According to a thread in the FarmVille Forums, Zynga has removed the extra nurseries said to be allowed back on June 14th, 2010. Here is FarmVille Community Manager Lexlicious on the topic back in June..

“Since the release of Nursery Barns, 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 and the ability to have more than one Nursery is a glitch. 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, as this issue will be addressed in the future. This fix may result in the loss of any Nursery Barns in excess of two. We appreciate your patience and understanding with us during this time.” Source : FarmVille Forums
Since then Farmers have been spotted with as many as 10 nurseries on their farms, this of course being extremely abusive. Although Farmers are claiming they have received all foals back in their gift box and a 100 FVC credit has been added. Here is FarmVille Community Manager Voltai on the matter…
“Players no longer have their second/extra nursery barns, but are credited 100 Farm Cash and have the animals from those barns in their gift boxes.
Side-Effect: Some players are missing certain animals that should have been distributed to their gift boxes and/or also missing chicken coops/dairy barns/etc.
Status: This is an intended function after removing multiple Nursery Barns for game stability reasons. Some users may be missing additional buildings/animals. We are working to resolve this.” Source : FarmVille Forums
Other Source : FarmVille Forums