I cant help you with the ethical side of the argument without falling back on the religious tenet of human superiority. Lets leave ethics aside for a few seconds and talk about basic biology.
Youre talking about farming humans for food. Who would be farming the humans? Fellow humans, I guess. So youre basically asking about cannibalism, right? From a biological standpoint, cannibalism
is risky/ dangerous. Im not just talking about human cannibalism, but basically about any mammal eating a mammal of the same species. Have you ever heard of prion diseases? Mad Cow disease is one. And it has been associated with cannibalism:
http://news.bbc.co.uk/1/hi/health/me...tes/355601.stm
http://www.sciencedaily.com/releases...0411071024.htm
http://www.leakeyfoundation.org/news..._x.jsp?id=3342