(Krishna honoring the cow)
"Kṛṣṇa said at that time that the cows are worshiped even by the demigods, and He practically demonstrated how to protect the cows. At least people who are in Kṛṣṇa consciousness should follow in His footsteps and give all protection to the cows. Cows are worshiped not only by the demigods. Kṛṣṇa Himself worshiped the cows on several occasions, especially on the days of Gopāṣṭamī and Govardhana-pūjā."
(Nectar of Devotion, Chapter 21)
"A human being has no need to kill animals because God has supplied so many nice things. If one indulges in meat-eating anyway, it is to be understood that he is acting in ignorance and is making his future very dark. Of all kinds of animal killing, the killing of cows is most vicious because the cow gives us all kinds of pleasure by supplying milk. Cow slaughter is an act of the grossest type of ignorance. In the Vedic literature the words gobhiḥ prīṇita-matsaram indicate that one who, being fully satisfied by milk, is desirous of killing the cow, is in the grossest ignorance. There is also a prayer in the Vedic literature that states:
namo brahmaṇya-devāya go-brāhmaṇa-hitāya ca
jagaddhitāya kṛṣṇāya govindāya namo namaḥ.
'My Lord, You are the well-wisher of the cows and the brāhmaṇas, and You are the well-wisher of the entire human society and world.' The purport is that special mention is given in that prayer for the protection of the cows and the brāhmaṇas. Brāhmaṇas are the symbol of spiritual education, and cows are the symbol of the most valuable food; these two living creatures, the brāhmaṇas and the cows, must be given all protection—that is real advancement of civilization. In modern human society, spiritual knowledge is neglected, and cow killing is encouraged. It is to be understood, then, that human society is advancing in the wrong direction and is clearing the path to its own condemnation. A civilization which guides the citizens to become animals in their next lives is certainly not a human civilization. The present human civilization is, of course, grossly misled by the modes of passion and ignorance. It is a very dangerous age, and all nations should take care to provide the easiest process, Kṛṣṇa consciousness, to save humanity from the greatest danger."
(Bhagavad-gītā As It Is 14.16, Purport)
"Lord Śrī Kṛṣṇa descended Himself at Vrajabhūmi with all His transcendental entourage and paraphernalia. Śrī Caitanya Mahāprabhu therefore confirmed that no one is as fortunate as the residents of Vrajabhūmi, and specifically the cowherd girls, who dedicated their everything for the satisfaction of the Lord. His pastimes with Nanda and Yaśodā and His pastimes with the cowherd men and especially with the cowherd boys and the cows have caused Him to be known as Govinda. Lord Kṛṣṇa as Govinda is more inclined to the brāhmaṇas and the cows, indicating thereby that human prosperity depends more on these two items, namely brahminical culture and cow protection. Lord Kṛṣṇa is never satisfied where these are lacking."
(Śrīmad-Bhāgavatam 1.8.21, Purport)
"The vaiśyas, the members of the mercantile communities, are especially advised to protect the cows. Cow protection means increasing the milk productions, namely curd and butter. Agriculture and distribution of the foodstuff are the primary duties of the mercantile community backed by education in Vedic knowledge and trained to give in charity. As the kṣatriyas were given charge of the protection of the citizens, vaiśyas were given the charge of the protection of animals. Animals are never meant to be killed. Killing of animals is a symptom of barbarian society. For a human being, agricultural produce, fruits and milk are sufficient and compatible foodstuffs. The human society should give more attention to animal protection. The productive energy of the laborer is misused when he is occupied by industrial enterprises. Industry of various types cannot produce the essential needs of man, namely rice, wheat, grains, milk, fruits and vegetables. The production of machines and machine tools increases the artificial living fashion of a class of vested interests and keeps thousands of men in starvation and unrest. This should not be the standard of civilization."
(Śrīmad-Bhāgavatam 1.9.26, Purport)
"The king cannot tolerate insults to the most important animal, the cow, nor can he tolerate disrespect for the most important man, the brāhmaṇa. Human civilization means to advance the cause of brahminical culture, and to maintain it, cow protection is essential. There is a miracle in milk, for it contains all the necessary vitamins to sustain human physiological conditions for higher achievements. Brahminical culture can advance only when man is educated to develop the quality of goodness, and for this there is a prime necessity of food prepared with milk, fruits and grains."
