Grazing is not as simple as letting animals wander around in a big field eating grass all day. Animals should not be allowed to eat grass beyond the leafy part of the blade and should be moved off of short grass until it is replenished. If grass is eaten too short, its root system will be weakened and weeds will begin to compete–possibly destroying a pasture.
Here are some guidelines for different types of grasses, giving the length at which grazing should start and at which it should stop:
Fescue: Start at 6-8″, Stop at 3-4″
Timothy: Start at 10-12″, Stop at 3-4″
Orchardgrass: Start at 9-10″, Stop at 3-4″
Bluegrass: Start at 4-6″, Stop at 1-1.5″
More info: http://www.wvu.edu/~agexten/forglvst/overgraz.htm