demand will be lower.

Corn fell Corn 7 cents per bushel to $6.36 after gaining 40 cents per bushel Tuesday and soybeans slipped 2 cents per bushel to $12.33.

The USDA raised Iowa’s projected corn yield to 169 bushels per acre, two bushels per acre more than its Sept. 1 estimate and four bushels per acre above the 2010 Iowa yield. The forecast yield will fall short of the record 182 bushels per acre set in 2009 and the 171 bushels per acre in 2008 and 2007.

Iowa’s average soybean yield was lowered to 50.5 bushels per acre from the 51 bushels per acre in 2010. The total soybean harvest in Iowa is projected to be 467 million bushels this year, down from 496 million bushels in 2010.

Nationally, the corn harvest is projected at 12.4 billion bushels, virtually unchanged from the 2010 harvest. The national average yield is projected to be 148 bushels per acre, down from 153 bushels per acre in  2010 and 164 bushels per acre in 2009.

The USDA is projecting the national soybean yield at about 3.1 billion bushels, down from 3.3 billion last year as about 3 million acres were shifted from soybeans to corn production.

The USDA said demand for U.S. grains — corn, soybeans and wheat – is expected to drop next year in the face of competition from grains coming from Central Asia.