The spawn rate in the beta is ridiculously high for both animals and hostile mobs. You can't walk outside at night without a massive amount of hostiles chasing you, and even during the day it's not safe because of creepers that have spawned all over the place. And there's no need to breed animals anymore; you can just run outside and slaughter the entire range of things that has spawned in the active area, and within a few minutes there will be a new batch to start all over with.