GRAINS-Soybeans, corn fall on US harvest pressure

By Julie Ingwersen

CHICAGO, Sept 29 (Reuters) - U.S. corn and soybean futures were lower on Monday in choppy trade, pressured by the expanding Midwest harvest of both crops, but uncertainty about quarterly government stocks data due on Tuesday kept a floor under prices, analysts said.

Wheat futures bounced from early declines and were posting modest gains.

As of 1:05 p.m. CDT (1805 GMT), Chicago Board of Trade December corn futures CZ25 were down 1 cent at $4.21 per bushel. November soybeans SX25 were down 4 cents at $10.09-3/4 per bushel while December wheat WZ25 was up 1/4 cent at $5.20 a bushel.

Warm and dry weather over the U.S. Corn Belt promoted harvest progress over the weekend, and forecasts called for more of the same this week. The U.S. Department of Agriculture has projected a record-large U.S. corn crop.

Ahead of the USDA's weekly crop progress report due later on Monday, analysts surveyed by Reuters on average expected the agency to report the U.S. corn harvest as 20% complete and soybeans as 19% harvested.

However, anecdotal reports of disappointing corn yields in some areas have underpinned futures as market players await the USDA's next monthly crop production estimates on October 9.

"So far, the yields on corn in some of the Corn Belt are not as big as the guys thought," said Don Roose, president of Iowa-based U.S. Commodities. "There is not much new selling by the producer. They are concentrating on harvest, not selling," Roose said.

Brokers were also hesitant to take on large new positions ahead of the USDA's annual small grains and quarterly grain stocks reports , both due on Tuesday.

"The trade expects very modest reductions in wheat production in the small grains summary report tomorrow, but the stocks reports are where USDA is known for its surprises. That has traders remaining cautious today," StoneX chief commodities economist Arlan Suderman wrote in a client note.

Meanwhile, U.S. soybean exporters are missing out on business to China amid a trade war between the two nations, with rival suppliers from Latin America taking their share.

After Buenos Aires briefly suspended grain export taxes last week, around 40 Argentine soybean cargoes were registered for export in November and December, mostly headed to China, two traders told Reuters. These purchases are directly eating into the prime U.S. marketing season.