GRAINS-Soy firm on export demand, uncertainty about U.S. yields

* Soybeans rise for three out of four sessions

* Corn lower as market awaits U.S. harvest news

* Wheat sags after one-month top; Australia, Argentina eyed

(Recasts throughout; updates prices, adds quotes, changes byline, dateline from previous PARIS/SINGAPORE) CHICAGO, Sept 18 (Reuters) - U.S. soybean futures firmed on Monday on export demand and uncertainty about yield prospects as the harvest begins in the heart of the Midwest crop belt, analysts said. Corn fell on expectations of continued, burdensome U.S. supplies, and wheat sagged on technical selling. As of 12:25 p.m. CDT (1725 GMT), CBOT November soybeans were up 1 cent at $9.69-3/4 per bushel. December corn was down 3-1/2 cents at $3.51-1/4 a bushel and December wheat was down 5-1/4 cents at $4.43-3/4 a bushel. Soybean futures climbed to session highs after the U.S. Department of Agriculture (USDA) through its daily reporting system said private exporters sold a total of 387,000 tonnes of U.S. soybeans, including 261,000 tonnes to China, the world's top soy buyer. "Export shipments through the first two weeks of the marketing year already exceed the seasonal pace needed to reach USDA's target by 45 million bushels. Said another way, we have to produce a big crop to keep up with this demand," INTL FCStone chief commodity economist Arlan Suderman said in a note to clients. Uncertainty about whether the U.S. soybean harvest will meet the USDA's forecast for a record crop totaling 4.3 billion bushels added support. "There is a growing sentiment that we might not be able to reach the pod weights that the USDA set up for us in their estimate last week," said Ted Seifried, chief market strategist with Zaner Ag Hedge. U.S. and global supplies of corn, on the other hand, should remain ample. The USDA last week forecast a U.S. corn yield of 169.9 bushes per acre and 2017-18 ending stocks at a hefty 2.3 billion bushels. That factor weighed on CBOT corn futures. "Even it that yield number does come in a little lower on corn, we are still looking at probably above a 2 billion-bushel carry-over," Seifried said. Analysts expected the USDA later on Monday to report the U.S. corn harvest as 10 percent complete and the soybean harvest 5 percent complete. CBOT wheat futures fell, reversing from early strength. The CBOT December contract turned lower on a lack of follow-through buying after reaching a one-month high at $4.50-3/4 a bushel. Wheat drew underlying support from reduced forecasts for Australia's harvest and excessive rains in Argentina, but big harvests in Black Sea exporters Russia and Ukraine continued to hang over the market.

CBOT prices as of 12:25 p.m. CDT (1725 GMT):

Last Net Pct Volume

change change

CBOT wheat WZ7 443.75 -5.25 -1.2 41268 CBOT corn CZ7 351.25 -3.50 -1.0 81893 CBOT soybeans SX7 969.25 0.50 0.1 72986 CBOT soymeal SMZ7 313.20 1.80 0.6 32373 CBOT soyoil BOZ7 34.51 -0.30 -0.9 43263

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 Naveen Thukral in Singapore and Gus Trompiz in Paris; editing by Adrian Croft and G Crosse)