Taking a step in an interesting direction, FarmVille
has started allowing cows to breed. How? Well, you must find a lonely bull wandering around on a friend's farm. Once you have, toss that bull into the dairy farm with all your other cows. If you leave him out, you can brush him for 80 coins every three days, but having him breed is much more lucrative in the long run.
Much like the mystery eggs from chickens, you will have a chance to receive a calf every time you harvest from the dairy farm. The color of the cows inside the farm are what determines the color of the calf. For example, if you have a bunch of brown cows in your dairy farm and only a few white cows, there is a higher chance that the calf will be brown.