SB 8.8.1
sri-suka uvaca
pite gare vrsankena
pritas te ’mara-danavah
mamanthus tarasa sindhum
havirdhani tato ’bhavat
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Sukadeva Gosvami continued: Upon Lord Siva’s drinking the poison, both the demigods and the demons, being very pleased, began to churn the ocean with renewed vigor. As a result of this, there appeared a cow known as surabhi.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The surabhi cow is described as havirdhani, the source of butter. Butter, when clarified by melting, produces ghee, or clarified butter, which is inevitably necessary for performing great ritualistic sacrifices. As stated in Bhagavad-gita (18.5), yajña-dana-tapah-karma na tyajyam karyam eva tat: sacrifice, charity and austerity are essential to keep human society perfect in peace and prosperity. Yajña, the performance of sacrifice, is essential; to perform yajña, clarified butter is absolutely necessary; and to get clarified butter, milk is necessary. Milk is produced when there are sufficient cows. Therefore in Bhagavad-gita (18.44), cow protection is recommended (krsi-go-raksya-vanijyam vaisya-karma svabhava jam).