“During the reign of Mahārāja Yudhisthira, the clouds showered all the water that people needed, and the earth produced all the necessities of man in profusion. Due to its fatty milk bag and cheerful attitude, the cow used to moisten the grazing ground with milk.”
Because the cows were very cheerful, the milk was dropping from the milk bag that the grazing ground became muddy. It was muddy not with water but with milk. So much milk was being delivered by the cows. Because the cows were always in “Cheerful attitude”. Cows also have got intelligence, when they know that they will be killed they do not supply sufficient milk. They cannot supply because of fear, just like if one’s mind is full of anxiety, they cannot work properly.
This is nature’s economic development. Artificially one cannot increase the production of milk. But according to the instruction of scripture, if we keep them cheerful without any fear, they will deliver double milk. Therefore in the Bhagavad-gītā it is recommended that cows should be protected in the human society.
Ref >> Srila Prabhupada Vani.