An animal of Pesulei ha'Mukdashin conceives before Pidyon and bears a child afterwards, Rav Huna contends that if one failed to sanctify the as yet unborn babies, before redeeming the mother, one should put them in a small room and allow them to die.
Our Daf Yomi shiur wonders if anyone has a good Svara why we have to allow the animals to die in such a cruel manner? Why can't they just be killed humanely?
The Gemara makes it clear that such offspring retain a degree of Kedushah which cannot be redeemed in any manner. It is therefore prohibited to intervene and actively cause their deaths. On the other hand, they cannot be brought on the Mizbe'ach. If we allow them to live and grow older, people will inadvertently work and shear the animals. We have no choice but to cause their deaths, "hands off."
Kollel Iyun Hadaf