Hybrid Halacha

Question #1: Grapes

Why is kilayim of grapes different from all the other kilayim prohibitions?

Question #2: Great Auks

Is it permitted to crossbreed auks and ducks?

Question #3: The Grand, Green Movers

I am green. Instead of trucks, may I use elephants, water buffalo and draft horses together to move my house?

Question #4: Accused of Graft!

Where does the Torah prohibit grafting trees?


In parshas Bereishis, the various species of animals and plants were instructed to reproduce lemi’neihem, according to their species, meaning that they were not to hybridize (crossbreed) with other species. These sources in parshas Bereishis bring to mind the several mitzvos taught later in the Torah not to mix species(Chullin 60a; Tosafos, Sanhedrin 60a; Ramban, Bereishis 1:26; Rashbam, Vayikra 19:19).

The word kilayim is translated by Onkelos and ibn Ezra (Vayikra 19:19) as “mixture,” although other commentaries understand that this word originates from the same Hebrew root as the word “prison,” beis ke’le (see Yeshayah, 42:22). This approach is quoted in the name of the Raavad (by the Rashas, in his commentary to the Yerushalmi, Kilayim 3:5) and by Rav Hirsch (Vayikra 19:19), who explains that the shoresh כ ל א means to keep or hold something back, and that the plural structure kilayim is similar to yadayim “hands” or raglayim “feet,” and means a pair. Therefore, the word kilayim means to treat as a pair two items (similar to our pair of hands) that are required to be kept separate.

Having explained the source of the word kilayim this way, there is no need to assume that Onkelos or ibn Ezra disagree. They are merely elucidating the word in the context of the posuk, where it means a forbidden mixture, whereas Rav Hirsch is explaining the etymological basis for this meaning.

Kilayim versus hybridization

It is important to clarify a common misconception. The prohibition of kilayim is not necessarily the creation of a new species — it is the appearance that one is mingling two species together. My desktop dictionary defines hybrid as “the offspring produced by breeding plants or animals of different varieties, species, or races.” Hybridization always involves making changes in the DNA of a species; most instances of kilayim do not. Planting seeds of different species in close proximity does not affect their genetic makeup – thus, technically, no hybridization transpires — yet it may be prohibited min haTorah. Similarly, wearing a garment manufactured from woolen and linen thread does not affect the two parent species or the DNA of the thread in the slightest.

Kilayim prohibitions

The Torah teaches about kilayim in two places, in parshas Kedoshim and in parshas Ki Seitzei. There the Torah mentions a total of six lo saaseh prohibitions, each of which is counted among the 365 lo saasehs that are included in the 613 mitzvos.

In parshas Kedoshim (Vayikra 19:19), the Torah teaches:

(1) Observe my laws! Do not mate your animal with a diverse species (kilayim).

(2) Do not plant your field with a diverse species.

(3) A garment containing diverse species called shatnez you shall not put upon yourself.

In parshas Ki Seitzei (Devorim 22:9-12), the Torah states:

(4) Do not plant diverse species in your vineyard

(5) lest what grows become sanctified (tukdash); the seed that was planted together with the growth of the vineyard.

(6) Do not plow with an ox and a donkey together.

(3, again) Do not wear shatnez, wool and linen together.

Of the six lo saaseh prohibitions counted here, the Torah calls four of them kilayim (which we translated as “diverse species”). The fifth, “lo sacharosh,” do not plow, prohibits different species of animals working together, such as plowing or pulling wagons, but is not called kilayim by the Torah.

The sixth is a prohibition against using what grew as kilayim in a vineyard. There is no prohibition in using any of the other mixtures, meaning that, although it is forbidden to hybridize different species of animals, crossbreed fruit, or plow with different species of animals, it is permitted to eat a crossbred fruit or what grew in a kilayim field other than a vineyard. Similarly, it is permitted to use a shatnez garment as long as I don’t wear it.

Grapes are different!

This leads us to our opening question: “Why is kilayim of grapes different from all the other kilayim prohibitions?” The product of kilayim of other species, including the fruit created by grafting and the mule created by mating a male donkey (jack) with a mare (female horse), are permitted to be used, even if a Jew created them in violation of the halacha. Only in the instance of kil’ei hakerem is there a prohibition to use what is produced.

