Pigs on the land are not destructive if you manage the grazing properly. This is the same as with sheep, goats, horses, cattle, chickens and people. Look up managed intensive rotational grazing techniques:
http://www.google.com/search?q=managed%20intensive%20rotational%20grazing&ie=utf-8&oe=utf-8 http://en.wikipedia.org/wiki/Managed_intensive_rotational_grazing We graze pigs, sheep and poultry together. They do a wonderful job. Our pig's diet consists almost entirely of pasture during the warm months and hay in the winter plus about 7% dairy for lysine (a protein). We also raise some beets, pumpkins, apples and other things and occasionally get a bit of boiled barley from a local brew pub and dated bread from a local bakery that makes a great training treat for loading (weekly task) to go to butcher. We have about 300 pigs on pasture. Works great.
Start by setting up a very good four wire (or more) hot perimeter fence. Then run division fences off of that to sub-divide the fields into grazing paddocks. As the forages get grazed down seed with alfalfa, clovers, kale, rape or what ever your choices are to improve forage quality. Rotate to leave parasites behind.
The chickens are our natural pest control. They break up manure paddies, smooth the soil, eat insects and provide eggs for weaner pigs. The poultry naturally tend to follow the bigger livestock just like in nature so they do not need to be fenced.
See:
http://flashweb.com/?s=rotational+grazing for more about how we manage it.
Cheers
-Walter
Sugar Mountain Farm
Pastured Pigs, Sheep & Kids
in the mountains of Vermont
Read about our on-farm butcher shop project:
http://SugarMtnFarm.com/butchershop