LEV740 The Pentateuch itself does not give any reason for the prohibition against plowing with an ox and ass together. Conceivably it belongs to the group of laws which oppose the mixing of diverse kinds, as we have in connection with sha'atnez and kelayim [this verse]. Indeed, Maimonides sees the prohibition against working animals of two different species together as a preventative against their having intercourse. (Sefer Ha-Chinukh, mitzvah 550). It is more likely, however, that the reason for this law is the general principle of tza'ar ba'alei chayyim: the prevention of pain to animals. For an animal to have to live or work together with others not of its own species is in itself a source of uneasiness and anxiety. Or as Ibn Ezra points out, yoking animals of unequal strength together is to certainly cause pain to the weaker animal and frustration to the stronger one (See his comments on Deuteronomy 22:10. See also the interesting theory of the Ba'al ha-Turim (on Deuteronomy 22:10), who states that since the ox, as a kosher animal, ruminates constantly, while the ass, as a non-kosher animal, does not, it will appear to the ass that the ox is "eating" all day, and as a result it will become aggravated with envy.)