Hybrid or Kil’ayim?

Parshas Kedoshim contains one of the two places in the Torah where the mitzvos of kil’ayim are taught.

Question #1: Kil’ei beheimah

May one attempt to crossbreed a mule with a stallion?

Question #2: Kil’ei zera’im

May I plant the vegetables in my garden close together?

Question #3: Kil’ei hakerem

Is there any way that I can plant vegetables near my vineyard?

Question #4: Harkavas ilan

Must I be careful before I purchase a fruit tree?

Many people assume that the halachic definition of the mitzvah of kil’ayim is the crossbreeding of different species of plants or animals, but, as we will soon see, not all of the laws of kil’ayim have to do with what a farmer or a scientist would call crossbreeding or hybridization.

My desktop dictionary defines hybrid as “the offspring produced by breeding plants or animals of different varieties, species, or races.” Thousands of years ago, mankind crossbred horses and donkeys to create mules and hinnies. This hybrid, called a pered (female pirdah) is already mentioned many times in Tanach. As a pack animal, the mule — produced from a male donkey, called a “jack” and a mare (female horse) — has many advantages over either of its parents. It is usually as strong as a horse, sturdy, sure-footed, and, notwithstanding its reputation for being “stubborn as a mule,” is more reliable for hauling than draft horses. (A hinney, which has less commercial value, is produced from a stallion (male horse) and a female donkey, called a “jenny.” The word “hinney” comes from its parents – a horse and a jenny.)

Other crossbred animals

Artificial insemination has been used to crossbreed camels and llamas with the goal of producing a larger quantity of quality llama wool. Mankind has created such interesting creatures as ligers, crossbreeds of male lions and tigresses, tiglons (sometimes called tigons) from male tigers and lionesses, leopons (male leopards and lionesses), wholpins (whales and dolphins) and geeps (goats and sheep). Most of these have resulted in limited, if any, commercial value, although it was thought by some that they might.

Crossbreeding animal species is one of the prohibitions of the Torah when it declares behemtecha lo sarbia kil’ayim (Vayikra 19:19). It is one of the unusual mitzvos that even a non-Jew is required to observe (Rambam, Hilchos Melachim 10:6).

The prohibition is only to create the crossbreed; one may use a mule or any other crossbred animal. However, not only is it prohibited to crossbreed a horse with a donkey, it is even forbidden to attempt to breed a mule or hinney with either a donkey or a horse (Mishnah, Kil’ayim 1:6). In fact, it is rare that such an attempt will produce offspring, although it is claimed anecdotally that this can happen upon occasion. Nevertheless, someone who attempts to crossbreed them violates a Torah prohibition.

Crossbreeding of plants

Crossbreeding of plants, or, as it is usually called, cross-fertilization or cross-pollination, is when one pollinates the flower of one species with pollen from a different species, to produce offspring with some characteristics of each. Many fruits have been developed this way, although I want to share that a nectarine is not a crossbreed of a peach and a plum, as often mistakenly thought. A nectarine is an ancient variety of peach (Prunus persica) that has a smooth skin. Botanists consider it to be the same species as peaches.

What is interesting is that, in the discussions about kil’ayim in the Torah, the Mishnah and the writings of Chazal, nowhere does it say that it is prohibited to cross-pollinate from one plant species to another. This does not mean to say that there is no prohibition of kil’ayim germane to trees or plants. Quite the contrary, there are three such prohibitions min haTorah. They are referred to as kil’ei zera’im, kil’ayim in plants; kil’ei hakerem, kil’ayim in vineyards; and kil’ayim in trees, usually referred to as harkavas ilan. But, as we will soon see, none of these three prohibitions has anything to do with crossbreeding.

Kil’ei zera’im

Kil’ei zera’im is planting two or more different species of grains, vegetables or other edible herbaceous plants in close proximity. Exactly what defines “close proximity” is a very complicated halachic topic, and depends on factors such as the shape and size of the vegetable patch, and what variety of produce one is planting. We should note that, from a botanical point of view, planting two species in close proximity will not cause hybridization because it does not affect the genetic makeup.