This prohibition is derived from a careful reading of the pasuk, where the Torah states: Do not plant diverse species in your vineyard lest what grows become sanctified (tukdash); the seed that was planted together with the growth of the vineyard. None of the other kilayim prohibitions include an additional lo saaseh that applies to what grows afterward. The Torah’s method of conveying this law is the word tukdash. The Rashbam explains the word tukdash to mean sanctified — the produce becomes prohibited like kodashim are prohibited for personal use. Although other rishonim have different explanations of the word tukdash, all agree that the produce that grows there is prohibited for use min haTorah.

Why are vineyards different?

Why does the prohibition against benefitting exist only with regard to kilayim in a vineyard? The Chizkuni (Devorim 22:9) explains because otherwise this type of kilayim can slip by unnoticed; the wheat that grows in a vineyard does not look different from wheat grown in a wheat field, as opposed to shatnez and animal husbandry, where the item worn or produced is noticeable that it includes two different “species.” Note that the grafting of a tree is similarly highly noticeable, at least initially.

Animal Hybrids

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 and a mare, may have advantages over either of its parents. It is usually as strong as a horse, yet sturdier and more sure-footed, and — notwithstanding its reputation for being “stubborn as a mule” — is often more reliable for hauling than draft horses. (A hinney, which has less commercial value, is produced from a female donkey [jenny] and a stallion.)

Not only is it prohibited to crossbreed a horse with a donkey, it is even forbidden to mate a mule or hinney with either a donkey or horse (Mishnah, Kilayim 1:6). In fact, it is rare that such an attempt will produce offspring, although it is claimed anecdotally that there are occasions in which a mule or hinney is fertile and reproduces.

Other crossbred animals

Artificial insemination has been used to crossbreed all sorts of species. Camels and llamas have been crossbred 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 individuals unconcerned about the halachic ramifications thought that offspring of such crossbreeding might provide an economic advantage. Crossbreeding animal species is one of the prohibitions of the Torah, when it declares behemtecha lo sarbia kilayim (Vayikra 19:19).

Great auks

At this point, let us discuss the second of our opening questions: “Is it permitted to crossbreed auks and ducks?”

Although both varieties of birds spend much time in and over water, I presume that there would be a kilayim prohibition involved in attempting to crossbreed them. There is an additional problem in attempting to crossbreed great auks with ducks, since great auks have been assumed extinct for well over a century.

Pulling your weight

A similar question would be whether a circus show could use species of different animals to haul together. We know that the Torah prohibited this when it wrote lo sacharosh beshor vachamor yachdav, Do not plow with an ox and a donkey together. The juxtaposition of this mitzvah between other kilayim prohibitions implies that this is an extension of the laws of kilayim.

Some rishonim explain that the prohibition of having two different species haul a load together is a type of min haTorah gezeirah to avoid housing them together at night, which (they contend) would be prohibited as crossbreeding animals. We do find other instances of Torah prohibitions whose purpose is to prevent a more serious violation of the Torah. The classic example of this is the prohibition of bal yematzei, owning chometz on Pesach, which the Torah itself states is to avoid violating the more serious prohibition of eating chometz on Pesach (Shemos 12:19). A similar idea is yichud, which the Gemara (Kiddushin 80b) implies is a Torah violation, whose purpose is to discourage the more serious violation of arayos.

Unfair labor practices?

Notwithstanding that the Torah and the Mishnah both imply that the prohibition of lo sacharosh is because of kilayim, many early authorities explain this law because of other reasons. The ibn Ezra  explains that this is prohibited because, although a donkey is an excellent work animal, it is not “strong as an ox.” In other words, the “reason” for this mitzvah is to teach us to be concerned not to overburden the donkey.

I want to show a reverse case. During a tour I once took of a reconstructed nineteenth-century farm, the plow was being pulled by a draft horse together with a mule. The curators explained to me that they own both horses and mules, and teaming up to work together depends on the animal’s temperament, not necessarily its species. They can sometimes successfully team together a particular mule and a particular horse, and sometimes two horses or two mules will not pull their weight together. Apparently, in the animal world, your coworker is as important a factor in job satisfaction as it is in the human world.

