We used to castrate our john mules at one or two months of age, but we had several problems with bleeders and in one case the mule eviscerated and died. Now we wait until they are at least 5 or 6 months old before calling out our vet. By this age the stomach wall has strengthened and the danger of a tragic outcome is greatly reduced. On the other hand, if you put it off much beyond the age of 9 months, the john can become difficult to handle.
Dr. Suzy Burnham of Texas A&M advocates "anesthesia with amnesia," because a mule remembers pain for a long time and will associate it with the people who inflicted it, whether intentionally or not. Some mules blame their present pain on the person they remember handled them last. My husband Jerry often receives this treatment after a young mule's castration, because he usually has the job of presenting the youngster to the vet for the sedation needle. Some mules forgive him quickly, others carry a grudge for a long time.
Find a veterinarian who has taken time to learn about the differences between castrating a donkey or mule compared to a horse. An experienced veterinarian will use sutures with mule or donkey colts, which are more prone to bleeding than horse colts. This knowledge can make the difference in whether or not the castrated colt survives.
Another difference is in sedatives for mules or donkeys. If the vet says these animals are no different from horses, look for another vet. A mule generally requires more sedative to put him down before he may be operated on. The last colt we castrated required three times the anaesthetic that a horse colt would have needed. Even then he was down only long enough for the surgery, and up immediately after.