GRAINS-U.S. corn, wheat, soy firm ahead of USDA monthly reports

* USDA August supply/demand reports due Thursday at 1600 GMT

* Analyst expect USDA to cut U.S. corn, soy yield

* USDA seen reducing U.S. spring wheat production estimate

(New throughout; updates prices, adds quotes, changes byline, dateline, previous PARIS/SYDNEY) CHICAGO, Aug 9 (Reuters) - U.S. wheat, corn and soybean futures were modestly higher on Wednesday as traders awaited key supply/demand reports due from the U.S. Department of Agriculture on Thursday. The reports will include the USDA's first assessments of the 2017 U.S. corn and soybean crops based on field surveys, and analysts expect the government to lower its yield forecasts for both crops. The USDA's current yield forecast for corn, based on historical trends, is 170.7 bushels per acre (bpa), while the average estimate among analysts surveyed by Reuters was 166.2 bpa. "Our crop ratings are well under a year ago. We started out in the eastern Corn Belt with too wet conditions. We've got droughty areas not just in the Dakotas, but from northwest Iowa into central Illinois," said Don Roose, president of U.S. Commodities. "I guess it all adds up that the government takes it (the corn yield estimate) down. But with the uncertainty, they might not take it as low as people think," Roose said. For soybeans, the average analyst estimate of the U.S. yield was 47.5 bpa, compared with the USDA's trend-based forecast of 48.0 bpa. As of 12:37 p.m. CDT (1737 GMT), Chicago Board of Trade December corn was up 1-1/4 cents at $3.85 per bushel and September wheat was up 2-1/2 cents at $4.59-1/2 a bushel. CBOT November soybeans were up 2-1/4 cents at $9.75-1/2 a bushel. Soybean futures were briefly pressured after the USDA said private exporters canceled a sale of 130,000 tonnes of U.S. old-crop soybeans to unknown destinations. CBOT wheat drew support from strength in Minneapolis Grain Exchange spring wheat futures, which firmed on continued worries about drought curbing spring wheat yields in the northern U.S. Plains and Canada. MGEX September spring wheat was up 8 cents at $7.39-1/4 a bushel after reaching $7.40, its highest since July 31. Analysts expect the USDA on Thursday to lower its forecast of U.S. all-wheat production, with the biggest declines seen for high-protein spring wheat. Nonetheless, large crops in Russia and Ukraine along with a rebound in French production kept the focus on ample global supplies overall.

CBOT prices as of 12:36 p.m. CDT (1736 GMT):

Net Pct Volume

Last change change

CBOT wheat WU7 459.50 2.50 0.6 53096 CBOT corn CZ7 384.75 1.00 0.3 99304 CBOT soybeans SX7 975.50 2.25 0.2 81685 CBOT soymeal SMZ7 314.30 0.00 0.0 31370 CBOT soyoil BOZ7 34.46 0.24 0.7 36797

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 Colin Packham in Sydney and Gus Trompiz in Paris; Editing by Susan Thomas and James Dalgleish)