We do about six months using pasture/hay and whey during the warm months and a bit longer during the cold months as some calories are burned to keep warm.
In my experiments I found that pasture alone is about eight months.
Grain alone from the same genetics is about six months.
Since we breed our own and supply a lot of people with piglets who do various management and feeds I've gotten to see these different groups over time and the above is fairly consistent.
Managed intensive rotational grazing and garlic are good ways to control parasites. If you're pig's got worms it will grow slower. If your pig has an imbalanced diet, e.g., lacking in lysine, then it will grow slower. If your pig is lacking in calories then it will grow grow leaner. Breed also makes a difference - some grow faster or slower than others, some are short and some are long bodied.
More important than the months to butcher is the question of dollars to raise the pig and the quality of the meat. I find that the pasture/hay plus dairy gives the highest quality meat of all the diets we've tried over the years, much better than grain. For us it is also much less expensive since we have pasture and dairy but don't have much grain. Choices depend on your situation.
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
