About a third of the 2009 US corn crop became ethanol

The US Department of Agriculture reported corn production was 12 billion bushels for the 2008-09 market year (September through August).[[Baker, Allen, Edward Allen, and Heather Lutman. “Feed Outlook, Oct. 14, 2009,” (PDF) October 14, 2009.]] Over the same period, 4.2 billion bushels of corn were converted into ethanol, [[Baker, Allen, and USDA Economic Research Service. “Feed Grains Database 2009: Yearbook Tables. Table 31—Corn: Food, seed, and industrial use.” United States Dept of Agriculture Economic Research Service. The Economics of Food, Farming, Natural Resources, and Rural America.]] over one-third of the production level. 

While supporters have argued that producing and using corn ethanol reduces greenhouse gas emissions compared to gasoline, the energy required to make corn ethanol and the potential land use changes from increased corn farming, at home and abroad, call into question how much, if any, reductions are actually achieved.  Some “advanced” biofuels such as cellulosic ethanol generally appear to offer a better climate equation.