Animals like horses and cows were offered in sacrifice to test the proper execution of the sacrifice. Otherwise, there was no purpose in killing the animal. Actually the animal was offered in the sacrificial fire to get a rejuvenated life. Generally an old animal was sacrificed in the fire, and it would come out again in a youthful body. Some of the rituals however, did not require animal sacrifice. In the present age, animal sacrifices are forbidden. As stated by Śrī Caitanya Mahāprabhu:
kalau pañca vivarjayet
“In this Age of Kali, five acts are forbidden: the offering of a horse in sacrifice, the offering of a cow in sacrifice, the acceptance of the order of sannyāsa, the offering of oblations of flesh to the forefathers, and a man’s begetting children in his brother’s wife.” (Cc. Ādi 17.164) Such sacrifices are impossible in this age due to the scarcity of expert brāhmaṇas or ṛtvijaḥ who are able to take the responsibility. In the absence of these, the saṅkīrtana-yajña is recommended. Yajñaiḥ saṅkīrtana-prāyair yajanti hi sumedhasaḥ (Bhāg. 11.5.32). After all, sacrifices are executed to please the Supreme Personality of Godhead. Yajñārtha-karma: such activities should be carried out for the Supreme Lord’s pleasure. In this Age of Kali, the Supreme Lord in His incarnation of Śrī Caitanya Mahāprabhu should be worshiped with His associates by performance of saṅkīrtana-yajña, the congregational chanting of the Hare Kṛṣṇa mantra. This process is accepted by intelligent men. Yajñaiḥ saṅkīrtana-prāyair yajanti hi sumedhasaḥ. The word sumedhasaḥ refers to intelligent men who possess very good brain substance.