how to level up cowgirls?
You can talk to them, give them their favourite food, and beyond that, they get one friendship point for each of their body parts that you max out in size. The maximum friendship level should be 9.
Edit: there is a system to where the cowgirl puts the nutrition you give her, but in short, just feed her a lot and it should work. The easiest body part to max out are the boobs. Just feed her slowly while she is producing milk and they should grow.