The horse sacrifice yajña or cow sacrifice yajña performed by the Vedic regulations shouldn't be misunderstood as a process of killing animals. On the contrary, animals offered for the yajña were rejuvenated to a new span of life by the transcendental power of chanting the Vedic hymns, which, if properly chanted, are different from what is understood by the common layman. The Veda-mantras are all practical, and the proof is rejuvenation of the sacrificed animal.
There is no possibility of such methodical chanting of the Vedic hymns by the so-called brāhmaṇas or priests of the present age. The untrained descendants of the twice-born families are no more like their forefathers, and thus they are counted amongst the śūdras, or once-born men. The once-born man is unfit to chant the Vedic hymns, and therefore there is no practical utility of chanting the original hymns.
And to save them all, Lord Śrī Caitanya Mahāprabhu propounded the saṅkīrtana movement or yajña for all practical purposes, and the people of the present age are strongly recommended to follow this sure and recognized path.