Replacing cows as a food source is, well, a cow of a different colour. You cannot have alternate forms of power for cows (the rocket-powered cow being an exception :-P ).
You're thinking about it all wrong. We don't need alternative forms of power for cows; we need them for humans; i.e. something to replace cows.
Unlike finding alternative forms of transport or electricity, the solution to this problem is simple, obvious, environmentally and economically sound and can be implemented almost immediately. Instead of growing crops to feed cows, and then eating the cows, what we should do is cut out the middleman and just grow crops for ourselves. This is around ten times more efficient, not to mention healthier and more humane.
And eventually, we can all learn to love tofu burgers.