Oct. 23 (Bloomberg) -- Corn jumped to a four-month high, heading for the biggest weekly gain this year, as wet, cold weather prolongs harvest delays in the U.S., threatening to curb yields for the world’s largest grower. Soybeans also gained.

Three inches (7.6 centimeters) of rain fell in parts of the Midwest in the past two days and the region is facing cold, wet weather for the next two weeks, according to the National Weather Service. The pace of the corn and soybean harvests as of Oct. 18 was less than half the average for the previous five years, the U.S. Department of Agriculture said this week.

“It’s going to be very difficult to get back into the fields before November,” said Don Roose, the president of U.S. Commodities Inc in West Des Moines, Iowa. “Prices are rising on the fear of losing additional bushels to the delayed harvests.”

Corn futures for December delivery rose 4 cents, or 1 percent, to $4.075 a bushel at 10:22 a.m. on the Chicago Board of Trade. A close at that price would mark an 9.4 percent gain for the week, the steepest this year. The grain earlier touched $4.115, the highest for a most-active contract since June 22.

Soybean futures for January delivery rose 4 cents, or 0.4 percent, to $10.11 a bushel in Chicago, after earlier reaching $10.2925, the highest since Aug. 14. The contract is headed for its third straight weekly gain.

About 17 percent of the corn crop was collected by Oct. 18, compared with 28 percent a year earlier, USDA data show. The pace was behind the average of 46 percent for the same time of the season in the past five years.

The soybean harvest was 30 percent completed, compared with 23 percent a week earlier and 64 percent a year earlier, the USDA said. The five-year average was 72 percent.

Crop-Damage Risk

Harvest delays raise the risk of rain, wind and eventually snow damaging crops left too long in fields, Roose said. Before today, corn jumped 17 percent this month and soybeans gained 8.6 percent as speculators increased bets on crop losses, he said.

“I’m not sure we have lost as much crop as the markets have dialed into prices,” Roose said. “You have to recommend farmers make some sales on this rally.”

U.S. corn production will rise to 13.018 billion bushels, up 7.6 percent from last year and the second-biggest crop ever, as yields reach records, the USDA said on Oct. 9. Soybean output will jump to a record 3.25 billion bushels, 9.5 percent more than last year.

Corn is the biggest U.S. crop, valued at $47.4 billion in 2008, followed by soybeans at $27.4 billion, government data show. The U.S. is the world’s largest producer and exporter of both.

To contact the reporter on this story: Jeff Wilson in Chicago at jwilson29@bloomberg.net

Last Updated: October 23, 2009 11:24 EDT