So, lets look into the assumptions, as analyzed by MathPro, a small, independent consulting firm specializing in energy economics with clients in industry, government and law firms:

The assumptions:  P&P researchers assume modern processing plants produce nitrogen fertilizer but otherwise assume average production technology produces corn.  The USDA assumes high-yield corn figures whereas P&P assume average-yield.  Both the USDA and P&P assume:

• modern processing plants convert corn to ethanol
• modern processing plants produce nitrogen fertilizer
• the government allocates energy credits to ethanol co products.  What this means is the government subsidizes ethanol production for the energy contained in replaced animal feed because farmers substitute left-over ethanol-production food (namely, distiller's dry grains and solubles) for other animal feed.

However, the USDA uses a 40% credit; whereas, P&P uses only 9% to "reflect the actual nutrients in the animal feed," says Pimentel.  A 40% credit makes ethanol energy costs look better than a 9% credit.

Moreover, the 2002 USDA and the P&P estimates in the table below used energy-credit allocation, which although complicated, is well defined and in common use.  The 2004 USDA estimate did not, using instead, according to Dave Hirshfeld of MathPro, an "unexplained" calculation having nothing to do with feed substitution, but producing a "more optimistic result than any other such estimate ever produced."

The P&P estimate includes, as energy inputs to corn production, the farmer's labor, farm machinery and irrigation.  In addition, they include, as energy inputs to ethanol production, processing water and equipment.  This is a similar approach agricultural economists use in crop-production assessments.  The USDA estimates do not include any of these line items, which provides "a positive energy return desired by the pro-ethanol lobby, but it is not accurate," says Pimentel.

Here's the entire table, with all the input energy line items, and the resulting totals.

(Answered Aug. 1, 2006)