Keep the cow in a smallish pen set up within the pasture where it and the dogs will stay so the dogs can wander around the pen to give the animals time to become accustomed to each other. When the dogs and cow can approach the
fence and touch noses through the
fence without getting excited, they can share the same pasture. Since a cow's basic instinct is to attack a strange dog, they have to be kept togather long
enough for the dog to no longer be strange to the cow. It helps a lot if you have other
cattle or sheep that are used to the dogs and don't react negatively to it.