St. Louis - Rising demand for corn from ethanol producers is pushing U.S. reserves to the lowest point in 15 years, a trend that could lead to higher grain and food prices this year. The Agriculture Department on Friday left its estimate for corn reserves unchanged from the previous month. The reserves are projected to fall to 675 million bushels in late August, when the harvest begins, or roughly 5 percent of all corn consumed in the United States. That would be the lowest surplus level since 1996.The limited supply is chiefly because of increasing demand from ethanol makers, which