Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


School of Animal Science

First Advisor

Robert A. Godke


The effects of somatotropin on ovarian reproductive function has become a keen research area in mammalian species over the past decade. However, to date, very little information is available as to the effects of somatotropin administration on reproduction in the domestic mare. Therefore, the purpose of these experiments was (1) to determine the potential effects (if any) of administration of exogenous equine somatotropin on ovarian follicular development and plasma and follicular fluid hormone concentrations in cyclic and noncyclic mares, (2) to determine the mechanism(s) of action of somatotropin treatment on these parameters in the mare, (3) to develop an efficient protocol for stimulating ovarian follicular development using somatotropin treatment to mares and (4) to develop a repeatable method for the induction of ovulation in seasonally anestrous mares treated with somatotropin. It was determined that somatotropin treatment increased the number of ovarian follicles in both cyclic and noncyclic mares. In addition, plasma and follicular fluid levels of IGF-I were increased in mares treated with somatotropin. In seasonally anestrous mares, sornatotropin treatment in conjunction with daily administration of a GnRH agonist was effective in inducing a 5-fold increase in the number of mares ovulating when compared with treatment with the GnRH agonist alone. In equine granulose cells cultured in vitro, both somatotropin and IGF-I were successful in enhancing cellular proliferation and steroidal output, providing evidence that these compounds act directly at the ovarian level to affect follicular development. In addition, it was determined that treatment to cyclic mares with somatotropin caused a significant delay in the process of follicular atresia, suggesting that it may be through this mechanism that somatotropin increases the number of ovarian follicles in the mare. Further studies are needed in this area of equine reproduction so that the exact mechanism(s) through which sornatotropin affects follicular development can be determined.