Corn stalks require lots of energy to plant, water, and harvest the corn stalks. The energy you get from the corn itself isn't enough to make up for all the money you have to spend to produce and ship corn.
Ignore the planting and watering part. The energy required just to collect and transport those corn stalks to the factory in many cases is more than they yield in energy from the fuel produced. That's the big problem with the bio-fuel programs. Unless the factory is right at the source of the biomass the energy produced is less the energy spent producing the fuel.