Albert Barnes Commentary Deuteronomy 15:19-23

"All the firstling males that are born of thy herd and of thy flock thou shalt sanctify unto Jehovah thy God: thou shalt do no work with the firstling of thy herd, nor shear the firstling of thy flock. Thou shalt eat it before Jehovah thy God year by year in the place which Jehovah shall choose, thou and thy household. And if it have any blemish, [as if it be] lame or blind, any ill blemish whatsoever, thou shalt not sacrifice it unto Jehovah thy God. Thou shalt eat it within thy gates: the unclean and the clean [shall eat it] alike, as the gazelle, and as the hart. Only thou shalt not eat the blood thereof; thou shalt pour it out upon the ground as water." — Deuteronomy 15:19-23 (ASV)

Compare to Exodus 13:11 and following. The directions of the preceding legislation (see Numbers 18:15 and following) are here assumed, with the injunction added that the animals thus set apart to God (Deuteronomy 15:19) were not to be used by their owners for their earthly purposes.

It is further allowed that firstborn animals which had a blemish should be regarded as exceptions, and instead of being given to God, might be used as food (Deuteronomy 15:21–22).

The application of the firstborn of cattle is here directed as in Deuteronomy 12:6, Deuteronomy 12:17, and Deuteronomy 14:23: they are to be consumed in the sacred Feasts at the sanctuary.