Corn prices declined Thursday for the first time in 12 days as investors bet that a rally that culminated in record prices, prompted by the worst floods since 1993 in the Midwest, was overdone, Bloomberg reported.

Prices for corn, used in food, animal feed and ethanol production, advanced 25 percent this month, bringing the year’s gain to 69 percent. Corn’s 14-day relative strength index, a gauge of momentum, has held above 70 since June 10, signaling that prices may decline.

Corn has been the best performer this year in the UBS Bloomberg CMCI index of 26 commodities, outperforming energy products, soybeans and metals. Crop conditions this month in the United States are the worst since 1996 and mirror the deterioration in 1993, when widespread flooding reduced corn production.

Some fields in Iowa, got more than 15 inches of rain in the past two weeks, National Weather Service data show. Potentially smaller crop production following the floods may reduce profits of grain handlers and food processors, such as Archer-Daniels-Midland Co. and Cargill Inc.

Global corn inventories as of Aug. 31 will drop to the lowest level in 24 years, the U.S. Department of Agriculture said.

The weather is endangering a U.S. corn crop already expected by the government to decline from last year’s record harvest after farmers planted 8.1 percent fewer acres, analysts said. Domestic corn production will drop 10 percent this year, to about 11.7 billion bushels, the USDA said last week.

U.S. inventories before the 2009 harvest will total 673 million bushels, representing about 20 days of supplies at current consumption rates. That’s down from 40 days of stockpiles estimated for this year and the lowest since 1996, when reserves were projected to last 18 days, USDA data show.