Halacha

הלכה א
הַמַּרְכִּיב זָכָר עַל נְקֵבָה שֶׁאֵינוֹ מִינוֹ בֵּין בִּבְהֵמָה בֵּין בְּחַיָּה וּבְעוֹף וַאֲפִלּוּ בְּמִינֵי חַיָּה שֶׁבַּיָּם הֲרֵי זֶה לוֹקֶה מִן הַתּוֹרָה בְּכָל מָקוֹם בֵּין בָּאָרֶץ בֵּין בְּחוּצָה לָאָרֶץ שֶׁנֶּאֱמַר (ויקרא יט יט) "בְּהֶמְתְּךָ לֹא תַרְבִּיעַ כִּלְאַיִם". וְאֶחָד בְּהֵמָה חַיָּה וְעוֹף שֶׁלּוֹ אוֹ שֶׁל חֲבֵרוֹ. וְאֵינוֹ לוֹקֶה עַד שֶׁיַּכְנִיס בְּיָדוֹ כְּמִכְחוֹל בִּשְׁפוֹפֶרֶת. אֲבָל אִם הֶעֱלָם זֶה עַל זֶה בִּלְבַד אוֹ שֶׁעוֹרְרָן בְּקוֹל מַכִּין אוֹתוֹ מַכַּת מַרְדּוּת:
כסף משנה
1.
When a person causes a male to enter into relations with a female of a different species1Even if both species are kosher (see Minchat Chinuch, mitzvah 244); see Sifei Cohen, Yoreh De'ah 294:1. - whether a domesticated animal, a wild beast, a fowl, even different types of sea-animals2But not fish (Kessef Mishneh). - he is liable for lashes according to Scriptural Law.3I.e., he transgresses a negative commandment, included as one of the 365 negative commandments by Sefer HaMitzvot (negative commandment 217) and Sefer HaChinuch (mitzvah 244). [This4In contrast to the laws regarding sowing mixed species of crops (Chapter 1, Halachah 1) and sowing mixed species in a vineyard (Chapter 5, Halachah 2). applies] in all places in Eretz Yisrael and in the Diaspora, as [Leviticus 19:19] states: "You shall not mate your animal with another species."
[This applies] whether the animal, beast, or fowl belongs to him or to a colleague.5Although the prooftext states "your animal," this is not meant as an exclusion (Sifra to that verse). He is not liable for lashes until he actually inserts one animal's organ into the others. If, however, he merely placed one on top of the other or encouraged them verbally, he is given stripes for rebellious conduct.6The punishment given for violating a Rabbinic ordinance.

הלכה ב
מֻתָּר לְהַכְנִיס שְׁנֵי מִינִין לְסַהַר אֶחָד. וְאִם רָאָה אוֹתָם רוֹבְעִים זֶה אֶת זֶה אֵינוֹ זָקוּק לְהַפְרִישָׁן. וְאָסוּר לְיִשְׂרָאֵל לִתֵּן בְּהֶמְתּוֹ לְעַכּוּ''ם לְהַרְבִּיעָהּ לוֹ:
כסף משנה
2.
It is permitted to place two species of animals in one corral.7For the owner is not involved in their mating. The Rama (Yoreh De'ah 297:3) states that in a place where there is a suspicion that others will cause the two animals to mate, it is forbidden to place them in the same corral. If one sees them mating, he is not obligated to separate them. A Jew is forbidden to give his animal to a gentile to have him mate it with a forbidden species.8Based on Bava Metzia 90a, the Radbaz and the Kessef Mishneh state that the difficulty is that it is forbidden to give a gentile instructions to perform a prohibited activity. According to the Rambam (Hilchot Melachim 10:6), it is forbidden for a gentile to crossbreed species. The Rama (loc. cit.:4), however, states that if the act is performed for the benefit of the gentile, there is no prohibition.

