Deuteronomy 22:10

10. Thou shalt not plough 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 (2Co 6:14), may or may not have been the case. But the prohibition prevented a great inhumanity still occasionally practised by the poorer sort in Oriental countries. An ox and ass, being of different species and of very different characters, cannot associate comfortably, nor unite cheerfully in drawing a plough or a wagon. The ass being much smaller and his step shorter, there would be an unequal and irregular draft. Besides, the ass, from feeding on coarse and poisonous weeds, has 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 ass and to pull only with one shoulder.

Robert Jamieson, A. R. Fausset & David Brown, 1871 · public domain

Verse 10 Thou shalt not plough with an ox and an ass - It is generally supposed that mixtures of different sorts in seed, breed, etc., were employed for superstitious purposes, and therefore prohibited in this law. It is more likely, however, that there was a physical reason for this; two beasts of a different species cannot associate comfortably together, and on this ground never pull pleasantly either in cart or plough; and every farmer knows that it is of considerable consequence to the comfort of the cattle to put those together that have an affection for each other. This may be very frequently remarked in certain cattle, which, on this account, are termed true yoke-fellows. After all, it is very probable that the general design was to prevent improper alliances in civil and religious life. And to this St. Paul seems evidently to refer, 2Cor 6:14 : Be ye not unequally yoked with unbelievers; which is simply to be understood as prohibiting all intercourse between Christians and idolaters in social, matrimonial, and religious life. And to teach the Jews the propriety of this, a variety of precepts relative to improper and heterogeneous mixtures were interspersed through their law, so that in civil and domestic life they might have them ever before their eyes.

Adam Clarke, 1831 · public domain