CONTROL OF FOLLICULAR DEVELOPMENT AND OVULATION RATE IN PIGS

There is considerable evidence that nutritional and metabolic control of follicular growth is mediated by metabolic hormones and growth fact ors, particularly with processes mediated by insulin-like growth facto r I (IGF-I) and its binding proteins (IGFBPs). From knowledge that hor mones and growth factors which can be affected by diet also positively affect ovarian function, the concept has emerged that metabolic modif iers of gonadotrophin action, rather than gonadotrophins themselves, c ould affect follicle development. While ovulation rate can be enhanced under certain conditions in cyclic gilts, assessing influences of met abolic modifiers on the post-lactational sow is confounded by variabil ity in the return to oestrus after weaning. In a series of studies inv olving insulin administration between weaning and oestrus, successive experiments produced different results, but several measures of reprod uctive performance were enhanced. Administration of somatotrophin (ST) has also been shown to increase follicular development in both gilts and sows. Both insulin and ST increase IGF-I production by pig ovarian follicles, and insulin is more effective than IGF-I in reducing atres ia and increasing progesterone in cultured pig follicles. Whether incr eases in litter size are achieved after an increase in ovulation rate involves many factors, including the quality of ova and whether the in crease in ovulation rate exceeds the uterine capacity to maintain preg nancy. Given the variation in genetics and management practices, devel opment of treatments to enhance follicle quality leading to maximal li tter size is challenging.