Srila Prabhupada

The cows, oh, as soon as they see Kṛṣṇa, they become . . . they lick up His feet and body, and every cow has got a different name. As soon as He will call, the cow will come immediately, and dropping milk. And those cows are also spiritual. Surabhī. It is described in the Brahma-saṁhitā, surabhīr abhipālayantam (Bs 5.29).
Surabhī. Surabhī cow means nonexhaustive. You can milk as much milk you want and as many times. In the material world the cow is limited. There is time that you can milk, morning and evening, and so much quantity, not more than that. But surabhīr means you can milk those cows anytime you like, and you can draw milk as many as you like, as much as you like. This is called surabhī. urabhīr . . . in the description of Brahma-saṁhitā: cintāmaṇi-prakara-sadmasu kalpa-vṛkṣa-lakṣāvṛteṣu surabhīr abhipālayantam. Surabhīr abhipālayantam. So therefore He is Govinda. He gives . . . He is pleasure for everyone.
(661211 - Lecture CC Madhya 20.152-154 - New York)

Especially if we are human being, the cow is supplying us milk, the most important foodstuff. So instead of giving protection to the cow, if we kill, do you think that is very sensible thing? No. I am supplying something very nice, and if you kill me, is that very good gratitude?
So at least in human life, these senses should be there. Cow protection is recommended in the Vedic literature because it is giving the most valuable foodstuff, milk. Apart from other sentiments, it is supplying, and in exchange of nothing. She simply eats some grasses from the ground. That's all. You don't have to provide cows with foodstuff. The things which you refuse, you take the grains and you supply the skin.
You take the fruit pulp, you supply the skin. You take the, I mean to say, from paddy. You take the rice. You supply the straw, and she delivers you a very nice foodstuff. And I have discussed all these point in my Śrīmad-Bhāgavatam, that human economic problem can be solved simply by having some land and some cows. That's all.
(680504 - Lecture SB 05.05.01-3 - Boston)