Like wyomama said. I'm speaking from memory here, and my memory has been known to be just a tad faulty, but I THINK the problem with cattle (and cattle alone - not sheep or goats or people or cats or dogs or pigs or horses or rabbits or - well, only cattle) is that the placentas fuse, and that's where the mixing of hormones takes place. Other animals (e.g. people) the placentas will usually remain separate; or with animals that do litters (rabbits, cats, pigs) they have evolved so they don't have a problem with it).