The Renewable Fuels Standard calls for an annual production of 7.5 billion gallons (US) of renewable fuels by calendar year 2012. Starch-based ethanol from corn (maize) has the potential to capture a large portion of this market. Approximately 4.9 billion gallons of ethanol was produced in the United States in 2006, requirng about 11 million acres. Expected increases in demand for corn will require increased acreage of corn and improved efficiency of ethanol production. For example, if the US corn area in 2008 is equal to that in 2007, some 30 percent of corn in 2008 will need to follow a crop other than soybean. Most of that will be corn following corn, which may increase costs related to tillage, soil fertility, pest management, residue management and logistics. Corn following corn often yields less, with greater yield variability, than does corn that follows soybean, though there is some evidence that this instability might be decreasing as new genetic traits and management techniques are introduced. Hybrids differ in ethanol yield, with a range from 2.61 to 2.84 gallons per bushel, or four to five percentage points around the mean. Part of the corn-to-ethanol production function is determined by starch concentration, but ethanol yield from corn grain remains difficult to measure. Using corn residue as a liquid fuel feedstock will also change the dynamics of continuous corn, in ways that are not yet well characterized.