How do I get my black stallion foal to grow up in the nursery barn?

Check list for this process
Empty Nursery barn (Click here for this explanation)
One black stallion foal


Process.

1. Put one ready calf or foal in empty nursery barn to make it ready (use move function to move ready animal or else it will harvest it instead of moving it)

2. Now place your Black stallion Foal in ready nursery barn.

3. Take out the Foal or calf you originally placed in the nursery to make it ready.

4. Tend or harvest nursery Barn with just the black stallion foal in it.

If you have more then one black stallion foal, do not put them all together in nursery barn. It will reduce your chances of growing up Black stallion and will increase chances of getting you another adult animal from your nursery barn. Also by just putting one Black stallion foal in barn, it will be much easier since you don't have to move many animals in and out of nursery barn so you can take more chances to grow up Black stallion.

Some people reported if you sell your old Nursery barn and make new one and do above process you will get an adult Black stallion in no time.  You do not need to sell your nursery barn to buy another one.  As long as your nursery barn is complete you can start a new one, I have four barns already.

UPDATE FROM ZYNGA CUSTOMER HELP WEBSITE: Multiple Nursery Barns Updated 06/15/2010 04:06 PM A glitch has come to our attention that allows users to have more than one Nursery Barn. This is not the intended functionality of this feature. We are intending on fixing this issue in the future; however, we have decided to allow users to have the ability to possess two Nursery Barns on their farm. Please know that if you continue to take advantage of this glitch and choose to have more than two nurseries on your farm, you are building at your own risk. This issue will be addressed in the near future and this fix may result in the loss of any Nursery Barns in excess of two.