Whilst they are this young you certainly need to feed a kilo a day, rising gradually to 2 / 2 1/2 at 16 weeks. Pigs being monogastrique do not digest celulose like polygastriques (ruminants?). I do know of people here (France) who do not subscribe to this theory and whose pigs (imho) show a marked lack of growth due to this.
The highest growth levels are in the first 16 weeks, best to feed as much protein as reasonable. They will love the brambles and especially the roots.
Hope this helps, and sorry if it reads like a lecture. HM