How to get your baby animals to become adults everytime

Today I tended three of my four nursery barns, and so far every single one of my nurseries produced an adult animal, two holstein cows and one cream draft horse.  Unless farmville has done something so that we produced adult animals everytime, the other possibility could be by not adding a new calf or foal to your nursery every time you recieve a new adult animal.  Give this a try and let me know if you are getting the same results.