הלכה ג
מִי שֶׁעָבַר וְהִרְכִּיב בְּהֶמְתּוֹ כִּלְאַיִם הֲרֵי הַנּוֹלָד מֵהֶם מֻתָּר בַּהֲנָאָה. וְאִם הָיָה מִין טְהוֹרָה עִם מִין טְהוֹרָה מֻתָּר בַּאֲכִילָה כְּמוֹ שֶׁבֵּאַרְנוּ בְּמַאֲכָלוֹת אֲסוּרוֹת:
כסף משנה
3.
When a person transgressed and mated his animal with a mixed species, it is permitted to benefit from the offspring.9I.e., if the mated species are not kosher. If the mated species were both kosher, [the offspring] is permitted to be eaten, as stated in Hilchot Ma'achalot Assurot.10Hilchot Ma'achalot Assurot 1:13.
Here we see another difference with produce that grows from mixed species. The latter is forbidden, while in this instance, the mixed offspring is permitted. Implied is that the prohibition against mixed produce affects the cheftza, the actual substance of the forbidden entity, while the prohibition against mating mix species applies only to the gavra, the person performing the mating.

הלכה ד
שְׁנֵי מִינֵי בְּהֵמָה אוֹ חַיָּה שֶׁדּוֹמִין זֶה לְזֶה אַף עַל פִּי שֶׁמִּתְעַבְּרִין זֶה מִזֶּה וְדוֹמִין זֶה לָזֶה הוֹאִיל וְהֵם שְׁנֵי מִינִין הֲרֵי הֵם כִּלְאַיִם וְאָסוּר לְהַרְכִּיבָן. כֵּיצַד. הַזְּאֵב עִם הַכֶּלֶב וְהַכֶּלֶב הַכּוּפְרִי עִם הַשּׁוּעָל וְהַצְּבָאִים עִם הָעִזִּים וְהַיְעֵלִים עִם הָרְחֵלִים וְהַסּוּס עִם הַפֶּרֶד וְהַפֶּרֶד עִם הַחֲמוֹר וְהַחֲמוֹר עִם הֶעָרוֹד אַף עַל פִּי שֶׁדּוֹמִין זֶה לָזֶה הֲרֵי הֵן כִּלְאַיִם זֶה בָּזֶה:
כסף משנה
4.
[Although] two types of animals or beasts resemble each other11In the text of the Mishneh Torah, this phrase is repeated a second time. and one can impregnate the other, since they are two species, they are considered as mixed species and it is forbidden to mate them.
What is implied? A wolf and a dog, a hunting dog12Our translation is taken from the Rambam's Commentary to the Mishnah (Kilayim 1:6). and a fox, deer and goats, mountain goats and sheep, horses and mules, mules and donkeys, and donkeys and wild asses13Our translation is taken from the Rambam's Commentary to the Mishnah (Kilayim 1:6). - although they resemble each other, they are considered as mixed species.

הלכה ה
מִין שֶׁיֵּשׁ בּוֹ מִדְבָּרִי וְיִשּׁוּבִי כְּגוֹן שׁוֹר הַבָּר עִם הַשּׁוֹר וְהָרַמָּךְ עִם הַסּוּס מֻתָּר לְהַרְכִּיבָן זֶה עִם זֶה מִפְּנֵי שֶׁהֵן מִין אֶחָד. אֲבָל אַוָּז עִם אַוָּז בַּר כִּלְאַיִם זֶה בָּזֶה מִפְּנֵי שֶׁהָאַוָּז (בֵּיתִּיִי) בֵּיצָיו מִבִּפְנִים וְאַוָּז בַּר בֵּיצָיו מִבַּחוּץ מִכְלָל שֶׁהֵם שְׁנֵי מִינִין. וְהַכְּוִי כִּלְאַיִם עִם הַחַיָּה וְעִם הַבְּהֵמָה וְאֵין לוֹקִין עָלָיו מִפְּנֵי שֶׁהוּא סָפֵק:
כסף משנה
5.
When a species has both a wild species and a domesticated species, e.g., a wild ox14The Radbaz notes that the term "wild ox" is sometimes used to refer to a buffalo which is considered a separate species and not part of the ox species. He states that here the intent is different and the Rambam is referring to a species of domesticated animal. See also Siftei Cohen 297:6. and an ox or a wild horse and a horse, it is permitted to mate them together, because they are one species. A duck and a wild duck, by contrast, are considered as mixed species with each other. [The reason is that] the domesticated duck has its testicles inside its body, while those of the wild duck are outside. This shows that they are different species.
[Mating a] ko'i15A ko'i is a hybrid born from breeding a deer and a goat. There is an unresolved question among our Sages if it is considered as a domesticated animal (behemah) or a wild beast [(chayah) the Rambam's Commentary to the Mishnah, Bikkurim 2:8]. In Hilchot Ma'achalot Assurot 1:13, he states that the term refers to any hybrid that comes from mating a domesticated animal with a wild beast. with a beast or a domesticated animal is considered as mating mixed species. One, however, is not liable for lashes, because [a ko'i's] status is one of doubt.16In Hilchot Nazirut 2:10-11, the Rambam writes:

