We don't need to buy any grain. I have raised multiple batches of pigs on just pasture. The sheep, of course, only need pasture. The chickens do fine with just pasture in the warm months since they are there to eat the insects anyways. Ducks and geese just need the pasture. In the winter we feed meat to the chickens as well as kitchen scraps.
There are a lot of free things you can get that are part of the "waste" stream but are good food. Stick with the pre-consumer "wastes" not post-consumer (e.g., plate scrapings). Pasture/hay+dairy is almost all of what our pigs thrive on and we have a herd of about 300.
You can grow, without any diesel or tractors, a lot of stuff. Depends on your climate what you should choose. For us pumpkins, sunchokes, turnips, kale, rape, etc are all great producers and make for excellent fall and winter fodder.
Peak oil problems will crush Big Ag because they're founded on energy intensive operations from start to finish. Small producers will do well - they're more adaptable.
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 http://SugarMtnFarm.com/csa