Personally, I would actually be open to that. On a moral level, I'm thinking, if the cow has gotten naturally pregnant and is provided food+shelter and hasn't been bred to provide unhealthy levels of milk and has enough milk to feed her young, too, then I feel like a talking cow would be on board. So, if performed with an abundance of caution and respect, I consider that moral.

But if I remember correctly, I read once that this would result in ⅕ of the milk output of a dairy cow in industrial production. And you'd only get this milk while and shortly after the cow is pregnant.
So, in essence, that would result in so little dairy, it would be pretty much a vegan diet anyways...