When provided with fresh pasture, pigs will actively graze. This reduces grain requirements, and distracts them from aggressive rooting. Of course, pigs WILL root a good bit of the pasture. But the key is to rotate them precisely every two weeks, and allow the pasture to rest and recover. As long as it is no longer disturbed, rooted pasture will mostly re-root itself and continue to grow. Between the hog manure fertilizer and the newly available seed bank, pasture recovers after a month or so with adequate rainfall (3/4 inch per week). If you live in a part of the country where this sort of rainfall is unlikely, then consider rotating once every week.