The faraa was the firstborn of the offspring. They would slaughter it as soon as it was born to them.
حدثنا الحسن بن علي حدثنا عبد الرزاق أخبرنا معمر عن الزهري
الفرع أول النتاج كان ينتج لهم فيذبحونه
[Its chain is daif] and it is in Musnaf Abdul-Razzaq, no. 7998 * its chain is daif because of the anan style narration of Zuhri, but its meaning is sahih by consensus.