Albert Barnes Commentary Deuteronomy 21:3

"and it shall be, that the city which is nearest unto the slain man, even the elders of that city shall take a heifer of the herd, which hath not been wrought with, and which hath not drawn in the yoke;" — Deuteronomy 21:3 (ASV)

The requirements regarding the place and victim are symbolic. The heifer represented the murderer, at least to the extent that it died in his place, since he himself could not be found. As bearing his guilt, the heifer therefore had to be one of full growth and strength, and one that had not yet been ceremonially profaned by human use.

Christian commentators find here a type of Christ and His sacrifice for humanity. However, the heifer was not strictly a sacrifice or sin-offering. The transaction was rather figurative and was designed to impress the lesson of Genesis 9:5.