[4] Opponents of corn ethanol production in the U.S. often quote the 2005 paper [5] of David Pimentel, a retired Entomologist, and Tadeusz Patzek, a Geological Engineer from UC Berkeley.
This unfermented component is called distillers grain, which is high in fats and proteins, and makes a good animal feed supplement.
He wrote that Pimentel assumes that corn ethanol entirely replaces gasoline and so the quantity of by-products is too large for the market to absorb, and they become waste.
[11] In Brazil where sugar cane is used, the yield is higher, and conversion to ethanol is more energy efficient than corn.
[13] A 2008 study by the University of Nebraska found a 5.4 energy balance for ethanol derived specifically from switchgrass.
[14][15] This estimate is better than in previous studies and according to the authors partly due to the larger size of the field trial (3-9 ha) on 10 farms.
In tropical regions with abundant water and land resources, such as Brazil and Colombia, the viability of production of ethanol from sugarcane is no longer in question; in fact, the burning of sugar-cane residues (bagasse) generates far more energy than needed to operate the ethanol plants, and many of them are now selling electric energy to the utilities.
However, while there may be a positive net energy return at the moment, recent research suggests that the sugarcane plantations are not sustainable in the long run, as they are depleting the soil of nutrients and carbon matter [citation needed] On the other hand, productivity of sugar cane per land area in Brazil has consistently grown over the decades; sugar cane has been shown to be less depleting to the soil than cattle and yearly cultures;[18] and there are many regions in the country where sugar cane has been cultivated for centuries.
[19] Those facts suggest that related soil depletion processes are very slow and therefore ethanol from sugar cane may be far more sustainable in the long run than common fossil fuel alternatives.