I think the author meant that she could produce milk (like a regular cow) without getting pregnant. I mean it's a fantasy world but the cow girl also mentioned about falling in love before being able to produce milk. In actuality, oxytocin (love hormone) allows the milk to be let down or released, and prolactin is the hormone that tells the glands to produce milk. Might not apply to cos girls in a fantasy world though."I can produce milk like a regular cow, even without getting pregnant" - Of course, regular cows only produce milk when they're pregnant or have a calf to feed... this would've made more sense if the author had just left out the reference to regular cows.