I think the ethics of farming is another place where problems in utilitarianism crop up.

There's a Parfitian argument that, since none of these animals would have existed otherwise, then killing them for food is no problem. But this would also apply to farming people, whether for food or chattel slavery, which we find repugnant. Obviously, though, this world is just as utility maximizing as Hanson's Malthusian em soup universe, neither of which seem particularly "good" (in fact, it is the em soup, just with fleshy people).

I don't have a "solution" to this, I think it just demonstrates one of the edges of utility theory's map.