*********,
I wanted to put my answer to you in writing. The question was concerning neutering a female dog, and my answer was that it was unequivocally forbidden. I will first reproduce Maimonides’s codification here, and then point out some notes introduced in the Shulhan Aruch and the commentaries, and then I will present the reasons as to why the decisors reject the Taz’s loophole for allowing neutering, and why I believe Rabbi ***** is mistaken in his analysis and conclusion. I would also like to say at the outset that it is beyond the scope of this response to go into the permissibility of saving an animal’s life through sterilization, as it is even the case that we may save a man’s life if it necessitates castrating him. I am only addressing the question of neutering the dog because the owner wants to prevent it from producing litters.
Forbidden Relations, Chapter 16:
It is forbidden to destroy a male’s reproductive organs. This applies to humans and also to animals, beasts, and fowl, both from a kosher species and from a non-kosher species, in the land of Israel and in the Diaspora… Whoever castrates [a person or an animal] should be lashed according to Scriptural Law everywhere. Even a person who castrates a person who has been castrated should be lashed… A person who castrates a female – whether a human or other species – is not liable.
Note this last line. This is not permissible. It is just not punished by a human court.
From Rabbi Avi Grossman, here.