In certain matters, a ko'i resembles a wild beast. In other matters, it resembles a domesticated animal. In still other matters, it resembles both a wild beast and a domesticated animal and in still other matters, it resembles neither a domesticated animal, nor a wild beast....
What is implied? [When] a ko'i [is slaughtered, its] blood must be covered as the blood of a wild beast must. Its fat is forbidden as is the fat of a domesticated animal. It is considered a union of mixed species if it is mated with either a domesticated animal or a wild beast, as if it were neither a wild beast or a domesticated animal. And it must be ritually slaughtered as is required for both a domesticated animal or a wild beast. Similarly, there are other halachic considerations that apply with regard to it and they will all be explained in their appropriate place.

See Hilchot Shechitah 12:8-9; Hilchot Bikkurim 9:5; 10:7; Hilchot Sha'ar Avot HaTumah 1:6, et al, which mention other laws applying to this animal.

הלכה ו
הַיְלוּדִים מִן הַכִּלְאַיִם אִם הָיוּ אִמּוֹתֵיהֶן מִין אֶחָד מֻתָּר לְהַרְכִּיבָן זֶה עַל זֶה. וְאִם הָיוּ שְׁנֵי מִינִין אָסוּר לְהַרְכִּיבָן זֶה עַל זֶה וְאִם הִרְכִּיב לוֹקֶה. וְכֵן אִם הִרְכִּיב זֶה הַנּוֹלָד אֲפִלּוּ עַל מִין אִמּוֹ לוֹקֶה. כֵּיצַד. פֶּרֶד שֶׁאִמּוֹ חֲמוֹר מֻתָּר לְהַרְכִּיבוֹ עַל פְּרֵדָה שֶׁאִמָּהּ חֲמוֹר. וְאָסוּר לְהַרְכִּיבוֹ אֲפִלּוּ עַל הַחֲמוֹר. אֲבָל פֶּרֶד שֶׁאִמּוֹ סוּס אָסוּר לְהַרְכִּיבוֹ עַל פְּרֵדָה שֶׁאִמּוֹ חֲמוֹר. וְכֵן כָּל כַּיּוֹצֵא בָּזֶה. לְפִיכָךְ הָרוֹצֶה לְהַרְכִּיב פֶּרֶד עַל פְּרֵדָה אוֹ לִמְשֹׁךְ בִּשְׁתֵּי פְּרֵדוֹת בּוֹדֵק בְּסִימָנֵי אָזְנַיִם וְזָנָב וְקוֹל אִם דּוֹמִין זֶה לָזֶה בְּיָדוּעַ שֶׁאִמָּן מִמִּין אֶחָד וּמֻתָּרִין:
כסף משנה
6.
Offspring from mixed species whose mothers are from the same species may be mated.17As the Rambam emphasizes in his Commentary to the Mishnah (Kilayim 8:4), the fundamental principle is that the species of the mother is the determining factor whether the union is considered as mixed species or not. If [the mothers] are from two species, it is forbidden to mate them and one who mates them is liable for lashes. Similarly, if one mated the offspring [of such a union with an animal of any other species,] even of its mother's species, he is liable for lashes.18The Kessef Mishnehsuggests that since the determining factor is the species of the mother, the text should read "he is not liable for lashes." According to that interpretation, the bracketed additions in the translation are inappropriate.
What is implied? A male mule19Which comes from mating a horse and a donkey. whose mother is a donkey may be mated with a female mule whose mother is a donkey. It is forbidden to mate it with [an animal of any other species,] even a donkey. It is, however, forbidden to mate a male mule whose mother is a horse with a female mule whose mother is a donkey. Similar laws apply in all analogous situations.
Therefore a person who wants to mate a male and female mule or to have [a carriage] drawn by two mules, he should check their identifying signs: their ears, their tails, and their voices. If they resemble each other, it is clear that their mother is from the same species and [the above activities] are permitted.

