* U.S. to issue monthly supply/demand data on Sept. 12
* Analysts expect cuts to U.S. corn, soy output
* U.S. corn harvesting starts early in some areas
CHICAGO, Sept 7 (Reuters) - U.S. soybean futures fell on Thursday as grain traders waited for farmers to harvest their fields and the government to release updated crop estimates next week.
Most-active soybean futures ended down 16-3/4 cents at $13.59-1/2 a bushel at the Chicago Board of Trade. CBOT wheat futures declined 9-1/4 cents to $5.99-3/4 a bushel, while corn edged up 1/2-cent to $4.86-1/4 a bushel.
The U.S. Department of Agriculture is slated to issue monthly crop data on Sept. 12, giving traders the latest outlook for production after recent heat and dryness lowered U.S. corn and soy condition ratings.
Analysts on average expect the USDA to trim U.S. production and yield estimates for both crops from August in the World Agricultural Supply and Demand Estimates (WASDE) report.
Farmers started harvesting corn sooner than normal in parts of the western U.S. Midwest following the hot, dry weather. The crop's rapid finish may threaten grain quality or yields.
"Harvest is coming on fast, which should be a warning," said Jim Gerlach, president of A/C Trading. "When your crop dies down instead of dries down, it costs you some yield."
"I think you're in a hurry up and wait, consolidation mode," Gerlach said. "I doubt you're going to move either side of that, at least until we get that September crop report or we move deeper into harvest."
Chinese demand for Brazilian soybeans is a negative for the U.S. market, brokerage CHS Hedging said. Brazil and the U.S. compete for export sales to global buyers, and China is the world's top soybean importer.
"China bought a significant amount of Oct/Nov Brazilian beans this week," CHS Hedging said. "Do they have much left in the tank to buy U.S. fall beans?"
In other news, Russia carried out another drone attack on Ukrainian port facilities on the Danube River, damaging grain silos on a vital export route for Kyiv, Ukraine said. Both countries are major crop exporters. (Reporting by Tom Polansek in Chicago; Additional reporting by Gus Trompiz in Paris and Naveen Thukral in Singapore; Editing by Rashmi Aich, Emelia Sithole-Matarise, Paul Simao and Cynthia Osterman)
