When secondary and tertiary consumers consumes, energy is lost within the system.
That explains why food webs at most only goes on up to 4 to 5 levels with humans at the top. So cows which consumes plant, would have less energy than the plant, since it is a larger organism and would need more energy to sustain it's life through respiratory and other metabolic activities.
It is also not very stable for our economy since it would mean a more competitive market for energy and it use as a source of food. We can see this in the recent rise in gas prices and food prices as well as the recession within the economy. In addition, it is also not very effective. Take ethanol for example. Statistic have been done and shown that if every family in the world has yield a field of corn it still would not be enough to satisfy the demand for ethanol. Furthermore, ethanol actually hurts the ecosystem more than oil. The major fear of oil is that it releases carbon monoxide, the stuff that comes from the exhaust of our cars. However, ethanol, when burn, actually releases more carbon than oil.