הלכה ז
כָּל הָעוֹשֶׂה מְלָאכָה בִּשְׁנֵי מִינֵי בְּהֵמָה אוֹ חַיָּה כְּאֶחָד וְהָיָה אֶחָד מֵהֶן מִין טְהוֹרָה וְהַשֵּׁנִי מִיִן טְמֵאָה הֲרֵי זֶה לוֹקֶה בְּכָל מָקוֹם שֶׁנֶּאֱמַר (דברים כב י) "לֹא תַחֲרשׁ בְּשׁוֹר וּבַחֲמֹר יַחְדָּו". אֶחָד הַחוֹרֵשׁ אוֹ הַזּוֹרֵעַ אוֹ הַמּוֹשֵׁךְ בָּהֶם עֲגָלָה אוֹ אֶבֶן כְּאֶחָד אוֹ הִנְהִיגָם כְּאֶחָד וַאֲפִלּוּ בְּקוֹל לוֹקֶה שֶׁנֶּאֱמַר יַחְדָּו מִכָּל מָקוֹם. אֲבָל הַמְזַוְּגָן פָּטוּר עַד שֶׁיִּמְשֹׁךְ אוֹ יַנְהִיג:
כסף משנה
7.
Anyone who performs labor with two species of animals or wild beasts together when one of them is kosher and the other is not kosher20I.e., according to the Rambam - other Rishonim differ - according to Scriptural Law, one is permitted to work with two animals from two different kosher species. It is only mating them which is forbidden. The Rambam's understanding is based on his interpretation of the Mishnah (Kilayim 8:2) which he understands to be divided in half, the first clauses referring to the prohibition of mating stated in the previous halachah, and the latter clauses to the prohibition against working with different species. The commentaries note that the Rambam's perspective is shared by the Tikkunei Zohar, Tikkun 14. is liable for lashes21I.e., he transgresses a negative commandment, included as one of the 365 negative commandments by Sefer HaMitzvot (negative commandment 218) and Sefer HaChinuch (mitzvah 550). in all places,22I.e., not only in Eretz Yisrael (see Kiddushin 36b). as [Deuteronomy 22:10] states: "Do not plow with an ox and a donkey together."
Whether one plows, seeds, has them pull a wagon, or a stone, or led them together even with his voice [alone],23In which instance, one might think that one is not liable because he did not perform a deed. he is liable for lashes. This is derived from the term "together."24For that implies a combined activity [the Rambam's Commentary to the Mishnah (Kilayim 8:2)]. If, however, one [merely] yokes them [to a wagon], he is exempt25For he has not had them perform a forbidden task. unless he pulls them or leads them.26The Radbaz and the Kessef Mishneh emphasize that the animals from the two must be yoked or tied together in some way for one to be liable for leading them. If they are separate, he is not liable.

