SB 8.8.2
tam agni-hotrim rsayo
jagrhur brahma-vadinah
yajñasya deva-yanasya
medhyaya havise nrpa
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
O King Pariksit, great sages who were completely aware of the Vedic ritualistic ceremonies took charge of that surabhi cow, which produced all the yogurt, milk and ghee absolutely necessary for offering oblations into the fire. They did this just for the sake of pure ghee, which they wanted for the performance of sacrifices to elevate themselves to the higher planetary systems, up to Brahmaloka.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Surabhi cows are generally found on the Vaikuntha planets. As described in Brahma-samhita, Lord Krsna, on His planet, Goloka Vrndavana, engages in tending the surabhi cows (surabhir abhipalayantam). These cows are the Lord’s pet animals. From the surabhi cows one can take as much milk as one needs, and one may milk these cows as many times as he desires. In other words, the surabhi cow can yield milk unlimitedly. Milk is necessary for the performance of yajña. Sages know how to use milk to elevate human society to the perfection of life. Since cow protection is recommended everywhere in the sastras, the brahmavadis took charge of the surabhi cow, in which the demons were not very interested.