Generally speaking, a diverse farm operation, including animals, produces more food value/acre than a comparable specialized farm. That's because a diversified farm has more than one use for a particular piece of land, e.g., using the orchard for sheep grazing (grass & windfalls) and hay production.
To answer your question, yes. Typical nitrogen fixers are clover or alfalfa. Typically they are planted with grass to provide pasture/hay. The animals graze the pasture and their dropping help fertilizer the ground. An additional benefit to using the ground this way is that it helps to break disease and pest cycles.
For our local food co-op, I deal with two organic farmers. One provides us with wheat flour, rapeseed and sunflower oil: he has beef cattle to feed manure into the system. His neighbours don't have animals, and put chick peas and lentils (that we buy from them) into the crop rotation.
:-)
