The Vedas forbid the killing of cows. Cows have many names, one of which is "anagha" which means "that which should never be killed", and the cow is considered one of the seven mothers who should always be honored. The bull is one of the seven fathers. Cow is mother since she gives her milk to humanity, and bull is a father since (traditionally) his strength helps to grow grains in the field.
In the Manu Samhita there are stringent punishments for killing or mistreating cows and bulls.