GRAINS-Corn down nearly 3 pct after USDA raises U.S. yield estimate

* USDA raises U.S. corn yield estimate to 169.9 bpa

* USDA raises soybean yield to 49.9 bpa, topping expectations

* Wheat turns higher after early declines

(Recasts throughout; updates prices, adds quotes, USDA data; changes byline, dateline, previous HAMBURG) CHICAGO, Sept 12 (Reuters) - Chicago corn futures fell nearly 3 percent on Tuesday, the biggest drop for a most-active corn contract in a month, after the U.S. Department of Agriculture raised its 2017 corn yield forecast, bucking most trade expectations for a reduction. Soybean futures also fell, pressured by a U.S. yield forecast that topped the highest in a range of trade expectations. Wheat futures firmed, rallying from early declines, after the USDA tightened its forecast of global wheat ending stocks for the 2017/18 marketing year. As of 12:50 p.m. CDT (1750 GMT), Chicago Board of Trade December corn was down 9-1/2 cents at $3.48 per bushel after dipping to $3.45-1/2, its lowest since Aug. 31. CBOT November soybeans were down 14-3/4 cents at $9.45-1/4 a bushel while December wheat was up 2-1/4 cents at $4.37 a bushel. Corn fell after the USDA in a monthly report raised its estimate of the average U.S. 2017 corn yield to 169.9 bushels per acre (bpa), from its August estimate of 169.5 bpa and above an average of analyst expectations for 168.2 bpa. "Two months in a row, the government throws a bearish surprise at the trade. There's nothing bullish about these numbers: yields are bigger than the trade thought," said Don Roose, president of Iowa-based U.S. Commodities. USDA also raised its soybean yield estimate to 49.9 bpa, from 49.4 in August, topping a range of trade expectations. "Soybeans got to enjoy some beneficial weather in the Corn Belt in August, and yield was increased as a result. This bump is not unexpected, but the 0.5 additional bushels per acre keeps the balance sheet's ending stocks at 475 million bushels, even with greater export demand," said Alex Norton, analyst at Beeson Inc. The increases came in spite of dry conditions in portions of the Midwest crop belt. "The U.S. yields show us just how resilient crops are these days to weather-related stress events. Technology has improved over the past five years," said Terry Reilly, senior commodity analyst with Futures International. CBOT wheat firmed on long wheat/short corn spreads and on the USDA's forecast for slightly smaller global inventories by the end of the 2017/18 marketing year. The USDA trimmed its world wheat ending stocks forecast to 263.14 million tonnes, from 264.69 million in August.

CBOT prices as of 12:56 p.m. CDT (1756 GMT):

Last Net Pct Volume

change change

CBOT wheat WZ7 437.00 2.25 0.5 64646 CBOT corn CZ7 348.25 -9.25 -2.6 220892 CBOT soybeans SX7 945.25 -14.75 -1.5 146625 CBOT soymeal SMZ7 298.70 -4.90 -1.6 56437 CBOT soyoil BOZ7 34.95 -0.20 -0.6 67605

NOTE: CBOT September wheat, December corn and November soybeans shown in cents per bushel, December soymeal in dollars per short ton and December soyoil in cents per lb.

(Additional reporting by Tom Polansek in Chicago, Michael Hogan in Hamburg and Naveen Thukral in Singapore; Editing by Alexander Smith and Lisa Shumaker)