According to Halacha, Jews are not supposed to castrate animals, and many poskim say this is a noahide law as well.
My question is whether this prohibition on castration of animals is just a prohibition on physical castration, or is chemical castration of animals also prohibited?
The Shulchan Aruch (EH 5:12) permits chemical castration of female animals but not of males
A person who gives a cup of root drink to a person or any living thing in order to render him impotent, this is forbidden. But, he (the giver of the drink) is not lashed. However, for a woman it is permitted to drink this root drink in order that she become impotent so she cannot give birth.
At the time, a potion was the way to chemically castrate, today there are more modern methods (e.g., zeutering, see here for details and a halachic analysis). Permitted methods for Jews would also be permitted for non-Jews.
Finally, I note there are workarounds available (see e.g., here) based on the Rema in 5:14.
פסקי תוספות בכורות פ"ט
אסור לסרס אפילו על ידי שתיית סם
Piskei Tosafot Bechorot
One may not castrate even through drinking a potion
