The effects of equine somatotropin on pituitary and testicular function in the stallion during the nonbreeding season

An experiment was conducted to determine the effects of equine somatotropin on the reproductive axis of the stallion during the nonbreeding season. Adult stallions were treated with equine somatotropin (20 μg/kg body weight [BW]; n = 5) or saline (n = 4) daily for 21 days starting in January. During the last week of treatment, stallions were subjected to low- and high-dose injections of luteinizing hormone (LH), as well as low- and high-dose injections of gonadotropin-releasing hormone (GnRH) and thyrotropin-releasing hormone (TRH). Two months after the onset of somatotropin treatment, semen was collected from all stallions every other day for 14 days. Treatment with equine somatotropin increased (P < .001) daily IGF-1 concentrations but had no effect (P > .1) on concentrations of LH, follicle-stimulating hormone (FSH), or testosterone. The testosterone responses to injections of LH were similar (P > .1) between treatments. Likewise, the LH, FSH, prolactin, and testosterone responses to the injections of GnRH/TRH were similar (P > .1) between groups. At seminal collections, stallions treated with somatotropin exhibited greater volumes of gel-free semen (P < .01) and gel (P < .05) and had decreased time until ejaculation (P < .05). In conclusion, somatotropin treatment for 21 days may alter the long-term accessory gland contribution to seminal volume but does not appear to alter pituitary gonadotrope function or testicular testosterone secretion.