The mitzva of the firstborn animal has two dimensions. One is its sanctity: it must be brought as an offering to God. The other is that it is one of the twenty-four gifts the Torah commands us to give the kohanim so they can be free to serve God.
The firstborn’s sanctity applies regardless of ownership — whether the animal belongs to a kohen, a levi, or a Yisrael. If the animal belongs to a Yisrael or levi, its firstborn must be given to a kohen. If it belongs to a kohen, he must separate it from his other animals and offer it on the altar; he and his household then eat its meat, as they would a firstborn received from a Yisrael.
Since the firstborn is a gift to the kohen, even if it develops a blemish and becomes unfit as an offering, it is still a mitzva to give it to him. The kohen’s family may eat it anywhere, even in a state of tum’a. The kohen may also sell its meat to a Yisrael, provided he does not sell it from a butcher shop, as it is unbecoming for the meat of a firstborn to be sold in a store (SA YD 306:6).
The firstborn is not given to the kohen while very young, so as not to burden him with its care. The owner keeps it until it has grown somewhat: thirty days for a sheep or goat, fifty days for cattle. If no kohen has been found after that, the owner continues to care for it until one is located (SA YD 306:2). In any event, the firstborn must reach the kohen within its first year, so he can offer it before it turns one, as it is stated: “Before the Lord your God you shall eat it year by year” (Devarim 15:20). Even a blemished firstborn must reach the kohen within that first year (SA YD 306:7).
Because the firstborn is sacred, one may not use it for any purpose, whether unblemished or blemished — neither plowing with a firstborn ox nor shearing a firstborn sheep. If one sheared it in violation, benefit from that wool is prohibited. If, however, the firstborn sheep developed a blemish and was slaughtered for eating, one may benefit from the wool attached to its hide (SA YD 308:1).
