The U.S. corn surplus estimate is up 5 percent on slowing global demand, with current U.S. Department of Agriculture numbers at 632 million bushels on August 31, according to the latest reports. The inventory increase, up from January's 602 million bushel estimate, is still significantly less than 2012's 989 million bushels.

The average estimate of 31 analysts in a Bloomberg survey was 616 million bushels. Corn futures have dropped 16 percent since reaching a record in August 2012 as global supplies improved and high prices slowed demand for U.S. exports and ethanol. 

U.S. exporters will sell 900 million bushels of corn in the year that ends August 31, down from 950 million forecast in January and the lowest since 1972, according to reports. In 2012, exporters shipped 1.543 billion bushels. Sales since September 1 are 52 percent behind a year earlier, say USDA data.

An estimated 4.5 billion bushels of corn will be used to produce ethanol, unchanged from the January forecast and less than 5.011 billion in 2012, according to the USDA. Feed demand was forecast at 4.45 billion bushels, also unchanged from January and down from 4.548 billion bushels in 2012.

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World output in the crop year that began October 1, 2012, will be 854.38 million tons, up from 852.3 million tons forecast in January and down from 882.47 million in 2012. Global consumption is forecast to fall to 867.34 million tons from 879.39 million tons in 2012.

Worldwide inventories at the end of the marketing year will be 118.04 million tons, up from 115.99 million predicted in January and down from 131.01 million tons in 2012, according to the USDA.