This experiment determined whether pulsatile administration of gonadotropin-releasing hormone (GnRH) would induce estrus and ovulation in seasonally anestrous primiparous sows and compared endocrine responses of GnRH-induced sows with those of primiparous sows that exhibited spontaneous estrus after weaning. Seventeen primiparous Landrace X Large White sows farroed in August 1982, lactated 23.8 ± 0.4 days (mean ± SEM), and weaned 9.0 ± 0.3 pigs per litter. Blood for determination of progesterone, luteinizing hormone (LH), and estradiol-17β (E) was collected every 6 h from 1 day before to 12 days after weaning. Twelve sows exhibited spontaneous estrus 135 ± 9 h after weaning, and these sows were considered to be normal. Five sows were anestrous for at least 23 days postweaning and failed to ovulate, as indicated by concentrations of progesterone that were less than 1.0 ng/ml in blood samples collected daily during this period. From Day 0 to Day 30 postweaning, levels of estradiol in anestrous sows varied between 3 and 30 pg/ml, concentrations of LH were low, and preovulatory-like LH surges did not occur. Beginning on Day 30 postweaning, four anestrous sows were given 1.5 µg GnRH (i.v.) hourly until onset of estrus and blood was collected every 6 h during GnRH treatment. The average interval from beginning of GnRH treatment to onset of estrus was 84 ± 5 h (range 72 to 96 h). Patterns of estradiol and LH secretion around estrus were similar in normal sows and those treated with GnRH. Estradiol remained elevated above basal concentrations (25 pg/ml) for 49.6 ± 4.0 h in normal sows compared to 48.0 ± 0 h in GnRH-treated sows. Average interval (h) from peak estradiol to the preovulatory LH surge was similar in normal (23.5 ± 4.2) and GnRH-induced sows (18.0 ± 4.2). We conclude the administration of hourly pulses of GnRH to weaned anestrous primiparous sows imposed a proper stimulation to the anterior pituitary that in turn led to an increased gonadotropin secretion that induced estrus and ovulation.



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