You'll need to have a land and cows in order to produce $MILK (no need to have bulls for that), and milk production will be determined by the rarity and the amount of Cows you have in your land.
$MILK production of a Cow NFT will depend on it's MilkPower and the Total MilkPower of the Game.
Cows Milk Production by Rarity
MilkPower of an Cow NFT can be decreased 0.5% daily