הלכה ח
אֶחָד שׁוֹר וַחֲמוֹר וְאֶחָד כָּל שְׁנֵי מִינִין שֶׁאֶחָד טָמֵא וְאֶחָד טָהוֹר בֵּין בְּהֵמָה עִם בְּהֵמָה כַּחֲזִיר עִם הַכֶּבֶשׂ. אוֹ חַיָּה עִם חַיָּה כְּיַחְמוּר עִם הַפִּיל. אוֹ חַיָּה עִם בְּהֵמָה כְּכֶלֶב עִם הָעֵז אוֹ צְבִי עִם הַחֲזִיר וְכַיּוֹצֵא בָּהֶם עַל כָּל אֵלּוּ לוֹקֶה מִן הַתּוֹרָה שֶׁחַיָּה בִּכְלַל בְּהֵמָה הִיא כְּמוֹ שֶׁבֵּאַרְנוּ בְּהִלְכוֹת מַאֲכָלוֹת אֲסוּרוֹת. אֲבָל מִדִּבְרֵי סוֹפְרִים שְׁנֵי מִינִין שֶׁהֵן כִּלְאַיִם בְּהַרְבָּעָה אֲסוּרִים לַחֲרשׁ בָּהֶן כְּאֶחָד וּלְמָשְׁכָן וּלְהַנְהִיגָן. וְאִם עָשָׂה בָּהֶן מְלָאכָה כְּאֶחָד אוֹ מָשַׁךְ אוֹ הִנְהִיג מַכִּין אוֹתוֹ מַכַּת מַרְדּוּת. וְאָסוּר לְהַנְהִיג בְּהֵמָה מִן הַיַּבָּשָׁה עִם חַיָּה שֶׁבַּיָּם כְּגוֹן עֵז עִם שִׁבּוּט וְאִם עָשָׂה פָּטוּר:
כסף משנה
8.
[The prohibition involves not only] an ox and a donkey, [but also] all other different species when one is non-kosher and the other, kosher, whether a domesticated animal with another domesticated animal, e.g., a pig and a lamb, or a beast together with a beast, e.g., a wild deer and an elephant, or a beast with a domesticated animal, e.g., a dog with a goat, or a deer with a pig or the like. For all of the above [types of combinations], one is liable for lashes according to Scriptural Law, for a beast is considered as an animal, as we explained in Hilchot Ma'achalot Assurot.27Hilchot Ma'achalot Assurot 6:1 quotes Deuteronomy 14:4-5 which states: "These are the animals that you may eat: an ox... a gazelle and a deer," including both domesticated animals and beasts in the general category "animals."
According to Rabbinic Law, however, whenever it is forbidden to mate two species, it is also forbidden to plow with them together, to pull them, or to lead them. If one performs labor with [two such species] together, pulled them, or led them, he is liable for stripes for rebellious conduct. It is forbidden to lead an animal from dry land together with a sea-animal, e.g., a goat with a large fish. If one did so, he is exempt.28Bava Kama 55a raises the question whether one is liable for stripes for rebellious conduct for such an action or not, for the two cannot function as a team in the ordinary sense, sense the fish cannot leave the water and the goat will not enter it. Since the question is left unresolved, the Rambam maintains that the person is not liable.

הלכה ט
עֲגָלָה שֶׁהָיוּ מוֹשְׁכִין אוֹתָהּ כִּלְאַיִם הַיּוֹשֵׁב בַּעֲגָלָה לוֹקֶה. וְאַף עַל פִּי שֶׁלֹּא הִנְהִיג. שֶׁיְּשִׁיבָתוֹ גּוֹרֶמֶת לַבְּהֵמָה שֶׁתִּמְשֹׁךְ הָעֲגָלָה. וְכֵן אִם הָיָה יוֹשֵׁב אֶחָד בַּעֲגָלָה וְאֶחָד מַנְהִיג שְׁנֵיהֶן לוֹקִין. וַאֲפִלּוּ מֵאָה שֶׁהִנְהִיגוּ כִּלְאַיִם כְּאֶחָד כֻּלָּן לוֹקִין:
כסף משנה
9.
When a wagon is being drawn by mixed species, one who sits in the wagon is liable for lashes. Even though he did not lead them,29I.e., he was holding the reins of the animals. [he is liable,] because sitting there causes the animal to pull the wagon. Similarly, if one person was sitting in a wagon and another leading the animals, they are both liable for lashes. Even 100 people who lead a team of mixed species are liable for lashes.

