Relationships between endogenous gonadotropin-releasing hormone, gonadotropins, and follicular development after weaning in sows

Two experiments were conducted to examine relationships between endogenous gonadotropin-releasing hormone (GnRH), concentrations of gonadotropin in the anterior pituitary and blood and ovarian follicular development in weaned sows. In the first experiment, 48 sows were slaughtered during 0 to 5 days after weaning. Because effects of breed, parity, farm and season of slaughter were confounded with day after weaning, data were analyzed solely on the basis of follicular development. Sows were divided into 4 groups based on the size and number of the largest ovarian follicles. Group 1 consisted of 11 sows with largest follicles <5 mm in diameter. Sows (n=11) in Group 2 had few (≤10) follicles 5 to 10 mm in diameter, and sows (n=11) in Group 3 had many (≥14) follicles 5 to 10 mm in diameter. Group 4 consisted of sows (n=12) with follicles >10 mm in diameter. Data from 3 sows that had ovulated were omitted from the analyses. Sows in Groups 1, 2, 3 and 4 were killed at an average of 0.7 ± 0.4, 2.5 ± 0.2, 4.7 ± 0.1 and 4.8 ± 0.2 days after weaning, respectively. Concentrations of GnRH in medial basal hypothalamus (MBH), stalk median eminence (SME) and hypophyseal portal area (HPA) were greater in sows with greater follicular development (Groups 3 and 4). Concentrations of luteinizing hormone (LH) in anterior pituitary (AP) and serum were not significantly different among groups. In the second experiment, 27 second parity Duroc sows were slaughtered 0, 60 or 96 h after weaning (9 sows per group). Concentrations of both LH and FSH in serum samples obtained at 6-h intervals from weaning to slaughter increased after weaning, and by 96 h levels of LH indicated a preovulatory surge was beginning. Follicular diameter increased with time after weaning, with increases in number of follicles ≥5 mm in diameter, and decreases in number of follicles <5mm in diameter. Concentrations of LH, but not FSH, in AP were greater at both 60 and 96 h than at 0 h after weaning (P<0.01). Concentrations of GnRH in MBH and SME were greater at 60 and 96 h than at 0 h (P<0.05). The concentration of GnRH in HPA was increased only at 96 h after weaning. In both experiments, concentration of GnRH increased after weaning, concurrently with postweaning increases in serum FSH, serum and pituitary LH and follicular development. Thus we propose that initiation of estrus and ovulation after weaning may be associated with changes in production of GnRH.