Ruling on castrating animals
Question

                What is the ruling in Islam on castrating animals such as sheep, lambs, horses, cattle and others, with the aim of making them grow fat or for other reasons? If that is haraam, are there any cases in which Islam would permit it?.
Praise be to Allah.There is nothing wrong with 
    castrating animals such as sheep and cattle if it is done for a reason, such 
    as making them grow fat and making the meat better. The Prophet  (peace 
    and blessings of Allaah be upon him) sacrificed castrated sheep, as it was 
    narrated by Ahmad and Ibn Maajah (3122) that the Prophet  (peace and 
    blessings of Allaah be upon him) sacrificed two large, castrated rams. 
    Classed as saheeh by al-Albaani in Saheeh Ibn Maajah. 
The fuqaha’ differed 
    concerning this issue, as it is mentioned in al-Mawsoo’ah al-Fiqhiyyah 
    (19/112): The Hanafis stated that there is nothing wrong with castrating 
    animals, because it is beneficial for the animal and for people. 
According to the Maalikis, 
    It is permissible to castrate animals that are eaten, and that is not 
    makrooh, because it makes the meat better. 
The Shaafa’is 
    differentiated between animals that are eaten and those that are not eaten. 
    They said: It is permissible to castrate those whose meat is eaten when they 
    are young, but it is haraam in other cases. They stipulated that castration 
    should not lead to the death of the animal. 
As for the Hanbalis, in 
    their view it is permissible to castrate sheep because it makes their meat 
    better, and it was said: It is makrooh in the case of horses and other 
    animals. End quote. 
A hadeeth was narrated 
    which forbids castration of animals, and horses in particular, but it is 
    da’eef (weak). 
That was narrated by Ahmad 
    (4769) from Ibn ‘Umar (may Allaah be pleased with him), who said: The 
    Messenger of Allaah  (peace and blessings of Allaah be upon him) forbade 
    castrating horses and other animals. Ibn ‘Umar said: Because that is how 
    they increase in number. Shu’ayb al-Arna’oot said in Tahqeeq al-Musnad: 
    Its isnaad is da’eef (weak); it was narrated in mawqoof form and in marfoo’ 
    form, and the mawqoof form is the one which is saheeh.   
Maalik and others regarded 
    castration of horses as makrooh. He said: There is nothing wrong with 
    castrating them if they are to be eaten. Al-Muntaqa by al-Baaji 
    (7/268). 
Shaykh Ibn ‘Uthaymeen (may 
    Allaah have mercy on him) said: As for castration, it is permissible if it 
    serves an interest, but necessary steps must be taken to avoid causing pain 
    to the animal. End quote from Liqa’ al-Baab il-Maftooh 
    (37/15). 
Conclusion: There is 
    nothing wrong with castrating animals whose meat is eaten, according to the 
    majority of scholars, so long as that is done for a reason and care is taken 
    to avoid causing pain to the animal. 
See also question no. 
    10502 
    for more information. 
And Allaah knows best.