kali-kāle taiche śakti nāhika brāhmaṇe
ataeva go-vadha keha nā kare ekhane
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; nā—does not; kare—execute; ekhane—at the present.
"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.
Link to this page: https://prabhupadabooks.com/cc/adi/17/163