Chicago - Nine months after a U.S. drought sparked a surge in global crop prices, corn has joined soybeans and wheat in a bear-market slump as demand slows and farmers prepare to boost output in 2013.

Corn futures in Chicago plunged 13 percent since the U.S. Department of Agriculture said March 28 that inventories were bigger than analysts forecast and that farmers will plant the most acres this year since 1936. Corn, soybeans and wheat, the biggest U.S. crops along with hay, have tumbled into bear markets with drops of more than 20 percent from 2012 highs.

Exports of corn from the United States, the world's largest grower, are at a 41-year low, while high grain costs last year forced beef producers including Cargill Inc. to close plants and ethanol makers such as Valero Energy Corp. to shutter distilleries. South American farmers are adding to supply with record harvests, and global food prices tracked by the United Nations dropped for five straight months through February.