This mitzvah applies only in Eretz Yisroel. Thus, someone in chutz la’aretz may plant his backyard garden with a wide variety of vegetables without any halachic concern, whereas in Eretz Yisroel someone planting a garden patch must be very careful to keep the different species separate.

Kil’ayim in a vineyard, kil’ei hakerem

Kil’ei hakerem is the prohibition against planting grains or vegetables in, near, above or below a vineyard. Again, this forbidden planting will not affect the genetic makeup of any of the plants involved. It is also clear that this was not the concern in halacha as we see from many of the halachic details. Here is one example: Although it is prohibited to plant grains or vegetables near a vineyard, there is a way to permit it by separating the vegetable patch from the vineyard with a halachic wall between them. For example, if one places two poles and a wire across the top, a tzuras hapesach, between the vegetable patch and the vineyard, it is permitted to plant vegetables right next to the vineyard (Eruvin 11a). This is similar to what we do when we construct an eruv to permit carrying on Shabbos. It is quite clear that, botanically, the tzuras hapesach does not accomplish anything to prevent the mingling of the species. Yet, with the tzuras hapesach, it is permitted to plant the crop; without the tzuras hapesach, it is a Torah prohibition to do so! This certainly cannot be explained on a scientific basis.

Even one grapevine is problematic near a crop plant, so care must be taken even in the home garden. For example, a pot with herbs or a vegetable under a trellised grapevine could forbid the grapes and the produce of the pot!

Unlike other forms of kil’ayim, the produce of kil’ei hakerem is forbidden to use.

The prohibition of planting grains or vegetables in a vineyard applies in chutz la’aretz, but only miderabbanan (Kiddushin 39a).

Harkavas ilan – grafting trees

The laws of kil’ayim also prohibit grafting one species of tree or plant onto the wood stock, or lower trunk, of another species. Although a town dweller may feel that this is a rare occurrence, in fact, contemporary plant nurseries and tree farmers usually graft branches of a species that produces delicious fruit onto the hardier stock of a different species.

For example, most peach and nectarine trees are produced by grafting a peach or nectarine branch onto the stock of a hardier tree, such as an almond. Someone who performs this, either in Eretz Yisroel or in chutz la’aretz, violates a Torah prohibition whether he is Jewish or not (Rambam, Hilchos Melachim 10:6). Most authorities rule that one may not own, water or prune a kil’ayim tree, whether or not it is in Eretz Yisroel or in chutz la’aretz (Shulchan Aruch Yoreh Deah 295:7 and Piskei Teshuvah).

However, many observant Jews purchased agricultural properties that contained kil’ayim trees and did not cut down those trees. Was there any justification for their actions? Numerous halachic responsa discuss what was apparently a widespread practice in the eighteenth and nineteenth century. Whereas most poskim rule that these Jews violated the halacha, some authorities justify the practice of owning these trees, at least in chutz la’aretz (Shu’t Chasam Sofer, Yoreh Deah #288; Aruch Hashulchan, Yoreh Deah 295:17-18).

Because so many trees are grafted nowadays, someone who owns a fruit tree should have a horticultural expert check whether its stock is from the same species or not. Often, even a non-expert can detect if a tree was grafted onto a different species by simply scrutinizing the tree. If the bark somewhere near the bottom of the tree looks different from the upper part of the tree, this indicates that the upper part of the tree was grafted. Often one can see a line separating the grafted scion from the rootstock, or a difference in thickness between the top and bottom. Before purchasing a new tree at a nursery, examine the trunk carefully for signs of grafting.

The prohibition of planting vegetables and other edible crops together applies only in Eretz Yisroel, whereas grafting trees applies equally min hatorah in chutz la’aretz and in Eretz Yisroel.

Although planting and caring for a kil’ayim tree is forbidden, the fruit from such a tree is permitted. Thus, one may purchase fruit in a market without worrying about kil’ayim.


