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SB 6.4.9

annaṁ carāṇām acarā
 hy apadaḥ pāda-cāriṇām
ahastā hasta-yuktānāṁ
 dvi-padāṁ ca catuṣ-padaḥ
annam — food; carāṇām — of those that move on wings; acarāḥ — the nonmoving (fruits and flowers); hi — indeed; apadaḥ — the living entities without legs, like the grass; pāda-cāriṇām — of the animals who move on legs, like the cows and buffalo; ahastāḥ — animals without hands; hasta-yuktānām — of the animals with hands, like the tigers; dvi-padām — of human beings, who have two legs; ca — and; catuḥ-padaḥ — the four-legged animals like the deer.
By nature’s arrangement, fruits and flowers are considered the food of insects and birds; grass and other legless living entities are meant to be the food of four-legged animals like cows and buffalo; animals that cannot use their front legs as hands are meant to be the food of animals like tigers, which have claws; and four-legged animals like deer and goats, as well as food grains, are meant to be the food of human beings.

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-padāṁ ca catuṣ-padaḥ: the four-legged animals (catuṣ-padaḥ), as well as food grains, are eatables for human beings (dvi-padām). 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 brāhmaṇas, kṣatriyas and vaiśyas — do not eat meat. Sometimes kṣatriyas go to the forest to kill animals like deer because they have to learn the art of killing, and sometimes they eat the animals also. Śūdras, too, eat animals such as goats. Cows, however, are never meant to be killed or eaten by human beings. In every śāstra, 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-saṁhitā says, pravṛttir eṣā bhūtānāṁ nivṛttis tu mahā-phalā: 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 śāstra especially recommends, kṛṣi-go-rakṣya: the vaiśya 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.