Chicago Board of Trade corn and soybean futures closed at their lowest in roughly a month Friday, pushed down by seasonal pressure from the start of the U.S. harvest, traders said.
Wheat followed the weaker trend on profit-taking after Thursday's rally and news of favourable planting weather in the U.S. Plains.
At the CBOT, December corn settled down 8-1/2 cents at $4.51 per bushel, its lowest close since Aug. 13, but held above psychological support at $4.50.
November soybeans ended down 24-1/4 cents at $13.15-1/4 a bushel, the lowest since Aug. 22, and December wheat fell 10-3/4 cents to $6.46-1/4 a bushel.
For the week, the benchmark November soybean contract fell 66-1/2 cents a bushel, or 4.8 percent, its first drop in seven weeks and biggest since May 2012.
"It's harvest time and we've got seasonal pressure. Also, we did get the finishing rains for some of the late crop. The market really stopped its rally on soybeans when the temperatures cooled and the rains came," said Don Roose, president of U.S. Commodities in West Des Moines, Iowa.
Additional pressure came from funds liquidating net long positions in CBOT soybeans.
During the session, Informa Economics, a closely watched research firm, raised its estimate of U.S. 2013 soybean production to 3.224 billion bushels, above the USDA's current forecast for 3.149 billion.
Informa also projected that U.S. soybean plantings for 2014 would reach 83.6 million acres, topping the 2009 record of 77.5 million acres by 8 percent.
BUMPER CORN HARVEST
December corn posted its third straight weekly decline, falling 1.7 percent as the market reacted to the prospect of a huge U.S. crop. The U.S. Department of Agriculture last week forecast corn output at a record 13.8 billion bushels, and a weekly crop report showed the harvest was 4 percent complete by Sunday.
Early yield reports were promising, and cash values have been falling in the interior Midwest.
"Harvest results for corn are coming in much better than expected," said Rich Nelson, an analyst with Allendale Inc in McHenry, Illinois.
Informa raised its estimate for the 2013 corn crop to 13.889 billion bushels, above the USDA's forecast of 13.843 billion.
However, the firm forecast that U.S. corn plantings would drop to 92.7 million acres in 2014, down about 5 percent from the USDA's current figure for 2013.
WHEAT UP IN WEEK
CBOT wheat fell on technical selling and profit-taking, giving back all of the previous day's 1.6 percent gain. But the December contract ended up 0.7 percent for the week, buoyed by better-than-expected export demand for U.S. wheat. The USDA said on Thursday that exporters shipped more last week to global buyers than in any week in at least 23 years, with most of the grain headed for China and Brazil.
The setback in corn and soybeans pressured values on Friday, along with improving prospects for the 2014 U.S. winter wheat crop, which was 12 percent seeded as of Sept. 15.
"We got some good rainfall this week for the Plains, Oklahoma and Texas. It's great wheat planting weather," Nelson said.
(By Julie Ingwersen; Additional reporting by Nigel Hunt in London and Colin Packham in Sydney. Editing by William Hardy, Andrew Hay and Andre Grenon)