Although space does not allow us to discuss this fascinating topic, there is a huge amount of halachic literature discussing the very common instance of using an esrog from a tree that was grafted onto a non-esrog tree. Most authorities rule that this esrog may not be used to fulfill the mitzvah on Sukkos.


Rav Hirsch (Vayikra 19:19) explains that the root word ke’le means to keep or hold something back, and that the plural form kil’ayim is similar to yadayim or raglayim and means a pair. Therefore, the word kil’ayim means to pair together two items that should be kept apart. This is to teach us that although we are given the world to develop, we must follow the rules that Hashem established for us to do so.

Mixed Breeds

muleQuestion: Mule Inventors

“Who invented, or should I say ‘discovered,’ the mule?”

Question: The Hybrid or the Hybridization?

“Is it permitted to use the product of a prohibited hybridization (crossbreeding) of animals?”

Question: Buy me a Mule!

“May I purchase a mule from a gentile? May I hire him to produce it for me?”

Question: Crossbreeding Pro

“Before I became frum, I was well experienced at hybridizing and raising crossbred birds. Is there any way that I can use this skill to earn a livelihood, now that I have become a baal teshuvah?”

Question: Roommates

“Is the zoo permitted to house different species together?”


Two mitzvos of the Torah deal with the mixing of animal species. In parshas Kedoshim, the Torah teaches: Behemtecha lo sarbia kil’ayim, “Do not crossbreed your animal” (Vayikra 19:19). This prohibition applies to beheimah, usually translated as domesticated species; chayah, usually but somewhat inaccurately translated as wild or non-domesticated* species; birds; and sea chayos, such as sea mammals (Mishnah, Baba Kama 54b and Gemara 55a). Violating this proscription is punishable by malkus, as is true for most lo saaseh violations of the Torah, but only if one mates them physically. Encouraging the mating process less directly is prohibited and is the source of a dispute between early authorities whether it is prohibited min haTorah (Drishah, Yoreh Deah 297:1) or only miderabbanan (Taz, ad locum). It is permitted to house two species together, and one has no requirement to separate them if they mate on their own (Yerushalmi Kelayim 8:2, quoted by Shulchan Aruch Yoreh Deah 297:3). (Those checking the references should note that there are two chapters in Shulchan Aruch Yoreh Deah numbered 297, and the laws we are discussing are in the second of those chapters.)

Lo Sacharosh

There is also another mitzvah of the Torah, mentioned in the context of Kelayim prohibitions in parshas Ki Seitzei: Lo sacharosh beshor uvachamor yachdav, You may not plough with an ox and a donkey together (Devorim 22:10). This mitzvah prohibits working two species of animals together. According to the opinion of the Rambam, the Torah prohibition of this law is violated only when one species is kosher and the other is non-kosher – other circumstances are prohibited, only because of a rabbinic injunction. Other authorities dispute this ruling of the Rambam, contending that working two species together is prohibited min haTorah, even when both are kosher or both are non-kosher. There is much to discuss about this topic, but we will leave it for a different article.

Which species?

The Mishnah (Kelayim 1:6) lists several combinations of species that one may not crossbreed, such as wolves and dogs, or mules and donkeys, and the Gemara (Bava Kama 55a) notes several other examples, including two varieties of geese where some physical differences determine that they are different species for halachic purposes. On the other hand, the Gemara (Bava Kama 55a) mentions that Persian camels and Arabian camels are not Kelayim together, even though the length of the neck of the two breeds are noticeably different. Furthermore, the Rambam rules that a species with wild and domesticated varieties, such as wild and domesticated oxen or horses, may be crossbred, even when the domesticated variety has some obvious differences from the wild variety (Rambam, Hilchos Kelayim 9:5).

We are left with a question: how does halachah define what is considered a variety of a species versus what is considered a different species? One may crossbreed or work together two animals that are considered two different varieties, but one may not crossbreed or work together two animals that halachah considers different species. However, the Mishnah never provides defining characteristics that we can use. It is also interesting to note that the Gemara (Bava Kama 55a) states that even two species that freely mate together in the wild may not be hybridized. Thus, an animal’s social life, also, does not determine what is considered its species.

