* USDA set to issue grain stocks, plantings data
* US soybean seedings estimate expected to rise
* Traders may refocus on U.S. weather after data release (Adds U.S. trading, changes byline, dateline from PARIS/SYDNEY)
By Tom Polansek
CHICAGO, June 30 (Reuters) - U.S. grain and soybean futures dipped on Thursday ahead of the release of closely watched U.S. crop data that are expected to show a buildup of inventories and an increase in the government's estimate for domestic soy plantings.
The U.S. Department of Agriculture, in a quarterly report due at 11 a.m. CDT (1600 GMT), will likely reveal that corn stocks as of June 1 had reached 4.528 billion bushels, according to a Reuters poll of analysts. That would represent the largest inventories on that date since 1988 and the fifth-highest in records dating to the 1920s.
For soybeans, inventories probably were 829 million bushels, which would be the highest June 1 figure since 2007, analysts said in the poll. Soy plantings are expected to rise to 83.834 million acres from the USDA's March estimate of 82.236 million.
Larger plantings could be bearish for soybean futures, said Kevin Van Trump, president of Missouri-based consultancy Farm Direction, because it would indicate the potential for a bigger harvest.
"Soybean acres here in the U.S. will take center stage today!" Van Trump told clients in a note.
November soybean futures, which represent the crop that was planted this spring for harvest in the autumn, were down 7-1/2 cents at $11.05 a bushel by 10:15 a.m. CDT.
December corn futures lost 3-1/2 cents to $3.79-1/2 a bushel, and December wheat slipped 1 cent to $4.64-1/4 a bushel.
"Trade is expecting to see higher volumes in storage than a year ago across the board," said Karl Setzer, risk management team leader for MaxYield Cooperative in Iowa.
For wheat, stocks on June 1 likely were 982 million bushels, which would be the most since 1988, analysts said.
Once the USDA releases its stocks and plantings data, some traders may shrug off the numbers to focus on U.S. weather that can determine the size of the upcoming harvests.
Corn and soybean prices have been swayed this month by weather forecasts, against a backdrop of the risk that a La Nina weather pattern could trigger a dry summer in the U.S. Midwest. (Additional reporting by Colin Packham in Sydney and Gus Trompiz in Paris; Editing by William Hardy and Jonathan Oatis)