“The cows in Krishna’s abode are also liberated souls. They are called surabhi cows. There are many popular pictures showing how Krishna loves the cows, how He embraces and kisses them. That passive relationship with Krishna is called santa. Their perfect happiness is achieved when Krishna comes and simply touches them.”
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.
Science of Self-Realization, Chapter 8