Effects of exogenous somatotropin (ST) on gonadal function in ruminants and swine

During the past 15 years, many investigators have examined the effects of s omatotropin (ST) on growth and lactation in farm animals. Throughout this p eriod, concerns about potential effects of ST on reproduction have been exp ressed. The objective of the present review will be to focus on the effects of exogenous ST on the hypothalamic-pituitary-gonad al axis. Plasma proges terone is increased when recombinant bovine (rb)ST is given to cattle, earl y in lactation, and also to sheep. Also, the release of progesterone from c ultured swine and human luteal cells is increased by ST. Treatment with rbS t increases the numbers of small follicles, but does not increase the ovula tory rate of ruminants. Doses of ST similar to those used to increase milk production do not affect the secretion of testosterone or spermatogenesis i n rams or bulls. Stimulatory and inhibitory effects of exogenous ST on repr oductive function of gilts have been reported. Daily injections of porcine ST (pST) delayed puberty and expression of estrus after puberty. Daily admi nistration of PST increased the number of small follicles, but not of mediu m follicles, whereas administration of PST by using a sustained release imp lant increased the number of medium follicles. Size and weight of reproduct ive organs and concentration of testosterone are not affected when pST is a dministered for at least 42 d, However, PST enhanced testicular development and spermatogenesis when given to neonatal boars. In summary, administrati on of exogenous ST at doses known to alter milk production and carcass comp osition may have subtle positive and/or negative effects on the reproductiv e systems of cattle and swine; however, these effects appear to be transien t. (C) 1999 Elsevier Science Inc. All rights reserved.