(Adds analyst quote, price reaction, spring wheat)
WASHINGTON, Sept 29 (Reuters) - U.S. soybean and corn stockpiles ballooned in the marketing year ended Sept. 1 as the massive supply base outstripped robust demand, the U.S. government said on Friday.
In its quarterly stocks report, the U.S. Agriculture Department reported that domestic corn ending stocks rose 32 percent from a year earlier to their highest level since 1988. Soybean ending stocks, which were 53 percent higher than Sept. 1, 2016, came in at a 10-year high.
Usage of corn and soybeans during June, July and August was the second highest ever for the period, coming in just below record levels set during the summer of 2016. USDA said corn disappearance totaled 2.93 billion bushels and soybean disappearance totaled 665 million bushels.
Chicago Board of Trade corn and soybean turned higher after the report was released.
The corn and soybean stocks estimates came in below the low end of market forecasts in a Reuters survey. Analysts estimates ranged from 2.310 billion bushels to 2.450 billion bushels for corn stocks and from 321 million bushels to 363 million bushels for soybean stocks.
The lower-than-expected soybean stocks figure stemmed in part from a downward revision in USDA's estimate of the 2016 soy harvest. USDA lowered its assessment of the crop by 10.6 million bushels, to 4.296 billion, trimming yield by 0.1 bushels per acre and harvested acres by 40,000.
Wheat stocks as of Sept. 1 dropped to 2.253 billion bushels from 2.545 billion bushels as U.S. production dropped off amid rising global competition.
USDA pegged the 2017/18 wheat crop at 1.741 billion bushels, the smallest in 15 years. The wheat harvest forecast came in within the range of analysts' estimates but above the average, which was 1.718 billion bushels.
Wheat futures fell on the harvest outlook, with MGEX spring wheat leading the way down as the government's estimate of a crop of 416 million bushels of spring wheat other than durum came in near the high end of market outlooks and above the USDA's May forecast.
"We had expected a cut in spring wheat harvested acres," said Jack Scoville, analyst at the Price Futures Group. "Instead we got an increase. They managed to find some spring wheat out there somewhere. That was the most impressive thing."
MGEX spring wheat for December delivery dropped 2.2 percent and hit its lowest since Sept. 20.
(Additional reporting by Michael Hirtzer in Chicago; Editing by Paul Simao and Tom Brown)