Corn stocks as of September 1 were forecast at 2.035 billion bushels, USDA said in its monthly supply and demand report, up from its March forecast of 1.835 billion bushels.
Soybean stocks were pegged at 895 million bushels compared with the March estimate of 900 million bushels. Wheat ending stocks were seen at 1.087 billion bushels, up from the government's March forecast of 1.055 billion bushels.
Analysts had been expecting corn stocks of 1.991 billion bushels, according to the average of analysts' estimates in a Reuters poll. The average of estimates was 898 million bushels for soybean stocks and 1.072 billion bushels for wheat stocks.
Chicago Board of Trade corn futures, which had been trading about 1 percent lower before the report was released, briefly dropped to their lowest since Nov. 26 but then turned higher as traders digested the government data.