Hormone containing growth promoting implants in farmed livestock.

Growth promoting implants have been used in the production of cattle and sheep for over 40 years. Implants improve growth rate (+10 to 30%), feed efficiency (+5 to 15%) and carcass leanness (+5 to 8%). The history of this technology is mainly one of optimizing dose and hormone combinations, although matrices to optimize delivery rates of hormones from implants has received some attention. Estrogens are the first requirement for the growth response and in combination with androgens, growth is further enhanced. Several implant matrices are used, affecting pay-out rate and delivery time. The delivery time of most compressed implants is approximately 120 days and reimplantation after 60-120 days gives an additional response. Blood concentrations of implant hormones are increased and there appears to be a threshold blood level below which a growth response is not observed. Several proposed mechanisms are reviewed. The somatotropic axis appears most plausible for estrogens. Androgens may occupy muscle corticosteroid receptors. Regulated and proper use of implants assures their safety.