कल्पान्तरे विश्वासटिप्पन्यः

Source: Nikhil

bulls and kine (mainly the latter) are often called ághnya/ā “not to be slain” in the Véda, despite the same texts referring to the killing of kine. There are two ways by which scholars reconcile this:

It was not permitted to kill them while still fertile/productive (capable of producing offspring or of producing milk for their calves and for men), and ághnyā refers specifically to the productive cow rather than the vaşā́ (barren cow). If this is the case, it is not Vedically acceptable to eat purchased beef, since such meat almost always comes from a fertile cow far before it becomes barren. However, barren kine that you kill yourself would then be permissible to eat.
It was not permitted to kill them outside of rites, and the references to killing kine in the Véda all refer to specific rites. Here “rite” encompasses more than just a sacrifice to the Gods, since even by the Sū́tra period it is permissible to kill and eat kine outside sacrifice in certain circumstances: honoring a guest, honoring the Fathers, and weddings (ĀpGS 1.3.10).

Between the two, the first explanation is more convincing. The word ághnyā is never used to describe the barren cow, while it’s especially frequently used in the context of milch-kine. +++(धेन्वनडुहोर् भक्ष्यम् इति त्व् आपस्तम्बः)+++ A restriction on killing such productive animals makes sense even pragmatically or ā priōrī, while a restriction on killing them later doesn’t.