Too often, they are given to stimulate growth and muscle development in horses, especially young, growing halter, sale, and performance horses. But, this is not their purpose. Research has shown that anabolic steroids do not cause young horses in good condition to grow taller or bigger with more muscling. Instead, research has shown that anabolic steroids causes side effects in the reproduction of stallions and mares. Stallions have a reduced sperm production.
Mares become very aggressive, having stallion-like behavior.
In the fall, horse owners and breeders often are targeting weanlings and yearlings for halter futurities and getting long-yearlings ready for training for next spring performance showing. Unfortunately, some use anabolic steroids, thinking they will get more growth and muscling in colts and fillies. Most of the research with anabolic steroids has been done with older stallions. Three age groups of colts were used in these studies. Various reproductive parameters were studied.