When a bullock, or a sheep, or a goat — The three sacrificial quadrupeds. (See Leviticus 22:19 and Leviticus 17:3-6.)
Is brought forth. — From this expression it was enacted during the second Temple that the animal fit for a sacrifice had to be born naturally. One brought into the world by artificial aid was disqualified for the altar.
It shall be seven days under the dam. — Under seven days the animal is extremely weak, and unfit for human food, and hence must not be offered as the food of God, as sacrifices are called. (See Leviticus 22:25.) For the same reason children could not be circumcised before the eighth day from their birth. (See Exodus 22:29.) Because the text here says that the newly born animal is to be with the dam seven days, it was enacted that if the mother died before the seven days (in which case it could not be with the dam seven days), it was for ever disqualified for a sacrifice.