Bekhorot 21: three things to remember

Rabbi Shimon allows an animal under eight days old into the tithing pen, learning this from firstborn laws through a verbal analogy.
A knowledgeable shepherd must evaluate a murky discharge to confirm an expelled fetus, which exempts future offspring from firstborn sanctity.
Rabbi Yoḥanan rules that offspring of an animal bought from an Israelite are non-sacred, presuming the seller would have warned about firstborn status.