Rashi on the mule

At the end of parshas Vayishlach, the Torah recounts how Anah, Sei’ir Hachori’s grandson, shepherded donkeys for his father, and, while doing so, discovered yeimim (Bereishis 36:24), which Rav Saadia Gaon, Rashi and others translate as mules. Rashi and the Ibn Ezra explain that Anah’s “discovery” means he developed the science of crossbreeding a male donkey (called a jackass) and a mare (a female horse) which produces a mule. (See the Targum Onkelos and the Ramban, who explain the verse very differently.) Rashi explains that Anah, who himself descended from a scandalous relationship, was the first to crossbreed two different species, also a scandalous act.

This statement of Rashi presents two questions:

  1. What is wrong with Anah having crossbred donkeys and horses? This is not one of the seven Noahide laws.
  2. Rashi’s comment that Anah was the first to create a mule implies that this was a newfangled “invention” and not yet commonly used. Yet Rashi himself, in parshas Tolados, mentions that when Yitzchak became well respected, people said that “the manure of Yitzchak’s mules is more valuable than Avimelech’s gold and silver” (Bereishis 26:13). Obviously, this means that mules were commonplace in the days of Yitzchak. Can both of these statements of Rashi be accurate?

Furthermore, the statement of Rashi in parshas Tolados presents yet another question, since it implies that it is not considered unbecoming to mention that Yitzchak owned mules, notwithstanding the fact that the Torah prohibits a Jew from producing them. Why, then, are Anah’s mules considered to be so scandalous?

To answer the question why Rashi criticizes Anah for creating mules, when a ben Noach is permitted to crossbreed animals, we need some broader Talmudic background.

Bnei Noach and crossbreeding

Although the seven mitzvos are the most basic mitzvah requirements that apply to bnei Noach, there are other mitzvos that apply to them, at least according to some opinions. Some tanna’im rule that the laws prohibiting sorcery apply to them, and others understand that they are prohibited from grafting one species onto the rootstock of a different species.

There is a tanna, Rabbi Elazar, who contends that bnei Noach are forbidden to crossbreed animals of different species, even though this prohibition is not treated as severely as are the seven mitzvos (Sanhedrin 56b). The Gemara (Sanhedrin 60a) explains that Rabbi Elazar derives that bnei Noach are forbidden to crossbreed animals from  the pasuk (partially quoted above), Es chukosai tishmoru behemtecha lo sarbia kil’ayim (Vayikra 19:19), which Rabbi Elazar interprets to mean, “You should be careful to observe the laws that I previously prohibited: Do not breed your animals — one species with another!” However, there is no previous place in the Torah where we are commanded not to crossbreed animals. Rabbi Elazar reasons that this must mean that when Noach left the teivah and was commanded concerning other laws, he was also told that he may not crossbreed animals. Thus, it would appear that when Rashi, in our parshah, bemoans Anah’s activities, he is assuming the halachah is as understood by Rabbi Elazar that all of mankind is prohibited from crossbreeding two species.

Halachic conclusion

The Rambam rules that a ben Noach is prohibited from crossbreeding animals (Hilchos Melachim 10:6). According to his approach, Rashi’s comments about Anah introducing something forbidden into the world are halachically accurate.

Asking a gentile

May one ask or hire a gentile to create hybrid animals? According to the Rambam, who rules according to Rabbi Elazar, this is certainly prohibited, because one is thereby causing a gentile to violate the Torah (Drishah).

The authorities conclude that asking or hiring a non-Jew to crossbreed is prohibited, even according to those who disagree with Rabbi Elazar and contend that a gentile is permitted to crossbreed. The Shulchan Aruch (Yoreh Deah 297:4), as understood by most authorities, prohibits having a gentile crossbreed for a Jew, because of the prohibition of having a gentile perform something that a Jew is not permitted to do myself, which is called amirah lenachri (Rema, Shach and others, based on Bava Metzia 90a).

