Objectionable masculine behaviour, such as penile erection, mounting, copulation, and aggression toward other horses or humans, is not always eliminated completely by castration. A horse that has had both testes removed but still exhibits overt masculine behaviour is sometimes called a false rig. False rigs are often attributed to a horse which is “proud cut”, that is part of the epididymis is left behind at castration; however, the epididymis is unable to produce the hormone testosterone. Which makes this theory false. The adrenal cortex is also a possible culprit for over production of testosterone in a gelding resulting in unwanted behaviour. However, the serum concentration of testosterone in false rigs is no greater than that of geldings that display no masculine behaviour.
Masculine behaviour of a false rig should be attributed to innate behaviour that occurs during normal social interaction between horses rather than to non-testicular production of testosterone. After castration, approximately 20% to 30% of geldings display masculine behaviour toward mares and aggression toward other horses, and approximately 5% display aggression toward humans. And masculine behaviour after castration when performed before puberty compared to after puberty may still continue to develop or persist.
Incomplete castration can occur from time to time. If one or both testes are not present in the scrotum at the time of the procedure, the castration should not be performed until full consultation with an equine surgeon.
Diagnosing the cause of persistent masculine behaviour after castration is to establish whether the behaviour is hormonally induced. Rectal palpation and ultrasound examination is a good place to start. Further tests include serum testosterone and oestrone sulphate, both of which are produced by testicular tissue and are highly elevated in these horses compared with geldings.
Stallion-like behaviour after castration is highly unlikely to be caused by extragonadal production of androgens and is most likely to be caused by innate, psychic behaviour or incomplete castration of a partial abdominal cryptorchid. A horse that displays objectionable, psychic masculine behaviour can be treated by using stricter discipline or isolating the horse from other horses. To improve objectionable masculine behaviour in a horse that has been incompletely castrated, the retained testis must be removed via either a conventional surgical approach or laparoscopy.
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