yavasam jagdhy anudinam
naiva dogdhy audhasam payah
tasyam evam hi dustayam
dando natra na sasyate
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Although you are eating green grass every day, you are not filling your milk bag so we can utilize your milk. Since you are willfully committing offenses, it cannot be said that you are not punishable due to your assuming the form of a cow.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
A cow eats green grasses in the pasture and fills her milk bag with sufficient milk so that the cowherdsmen can milk her. Yajñas (sacrifices) are performed to produce sufficient clouds that will pour water over the earth. The word payah can refer both to milk and to water. As one of the demigods, the earthly planet was taking her share in the yajñas — that is, she was eating green grass — but in return she was not producing sufficient food grains — that is, she was not filling her milk bag. Prthu Maharaja was therefore justified in threatening to punish her for her offense.