There is a difference in halachah that results from the dispute why one may not hire a gentile to crossbreed for you. May one teach a gentile how to crossbreed animals for the gentile’s benefit (see Shach, Yoreh Deah 297:4)? According to the Rambam, this is prohibited, since one will be teaching him to do something that he may not do. However, according to those who contend that a gentile may crossbreed animals, it is permitted to advise or instruct the gentile how to do so, even if he uses a Jew’s animals, since he is not doing so in order to benefit a Jew.

Crossbreeding pro

At this point, we can address another one of our opening questions: “Before I became frum, I was well experienced at hybridizing and raising crossbred birds. Is there any way that I can use this skill to earn a livelihood, now that I have become a baal teshuvah?”

The answer is that one can practice breeding of the same species, assuming one can figure out what is considered the same species according to halachah. Whether one can be paid to train a gentile how to crossbreed two different species will depend on the above-quoted dispute. It would appear that the Shach rules that one may, whereas the Derishah and others prohibit. I refer an individual with this question to his own rav or posek.

Using a hybrid

Whether we rule according to Rabbi Elazar or the differing tanna, the halachah remains that even when an animal is created by prohibited hybridization, one may benefit from the crossbred animal (Taz, Yoreh Deah 297:2). Even according to Rabbi Elazar, one may purchase a mule, once it has been produced, and use it, and even a person who violated the halachah and created a mule may use it. Thus, Yitzchak may have purchased many mules to assist him, and the fact that people praised the quality of Yitzchak’s mules is not disturbing.

The beefalo

Relatively recently, a new hybrid was developed, which is a cross between the ordinary beef cattle and a North American bison, which Americans colloquially call a buffalo. Is it permitted to make this crossbreed? One major authority contends that whether one may crossbreed buffalo and cattle depends on whether one is required to perform kisuy hadam, the mitzvah of covering the blood of shechitah, after slaughtering a buffalo. Kisuy hadam is required only on fowl and chayos but not on beheimos, such as cattle. If there is no requirement to perform kisuy hadam on buffalo, this demonstrates that it is considered a beheimah. Since there are only three species of beheimah — sheep, goats, and cattle, then ruling that a buffalo does not require kisuy hadam means that halachah considers it to be a beheimah, and, if it is a beheimah, the process of elimination proves that it must be considered a variety of cattle, since it is certainly not a sheep or a goat.

Sefardim, Ashkenazim and buffalos

Is kisuy hadam required on a buffalo? This is a dispute between the Shulchan Aruch and the Rema, the Shulchan Aruch (Yoreh Deah 28:4) concluding that there is no requirement to perform kisuy hadam, whereas the Rema rules that one should do so without a brocha  since we are uncertain whether it is considered a chayah. The Aruch Hashulchan (Yoreh Deah 297:8) notes that this dispute between the Shulchan Aruch and the Rema will also affect whether one is permitted to crossbreed buffalo with ordinary cattle, since the Shulchan Aruch, by concluding that it is a beheimah, must hold that they are halachically considered to be the same species. On the other hand, since the Rema is concerned that buffalo might be a variety of chayah, one would not be permitted to crossbreed it with cattle.

Halachic conclusion: According to the Aruch Hashulchan, a Sefardi would be permitted to crossbreed buffalo with cattle, and an Ashkenazi would not.

Who invented the mule?

Was Anah the first one to create a mule, or did it precede him?

The Gemara (Pesachim 54a) cites a dispute among three tanna’im regarding who created the first mule. According to Rabbi Yosi, Adam created the first mule on the first motza’ei Shabbos of Creation. Rabban Shimon ben Gamliel disagrees, contending that Anah created the first mule. In a different beraisa, the Gemara quotes Rabbi Nechemiah, who contended that mules were created by Hashem at the very end of the Six Days of Creation. The passage Rashi quotes in parshas Vayishlach is indeed originally from Rabban Shimon ben Gamliel, in the Gemara. However, when Rashi in parshas Tolados quotes the Bereishis Rabbah about Yitzchok’s mules, presumably that passage accords with one of the other opinions among the tanna’im, who date the creation of the mule much earlier.

