Leviticus 27:26–27 provide further details regarding the vows of persons and animals. According to the verses, the firstborn man or animal cannot be subject to a vow. This is because the firstborn already belongs to the Lord (see Exodus 13:2; Exodus 34:19–20).
26 “But a firstborn of animals, which as a firstborn belongs to the LORD, no man may dedicate; whether ox or sheep, it is the LORD’s.