CC Ādi 17.163

kali-kāle taiche śakti nāhika brāhmaṇe
ataeva go-vadha keha nā kare ekhane
Synonyms: 
kali-kāle — in the Age of Kali; taiche — such; śakti — power; nāhika — there is none; brāhmaṇe — in the brāhmaṇas; ataeva — therefore; go-vadha — killing of cows; keha — anyone; — does not; kare — execute; ekhane — at the present.
Translation: 
“Formerly there were powerful brāhmaṇas who could make such experiments using Vedic hymns, but now, because of the Kali-yuga, brāhmaṇas are not so powerful. Therefore the killing of cows and bulls for rejuvenation is forbidden.