Only the firstling of the animals, which should be the LORD'S firstling, no man may dedicate it; whether it be ox, or sheep: it is the LORD'S.
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George Leo Haydock
First-born. Septuagint add "of beasts. "Men, though belonging to the Lord on that title already, (Exodus xiii. 2,) might still be more particularly consecrated to him by vow, as Samuel was. (Calmet)
A vow must be concerning some greater good to which we are not otherwise bound. Such vows are agreeable to God, and can never be broken without sin. See Genesis xxxi. 13 and 1 Timothy v. 12. (Worthington)