Bechoros 3:2-3

Bechoros 3:2

Rabban Shimon ben Gamliel says that if a Jew purchases an animal from a non-Jew that is nursing a young, one need not be concerned that it might be the offspring of a different animal. If he walked among the herd and saw both animals that had not given birth before and those that had given birth before nursing the young, he need not be concerned that the offspring of one animal went to another (i.e., we can assume that the mothers who had not given birth before are nursing the firstborns).

Bechoros 3:3

Rabbi Yosi ben Meshulam says that one who slaughters a firstborn animal clears the space to cut on either side using a hatchet; he may pluck the hair so long as he doesn’t remove it from the fleece (because it is prohibited to shear a firstborn). The same is true of one who pulls out the animal’s hair to examine a blemish.