Animal sacrfice, as performed in pre-Kali Age time (when it was recommended, after the commencement of the Kali Age is is no longer recommended, along with some other acts, such as brothers begetting children in one's wife if the actual husband was sterile or deceased, as in the case of Vedavyasa begetting children with his deceased brother's wives), had very particular rituals involved. Specialized brahmans to oversee the whole proceedings, to make sure everything went according to the directions. The animal sacrificed gained (visibly) a new animal life, or sometimes even a human life.
It cannot be performed properly now, and it had nothing to do with slaughterhouses killing en masse.