Sefer HaChinuch ספר החינוך

To not do work with two species of animals: To not plow with an ox and a donkey together. And the same is the case with any two species of animals, when one is pure and the other one is impure. And it is not specifically plowing that is forbidden, but rather the same is true for all types of work — for example, threshing or pulling a wagon [or] any of the types of work. And about this is it stated (Deuteronomy 22:10), “You shall not plow with an ox and a donkey together.”

[Concerning] the root of the commandment, Rambam, may his memory be blessed, wrote (Guide for the Perplexed 3:49, see also Ramban on Deuteronomy 22:10) that it is from the root of the prohibition of interbreeding animals as a forbidden mixture. As the way of workers of the land is to bring a pair (of animals) from one barn and lest [and there] he would graft them. And the root of the prohibition of interbreeding I wrote in its place in the Order of Kedoshim Tehiyu (Sefer HaChinukh 244). And after permission of the Master — the teacher mentioned — and concession to his good word, I will also answer my portion. And I say that from the explanations of this commandment is the matter of [causing] pain to animals, which is a prohibition of the Torah. And it is known that for species of animals and fowl, there is great anguish in dwelling with those that are not its species and — all the more so — to do work with them. And [it is] as we see with our eyes with the ones that are not under our hands (undomesticated species) that every bird dwells with its species; and all animals and other species also always cling to their [own] species. And any wise-hearted person will [learn] proper conduct from this, to never put together two people whose natures are different from one another and different in their behavior in any matter — like a righteous person and an evildoer [or] a slight person and an honorable one. Since if the Torah was exacting about the pain that there is to animals — that are not intelligent creatures — from this, all the more so [is this the case] with people that have an intelligent soul, in order to know their Maker.

From the laws of the commandment is that which they, may their memory be blessed, said (see Sefer HaMitzvot, Negative Commandments 218 and Mishneh Torah, Diverse Species 9:7) that the obligation of this commandment from Torah writ is only with two species like the ox and the donkey — that one of them is pure and the second one impure, [such] that their natures are very distant from one another. But with two species that are [both] pure or impure — like an ox and a goat, or a donkey and horse — it is not forbidden from the Torah. Rather it is [only] rabbinically that any two species that would [constitute a case of] forbidden interbreeding are forbidden — and both [domesticated] beasts and [undomesticated] animals are included in the prohibition. And it is the same whether one plows with them or seeds or pulls a wagon with them. And so [too,] if one [person] was sitting in the wagon and one was leading [them] — both of them are lashed, because his sitting in the wagon [also] causes the animal to pull. (See Mishneh Torah, Diverse Species 9:9.) And even if a hundred [people] led together, all of them are lashed. And [about] that which they said that it is permissible to do all work with a person and an animal, as it states, “with an ox and a donkey” — and not with a person and a donkey or a person and an ox (see Mishneh Torah, Diverse Species 9:10) — the explanation of the matter is according to the root that we mentioned. As the joining of an animal is not distressing for a person, since there is no connection with it at all. And behold, it is like he is doing work with a tree or a stone; and [so] it does not at all come into the designation that we spoke about. And also from the matter of the commandment is that which Rabbi Yitschak said at the end of Makkot 22a, that one who leads an ox of those disqualified from the consecrated is lashed — even though it is one body, the verse made it like two bodies to be lashed for. And the rest of its details are explained in the eighth chapter of Kilayim. (See Tur, Yoreh Deah 297.)

And this prohibition is practiced in every place and at all times by males and females. And one who transgresses this and seeds or plows or pulls or leads with a forbidden mixture — which are two species, one impure and one pure — is liable for lashes from Torah writ. And he is liable for lashes even if he led them together with his voice alone — like the way of animals that sometimes walk with the coaxing of people, as it is stated, “together” — in any case. They, may their memory be blessed, made a precise inference from the words of the verse to obligate lashes [even] with speech without a deed. And in [the case of] two species that were [both] impure or pure, that are forbidden to interbreeding with each other, he is rabbinically liable for lashes of rebellion.