Legumes include alfalfa and clover. Here is some info from the following link:
"Legumes can be a valuable part of a cattle feeding program. In fact, legumes can actually improve animal performance by encouraging intake, by providing greater feed efficiency and by fostering a better use of forage nitrogen content."
A certain amount of legumes will most likely grow in any complex natural pastureland. That doesn't mean they are the kind that produce big fat seed pods like lima beans or soy. Like the previous poster said, alfalfa, clover and other legumes found in the wild are certainly healthy for grazing animals. If they weren't they would eat around them.