Ralph Mowery wrote:
In my back yard there are about 50 or more holes.

You don't indicate where you live, but if you're in the Midwest USA, it
could be voles. See: http://en.wikipedia.org/wiki/Vole

One way to tell is to walk across your lawn...if you're walking on firm
lawn and then suddenly sink down a few inches, it's voles.

A local lawn pest service provider can help you get rid of them, if
that's what you have, by installing sonar devices (to scare them away
from your lawn and into your neighbor's lawn) or traps. I never tried
the castor oil solution mentioned in the wikipedia article.

Good luck!