Only ruminating mammals are able to digest the cellulose fiber in the grasses. Pretty much any herd animal should work. Since humans cannot digest cellulose, the best we can do to help is to manage the herds and harvest the dairy and meat they produce, and benefit from the improved soil and micro-climate, which allows us to rotate crops into the grazing lands.
We would still do well to add human waste to the soil, but we cannot replace the cellulose-digesting function of the herd animals.
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