Cows are fed clover. As with all processes in CountryLife, up to three units of clover can be added to the cow's manger at a time. Holstein cows turn a unit of clover into a unit of milk in 60 seconds.
Holstein cows are available as a item which can be gifted by neighbors, one per day. Additional Holstein cows can be purchased for 2,500 coins each, but you would have to be very shy about asking neighbors if you felt the need to buy one.