No energy conversion is 100% effective. That is the second law
of thermodynamics. Prarie dogs, if they did, in fact eat grass,
would need to expend energy to go to where the grass is, bite off
the grass, chew the grass, digest the grass and expel the
undigested waste. A large portion of grass is undigestible, as
evidenced by the large volume of prairie dog waste. Turning
vegetation into animal tissue is not very efficient. The energy
conversion efficiency for this is generally estimated to be
10%.