Lord Krishna is the well-wisher and protector of the Cows. His other name is Govinda, which means One Who gives pleasure to Cows and senses. Vedic times consider cow as one of the seven mothers of the humanity. She provides nutritious milk, which is very important food for human beings right from young days. Milk and milk products are most important food ingredients for humanity, hence it is very important to protect them. And bulls are very important for agriculture, Lord Balaram is considered Lord of agriculturist, He carries a plough, because of which He is called Haladhara. With the bulls we plow the land, hence they are like the father who produces food and cow is like the mother producing milk. The manure from them, is very important for agriculture, hence any civilized human being should be protecting these animals.
It is a Vedic tradition, to have Goshala or shelter for cows in the temples. Following, this tradition here at ISKCON, Kanpur, we have a Goshala which houses about 20 cows. The milk supplied by these cows is used for various Deity worship programs. The milk given by these cows is used for daily Abhisheka seva to Saligrama shila, it is also used for preparing various food items which are offered to deities as Naivedya and finally these food items are distributed to all the devotees and pilgrims in the temple as Maha Prasadam.
Srimad-Bhagavatam 1.17.9 Purport
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 Maharaja Pariksit and the Pandavas 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 Maharaja Pariksit. For protecting the cows and brahminical culture, the Lord, who is very kind to the cow and the brahmanas (go-brahmana-hitaya), will be pleased with us and will bestow upon us real peace.
Srimad-Bhagavatam 1.14.34 Purport
The Lord is the protector of cows and the brahminical culture. A society devoid of cow protection and brahminical culture is not under the direct protection of the Lord, just as the prisoners in the jails are not under the protection of the king but under the protection of a severe agent of the king. Without cow protection and cultivation of the brahminical qualities in human society, at least for a section of the members of society, no human civilization can prosper at any length.
Srimad-Bhagavatam 4.21.38 purport
Protection of Cows is the single-most important principle towards saving the whole world from both moral and spiritual degradation. Simply by protection of Cows all the wonderful benefits of religiosity and piety will be automatically achieved without any separate endeavour. In scriptures it is said, namo brahmanya-devaya go-brahmana-hitaya ca jagad-dhitaya krsnaya govindaya namo namah, “My Lord, You are the well-wisher of the Cows and the brahmanas, and You are the well-wisher of the entire human society and world.” (Visnu Purana 1.19.65) Lord Krsna, the Supreme Personality of Godhead, is the prime protector of brahminical culture and the Cow. Without knowing and respecting these pleasing personalities, one cannot realize the science of God, and without this knowledge, any welfare activities or humanitarian propaganda cannot be successful. SB 4.21.38 purport