To get the fermentation done you need to start with sugars. Cellulose and hemi-cellulose are long polymeric sugar (ex: glucose and xylose). If you have a technology that could use the entire plant and to make sugars then you would make a difference. At 20ton/ha and year producing biomass you could make 10+ton sugars/year and after fermentation you would have 7ton of "fuel". That is 7 ton fuel per hectare and year. So if Nebraska:s 20.000.000 hectares would be used (just as an easy calculation example - not the real thing ok?) you would get 140.000.000tons of fuel per year. No more corn but fuel only. That would cover a fair pice of what is needed.
What is missing here? Making sugars from cellulose/hemi-cellulose! That technology at the right sugar price is being done by several companies. My favorite is REAC Fuel....but there are others that can do the trick.