It is safe to castrate a horse after age 5; however, there are some important things to consider. Typically male horses are castrated in the first year or 2 of life before they are sexually mature and have begun breeding. Therefore, their testicles and the blood supply to them are smaller. As colts age and their testicles enlarge the risks associated with castration increase. Bleeding is more likely and often ligatures (stitches) are employed to help minimize this.
The presence of the suture material at the castration site increases the risk of infection. Also, mature stallions have a much larger scrotum than immature colts and this “extra” skin can limit drainage from the castration site predisposing to seroma/hematoma formation and possibly infection. So, my recommendations include “closed” castration of mature stallions (any 3 years of age) in a clinic setting. This provides a sterile surgical environment (sterile prep and drapes), something that is difficult to achieve in the field. This allows use of sutures and the excess scrotal skin can be removed and the castration site closed entirely.
Control of bleeding is much better achieved in this manner and recovery time is shorter and less complicated. Stallions that have a “closed” castration (as described above) can return to work 3-4 days post-operatively and have less swelling typically than traditional “open” castrations where the incisions are left open to close on their own.