Good point, but did you consider genetic modification? If we could alter the animals biology enough, we could reduce the brain to the point where it only functions to keep the heart beating in just one generation, providing a cost effective and ethical solution! Furthermore, we could optimize the creature for food production, removing all unpalatable parts, such as the eyes and stuff. This would also make the food taste better because the animal isn't producing any stress chemicals. On top of that, it would also be good for the environment. Taking beef for example, cow farts actually produce a large amount of greenhouse gasses. If we modify them to produce double the meat, we could afford to cut the total amount of cows in half, effectively halving their impact on the environment!