Bechorot 25 - Shearing a Firstborn
It is forbidden to shear a firstborn, even a blemished one. How is one to deal with the hair of the firstborn animal while slaughtering it? Rabbi Yose ben Meshulam gave a rule: one can clear the area on the neck for the shechitah incision. One may even pluck out the hair, but leave it in place, to avoid the impression that he is shearing it. Can one do it on a Yom Tov holiday? Is plucking not considered sharing, but it would still be prohibited on a Yom Tov, or is plucking permitted because he does not have the intention to shear, and in that case it would be allowed even on a Yom Tov? They asked Rav Huna, and he said that it is allowed.
If he does shear a firstborn, the wool is still prohibited for benefit. If some wool fell out and was placed in storage for safekeeping, and then the firstborn was slaughtered, the wool is now permitted. This is also true for wool that was sheared illegally while the firstborn was alive, but the teacher did not want to mention it.
Art: Tom Roberts - Shearing the Rams