What is it with UFOs and cows anyway. Does anybody ever report aliens in their hen house? Maybe probing their hogs? It's always cows.
Alien technology, while far advanced compared to our own, still has no anal probes small enough for chickens. And the aliens found out early on that hogs simply enjoy anal probes too much (dirty pigs!), so they stick to cows now.
They will continue abducting cows for heifer and heifer, and there's nothing we can do to stop them.