Renewing concerns about whether there will be enough corn to support the demand for both fuel and food, a new study has found that ethanol plants could use as much as half of America’s corn crop next year. Dozens of new ethanol plants are being built by farmers and investors in a furious gold rush, spurred by a call last year from the Bush administration and politicians from farm states to produce more renewable fuels to curb America’s reliance on oil. But the new study by the Earth Policy Institute, an environmental group, found that the number of ethanol plants coming on line has been underreported by more than 25 percent by both the Agriculture Department and the Renewable Fuels Association, the ethanol industry’s main lobbying group.