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 endeavor. This is because Cows are as beloved to the Supreme Personality of Godhead, Lord Krishna, as even saintly brahmanas.
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
For the Cowherd men and the Cows, Krishna is the supreme friend. His pastimes in Gokula, His dhama, are always favorable to the brahmanas and the Cows. His first business is to give all comfort to the Cows and the brahmanas. In fact, comfort for the brahmanas is secondary, and comfort for the Cows is His first concern. Because of His presence, all people would overcome all difficulties and always be situated in transcendental bliss. SB10.8.16
Krishna says in Srimad Bhagavatam, “I can be worshiped within the Cows by offerings of grass and other suitable grains and paraphernalia for the pleasure and health of the Cows, and one may worship Me within the Vaishnavas by offering loving friendship to them and honoring them in all respects.” SB11.11.43