The USDA's monthly supply/demand reports are due at 1330 GMT. Analysts expect USDA to raise its forecast of US corn ending stocks for 2012/13.

"Corn has fallen ahead of the USDA data and fundamentals suggest that the trade is expected to remain in the lower range in the near future," said Jonathan Barratt, chief executive of BarrattBulletin, Sydney-based commodity research firm.

Underscoring the concerns, USDA reported export inspections of US corn in the latest week at 7.861 million bushels, below a range of trade estimates for 9 million to 15 million.

Also, reports that a South Korean feedmaker issued an international tender for corn and feed wheat but excluded corn from the United States, the world's biggest supplier, put some downward pressure on the gain.

Some traders said the poor quality of US corn could be the reason behind the rejection rather than price.

Severe drought in the heart of the US Corn Belt this summer contributed to the spread of aflatoxin, the toxic byproduct of a mold that tends to spread in corn in drought years.

Chicago Board Of Trade March corn fell 0.2 percent to $7.28-1/4 a bushel. Corn hit a three-week low in the previous session.

CBOT March wheat lost 0.2 percent to $8.46-1/2 a bushel. On Monday, the contract fell below chart support at $8.51-1/2, a level that had held for four straight sessions last week. The contract dipped to an intra-day low of $8.46-3/4, its lowest level in three weeks.

Wheat fell ahead of the USDA data but poor weather conditions in the United States would underpin the contract, Barratt said.

The USDA is expected to slightly raise its forecast of 2012/13 ending wheat inventory.

The news that the US won only a small share of purchases by Saudi Arabia and Iraq also added to bearish sentiment.

CBOT January soybeans fell marginally to $14.74-1/2 a bushel.

Favourable crop weather in Brazil has boosted prospects for a record-large crop in that country, with harvest expected in early 2013.