Date of Award

2001

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

School of Animal Science

First Advisor

Donald L. Thompson, Jr

Abstract

A series of experiments was conducted to gain a better understanding of the effects of nutrition and hormonal manipulation on the reproductive processes of the horse. In Experiment 1, body condition score (BCS) and ultrasonic backfat measurements were highly correlated in mares, especially over the tailhead area. In Experiment 2, high BCS (HBCS) mares continued to ovulate during the winter anestrous period, whereas low BCS (LBCS) mares went deeply anestrous. Daily hormone concentrations were not affected by BCS, however, responses to hormonal challenges indicated cyclicity in HBCS mares and reproductive quiescence in LBCS mares. In Experiment 3, treatment of average BCS mares with gonadotropin releasing hormone analog (GnRHa) and thyrotropin releasing hormone (TRH) tended to increase (P < 0.09) the number of medium-sized follicles, and GnRHa increased (P < 0.0002) synthesis and storage of luteinizing hormone. In HBCS and LBCS mares (Experiment 4), equine growth hormone (eGH) increased (P < 0.0001) insulin-like growth factor-1 (IGF-1), and IGF-1 was higher (P < 0.02) in HBCS mares. Treatment with GnRHa after eGH increased (P < 0.002) the number of follicles and two LBCS mares ovulated; however, these mares soon reverted back to an anestrous state. In Experiment 5, leptin concentrations were higher (P < 0.009) in HBCS mares and decreased from September to January; however, eGH treatment did not affect leptin concentrations. Dexamethasone treatment increased (P < 0.001) leptin concentrations in HBCS mares. Also, pregnant mares had higher (P < 0.0001) leptin concentrations prior to parturition than after, indicating that leptin may be produced in part by the placenta. Lastly, leptin concentrations increased (P < 0.004) in foals not treated with eGH and remained unchanged in foals treated with eGH for 12 months. These results indicate that although hormones, such as eGH and GnRHa, may play a role in alleviating some of the deleterious affects of nutritional anestrous, the amount of body condition may be the most important factor affecting reproductive activity in the horse. Also, plasma concentrations of leptin are directly related to the amount of body condition indicating that leptin may also influence reproductive activity to a large degree.

ISBN

9780493563015

Pages

146

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