Ovarian control of gonadotrophin secretion in the prepubertal heifer

Abstract In order to investigate the development of ovarian feedback control of gonadotrophin secretion in the immature animal, three groups of eight Hereford X Friesian heifers were bilaterally ovariectomized at either 2, 6 or 12 weeks of age. Blood samples were collected for gonadotrophin analysis (15-min intervals for 6 h) on days −1, 4, 7, 14, 28, 55 and 110 relative to ovariectomy. The ovaries from these animals showed that follicular development was established soon after birth and the follicles were producing oestradiol. In all animals, ovariectomy was followed by a rise in plasma gonadotrophin concentrations although the response was delayed in heifers ovariectomized at 2 weeks compared to those ovariectomized at 6 or 12 weeks of age. Mean LH concentrations were significantly elevated 14, 7 or 4 days after ovariectomy and mean FSH concentrations were significantly higher than pre-treatment values 28, 7, or 4 days post-ovariectomy in heifers ovariectomized at 2, 6 or 12 weeks of age, respectively. The results suggest that an inhibitory mechanism, of ovarian origin, that controls gonadotrophin secretion develops between 2 and 12 weeks of age in the heifer.