Have plenty of high quality legume pasture. Pigs love clover, alfalfa, chicory, rape, turnips, and short vegetative grass. Pigs aren't ruminants. They won't bloat on pure stands of alfalfa or clover. Pigs won't gain on pasture without grain.
Sows can maintain weight on a very high quality legume pasture without grain.
Genetics differ greatly in their ability to utilize forages and fiber. Colored breeds are much better. Select gilts for breeding stock that thrived under pasture finishing. Keep management level of the sows and gilts very high. Gilts should be less than seven months old when selected.