* Soybeans dented by record crop forecast and steady rating

* Corn prices fall after bounce, wheat down for second day

CHICAGO, Aug 13 (Reuters) - Chicago soybean futures lost more ground on Tuesday, setting another four-year low a day after U.S. Department of Agriculture data reinforced the prospect of a bumper U.S. harvest.

Corn edged lower, giving up some of Monday's gains as a better than expected rating of U.S. crops tempered support from a surprise downward revision to projected U.S. corn stocks.

Wheat also eased against a backdrop of competition from cheaper Black Sea supplies and after a modest purchase by Egypt in a huge import tender.

The most active soybean contract on the Chicago Board of Trade (CBOT) was down 21-1/2 cents at $9.64-3/4 a bushel by 12 p.m. CDT (1700 GMT), the lowest since September 2020.

CBOT corn gave up 3-3/4 cents to $3.97-3/4 a bushel, and CBOT wheat was down 5-3/4 cents at $5.31 a bushel.

The USDA raised its estimates for corn and soybean production on Monday, putting its soybean crop projection at a record level.

With "non-threatening" weather predicted for the next few weeks, "there's no threat to that yield forecast and to that production estimate for the moment," said Mark Soderberg, senior agricultural market analyst at ADM Investor Services.

"We're trying to find a price level that stirs up better demand and probably more importantly, starts to discourage South American growers from planting more beans than they did last year."

He added that speculators were likely sitting on a record short position in soybeans.

But Soderberg noted a higher trend with Minneapolis wheat, with production there having been forecast below expectations.

A separate USDA report on Monday left weekly ratings for corn and soybean crop conditions unchanged, contrary to market expectations of a slight decline.

The USDA reported some corn sold to Mexico on Tuesday morning, but not enough to impress the market, said Soderberg.

Egypt's state grains buyer said it bought 280,000 metric tons of wheat in a tender on Monday, falling well short of its target of 3.8 million tons.

(Reporting by Renee Hickman in Chicago, Additional reporting by Gus Trompiz in Paris and Naveen Thukral in Singapore; Editing by David Goodman and Emelia Sithole-Matarise)