You must set apart to the LORD your God every firstborn male produced by your herds and flocks. You are not to put the firstborn of your oxen to work, nor are you to shear the firstborn of your flock.
19. All the firstling males. Another caution is added, that they should make no profit of the first-born; for they might have used the labor of the ox in plowing, or as a beast of burden; they might also have sheared the lambs, and have afterwards brought a deteriorated animal into the tabernacle. God commands, therefore, that what was due to Him should be honestly and absolutely paid.
Here is, 1. A repetition of the law concerning the firstlings of their cattle, that, if they were males, they were to be sanctified to the Lord (Deu 15:19), in remembrance of, and in thankfulness for, the sparing of the first-born of Israel, when the first-born of the Egyptians, both of man and beast, were slain by the destroying angel (Exo 13:2, Exo 13:15); on...
Commenting on Deuteronomy 15:19-23
All the firstling males that come of thy herd, or of thy flock, thou shalt sanctify unto the Lord thy God,.... According to the law in Exo 13:2. See Gill on Exo 13:2; See Gill on Exo 13:12; See Gill on Exo 13:13, thou shalt do no work with the firstling of thy bullock: as to plough or tread out the corn with it, which...