Mishnayos Bechoros Perek 4 Mishnah 1

עד כמה. להטפל בבכור – to be engaged with its (i.e., the firstling’s) raising prior to giving it to the Kohen.

בדקה עד שלשים יום ובגסה עד חמשים – In the Gemara (Tractate Bekhorot 26b), we derive it, from what is written (Exodus 22:28): “You shall not put off the skimming of the first yield of your vats. You shall give Me the first-born among your sons,” and adjacent to it (Exodus 22:29), “You shall do the same with your cattle and your flocks,” and they show an incongruity/objection, “your bull” which is mentioned earlier in Scripture to “the skimming of the first yield of your vats” which is early, and “your flocks” which is later than the “the first-born among your sons” but “the skimming of the first yield of your vats” which are the first-fruits are offered at fifty days, that the grain is ripened on Passover and we don’t bring the first-fruits until Atzeret/Shavuot, as it is written (Exodus 23:16): “and the Feast of the Harvest, of the first fruits of your work, [of what you sow in the field], and just as you do for the “skimming of the first yield of your vats, which you do not bring until the end of fifty days, so you do for your bulls that you do not bring it until fifty days, and in the manner that you act regarding the first-born of your sons, as it is written concerning it (Numbers 18:16): “Take as their redemption price from the age of one month up,” similarly you should do for your flock that you should not bring it until thirty days.

בדקה שלשה חדשים – because its care-taking (i.e., of small cattle) is more troublesome (see Talmud Bekhorot 26b and Talmud Bava Metzia 69a), for its teeth are thin and it is not able to eat grass. And if it won’t be with its mother it will die. But the Halakha is not according to Rabbi Yossi.

לא יתנו לו – for since its care-taking is cast upon an Israelite for fifty days, and the Kohen said to him “Within this time [period], give it to me and I will pasture/tend it.” It is similar to someone who rents it that he saves him from the trouble on the condition that he (i.e., the owner) will give it to him and not to another Kohen, and this is similar to a Kohen who helps/assists in the granary/threshing floor to thresh and to winnow in order that they will give him the heave-offerings/Terumot, and it is taught [in a Baraitha] that Kohanim and Levites who assist in the granary/threshing floor, they don’t give them have-offerings and tithes. But if [the animal] had a blemish and he said to him: “Give it to me and I will consume it, for since it was pointed out to him, e was not like the Kohen who assists in the granary.

בין תמים בין בעל מחם – whether at the time of the Temple or whether at the present time, every year it is his, he is permitted to uphold it. And a firstling with a blemish, we derive that it is given to a Kohen, and he consumes it with its blemish for since it is written (Numbers 18:18): “But their meat shall be yours,” is the plural language, whether it is pure firstling or whether it is a firstling with a blemish.