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 as well as food grains, are meant to be the food of human beings. 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, ksatriyas and vaisyas–do not eat meat. Sometimes ksatriyas 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. Sudras, too, eat animals such as goats. Cows, however, are never meant to be killed or eaten by human beings. In every sastra, 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 sastra especially recommends, krsi-go-raksya: 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.
Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Sixth Canto, Chapter 04 – Text 09