With the Temple destroyed, an unblemished firstborn cannot be offered. When one is born today, one must wait until it develops a disqualifying blemish; only then may the kohen slaughter it. Though caring for such an animal is a considerable burden, a kohen may not refuse to accept it — refusal would appear as contempt for the priestly gifts. He may not shut it away to starve, since that destroys sacred property. Nor may he cause a blemish: he may not, for example, press dough onto the firstborn’s ear hoping a dog will bite it along with the ear, nor may he ask a non-Jew to inflict a blemish. He must accept the firstborn, care for it, slaughter it if it develops a blemish on its own, and bury it if it dies without one, since neither its meat nor its hide may be used (SA YD 313:1–3; 309:1).
To avoid this problem, the custom is to sell animals that have not yet given birth to a non-Jew, as explained in the next section. If a Yisrael deliberately refrained from making such a sale in order to burden the kohen, the kohen is not required to accept the firstborn (Rema YD 306:4). If the Yisrael was merely negligent, without intent to provoke, a question arises: must the kohen accept and care for the firstborn, or may he argue that the Yisrael’s negligence leaves that responsibility with the Yisrael? (Ḥatam Sofer, YD §302; Pitḥei Teshuva, YD 306:2–3).
When a firstborn arrives unintentionally, the kohen must accept and care for it until it dies. Knowing this, kohanim were understandably eager for blemishes to appear — to the point that the Sages declared kohanim suspected of inflicting them. Accordingly, if a firstborn develops a blemish that a person could have caused, the kohen must produce a witness who saw it occur on its own; without that testimony, the blemish does not permit slaughter (SA YD 314:1).
Once a witness attests that the blemish occurred naturally, the animal is shown to an expert in the laws of blemishes. If none is available, it is shown to three Torah scholars, at least one familiar with these laws. If they rule that it has developed a permanent, non-healing blemish, it may be slaughtered (SA YD 309:2). If it was slaughtered without prior examination, its meat is prohibited — even if a scholar who examined it afterward confirms the blemish was permanent. The Sages imposed this penalty for bypassing the prior examination. Some are lenient with obvious blemishes (SA YD 310:1; Taz 310:1).