הלכה י
מֻתָּר לַעֲשׂוֹת מְלָאכָה בְּאָדָם וּבַבְּהֵמָה אוֹ חַיָּה כְּאֶחָד. כְּגוֹן אָדָם שֶׁחוֹרֵשׁ עִם הַשּׁוֹר אוֹ מוֹשֵׁךְ עֲגָלָה עִם חֲמוֹר וְכַיּוֹצֵא בָּזֶה שֶׁנֶּאֱמַר (דברים כב י) "בְּשׁוֹר וּבַחֲמֹר יַחְדָּו" לֹא בְּאָדָם וַחֲמוֹר אוֹ בְּאָדָם וְשׁוֹר:
כסף משנה
10.
It is permitted to perform a task with a human and an animal together or with a beast, e.g., a person can plow with an ox or pull a wagon with a donkey, or the like. [This is derived from the phrase:] "an ox and a donkey together," i.e., not a man and a donkey and not a man and an ox.

הלכה יא
בֶּהֱמַת פְּסוּלֵי הַמֻּקְדָּשִׁין אַף עַל פִּי שֶׁהִיא גּוּף אֶחָד עֲשָׂאָהּ הַכָּתוּב כִּשְׁנֵי גּוּפִין. מִפְּנֵי שֶׁהָיְתָה קֹדֶשׁ וְנַעֲשֵׂית כְּקֹדֶשׁ וּכְחל מְעֹרָבִין זֶה בְּזֶה וְנִמְצֵאת בְּהֵמָה זוֹ כִּבְהֵמָה טְמֵאָה עִם הַטְּהוֹרָה הַמְעֹרָבִין כְּאֶחָד. הֲרֵי הוּא אוֹמֵר (ויקרא כז יא) "וְאִם כָּל בְּהֵמָה טְמֵאָה אֲשֶׁר לֹא יַקְרִיבוּ מִמֶּנָּה קָרְבָּן לַה'". מִפִּי הַשְּׁמוּעָה לָמְדוּ שֶׁאֵינוֹ מְדַבֵּר אֶלָּא בִּפְסוּלֵי הַמֻּקְדָּשִׁין. לְפִיכָךְ הַחוֹרֵשׁ בְּשׁוֹר פְּסוּלֵי הַמֻּקְדָּשִׁין אוֹ הַמַּרְבִּיעַ הֲרֵי זֶה לוֹקֶה מִשּׁוּם כִּלְאַיִם. וְאִסּוּר זֶה מִדִּבְרֵי קַבָּלָה:
כסף משנה
11.
When an animal has been sanctified but disqualified [and then redeemed]30I.e., an animal was sanctified as an offering, but then developed a blemish that disqualifies it. It was redeemed and thus is considered as an ordinary animal in that after it is slaughtered, it may be eaten without any of the strictures associated with consecrated food. While alive, however, it is forbidden to work with it or shear it, like consecrated animals. See Rashi, Makkot 22a. The Ra'avad and Tosafot offer different interpretations. even though it is one animal, the Torah considers it as two bodies. For it is consecrated and it is as if it is both a consecrated and an ordinary animal mixed as one. Thus this animal is like a non-kosher animal and a kosher animal mixed together and [Leviticus 27:11] states: "If any impure animal that may not be offered as a sacrifice." According to the Oral Tradition, [the verse] was interpreted as referring to animals that had been sanctified, but disqualified. Accordingly, a person who plows with an ox that had been sanctified but disqualified or mates such an ox is liable for lashes because of the prohibition against mixed species. This prohibition is part of the received tradition.

זרעים הלכות כלאים פרק ט
Zeraim Kilayim Chapter 9