nisamyodayudham ca tam
gauh saty apadravad bhita
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
When the earth saw that King Prthu was taking his bow and arrow to kill her, she became very much afraid and began to tremble. She then began to flee, exactly like a deer, which runs very swiftly when followed by a hunter. Being afraid of King Prthu, she took the shape of a cow and began to run.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Just as a mother produces various children, both male and female, the womb of mother earth produces all kinds of living entities in various shapes. Thus it is possible for mother earth to take on innumerable shapes. At this time, in order to avoid the wrath of King Prthu, she took the shape of a cow. Since a cow is never to be killed, mother earth thought it wise to take the shape of a cow in order to avoid King Prthu’s arrows. King Prthu, however, could understand this fact, and therefore he did not stop chasing the cow-shaped earth.