(Śrīmad-Bhāgavatam 1.16.4, Purport)
"The principal sign of the age of Kali is that lower-caste śūdras, i.e., men without brahminical culture and spiritual initiation, will be dressed like administrators or kings, and the principal business of such non-kṣatriya rulers will be to kill the innocent animals, especially the cows and the bulls, who shall be unprotected by their masters, the bona fide vaiśyas, the mercantile community. In the Bhagavad-gītā (BG 18.44), it is said that the vaiśyas are meant to deal in agriculture, cow protection and trade. In the age of Kali, the degraded vaiśyas, the mercantile men, are engaged in supplying cows to slaughterhouses. The kṣatriyas are meant to protect the citizens of the state, whereas the vaiśyas are meant to protect the cows and bulls and utilize them to produce grains and milk. The cow is meant to deliver milk, and the bull is meant to produce grains. But in the age of Kali, the śūdra class of men are in the posts of administrators, and the cows and bulls, or the mothers and the fathers, unprotected by the vaiśyas, are subjected to the slaughterhouses organized by the śūdra administrators."
(Śrīmad-Bhāgavatam 1.17.1, Purport)
"Protection of bulls and cows and all other animals can be possible only when there is a state ruled by an executive head like Mahārāja Parīkṣit. Mahārāja Parīkṣit addresses the cow as mother, for he is a cultured, twice-born, kṣatriya king. Surabhi is the name of the cows which exist in the spiritual planets and are especially reared by Lord Śrī Kṛṣṇa Himself. As men are made after the form and features of the Supreme Lord, so also the cows are made after the form and features of the surabhi cows in the spiritual kingdom. In the material world the human society gives all protection to the human being, but there is no law to protect the descendants of Surabhi, who can give all protection to men by supplying the miracle food, milk. But Mahārāja Parīkṣit and the Pāṇḍavas were fully conscious of the importance of the cow and bull, and they were prepared to punish the cow-killer with all chastisement, including death. There has sometimes been agitation for the protection of the cow, but for want of pious executive heads and suitable laws, the cow and the bull are not given protection. The human society should recognize the importance of the cow and the bull and thus give all protection to these important animals, following in the footsteps of Mahārāja Parīkṣit. For protecting the cows and brahminical culture, the Lord, who is very kind to the cow and the brāhmaṇas (go-brāhmaṇa-hitāya), will be pleased with us and will bestow upon us real peace."
(Śrīmad-Bhāgavatam 1.17.9, Purport)
"The Lord is described thus: namo brahmaṇya-devāya go-brāhmaṇa-hitāya ca. The purport of this prayer is that the Lord specifically protects the brāhmaṇas and the cows, and then He protects all other members of society (jagad-dhitāya). It is His will that universal welfare work depends on the protection of cows and brāhmaṇas; thus brahminical culture and cow protection are the basic principles for human civilization. Kṣatriyas are especially meant to protect the brāhmaṇas, as is the supreme will of the Lord: go-brāhmaṇa-hitāya ca. As, within the body, the heart is a very important part, so the brāhmaṇas are also the important element in human society. The kṣatriyas are more like the whole body; even though the whole body is bigger than the heart, the heart is more important."
(Śrīmad-Bhāgavatam 3.22.3, Purport)
"Every day the influence of Kali-yuga increases/ And the poor souls, being oppressed by the age of Kali, cry in agony/ Every day ten thousand cows are slaughtered/ And the expert leaders eat untouchable foodstuffs/ Men of mudlike intelligence increase day by day/ And a husband and wife can dissolve their relationship simply by a word/ Everyone has become a witch by the influence of the age/ Everyone is unhappy due to the absence of the devotees' mercy."
(Srila Prabhupada's Vyasa-puja Offering to Srila Bhaktisiddhanta Sarasvati, Sixth Vaisistya, Verses 57-60, 1961)