Once the Xbox reaches 1.8, mobs passive mobs will no longer despawn. However, they will no longer respawn once they are killed either.
I might need to launch a wolf cull in that case. I've stopped taming wolves as that stops passive mobs spawning if you have too many, but wild/untamed ones constantly kill other passive mobs. If we don't kill them all off, they'll kill every other passive mob, if passive mobs don't respawn, we'll soon have noting left.