Srimad Bhagavatham : 6. 4. 9.
Chapter-4. ( The Hamsa-guhya Prayers )
Slokam : 9.
annam caranam achara hy apadah pada-carinam,
ahasta hasta-yuktanam dvi-padam ca catush-padah.
By nature’s law, or the arrangement of the Supreme Personality of Godhead, one kind of living entity is eatable by other living entities.
As mentioned herein, dvi-padam ca catush-padah: the four-legged animals (catush-padah), as well as food grains, are eatables for human beings (dvi-padam).
These four-legged animals are those such as deer and goats, not cows, which are meant to be protected.
Generally the men of the higher classes of society—the brahmanas, kshatriyas and vaisyas—do not eat meat.
Sometimes kshatriyas go to the forest to kill animals because they have to learn the art of killing.
Sudras, too, eat animals such as goats.
Cows, however, are never meant to be killed or eaten by human beings.
In every sastram, cow killing is vehemently condemned.
Indeed, one who kills a cow must suffer for as many years as there are hairs on the body of a cow.
Manu-samhita says, pravrttir esa bhutanam nivrttis tu maha-phala: we have many tendencies in this material world, but in human life one is meant to learn how to curb those tendencies.
Those who desire to eat meat may satisfy the demands of their tongues by eating lower animals, but they should never kill cows, who are actually accepted as the mothers of human society because they supply milk.
The sastram especially recommends, krshi-go-rakshya: the vaisya section of humanity should arrange for the food of the entire society through agricultural activities and should give full protection to the cows, which are the most useful animals because they supply milk to human society.