Well naturally Cows, similarly to humans, begin to lactate after they give birth to a child. Their child, when not used for veal or raising, would then drink the milk taking the uncomfortable pressure off. 

Humans milk cows for their profit. Not because we have a survival instinct to. After one-years-old, the human being has no need for human breast milk, no less COWS breast milk.

Drinking cow’s milk is a tradition, and a bad one at that. 

Anything that comes from an animal isn’t vegan. Because anything that comes from an animal isn’t willingly given. Because animals can’t give consent, and naturally they wouldn’t. It’s not natural for them to give their precious milk away and leave their babies with none. We are impinging on their freedom and natural way of life—NOT fulfilling our own. It’s not natural for us either.