All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
It is absolutely forbidden to remove the uteri of inedible animals because that involves torturing them and changing the form in which they were created. Muslim jurists have mentioned that it is absolutely forbidden to castrate inedible animals; whereas, doing this to edible small animals is permissible since it makes their meat tastier. An-Nawawi(May Allah bless his soul) said, " It isn`t permissible to castrate an inedible animal, be it young or old, but it is permissible to castrate edible small animals since doing so makes their meat tastier. However, this isn`t permissible with edible old animals."{Al-Majmou`, 6/177}.
In fact, this (castrating an animal) is similar to removing its uterus in the sense that both are of no considerable benefit. Whoever wants to be kind to animals should do that and leave them the way they were created by Allah, The Almighty. The Messenger of Allah(PBUH) is our role model as far as being kind and merciful to animals is concerned. Ibn Mas'ud (May Allah be pleased with him) reported: We were with the Messenger of Allah (PBUH) in a journey when he drew apart (to relieve nature). In his absence, we saw a red bird which had two young ones with it. We caught them and the red mother bird came, beating the earth with its wings. In the meantime the Prophet (PBUH) returned and said, "Who has put this bird to distress on account of its young? Return them to her." He (PBUH) also noticed a mound of ants which we had burnt up. He asked, "Who has set fire to this?" We replied: "We have done so." He (PBUH) said, "None can chastise with fire except the Rubb of the fire." [Abu Dawud].
Moreover, Ibn 'Umar (May Allah be pleased with them) said: The Messenger of Allah (PBUH) said, "A woman was punished in Hell because of a cat which she had confined until it died. She did not give it to eat or to drink when it was confined, nor did she free it so that it might eat the vermin of the earth."{ Al-Bukhari and Muslim }.
The Messenger of Allah(PBUH) has clarified that being kind and merciful to an animal entitles one to be rewarded by Allah, The Almighty. Abu Hurairah (May Allah be pleased with him) reported: Messenger of Allah (PBUH) said, "While a man was walking on his way he became extremely thirsty. He found a well, he went down into it to drink water. Upon leaving it, he saw a dog which was panting out of thirst. His tongue was lolling out and he was eating moist earth from extreme thirst. The man thought to himself: 'This dog is extremely thirsty as I was.' So he descended into the well, filled up his leather sock with water, and holding it in his teeth, climbed up and quenched the thirst of the dog. Allah appreciated his action and forgave his sins". The Companions asked: "Shall we be rewarded for showing kindness to the animals also?" He(PBUH) said, "A reward is given in connection with every living creature". [Al-Bukhari]. And Allah knows best.
A Fatwa about castrating and sterilizing cats to prevent inevitable mischief was published under No.(3458).