Most pig forage crops are above ground as they tend to be fast growing and soft. In my environment &
experience, Umbelliferae, are root of choice but Blackberrys are right up there. If you choose the right pig (Tamworth & Tamworth crosses are know as excellent rooters as they have a wonderful long & powerful nose) they will root regardless looking for grubs and other forage. Please count on giving them supplemental feed with a mineral ration, and then watch out for the associated salts and soil compaction. Nothing in life is free.