Commentary for Eduyot 7:6

ולד שלמים לא יקרב שלמים – but one puts it into the prison and it dies. It is because a decree, for if we you were to say that the offspring of a peace-offering has a remedy, one can wait for the mother until she gives birth and the flock would grow from the offspring and it would provide wool and labor.

Rabbi Joshua and Rabbi Papias testified concerning the offspring of a peace-offering, that it can be brought as a peace-offering, whereas Rabbi Eliezer says that the offspring of a peace-offering cannot be brought as a peace-offering.
But the sages say: it can be brought.
Rabbi Papias said: “I testify that we had a cow, which was a peace-offering, and we ate it at Passover, and its offspring we ate as a peace-offering at the [next] festival.
This mishnah discusses the offspring of a peace-offering. A peace-offering (shelamim) was a sacrifice that was usually brought either as a voluntary offering, or on festivals. The breast and the right hand leg would go to the priests and the remainder of the animal could be eaten by those who had brought it. The issue in our mishnah is the status of the offspring of an animal that had already been set aside to become a peace-offering. In other words, after the owner declared that he was going to bring the animal to the Temple as a peace-offering, it gave birth.
According to Rabbi Joshua and Rabbi Papias, the offspring of a peace-offering can be brought as a peace-offering. Rabbi Eliezer ruled that it may not be brought as an peace-offering. The Talmud explains that according to Rabbi Eliezer the animal is put into a pen and let to die through starvation. The reason is that if the halakhah were to allow the owner to bring it as a peace-offering he would have incentive to delay bringing the mother , who has already been declared a peace-offering, to the Temple. The owner might wait until she gives birth, perhaps several times, in order that he would be able to bring more peace-offerings (after all he benefits as well by having more meat to eat). This delay in bringing the animal to the Temple would violate a rule in Deuteronomy 23:22 which states that when you offer a voluntary sacrifice, do not delay in bringing it.
The Sages side in this dispute with the testimony of Rabbi Joshua and Rabbi Papias. At the end of the mishnah Rabbi Papias brings his own personal experience of having eaten a peace-offering at one festival and its offspring at the next.

ואכלנו ולדה שלמים בחג – on the holiday of Shavuot, for if he would wait and look forward to the holiday of Sukkot, it is found that he would violate a positive commandment, as it states (Deuteronomy 12:5,6): “There you are to go, and there you are to bring [your burnt offerings and other sacrifices, your tithes and contributions, your votive and freewill offerings, and the firstlings of your herds and flocks],” that implies on the first Festival that you would go there, bring all your vows that are upon you. However, the negative commandment of “[When you make a vow to the LORD your God] do not put off fulfilling it,” he does not transgress until three festivals have passed by.