The sacrifice in yajña, recommended, that is not for killing the animal, but it is a testing, how the Vedic mantras are being properly chanted. Because an old animal put into the fire, by Vedic mantras he would come out again with young life. That is sacrifice of animals in the yajña. Therefore in this age there is no such expert brāhmaṇa who can chant the mantras properly or he can behave because the life is very abominable. Therefore, because there is no expert brāhmaṇa, so these sacrifices are forbidden in this age.
Pradyumna: Aśvamedha-yajñas or gomedha-yajñas, or the sacrifices in which a horse or a bull is sacrificed, were not, of course, for the purpose of killing the animals. Lord Caitanya said that such animals sacrificed on the altar of yajña were rejuvenated and a new life was given to them. It was just to prove the efficacy of the hymns of the Vedas. By recitation of the hymns of the Vedas in the proper way, certainly the performer gets relief from the reactions of sins. But in case of such sacrifices not properly done under expert management, certainly one has to become responsible for animal sacrifice."
Prabhupāda: This is a long subject matter. But the sacrifice in yajña, recommended, that is not for killing the animal, but it is a testing, how the Vedic mantras are being properly chanted. Because an old animal put into the fire, by Vedic mantras he would come out again with young life. That is sacrifice of animals in the yajña. Therefore in this age there is no such expert brāhmaṇa who can chant the mantras properly or he can behave because the life is very abominable. Therefore, because there is no expert brāhmaṇa, so these sacrifices are forbidden in this age. Kalau pañca vivarjayet aśvamedhaṁ gavālambhaṁ devareṇa sutotpattiṁ sannyāsam (CC Adi 17.164). These things are forbidden in this age, because there is no proper men to conduct.
So in the Kali-yuga this sacrifice is recommended: yajñaiḥ saṅkīrtana-prāyair yajanti hi sumedhasaḥ (SB 11.5.32). Saṅkīrtana-yajña. This yajña will help you. Real thing is the heart, the mind. If this yajña, saṅkīrtana-yajña, cleanses your mind and heart, then you come to the platform of knowledge. Then automatically other things become effective. So in this age it is recommended that other yajñas will not be effective, neither it is recommended. But the saṅkīrtana-yajña-yajñaiḥ saṅkīrtanaiḥ—that is recommended. Everyone can join it, and he can become purified, he can become situated on spiritual platform, and his life becomes successful.