Pulsatile Administration of Gonadotropin Releasing Hormone to Lactating Sows: Endocrine Changes Associated with Induction of Fertile Estrus1

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Abstract
Two experiments were conducted to determine whether pulsatile administration of gonadotropin releasing hormone (GnRH) would induce estrus and ovulation in lactating, anestrous sows. In each experiment, six lactating sows received GnRH, i.v. (2.5 micrograms every 2 h, Exp. 1; 1.5 micrograms every h, Exp. 2) until 24 h after estrus or 7 days, whichever came first. In Experiment 1, three of six GnRH-treated lactating sows exhibited estrus 4.0 +/- 0.0 days after GnRH treatment began. All three GnRH-treated sows conceived at the estrus induced during lactation. Patterns of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in serum around estrus were similar between GnRH-treated sows that exhibited estrus during lactation and weaned control sows. However, in GnRH-treated sows that did not show estrus, a preovulatory-like surge in FSH, but not in LH, occurred on Day 4 of GnRH treatment. Prolactin concentrations in serum dropped from 25.6 +/- 2.4 ng/ml during lactation to less than 6 ng/ml within 12 h after weaning (Exp. 1). In Experiment 2, all six GnRH-treated sows exhibited estrus 3.8 +/- 0.3 days after initiation of GnRH treatment, and five sows conceived. Patterns of LH and FSH during lactation were similar between GnRH-treated and lactating control sows during the first 3 days of GnRH treatment. During 3 days before estrus, LH concentrations were lower (P less than 0.05) and FSH concentrations tended to be higher (P less than 0.20) in lactating GnRH-treated sows than in control sows whose litters had been weaned. Concentrations of estradiol in GnRH-treated sows were greater than those in controls during lactation but were similar in both groups on the day of estrus. In both experiments, concentrations of progesterone were greater in GnRH-treated than in control sows prior to and during estrus. We conclude that, despite some differences in hormone secretion between GnRH-treated lactating sows and sows with litters weaned, pulsatile administration of GnRH caused requisite endocrine changes for fertile estrus during lactation.