However, from a halachic perspective, there are several unusual factors here. For one, mules are the offspring of male donkeys and mares (female horses). The halacha is that teaming a mule and a horse is prohibited min haTorah because they are different species, notwithstanding that their size and strength may be functionally equivalent. In other words, the reason that the ibn Ezra presents for the prohibition of lo sacharosh does not fit the halacha. Furthermore, the mitzvah of lo sacharosh permits matching a large mature draft horse with an undersized pony colt, notwithstanding that the young and small pony will have a very difficult time pulling its weight alongside its powerful coworker.

Nevertheless, we could still accept the ibn Ezra’s approach to analyzing the “reason” for this mitzvah. As noted by the Sefer Hachinuch, we can never, and should never, claim to understand the “reason” for a mitzvah. Why Hashem commanded us to perform a specific mitzvah is not something for us to try to prove or to rationalize. Unfortunately, such rationalizing has often led to individuals not complying with mitzvos. We know that this error was perpetrated even by the greatest of the great – for example, by Shelomo Hamelech when he accrued more wealth and wives than the Torah permits. The Sefer Hachinuch explains that taamei hamitzvah does not mean “reasons” for mitzvos, but that the word taam should be translated here as “taste,” meaning that these are ideas, messages, or tastes that we can apply to ourselves as lessons when we observe or study these mitzvos. The Rambam also agrees that “reasons” or “tastes” of mitzvos do not always reflect the halachic reality. (Those who oppose this approach to taamei hamitzvah rally around Rav Hirsch, who usually espouses reasons for mitzvos only after a highly detailed analysis of all its laws, and suggests taamei hamitzvah only when they fit the halachic details of the mitzvah.)


At this point, I am returning to the last question that I asked: “Where does the Torah prohibit grafting trees?” If we look carefully at the pesukim of kilayim prohibitions, quoted above, we will note that nowhere does the Torah explicitly prohibit the grafting of one species of fruit tree onto another, which is called in Hebrew harkavah. If, indeed, this prohibition is not mentioned in the Torah, how do we know that it is prohibited?

By means of a complicated homiletic derivation, based on the first words of the pasuk, Observe my laws, the Gemara  (Kiddushin 39a; Sanhedrin 60a) derives that harkavah, grafting a fruit tree onto a different species, is prohibited min haTorah. The Rambam (Hilchos Kilayim 1:5) concludes that it is included under the lo saaseh of sadecha lo sizra kilayim, the prohibition of planting different species of grains together.

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 is 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 peach. In their opinion, the difference between nectarines and peaches is the difference between two people who have different complexions or perhaps variant orientations of skin pigment, and certainly not a halachic consideration. I am unaware of anyone who has attempted to study this as a halachic issue. The practical difference is whether it is permitted to graft a nectarine scion onto a peach stalk or vice versa.

What is interesting is that, in the discussions about kilayim 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 kilayim germane to trees or plants. Quite the contrary, there are three such prohibitions min haTorah. They are referred to as kil’ei zera’im (kilayim in plants), kil’ei hakerem (kilayim in vineyards), and harkavas ilan (kilayim in trees). But, as we will soon see, none of these three prohibitions has to do with crossbreeding.

The prohibition applies to herbaceous, as opposed to woody plants, meaning that it does not apply to trees and shrubs, but it does apply to vegetables and many herbs. Thus, one may plant seeds of different trees together, yet one is forbidden to plant a mix of vegetable seeds (Rambam, Hilchos Kilayim 1:6).

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 of the species.

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

Kilayim in a vineyard, kil’ei hakerem

Kil’ei hakerem is the prohibition against planting grains or vegetables in or near a vineyard. Again, this forbidden planting will not affect the genetic makeup of any of the plants involved. It is also quite clear that this was not the concern in halacha, as we see from many of the halachic details. For example: although it is prohibited to plant grains or vegetables near a vineyard, it is permitted to separate the vegetable patch from the vineyard by placing a halachic wall between them. For this, two poles and a wire at the top, a tzuras hapesach, between the vegetable patch and the vineyard suffices (Eruvin 11a), 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 prevent the mingling of the species. Yet, with the tzuras hapesach, it is permitted to plant the grain and, without it, there is a Torah prohibition to do so! This certainly cannot be explained on a scientific basis.


Observing and studying the laws of kilayim reminds us how all of Hashem’s creation always follows His instructions. This reminds the contemplative Jew that, if the plants heed Hashem’s word, how much more must we strive to obey His instructions.