The Torah is clear with regard to the requirements of the parent of a male firstborn that must be redeemed. A month after birth, a payment of five shekel must be made to the kohen in order to redeem the child (see Bamidbar 18:16). The same detail does not exist regarding a firstborn kosher animal. How long does the owner of the animal have to care for the firstborn before transferring it to the kohen?
This question is dealt with in the first Mishna of the fourth perek of Massekhet Bekhorot, which begins on today’s daf, whose focus is on how the firstborn animal is transferred to the kohen and what his responsibilities are once he receives it.
According to the opening Mishna of the perek:
Until when must an Israelite tend to and raise a firstborn? With regard to a small animal, thirty days, and with regard to a large animal, it is fifty days. Rabbi Yosei says: In the case of a small animal the period is three months. If the priest says to the Israelite during this period ‘give it to me’, he must not give it to him. But if the firstborn was blemished and the priest said to him ‘give it to me so that I may eat it’, then it is allowed. And in Temple times, if the firstborn was in an unblemished state and the priest said to him ‘give, and I will sacrifice it,” it was allowed.
Although we find that the Sages in the Gemara search for Biblical sources for these periods of time, in his Commentary to the Mishna the Rambam writes that the main reason for these rules is that all gifts given to the kohen must be given in a manner similar to a gift given to a king, that is, it must be fully developed. For this reason, the owner cannot give the kohen the animal while it still needs the efforts to nurture and raise it, nor is the kohen permitted to take the animal at that stage.
The exception to this rule is when the kohen says that he would like the animal for slaughter (if the animal has a mum – a blemish that precludes sacrifice) or for sacrifice (for an animal without a mum). If the animal is old enough to be slaughtered for personal use or as a sacrifice then there is no reason to refrain from giving it to the kohen.