By the way, it is possible that Rabban Shimon ben Gamliel accepts the essence of the statement about Yitzchok, but simply does not include the word mules in his version. Tosafos (Bava Metzia 85a) quotes the Midrash Rabbah that Rashi quotes in parshas Tolados, but with one change: In his version, people complimented the manure of Yitzchok’s animals, rather than specifically his mules. This approach would reflect the opinion of Rabban Shimon ben Gamliel.

Meet the mule

Although most people use the term mule to refer both to the offspring of a stallion (male horse) and a jenny (female donkey) and to the offspring of a jackass (male donkey) and a mare (female horse), this is technically inaccurate. A mule is the offspring of a jackass and a mare. The offspring of a stallion and a jenny is called a hinny. However, Chazal use the word pered to describe either a mule or a hinny; a mule is called pered ben susya, the offspring of a mare (see Chullin 114b) and a hinny is called pered ben chamorah, the offspring of a jenny. (The word pered, itself, is of Tanachic origin — for example, Avshalom rode on a pered — but there is no indication in Tanach regarding its specific parental origin.)

There are visible differences between a mule and a hinny, particularly in the appearance of their ears, tail and voice (Chullin 79a). Mankind has found mules useful, because they are very strong and often easier to train and work with than horses, and withstand difficult hardships better than do horses.

On the other hand, hinnies are sometimes no more useful than donkeys, and sometimes have a reputation for being of difficult temperament. In size and strength, they usually approximate donkeys. Since they are usually no more useful than donkeys, and they are virtually always sterile, it is far less common for farmers to breed them. In general, neither mules nor hinnies produce offspring, although there are anecdotal instances of female mules reproducing after mating with stallions or jackasses.

One is permitted to mate a male mule with a female one (Rambam, Hilchos Kelayim 9:6). However, whether one may mate a mule and a hinny is the subject of a dispute among tanna’im (Chullin 79a). The Rambam (Hilchos Kelayim 9:6) and the Shulchan Aruch rule that this is prohibited, just as it is prohibited to breed animals of different species. This is prohibited, even though it is almost certain that this match will not produce offspring.

Difference between pered and mule

Now that we are well educated about the difference between a mule and a hinny, we can answer another of our opening questions: “What is the difference between the Hebrew pered and the mule?” The answer is that the word pered is used by Chazal to mean either a mule or a hinny. Rashi, on the verse in parshas Vayishlach, says clearly that Anah crossbred a male donkey with a female horse, which means that he created a mule.


Speaking of mules reminds me of the passage of Gemara (Bechoros 8b) that recounts a puzzling conversation that transpired between the scholars of Athens and the tanna Rabbi Yehoshua. The Athenians asked Rabbi Yehoshua: “When salt spoils, with what do you salt it?” To this, Rabbi Yehoshua answered, “With the afterbirth of a mule.” They then asked him, “Does a mule have an afterbirth?” To this he replied, “Does salt spoil?”

What is meant by this short but very enigmatic debate?

The Athenian scholars were challenging the fact that the Jews maintain that we will eventually be redeemed. The scholars claimed: “You Jews did not keep your end of the deal with G-d, and therefore your deal is abrogated. Indeed, it was to have been a covenant forever, like salt, but your salt spoiled!”

To this, Rabbi Yehoshua replied: “Our children (our afterbirth) continue to study Torah, and that is our guarantee.”

The Athenians retorted: “But you are a mule. You do not have a future that will have a relationship with G-d.”

Rabbi Yehoshua responded: “You are mistaken. You claim that our covenant with Hashem is abrogated. This is not true. Salt does not spoil, and our covenant with Hashem is forever!” (See Commentary of the Vilna Geon to Aggados Hashas.)

* The Gemara (Chullin 59b) mentions several characteristics that distinguish beheimos from chayos, mostly dependent on the animal’s horns. Reindeer, although domesticated, are clearly chayos since they have branched antlers, whereas there are non-domesticated species that are almost certainly categorized as beheimah.