we had our adult pigs in our overgrown woods for 2 years. its been empty for 6 mths now but today the weeds are still 3ft high so to have any real effect to need to concentrate their rooting by confining them to a certain area at a time. we chose to let them enjoy the whole area but they have made a real mess, its difficult to walk over the ruts and the weeds have just grown back covering the bumps. the dense pine woodland section hasnt been dug up too much but younger hardwood areas have big clods everywhere. i dont think young weaners will do much workwise til they are 4 mths old, choose a hardy hairy breed if there are lots of stingers. we had tamworth, gos and tamxgos and they coped fine.
feeding wise, they wont get many calories from the weeds or grass as they cant digest it very well so just feed as normal. adjust their feed inline with their condition and exercise.
its a great way to rear pigs though, they are much happier in woodland.