Advances in how efficiently corn can be converted to ethanol have led to an improvement in how much energy is squeezed from corn, according to a study released Monday by the United States Department of Agriculture.

The study was based on a survey of 1,814 corn farmers in 19 states and ethanol producers in Minnesota, Iowa, Nebraska and South Dakota. It found that for every British Thermal Unit (BTU) needed to make ethanol, 2.3 BTUS of energy were produced.

That's up from 1.76 BTUs of energy produced in 2004, a significant improvement, said Ward Nefstead, one of the study's authors and a professor in the University of Minnesota's Applied Economics Department.

Genetic improvements to corn seeds have increased starch levels, allowing for more energy to be wrung from corn, he said. The use of alternative energy sources to power ethanol plants -- instead of natural gas -- has increased.

And farmers are applying nitrogen fertilizer more efficiently, too, reducing the amount of energy needed to grow corn.

Nitrogen use on a per bushel basis has declined by 20 percent since the mid-1990s, the study said. Meanwhile, ethanol yields have increased by about 10 percent over the past 20 years, so proportionately less corn is required to make ethanol.

Ethanol has gone from being an "energy sink" -- meaning it took more energy to produce than it yielded -- in its early days to being a "substantial net energy gain" nowadays, according to the USDA.

Mike Hughlett • 612-673-7003