There's a fantastic post by The Ethical Butcher on why choosing Heritage Breeds is crucial to developing sustainable meat production.
From the article:
Every farm that is raising and selling heritage meats is one more that IS NOT a disgusting feedlot. As I stated earlier, the needs of these breeds make factory farming a non-option. They basically demand a farmer to revert to old methods, seasonality and bio-diversity. All of this requires respect, attentiveness and a connection to the earth and its cycles from the farmer. By raising far fewer animals, the problem of pollution from the waste products are greatly reduced. A natural diet lessens pressure to produce the corn and soy used in commercial feed, and therefore also lessens the amount of fossil fuels needed to transport these foods. All that waste only results in diseased animals.* Because heritage breeds eat good food, are given space to live their lives the way nature intended, and allowed to form familial bonds and have farmers deeply committed to their health, I can only imagine these animals are much happier than their industrial counterparts. At this point, most heritage breeds are raised on very small farms and so often become local products by default. Supporting these local farms bolsters local economies. It's like killing 10 birds with one stone. On a purely visceral level, it is not even worth comparing the flavor and texture of heritage breed meat to that of industrial breeds.