that sounds utterly ridiculous.
It is ridiculous. But it's currently making my world absolutely unplayable.

You'll notice the "E: 13/17054"
For those of you who don't yet know, that means in my current view there are 13 mobs somewhere in the world in front of me, out of a total of 17054 mobs.
You will also notice the number is much higher now then I said originally. They is because they seem to be continuously spawning or something, and at my humble 0.05 fps every frame about 100-200 more sheep are spawned.
Only 13 mobs are in my view at the moment because I had the brilliant idea to attempt to tunnel into the hill and murder all of the sheep... Yeah that's not happening now. Is there any way to kill all of them? Does any map editor support spawning/despawning mobs?
As a final note, my belief is that this is the result of some bug with breeding, causing animals to breed constantly. I don't see it being the fault of the mob spawning system, as from my understanding of the system that makes absolutely no sense.