Gemara

(Beraisa): Nochrim are commanded about seven Mitzvos: monetary laws, blasphemy, idolatry, Arayos, murder, theft, and eating from a living animal.

R. Chidka says, they are also forbidden to castrate;

(Beraisa - Tana d'Vei Menasheh): Their seven Mitzvos are idolatry, Arayos, murder, theft, eating from a living animal, castration and Kilayim.

Tana d'Vei Menasheh learns castration from "Shirtzu va'Aretz u'Rvu Vah." The other Tana says that this is a blessing, not a Mitzvah.

Bava Metzia 90a - Question: Amirah l'Nochri (telling him to do Melachah on Shabbos) is forbidden. Is this a stringency of Shabbos (since one who desecrates Shabbos is liable to death), or does it apply to any Lav?

Answer - Question (from Eretz Yisrael): If Nochrim steal bulls from Yisraelim (their friends, who prefer castrated bulls for plowing), castrate them and return them, what is the law?

Answer (Shmuel's father): This is a ruse. The owner may not benefit from this. He must sell them. (Even though castration is only a Lav, one may not ask a Nochri to do it.)

Rejection (Rav Papa): In Eretz Yisrael, they hold like Rav Chidka, who forbids Nochrim to castrate. One who encourages them to do so transgresses "Lifnei Iver."

(Rava): Shmuel's father required selling the bulls for slaughter.

(Abaye): No, it suffices to fine the owner by forcing him to sell them.

Question: Obviously, selling it to one's (adult) son is (permitted,) like selling it to a stranger. May he sell it to his minor son?

Answer #1 (Rav Achi): No.

Answer #2 (Rav Ashi): Yes.

Mereimar and Mar Zutra (alternatively - two Chasidim) would exchange their bulls (after Nochrim castrated them) with each other.

Rishonim

The Rif and Rosh (Bava Metzia 52b and 7:6) bring the Gemara in Bava Metzia. (The Rif omits the opinions about selling to one's son.)

Rambam (Hilchos Isurei Bi'ah 16:13): One may not tell a Nochri to castrate a Yisrael's animal. If the Nochri took it by himself and castrated it, it is forbidden. If the Yisrael schemed, we fine him. He must sell it to another Yisrael. He may sell it to his adult son, but he may not sell or give it to his minor son.

Magid Mishnah: The Gemara says that (perhaps) the Isur to tell a Nochri to castrate is only according to the opinion that Nochrim are forbidden to do so. The Rambam rules that Nochrim are permitted. Nevertheless, he holds that mid'Rabanan one may not tell a Nochri to do anything forbidden to Yisraelim.

Rambam (Hilchos Kil'ayim 1:3): One may tell a Nochri to plant Kil'ai Zera'im (diverse seeds) for him.

Rebuttal (Ra'avad): The question of Amirah l'Nochri was not settled; we must be stringent.

Kesef Mishneh: Since Amirah l'Nochri is mid'Rabanan even regarding Shabbos, for which one is stoned, we may be lenient about a Safek mid'Rabanan. However, the Gemara concluded that a Nochri is not commanded about castration, so regarding Amirah l'Nochri, it is like any Lav, and the Rambam is stringent about castration! Perhaps he learns from Rav Mesharshiya, who gave a Perutah to a Nochri child to plant hops for him in a vineyard (Shabbos 139a). However, that was in Chutz la'Aretz, where we follow the lenient opinion (R. Tarfon) in Eretz Yisrael, which permits a Yisrael to do so! The Rambam must hold that Rav Mesharshiya does not follow (in Bavel) the lenient opinion in Eretz Yisrael, for if so, he would have planted hops through a Yisrael. Rather, a Yisrael may not do so, but one may tell a Nochri to do so. However, this still does not explain why the Rambam is stringent about castration, and why he permits only Kil'ai Zera'im, but not in a vineyard. Also, the Rambam brings Rav Mesharshiya's practice regarding Kilai ha'Kerem in Chutz la'Aretz, but here he surely discusses Eretz Yisrael, for in Chutz la'Aretz a Yisrael may plant Kil'ai Zera'im!

