SB 9.2.3

pṛṣadhras tu manoḥ putro
 go-pālo guruṇā kṛtaḥ
pālayām āsa gā yatto
 rātryāṁ vīrāsana-vrataḥ
Word for word: 
pṛṣadhraḥ tu — among them, Pṛṣadhra; manoḥ — of Manu; putraḥ — the son; go-pālaḥ — herding cows; guruṇā — by the order of his spiritual master; kṛtaḥ — having been engaged; pālayām āsa — he protected; gāḥ — cows; yattaḥ — so engaged; rātryām — at night; vīrāsana-vrataḥ — taking the vow of vīrāsana, standing with a sword.
Translation: 
Among these sons, Pṛṣadhra, following the order of his spiritual master, was engaged as a protector of cows. He would stand all night with a sword to give the cows protection.
Purport: 

One who becomes vīrāsana takes the vow to stand all night with a sword to give protection to the cows. Because Pṛṣadhra was engaged in this way, it is to be understood that he had no dynasty. We can further understand from this vow accepted by Pṛṣadhra how essential it is to protect the cows. Some son of a kṣatriya would take this vow to protect the cows from ferocious animals, even at night. What then is to be said of sending cows to slaughterhouses? This is the most sinful activity in human society.