USDA reported 87.9 million acres (35.1 million hectares) sown with corn this spring, nearly one million acres below its official prediction in March.

The area exceeds the amount planted last year but threw doubt on prospects for a supply glut.

As the world's leading corn producer, changes to the US outlook have repercussions for global food prices. The US grows more than half the world's exports of corn, used in products from animal feed to motor fuel.

USDA also said in the quarter ended in May that corn stockpiled in silos and elevators shrank at a rate 26% faster than the same period last year. This left stores surprisingly low and reflected growing consumption, a trend partly fuelled by rising US ethanol output.

"Now we don't have as much corn on hand. We don't have as many acres. We still have strong demand. It's significantly bullish for the market if these numbers can be believed," said Darin Newsom of Telvent DTN, a commodity information service.

Corn for December delivery, which reflects the current crop, instantly rose by a daily trading limit of 30 cents as the CBOT market opened, and settled at US$3.73½ a bushel, up 29½ cents or 8.6%. July corn rose 10.2% to US$3.58 at the open, settling up 9%.

The grain has hurt commodity investors this year, falling by 21% through Tuesday (June 29) as a balmy, dry April gave farmers a head start. But the USDA said conditions worsened by May as temperatures dropped and rain soaked much of the Corn Belt, "hampering the planting of the remaining acreage and threatening emerged plants."

Traders now question whether this year's harvest will top the 2009 record. With less farmland planted than expected, each stalk would have to yield more corn.

"We're working with less acres than we thought," said Don Roose of advisers US Commodities.

"Our cushion has shrunk a little bit. Now we will start to focus on what yields are," he said.

In June, USDA predicted corn yields at 163.5 bushels an acre, down slightly from last year's record level.

Soy rose 0.1% to US$9.48½ a bushel, after USDA reported a record 78.9 million acres planted with the oilseed. The rain that slowed corn aided beans, which can be sown later.

"Corn's loss was partly soy's gain," said Lewis Hagedorn, an analyst at JPMorgan.