Magid Mishneh (Hilchos Sechirus 13:3): Some texts of the Rambam permit a Yisrael to tell a Nochri 'muzzle my cow and thresh with it.' Amirah l'Nochri regarding a Lav was not resolved, so we are lenient. The true text of the Rambam forbids, just like he forbids telling a Nochri to castrate.

Mishneh l'Melech: Semag says that since the question was not resolved, one may not tell a Nochri 'muzzle my cow and thresh with it,' but we do not lash for this. Why is he stringent? We are lenient about a Safek mid'Rabanan! Other Meforshim are stringent because the latter Amora'im forbade castration through Nochrim, i.e. the Safek was resolved.

Rosh (ibid.): The Halachah follows Abaye. It seems that Rava accepted his words. We permit selling it to one's (minor) son.

Nimukei Yosef (DH v'Yizdaven): We force the Yisrael to sell the bull so he will not benefit from the Aveirah. Even though it sells for more money, this discourages the Nochri. He did the ruse in order that the Yisrael himself would plow with it.

Rosh: The Chasidim did not want the Nochrim to castrate their bulls. Still, they were stringent, for the Nochri wanted to help the Yisrael. The Ra'avad is lenient, for the question was not settled. I say that it was settled. All Amora'im except for Rav Papa forbid, even though the Halachah is unlike Rav Chidka.

Tosfos (DH Michlefei): The Chasidim were stringent, lest people suspect that they wanted the Nochrim to do so, or lest others tell Nochrim to do so.

Hagahos Ashri: We fine about castration, out of concern for scheming. Since the Nochri knows the Yisrael, stole the animal, castrated it and returned it, there are grounds to suspect that the Yisrael told him to do so.

Poskim

Shulchan Aruch (EH 5:14): A Yisrael may not tell a Nochri to castrate the Yisrael's animal. If the Nochri himself took the animal and castrated it, it is permitted. If the Yisrael schemed (Rema - or even if he did not scheme, but the Nochri knows him and did so to help him) we fine him and force him to sell it to another Yisrael. He may sell it to his adult son, but he may not sell or give it to his minor son.

Rema: One may give animals to a Nochri (to sell or raise them) for Machatzis Sechar (to share the profits), even though he knows that the Nochri will castrate them. This is because the Nochri does so for his own benefit.

Teshuvas Maimoniyos (l'Sefer Kedushah 2, cited in Beis Yosef DH Kosvu): We hold that a Nochri is not commanded about castration. Amirah l'Nochri does not apply, for the Yisrael did not hint that he desires this. This is like a Nochri who comes to extinguish on Shabbos. We need not protest.

Rema: One may sell animals and roosters to a Nochri, even though the Nochri will castrate them. Some forbid. If the buyer does not castrate them himself, rather he gives them to another Nochri to castrate them, all permit.

Terumas ha'Deshen (299, brought in Beis Yosef DH uvi'Terumas): The Ritzva and Rosh rule unlike R. Chidka. Semag rules like R. Chidka, but is lenient if the Nochri will not castrate them himself, rather, through his slave.

Question (Beis Yakov 2, cited by R. Akiva Eiger Shulchan Aruch 388:11): Here, the Rema rules like the Terumas ha'Deshen that Ein Shali'ach l'Davar Aveirah applies to Nochrim. In CM (388:15), the Rema rules Like the Tashbatz, who says that Yesh Shali'ach l'Davar Aveirah if the Shali'ach is established to transgress!

Beis Shmuel (16): One should be stringent like those who rule like R. Chidka (and not sell to Nochrim), and also like those who rule unlike R. Chidka (and forbid Amirah l'Nochri for all Isurim).

Beis Shmuel (17): The stringent opinion surely forbids giving for Machatzis Sechar. Normally, Lifnei Iver does not apply if the receiver could buy the Isur elsewhere. That is when he is limited (how much he will drink or offer to idolatry). Here, he will castrate all animals he gets!