When a person owns a Bechor (firstborn animal), he has to give it to a Kohen (priest). He doesn't give it to him right away when it's born; rather, he cares for the animal for a certain amount of time, until it's grown up a little and there's no major work that has to be done with it. Then he gives it to a Kohen. A Bechor is a gift given to the Kohanim, and therefore it has to be given over in a very noble, respectful way, like somebody presenting a gift to a king. So the Kohen cannot do any work or even appear to do any work for the animal. That would be problematic. It would make it look as if there's a payment being offered for it, so the Kohen can't do any work for the Bechor, the Mishnah says.
How long does the Yisrael (Israelite) have to care for the Bechor before he gives it to the Kohen? For a Bechor that is a Behemah Dakah, a small animal like a sheep or a goat, the owner has to care for it for 30 days after it's born. Those are considered to be the hard times. For a Bechor that is a Gasah, a large animal like a cow, he has to care for it for 50 days from its birth, and then give it over to the Kohen. The Gemara learned this through a connection to the Bikurim (first fruits), which is brought down in a Pasuk (verse), and they learn it out in that way.
Rabbi Yose says that for a Bechor of a small animal, the owner has to care for it for a period of three months. So Rabbi Yose disagrees about the reality. According to Rabbi Yose's opinion, if it's a small animal, it has to be cared for longer because a small animal has teeth that are very thin, teeth that are very weak, and therefore it has an inability to eat grass. It needs to be with its mother, nursing, for a longer period, and that's why it needs the full three months, as opposed to what the Tanna Kama (first opinion) said, which was only 30 days. But the Halacha (Jewish law) is not like Rabbi Yose; it only needs those 30 days.
We spoke about the situation in which a person is not allowed to hand it over to a Kohen, but what if the Kohen wants to take it before those time periods? Amar lo haKohen, betoch zman zeh, tenahu li, harei zeh lo yitnenu lo. The Kohen told the owner, “Within the time period for the Bechor, give me the Bechor now; I'll take care of it.” The owner is not allowed to give it to him. It appears as if the Kohen is doing the work for the Yisrael by caring for the Bechor and receiving the Bechor as payment. It is forbidden for the Kohen to do that because it looks like he's working for the Bechor. The Kohen is treated like a king, and therefore anything that appears to be like he's working for any of the Matanot Kehuna (priestly gifts) is forbidden. You're not allowed to have the Kohen work on your grain and then give him Terumah (a portion). That would be disrespectful to the Kohen.
If it was a Bechor, he would have told him to give it to me so that our food would be eaten. But if the Bechor was blemished, the Kohen would be allowed to slaughter it immediately and eat it right away. If the Kohen told the owner, “Within the time that we mentioned before, give me the Bechor; I'm going to slaughter it right away and eat it,” Mutar (it's permitted). In that situation, it's permitted to give him the animal because the Kohen is going to slaughter it right away. He doesn't need to care for it whatsoever. So it doesn't appear that he's working for the Yisrael in return for the Bechor. It doesn't give off that perception because he's slaughtering it right away. The only time it gives off that perception is if he's caring for it for an extended period of time.
In the times of the Beit HaMikdash, if the Bechor was unblemished—and when it's unblemished, it's offered on the Mizbeach (altar)—and the Kohen tells the owner, “Give me the Bechor, and I'm going to offer it right away to the Beit HaMikdash,” he's not going to care for it either. He's not going to spend all the time nurturing it, so there's no hard work for him either, and it doesn't give off that perception. Mutar, it's permitted to give him the animal. He can offer the Bechor as a korban (offering) right away when it is eight days old; he doesn't need to care for it whatsoever.
The Bechor that's handed over has to be eaten within the first year. Whether you're talking about an unblemished Bechor, which is brought as a korban in the Beit HaMikdash, or a blemished one, which is slaughtered and eaten by the Kohen, the Pasuk tells us, “before Hashem, your God, you should eat it year by year.” This tells us you have a one-year deadline to eat the Bechor and to do what you will with it.