@Thomas,
they can make ethanol from garbage really. Its just that the end product (in energy expended) is still higher than we recoup in the product.
Corn stubble is now being used for cattle feed (It gets ground up with the cob and the corn as a "greenchop". Then its ensiled and fed as a "Stew" to lactating cows. The need to consider that we use 70% of our cropland related to meat or dairyt production is getting too high so we are using everything in grain except the rattle of the elaves.
Switchgrass, new years twig (biomass) all kinds of carbo plants can make ethanol. I like the algae diesel as THE BEST non petrol based. Its energy need is mostly reaction with an epoxide (which is methanol (Which can be made from coal) and a hydroxyl . The diesel, as a complex ester and a left over glycerin is producing a real energy dense fluid and a great by product for fertilizers, explosives, plastics etc etc.