Is a gentile prohibited from castrating a human/animal as part of the Nohadite laws?
This matter is disputed in the Talmud and Poskim. Some Poskim rule that gentiles are not prohibited against castrating. Other Poskim rule that gentiles are prohibited against castrating. Practically, one is to be stringent like the latter opinion. [The practical ramification of this matter is a) Can one sell an animal to a gentile if one knows he will castrate it [this matter will be explained in a further Halacha], and b) What should one instruct gentiles to do, if they desire to keep the seven Nohadite laws.]
May one ask a gentile to castrate an animal:
 Rav Chidka and Tana Divei Menashe in Sanhedrin 56b and in Bava Metzia 90a rule that one of the seven Nohadite laws is not to castrate [they remove the prohibition of Birchas Hashem and replace it with Sirus], and hence gentiles are commanded against castrating, just like they are commanded to follow the other Nohadite laws; However, the Tana Kama in Sanhedrin ibid rules that gentiles are not commanded against castrating. [As they list Birchas Hashem as one of the seven Nohadite laws, which thus excludes the prohibition of Sirus from being included.]
 Implication of Michaber E.H. 5:14 and Stam opinion in Rama ibid, who permits selling to a gentile even if one knows that he will castrate [See Chelkas Mechokeik 5:8]; Rambam; Rosh [brought in Chelkas Mechokeik ibid]; Rashba; Hagahos Maimanis, brought in Beis Shmuel 5:16; Minchas Chinuch Mitzvah 291 in opinion of Rambam and Chinuch. Chelkas Mechokeik 5:8 concludes that we rule like this opinion, as so rule vast majority of Poskim; [The Chelkas Mechokeik ibid explains that the final understanding of the Gemara Bava Metzia ibid is what determines whether we hold like Rav Chidka or not, disputed as, and all those who rule that the prohibition of Amira Lenachri applies to all Issurim, dispute the ruling of Rav Chidka, and since the final ruling in the Shulchan Aruch follows that Amira Lenachri applies in all cases, it is a proof that we do not rule like Rav Chidka, and if so, then the Rama should not have recorded this opinion.] Gra 5:37 “We do not rule like Rav Chidka, as write all the Poskim”; Aruch Hashulchan 5:26 writes that most Poskim rule gentiles are not commanded, and so is the main Halacha, although some are stringent.
 2nd opinion in Rama ibid who forbids selling an animal to a gentile, as explained in Taz 5:10, Chelkas Mechokeik ibid and Terumos Hadeshen 299; Semag; Hagahos Ashri, brought in Beis Shmuel 5:16; Raavad, brought in Rosh, brought in Chelkas Mechokeik ibid
 Beis Shmuel 5:16; 18; brought in Aruch Hashulchan ibid
 Michaber E.H. 5:14
 Literally “Kutti”
 The reason: This is forbidden due to Amira Lenachri, as every matter in the Torah that is forbidden for a Jew to perform is likewise forbidden for a Jew to ask a Gentile to do for him. [Beis Shmuel 5:16; Beir Hagoleh; Shach Y.D. 141:17 and 23; Admur 243:1; 343:5; 450:20; Ribis 72; Sheila Usechirus 29; Michaber C.M. 338:6 [regarding muzzling animal]; Question brought in Bava Metzia 90a and above Poskim rule stringently-see Biur Hagr”a 5:32] Alternatively, the reason is because it is Biblically forbidden for even a gentile to destroy the reproductive organs of a human:animal, and it is part of the Nohadite laws. [Beis Shmuel 5:16; Rav Chidka in Bava Metzia 90a and Sannhedrin 56b]