In addition to the prohibition against breeding two species with one another, the Torah forbids working with two different species together. For example, one may not plow with an ox and a donkey together or harness a horse and a donkey to the same wagon. The verse states: “You shall not plow with an ox and a donkey together” (Devarim 22:10). The Torah uses the example of an ox and a donkey because these are domesticated species commonly put to work together. However, the prohibition applies to any combination of two animal species used together for labor. The prohibition is not limited to the driver who harnesses the horse and donkey to the wagon and directs them; anyone who assists in the work violates it as well. Even one who only calls out to drive them forward violates a Torah prohibition, since he causes them to continue pulling the wagon or plow (SA YD 297:11, 13–14).[3]
Jews alone are bound by this prohibition. If a Jew hires a gentile to transport merchandise to another city, and the gentile chooses to use a wagon harnessed to a horse and a donkey, the Jew violates no prohibition, since he has no stake in that arrangement (Arukh ha-Shulḥan YD 297:21). However, a Jew may not sit in a wagon that a gentile drives with a horse and donkey, because the passenger becomes a partner in the labor. If he were not sitting in the wagon, the driver would not command the animals to pull it (SA YD 297:12).
The prohibition of kilayim applies only when the two species are tied together. Therefore, one riding a horse may hold a rope attached to a dog. However, he may not tie the dog’s leash to the horse’s saddle, since doing so connects the two animals. At times, the dog may move ahead of the horse and pull it forward, causing the rider to violate the prohibition of animal kilayim (Arukh ha-Shulḥan YD 297:19).
Although we cannot fully grasp the depth of the mitzvot, especially the ḥukim, the Sages nevertheless sought to understand their reasons. Rambam explains that this prohibition is meant to keep different species apart, so that people will not be led to breed one species with another (Moreh Nevukhim 3:49). Sefer ha-Ḥinukh explains that the prohibition prevents animal suffering. Each species naturally prefers the company of its own kind and suffers when paired with another species. This is especially true during labor, because animals of different species do not have equal strength; when they do not pull the wagon or plow evenly, they suffer (Sefer ha-Ḥinukh 550).
