U.S. corn and soybeans tumbled as much as 2 percent on Monday and were headed for their largest losses in a month after dry conditions over the weekend allowed farmers to catch up on harvesting big U.S. crops.
Wheat eased for a third straight session at the Chicago Board of Trade, pressured by declines in corn and showers in the U.S. Plains that improved soil moisture for winter wheat planting.
Corn futures fell to a three-year low while soybeans eased to their lowest point in about two weeks.
"We're under harvest pressure as much as anything," said Don Roose, analyst at U.S. Commodities in West Des Moines, Iowa. "We're going to switch from the soybean harvest to focus on corn harvest."
A Reuters poll of analysts pegged the U.S soybean harvest at 78 percent complete and the corn harvest at 52 percent done as of Sunday, up from 63 and 39 percent respectively a week ago. The U.S. Agriculture Department was to update harvest progress in a report due at 3 p.m. CDT (2000 GMT).
Farmers hit the fields hard over the weekend, harvesting the crops amid forecasts for widespread rains this week that will delay harvest activity.
The government next week will release its supply and demand forecast for November. USDA's October report was scrapped due to the partial shutdown of the U.S. government - the first such cancellation in 147 years.
Anecdotal yield results have been largely better than expected and USDA could lift estimates for the crops in next week's report. The corn crop already is estimated to be the record-large and the soy crop the fourth largest in history.
"We're looking ahead to the November crop report," said Brian Basting, commodity research analyst at Advance Trading. "Most people are looking for significant increases in both corn and soybean yields."
Corn and soybean futures each were on pace for their largest daily declines since Sept. 30, when the last major USDA report was released.
CBOT corn for December delivery was down 9-1/4 cents, or 2.1 percent, at $4.30-3/4 per bushel. Prices earlier fell to $4.30-1/4, the lowest level since September 2010.
CBOT November soybeans were 18 cents lower at $12.74-1/2 as of 12:47 p.m. CDT (1747 GMT).
Analysts said they have seen evidence that lower prices have boosted demand, which they called an encouraging sign.
USDA on Monday said 83.6 million bushels of U.S. soybeans were inspected for export in the latest reporting week - nearly double the low end of analyst estimates. Corn export inspections totalled 26.5 million bushels, slightly below expectations.
"The (soybean) export inspection was just a stellar number," Basting said. "We're seeing really good demand."
U.S. ethanol makers have strong profit margins and last week the government pegged the U.S. ethanol grind as the largest since June 2012.
Wheat on worst streak since July
CBOT December wheat was down 7 cents at $6.83-3/4. Futures were on pace for their third consecutive session of lower prices and the worst streak since July.
"Planting across the Northern Hemisphere is looking a bit better than it was 10 days ago, so there's not so much weather premium into the new crop and for the old crop we've still got plenty of wheat around," said James Dunsterville, head analyst with Geneva-based AgriNews.
"Futures coming off may be a little bit of profit-taking because the winter planting is a little bit better, there's rain in Argentina. The Russians and Ukrainians have caught up a bit with their winter wheat sowing."