Corn fell to a one-week low and soy halted a four-session rally on speculation that demand increases from China may not be as big as expected, after the country signaled it will end the yuan's peg to the dollar.

Dollar-priced commodities may be cheaper for China as its currency strengthens. Corn jumped as much as 1.7% and soy rose 0.9% after the central bank said June 19 that it will end the dollar peg. China also may have enough supply for now, after buying 360,000 tonnes of corn in the past two weeks.

Corn futures for December delivery fell 2.75 cents, or 0.7%, to US$3.72 a bushel on the Chicago Board of Trade, after touching US$3.68, the lowest level since June 11. The price has dropped 10% this year, partly on sagging demand.

Soy futures for November delivery slipped 3 cents, or 0.3%, to US$9.36 a bushel in Chicago, the first decline since June 15. Futures gained 2.3% last week as wet weather delayed seeding in the US.

China's yuan announcement was a symbolic concession before the Group of 20 summit in Canada, which starts Wednesday (Jun 23), analysts said.

Lower energy prices also may have hurt corn and soy, which are used to make biofuels, Bayer said. Crude oil fell as much as 1.7% after a New Orleans judge lifted a six-month moratorium on deepwater drilling imposed by President Barack Obama. Gasoline futures dropped as much as 1% in New York.