CBOT December wheat lost 0.13 percent to $5.98 per bushel. December corn slipped 0.22 percent to $5.79. Corn has declined for four straight days, its longest losing streak since November.
January soybeans were little changed at $11.27-1/4, after dropping 0.8 percent on Monday in the biggest daily decline in more than a week.
"General macro-economic sentiments are still influencing grain prices," said Lynette Tan, an analyst at Phillip Futures in Singapore.
She said the market was anticipating more supply of most grains, and especially corn and wheat, in the monthly forecasts due to be released by the US Department of Agriculture on Friday, Tan said.
Wheat prices have been driven lower by slowing demand growth amid the spread of the euro zone sovereign debt crisis, and cheaper exports from the Black Sea region.
An abundance of rain-soaked Australian wheat is set to further dampen US and South American corn prices in 2012, as key corn importers switch to the cheaper, damaged grain for animal feed.
JOB CUTS
Grain prices have been hit by volleys of negative news about the global economy which have raised concerns about demand.
Standard & Poor's on Monday warned it may carry out an unprecedented mass downgrade of euro zone countries if EU leaders fail to reach an agreement on how to solve the region's debt crisis in a summit later this week.
An Asian Development Bank report also said the Asian region now faces greater risks from the debt crisis, and a fragile US economy, than just three months ago.
In a sign of the times, Cargill Inc said it would let go of 2,000 of its employees globally, citing a continued weak global economy.
Argentina's government is expected to approve the export of an additional 2 million to 3 million tonnes of the 2010/11 corn on Wednesday, a senior agriculture official said, which would add to plentiful supplies of the grain.
US crude futures were weaker on Tuesday, hurt by a cloudy economic outlook in Europe after S&P's warning.