Corn futures rose to a four-month high and soy gained the most in a week on speculation that unfavourable weather will affect grain production in the US.

Four inches of rain (10 centimetres) fell in parts of the Midwest over the past three days, causing floods that damaged some crops, according to World Weather Inc. From southern Missouri to North Carolina, rain is needed to ease stress on plants that have had less than 25% of normal precipitation the past two weeks, according to the forecaster.

"The weather has not been conducive for optimal yield potential," said Chad Henderson, a market analyst at Prime Agricultural Consultants Inc. in Brookfield, Wisconsin. "People are starting to realise that the rains are reducing yields, and dry weather across the southern US may also cut yields."

Corn futures for December delivery rose 1.25 cents, or 0.3%, to US$3.8575 a bushel on July 6 at 10:25 a.m. on the Chicago Board of Trade, after touching US$3.915, the highest level since March 1. The most-active futures rose 6.7% last week, the third gain in four weeks, after the government said US farmers planted less then they intended this year.

Soy futures for November delivery rose four cents, or 0.4%, to US$9.0975 a bushel. A close at that price would be the largest gain for the most-active futures since June 28.