It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) forbade the castration of horses and other animals. Sayyiduna Ibn Umar (may Allah be pleased with them both) said: Not castrating increases the progeny.
Hadith Takhrij«اسناده ضعيف لضعف عبد الله بن نافع ، (انظر مسند أحمد ترقيم الرسالة: 4769 ترقیم بيت الأفكار الدولية: 4769»
Brief Explanation
Benefits: … If animals are left upon their natural disposition and are not castrated, then their progeny also increases, and there is also reward and merit for giving them for mating.