GRAINS-Soybeans sink on South American rains, slowing demand

* Rains in Brazil curb soybean and corn prices

* Wheat pressured by bumper crops in Australia, Canada

* Market awaiting direction from USDA world crop report (Rewrites throughout with U.S. market open, adds quote, updates prices, changes byline, changes dateline from PARIS/SINGAPORE)

By Karl Plume

CHICAGO, Dec 8 (Reuters) - U.S. soybean futures fell for a third straight session on Tuesday on slowing export demand and as forecasts for more rain in South American crop areas tempered concerns about tightening supplies.

Corn and wheat followed soybeans lower, pressured by improved South American crop prospects and a firmer U.S. dollar.

Grain prices have scaled to multi-month or multi-year highs in recent weeks on strong demand, notably from China, and tightening supplies. Dry weather, meanwhile, had threatened Brazilian and Argentine crops that are due to begin replenishing global supplies in the coming months.

"The market had a job to do, and that was ration supplies. For the moment, the perception is that we've accomplished that job," said Ted Seifried, chief ag market strategist at Zaner Group.

"We will continue to be somewhat weak unless there is a dramatic change in South American weather back to a drier trend," he said.

Widespread showers in recent days are expected to benefit Brazilian crops, while rain in Argentina has aided planting.

More rain is expected in central and southern Brazil and northern Argentina in the six- to 10-day window, and in the 11- to 15-day period in center-west and northeast Brazil, according to Commodity Weather Group.

Chicago Board of Trade January soybeans were down 10-1/2 cents at $11.48 a bushel at 11:33 a.m. CST (1733 GMT). March corn fell 4-3/4 cents to $4.19-1/4 a bushel and CBOT March wheat dropped 7 cents to $5.70-1/2 a bushel.

Traders are turning their attention to the monthly U.S. Department of Agriculture (USDA) world supply and demand report on Thursday, which is expected to show smaller South American crops and tighter corn and soybean supplies.

Wheat futures remain capped by forecasts of bumper production in Australia and Canada, which have tempered talk of tightening global availability after a recent wave of importer purchases. (Additional reporting by Gus Trompiz in Paris and Naveen Thukral in Singapore Editing by David Goodman and Mark Potter)

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