Net energy gain from converting corn to ethanol is improving in efficiency, according to a report from USDA. The report is the result of a survey of corn growers for the year 2005 and of ethanol plants in 2008, according to

USDA measured all conventional fossil fuel energy used to produce 1 gallon of corn ethanol. For every British Thermal Unit of energy required to make ethanol, 2.3 BTUs of energy were produced, according to this most recent data. That compared favorably to the last study in 2004, which showed it took 2.3 BTUs of conventional fuel to produce 1.76 BTUs of ethanol energy.

According to the report, overall, ethanol has made the transition from an energy sink to a moderate net energy gain in the 1990s to a substantial net energy gain in the present. It said ethanol yields have increased by about 10 percent in the last 20 years, so proportionately less corn is required. In addition to refinements in ethanol technology, corn yields have increased by 39 percent over the last 20 years, requiring less land to produce ethanol.

The report can be found here