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SB 12.3.36

patiṁ tyakṣyanti nirdravyaṁ
 bhṛtyā apy akhilottamam
bhṛtyaṁ vipannaṁ patayaḥ
 kaulaṁ gāś cāpayasvinīḥ
Synonyms: 
patim — a master; tyakṣyanti — they will abandon; nirdravyam — lacking property; bhṛtyāḥ — servants; api — even; akhila-uttamam — most excellent in personal qualities; bhṛtyam — a servant; vipannam — incapacitated; patayaḥ — masters; kaulam — belonging to the family for generations; gāḥ — cows; ca — and; apayasvinīḥ — which have stopped giving milk.
Translation: 
Servants will abandon a master who has lost his wealth, even if that master is a saintly person of exemplary character. Masters will abandon an incapacitated servant, even if that servant has been in the family for generations. Cows will be abandoned or killed when they stop giving milk.
Purport: 

In India, the cow is considered sacred not because Indian people are primitive worshipers of mythological totems but because Hindus intelligently understand that the cow is a mother. As children, nearly all of us were nourished with cow’s milk, and therefore the cow is one of our mothers. Certainly one’s mother is sacred, and therefore we should not kill the sacred cow.