Ethanol futures fell to the lowest price in more than four weeks as concern that weather is harming South American corn crops eased.

Futures sank on speculation rain forecast for Argentina, Brazil and Paraguay will bring relief to fields that have suffered in dry, hot weather. Corn is used to make U.S. ethanol, which yesterday rose the most in more than a week.

“It’s giving back yesterday’s gains,” said Matt Janney, a trader at Citigroup Global Markets Inc. in Chicago. “Corn was down and that put pressure on it.”

Denatured ethanol for February delivery declined 3 cents, or 1.4 percent, to $2.112 a gallon on the Chicago Board of Trade, the lowest price since Dec. 19. Futures have slipped 10 percent in the past year.

In cash market trading, ethanol in Chicago slumped 4 cents, or 1.9 percent, to $2.10 a gallon and in New York the biofuel decreased 2 cents, or 0.9 percent, to $2.21, according to data compiled by Bloomberg.

Ethanol on the West Coast lost 2 cents, or 0.9 percent, to $2.215 a gallon and in the U.S. Gulf the biofuel slipped 0.5 cent to $2.22.

Corn for March delivery dropped 10.5 cents, or 1.7 percent, to $5.935 a bushel in Chicago. One such unit makes at least 2.75 gallons of ethanol.