Hare Krsna
Bull is the emblem of moral principle and the cow is the representative of the earth. When the bull and the cow are happy in joyful mood it is to be understood that the people of the world are also happy and are in joyful mood. The reason is that the bull helps production of grains in the agricultural field and the cow delivers milk the miracle of aggregate food values. The human society therefore maintains, these two important animals very carefully so that they can wander everywhere in cheerfulness.
(SB 1.16.18 purport)