Soybeans slid to five-month lows on good crop weather, with losses in old-crop contracts outpacing those in new-crop months as traders unwound bull spreads.
Chicago Board of Trade wheat slumped more than 3 percent to the lowest since 2010, pressured by adequate global supplies and an accelerating harvest.
Fund long liquidations dragged all corn contracts to fresh contract lows as trading resumed after the long U.S. Independence Day holiday weekend.
"Corn is lower as weather conditions continue to be superb over the vast preponderance of the growing area," said Sterling Smith, analyst at Citigroup.
Plentiful soil moisture and warm weather throughout much of the Midwest posed little threat to the developing corn and
soybean crops. Corn is entering its critical pollination stage of development this month.
"We will probably be looking at weather that will be ideal for pollination," Smith said.
Weekly crop condition ratings were expected to hold steady with the U.S. corn crop in the best shape in 15 years and soybeans seen at their best in 20 years, analysts said on Monday.
CBOT July corn closed down 7-3/4 cents, or 1.9 percent, at $4.09-1/4 per bushel after earlier hitting a low of $4.05-1/2, the lowest for a spot contract since August 2010. Actively traded December corn fell 9 cents, or 2.2 percent, to $4.06-1/4 a bushel.
CBOT July soybeans dropped 24-3/4 cents, or 1.8 percent, to $13.63 a bushel, while new-crop November futures shed 8 cents, or 0.7 percent, to $11.25-1/2 per bushel.
News that Chinese importers bought 347,000 tonnes of U.S. soybeans for 2014/15 marketing year delivery limited losses in new-crop contracts.
Wheat prices slumped despite worries about new-crop quality following adverse weather this spring. Global stocks are ample and most major producing countries are currently harvesting large crops.
Germany's DBV farmers' association estimated on Monday the 2014 winter wheat crop would reach almost 25 million tonnes,
versus 24.6 million in 2013.
September soft red winter wheat futures fell to a contract low of $5.56 a bushel and closed 22-3/4 cents lower at $5.56-3/4. The day's 3.9 percent drop was the steepest since March 2013.
Commodity funds sold an estimated net 11,000 corn contracts, 3,000 soybean contracts and 9,000 wheat contracts trade sources said.
