Most of you who have been around horses have probably heard the term proud cut.  This term is used when a horse that has been castrated still acts like a stud.  It was believed for many years that all of the tissue attached to the testicle, namely the epididymis, must be removed at the time of surgery.  If not, the epididymis produced enough hormones to cause the horses to still act like a stud.  However, it has been proven for over 30 years that this tissue does not produce any hormones and if the testicle is removed, the horse is correctly castrated.  Therefore, there is no such thing as a horse being proud cut. 

Although some have believed the adrenal gland produces enough male hormones to cause the behavior, this has also been shown to be incorrect.  Dr. Jim Schumacher indicates that most of the time, this behavior is due to an innate behavior during normal social interaction between horses rather than hormone production from another source.  However, it is possible a horse was castrated incorrectly or the horse had a testicle in the abdominal cavity that was not removed.  To determine if testicular tissue is present, a blood test called an HCG response test is used.  A blood sample is taken and then the hormone HCG is given to the horse and another blood sample is taken at 1 to 2 hours.  If the testosterone concentration increases, there is a 94% chance of testicular tissue remaining.  A one-time blood sample for estrone sulfate level is also 96% accurate in a horse over 3 years of age.  If testicular tissue is believed to exist, the best method of finding and removing it is with a laparoscope because the tissue is sometimes difficult to find without being able to look inside the abdomen.