Jan. 29 (Bloomberg) -- Cotton rallied, heading for the biggest monthly gain in almost two years, on mounting concern that output will drop in the U.S., the worldΆs biggest exporter.

Planting of cotton in the U.S. probably will plunge 16 percent this year to 10.32 million acres as farmers shift to more-profitable crops, including corn and soybeans, according to a Bloomberg survey. While cotton futures are up 28 percent since reaching a 31-month low in June, prices are still less than half of the record reached in 2011 because of a global surplus.

“Farmers are going to have to commit soon to what they are planting down south, so we may get caught with short supplies,” Sterling Smith, a futures specialist at Chicago-based Citigroup Global Markets Inc., said in an e-mail.

Cotton for March delivery rose 1.7 percent to 82.39 cents a pound at 2:30 p.m. on ICE Futures U.S. in New York. Prices are up 9.6 percent since Dec. 31, heading for the biggest monthly gain since February 2011.