Drinking cow blood will provide food while keeping the cow alive, which is like having your pie and eating it too. In Africa.
Yum Yum kids, nothing like a glass of warm blood for supper.
Africa Top Five: #2 - got blood? [India Ink]
Of course the worse vampires are sick people who get blood transfusions at hospitals. While the rest of us donate our blood to blood banks so they can tank up.
The Vampire bat of Mexico also prefers cow blood as elsewhere in the world, for which reason the human vampirism as an legend got started. Being that most people have not evolved fangs yet, we never really adapted to this method of animal conservation via just snacking on a little blood which the animal can replace. Other then in Africa.
However we can suck other people dry of their emotional energy by upsetting them to get mad or fearful, and then feed off that energy to give our self a stemulating lift when we feel the need for a power drink.
The human "emotional vampires" are the most common of all, trying to maintain life by living off the emotional energy of other people until we drain them dry and they die on us. Oh well, they should have been wearing a cross of something religious.