Deuteronomy 22:9

Defiled — or sanctified. Different crops become “common” at different times. The year’s corn was freed by the wave-sheaf and wave-loaves. The trees not for five years. The rule about the ox and the ass may rest partly on the ground of humanity, the step and the pull of the two creatures being so very unlike. St. Paul gives a spiritual sense to the precept in 2 Corinthians 6:14. “Be not unequally yoked together with unbelievers.” The ox was a clean animal and fit for sacrifice. The ass was unclean, and must be redeemed with a lamb. The clean and unclean must not till the holy land of Jehovah together.

All these precepts are part of the laws of holiness in Leviticus — rules of behaviour arising from the fact that Israel is the special people of a holy God.