The third perek (=chapter) of Masechet Bekhorot begins on today’s daf (=page). Its main focus is to analyze cases where we do not know whether the mother animal has ever given birth before, for example, where the animal was purchased from a non-Jew. Must we assume that the first animal that is born is a firstborn?
The Mishnah on today’s daf brings two opinions:
1. Rabbi Yishma’el says that we decide based on the age of the animal.
– If a goat gives birth in the first year of its life, the child certainly is considered a firstborn.
– If a sheep gives birth during the first two years of its life, the child certainly is considered a firstborn.
– If a cow or a donkey is three years old and gives birth, the child certainly is considered a firstborn.
Generally speaking, a goat can become pregnant at three months of age, although it is recommended to wait until the animal is nine moths old when it reaches two-thirds the weight of an adult animal. Pregnancy lasts five months, so it is possible for a goat to give birth within the first year of its life.
Sheep usually can become pregnant at 7-9 months of age and they give birth five months later at 12-14 months, that is to say, in their second year of life.
A cow can become pregnant at 12-14 months old and it gives birth after a nine month pregnancy at 23-25 months old, that is, in its third year.
2. Rabbi Akiva argues that since certain types of miscarriages are considered to be the “firstborn,” it is impossible to rely on the age of the animal as described by Rabbi Yishma’el. The rule that he presents is based on certainty. If we know that the animal has already once given birth, the animal will not be given to the kohen. If we know that it never gave birth, it remains in the possession of the owner. If we are unsure, we wait until it develops a mum – a blemish that precludes it from being brought as a sacrifice – and then it can be eaten by the owner.