GRAINS-Corn, soybeans rise on US yield uncertainty, firming cash markets

By Karl Plume

CHICAGO, Oct 17 (Reuters) - U.S. corn futures extended gains into a fourth session on Friday and were set for the first weekly rise in a month as slow farmer sales of newly harvested grain and reports of lower-than-expected harvest yields supported the market.

Soybeans rose for a third day as domestic demand remained brisk and as trade negotiations between the U.S. and top soy buyer China appeared to be back on track following moves by both sides that escalated trade tensions last week.

Wheat firmed and was poised for its first weekly advance in five weeks, supported by short covering and bargain buying following the market's recent drop to five-year lows.

Trading remained cautious as the U.S. government shutdown deprived the market of key data including corn and soybean harvesting progress and updated yield estimates.

U.S. farmers are widely expected to reap a record corn crop and a bumper soy crop, but reports of crop shortfalls in spots have cast doubt about the government's most recent estimates.

Sluggish farmer sales of newly harvested corn and soybeans also underpinned futures.

"For corn at these price levels, many people are underwater. They can't afford to sell it because they'd have a loss," said Don Roose, president of U.S. Commodities. "The farmer's not selling soybeans either."

Soybeans were also lifted by strong demand from U.S. processors and historically high crush rates have eased some concerns about a lack of demand for exports to China.

Global grain merchant ADM ADM.N is offering incentives for U.S. farmers to deliver soybeans to a major processing facility this month, sources said.

Chicago Board of Trade December corn CZ25 was up 2-1/4 cents at $4.24 a bushel by 12:15 p.m. CDT (1715 GMT) and November soybeans SX25 rose 8-1/2 cents to $10.19-1/4 a bushel. CBOT December wheat WZ25 added 1-1/2 cents to $5.04 a bushel.