I've generated a world and killed all the animals, but no more spawn. Why?
Explore the entire seed for two reasons: 1) I read that if chunks that generate animals haven't been "discovered," they won't spawn new animals. I walked my entire world but no new animals appeared. 2) I discovered on my tour that all the animals I hadn't killed had fallen down a hole. I tunneled them out and once I did, animals began spawning all over, including inside the fenced-in area I built. Back in the sheep shearing business!
The mechanics of the Minecraft spawn rates have changed. Animals only spawn when you "discover" a new chunk (a 16 by 16 block area). After you have killed all of the animals in the world they will be gone forever. There is only one way to get more animals and that is breeding. Look it up in the Minecraft Wiki.