* Wheat futures weighed down by large Russian supplies

* Market eyes US monthly supply-demand report for price direction

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(Adds latest prices and analyst comments, adds bullet, changes headline, changes byline and dateline from HAMBURG)

CHICAGO, Sept 11 (Reuters) - Chicago corn futures fell and wheat futures slid to the lowest level since May on Monday as traders scrambled to adjust their positions ahead of a key government crop report.

Corn futures eased ahead of Tuesday's monthly U.S. Department of Agriculture World Agricultural Supply and Demand Estimates (WASDE), with traders expecting cuts to forecasts for the U.S. corn harvest.

But uncertainty over the actual crop size weighed on the corn market, said Karl Setzer, brokerage research lead for Mid-Co Commodities.

"Even if the report comes out with a smaller yield on corn, the general feeling in the market is that USDA may have underestimated the number of acres planted," Setzer said. "So you could see a smaller yield number but end up with a larger crop if more acres were actually planted."

In the wheat market, robust global supplies and sluggish demand for pricier U.S. crops sent futures lower for their third straight session.

The Chicago Board of Trade's most active wheat contract was down 11-1/2 cents to $5.84-1/4 a bushel at 1558 GMT after touching a new contract low of $5.80-1/4 a bushel in midday trading - the lowest since May 31.

Much of the day's market pressure was focused on heavy competition from Russia, traders said. Russian wheat prices are low and trade below the unofficial minimum export prices being imposed by the Russian government, one trader said, adding that a quick resumption for a safe Ukraine shipping channel is not expected.

Soybean futures firmed on bargain buying and market expectations that recent heat waves across the U.S. Midwest and drought conditions may have a bigger-than-expected hit on crop yields this fall, traders said.

Soybeans climbed 6-1/2 cents at $13.69-1/2 a bushel while corn eased 1/4-cent at $4.83-1/2 a bushel. (Additional reporting by Zachary Goelman in New York City, Michael Hogan in Hamburg, Peter Hobson and Naveen Thukral in Singapore; Editing by David Goodman and Mark Porter)