Deuteronomy 22:10

Thou shalt not plow with an ox and an ass together.

Thou shalt not plow with an ox and an ass together. Whether this association, like the mixture of seeds, had been dictated by superstitious motives, and the prohibition was symbolical, designed to teach a moral lesson (2 Corinthians 6:14), may or may not have been the case. Maimonides, following the generality of Jewish writers, considers the reason of this interdict to have been, that the ox was a clean, while the donkey was an unclean animal. But the prohibition prevented a great inhumanity still occasionally practiced by the poorer sort in Oriental countries. An ox and a donkey being of different species, and of very different characters, cannot associate comfortably, nor unite cheerfully in drawing a plow or a wagon. The donkey being much smaller, and his step shorter, there must be an unequal and irregular load. Besides the donkey, from feeding on coarse and poisonous weeds, as a fetid breath, which its yoke-fellow seeks to avoid, not only as poisonous and offensive, but producing leanness, or, if long continued, death. And hence, it has been observed always to hold away its head from the donkey, and to pull only with one shoulder.

The classic writers on agriculture give the same precept as Moses; and yet the cruel and unnatural practice of yoking these two animals of different species was very prevalent, as appears from a familiar allusion to it by Plautus ('Aulularia,' art. 1:, sec. 4, where Euclio's dialogue with Megadorus says, 'Now if I were to give my daughter to you, it seems to me that, when we had formed this alliance, I should be the donkey and you the ox').