A big problem with agriculture-based bio-fuels, is that growing crops with modern technology uses a lot of fuel. Tractors and other machinery, fertilizer, and a million other things all take energy, which often comes in the form of fossil fuels.
My brother-in-law has a farm, and claims that if you look at the amount of energy used and produced by farms, agriculture hasn't been economically viable for more than half a century. We pump fuel out of the ground, have farms turn it into food, and we eat that. It works because the energy from fossil fuels is already there, and eating is important. But if farms turn fuel into more fuel, those farms need to be a lot more efficient, or we're just burning fuel for nothing.
My brother-in-law has a farm, and claims that if you look at the amount of energy used and produced by farms, agriculture hasn't been economically viable for more than half a century. We pump fuel out of the ground, have farms turn it into food, and we eat that. It works because the energy from fossil fuels is already there, and eating is important. But if farms turn fuel into more fuel, those farms need to be a lot more efficient, or we're just burning fuel for nothing.