Master of Science (MS)


School of Animal Science

Document Type



Two experiments were conducted to determine the effects of growth hormone (GH) or melatonin on the reproductive axis of the stallion. In Experiment 1, nine stallions were treated with GH (20 µg/kg BW) or saline for 21 d 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 combined injections of gonadotropin releasing hormone (GnRH) and thyrotropin releasing hormone (TRH). Two months after the onset of GH treatment, semen was collected from all stallions every other day for 2 weeks. Treatment with recombinant equine GH increased (P < 0.001) daily IGF-I concentrations, but had no effect (P > 0.1) on concentrations of LH, follicle stimulating hormone (FSH), or testosterone. The testosterone responses to injections of LH were similar (P > 0.1) between treatments. Likewise, the LH, FSH, prolactin, and testosterone responses to the injections of GnRH/TRH were similar (P > 0.1) between groups. Stallions treated with GH exhibited greater volumes of gel-free semen (P < 0.01) and gel (P < 0.05) and had decreased time until ejaculation (P < 0.05). In Experiment 2, nine stallions were given corn syrup containing either melatonin (0.06 mg/kg BW) or nothing for 90 d starting in July. Between d 68 and 75 of treatment, stallions were given injections of LH and combined injections of GnRH and TRH, similar to Experiment I. Semen was collected from all stallions for three days during the last week of treatment. Treated stallions exhibited decreased daily concentrations of prolactin (P < 0.01) and FSH (P < 0.05), and tended to have lower (P = 0.07) LH concentrations for the first 30 d. Testosterone concentrations were similar between groups. In treated stallions, the low dose administration of GnRH/TRH was not as effective (P < 0.01) at increasing plasma concentrations of FSH and testosterone, and the response in plasma prolactin concentrations to a high dose administration of GnRH/TRH was decreased (P < 0.01). Melatonin treatment did not alter seminal characteristics or libido. In conclusion, GH may alter the long-term accessory gland contribution to seminal volume, but does not appear to interact with other constituents of the reproductive axis in the stallion. Long-term melatonin administration decreases plasma concentrations of gonadotropins and prolactin, but the role of melatonin in perturbation of hypothalamic interaction with the pituitary deserves further study.



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Committee Chair

Donald L. Thompson, Jr.