We raise our animals using intensive rotational grazing, which means our goats and chickens in the growing season (Approx. April-November) don’t spend more than a few days in one place. In the winter I have them right on top of my garden where they can deposit their lovely fertilizer on the ground for me.
Keeping them moving creates a cleaner place for them to eat. The problem with a big field is they tend to go after all their favorite plants like a kid in a grocery store goes after the candy and then come back to home base where a person might supplement them with something else like grain (more candy).
Most of their manure gets concentrated at home base and the soil becomes exposed because of foot traffic. With better management practices like rotational grazing all of this traffic and manure gets distributed evenly so that the ground has a time to rest and recoup after the animal is moved. The result is a healthier animal and a healthier more nutritious pasture.