December corn eased 5-3/4 cents to $3.70-1/2 per bushel at 10:11 a.m. CDT (1511 GMT), near Monday's 10-month low of $3.70. CBOT November soybeans were off 9-3/4 cents to $9.28-1/2 per bushel, on pace to close at the lowest levels since June 30.
The US Department of Agriculture lifted condition ratings for the US corn crop late on Monday and slightly reduced ratings for soybeans. Analysts surveyed by Reuters anticipated unchanged ratings for both.
Rains were falling in parts of Nebraska and Iowa and the storm system was forecast to move east, likely increasing yield potential for corn and soybeans and further weighing on prices.
"Weather forecasters have a fairly corn-friendly outlook for the next week or so," said Tobin Gorey of the Commonwealth Bank of Australia. "And corn crop conditions for last week were a little better, according to the USDA's survey."
USDA surprised traders and sent crop prices tumbling last week when the agency predicted a record-large US soybean crop and the third-biggest corn harvest. Most analysts expected cuts in yield estimates due to relatively hot and dry weather during the growing season.
Wheat prices remained anchored by abundant global stocks and technical selling.
CBOT September wheat was down 9 cents to $4.32 per bushel, holding just above Monday's four-month low of $4.31. K.C. September hard red winter wheat fell 8-1/2 cents to $4.28-1/4 per bushel, the lowest since April, and MGEX September spring wheat eased 14-3/4 cents, or 2 percent, to $6.55-1/4.
The USDA in its weekly crop progress conditions report released after the close of trading on Monday said 40 percent of US spring wheat was harvested, up from the five-year average pace of 35 percent.
The National Oilseed Processors Association's monthly report was due at 11 a.m. CDT. Analysts were expecting a July crush of 143.004 million bushels, according to the average of five estimates in a Reuters survey. NOPA reported a crush of 143.715 million bushels for July 2016.