Nutrition influences age at puberty, duration of postpartum anestrus, and ovulation rate. Mechanisms mediating these effects, however, have not as yet been fully elucidated. Shifts in feed intake are accompanied by changes in circulating levels of insulin-like growth factor-I (lGF-I), and somatotropin (ST). Feed restriction increases ST, decreases IGF-l, and shifts the proportions of IGF binding proteins (IGFBP). Active immunization against growth hormone releasing factor (GRFi) has been used to examine the effects of alterations in the somatotropic axis, independent of feed intake, on reproduction in prepubertal gilts and heifers. GRFi decreased the number of large follicles present on the ovaries prior to puberty, delayed onset of puberty in heifers, and decreased ovulation rate in gilts. GRFi altered follicular turnover in prepubertal heifers. Replacement therapy with somatotropin restored follicle numbers. GRFi decreased serum and follicular fluid IGF-I and IGFBP-3 and increased IGFBP2. Feed restriction also reduced IGF-I in follicular fluid. We speculate that the population of follicles affected by GRFi or feed restriction is related to their sensitivity to IGF-I. Possibly, alterations of ovarian or peripheral IGF-I at critical times during prepubertal follicular growth result in decreased follicle numbers, culminating in delayed onset of puberty in heifers and decreased ovulation rate in gilts.



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