When the cattle are kept tight to a paddock they eat what they want and trample the rest. The cattle are then moved so the species of pasture they liked have a chance to regrow and the species of pasture they didn't like got trampled. The trampled pasture helps keep the soil temperature down which helps to keep water from evaporating which helps the pasture keep growing. The trampled pasture is also food for the soil microbes who live in a whole another world beneath the soil.
Michael J. Adamucci