Endocrine pathogenesis of postweaning anestrus in swine: Response of the persistently anestrous sow to hormonal stimuli

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Abstract
SUMMARY: The endocrine function of the individual components of the hypothalamo-hypophyseal-ovarian axis of the postweaning anestrous sow was evaluated by monitoring the sow's response to exogenous estradiol, gonadotropin releasing hormone (GnRH), and gonadotropins. Sows (4 to 6/group) not returning to estrus by 42.8 ± 3.1 days after weaning were assigned to 1 of the following treatments; 10 μg of estradiol benzoate (eb)/kg of body weight; 200 μg of GnRH, 1,000 IU of pregnant mare's serum gonadotropin (pmsg); 1,000 IU of human chorionic gonadotropin (hcg); or 4 ml of saline solution plus 2 ml of corn oil. A preovulatory-like surge of luteinizing hormone [(lh) > 12 hours in duration] was observed in all weaned sows treated with eb. All eb-treated sows exhibited estrus and ovulated but none conceived. Sows given GnRH had transiently increased (< 3 hours) lh concentrations that were not associated with estrus or ovulation. Treatment with pmsg caused an increase in peripheral concentrations of 17β-estradiol that was followed by an lh surge, estrus, ovulation, and conception. Treatment with hcg caused an increase in circulating concentrations of 17β-estradiol that was accompanied by a surge of lh in some sows and ovulation in all sows. Not all sows treated with hcg exhibited estrous behavior, but conception occurred in 2 of 3 sows that were mated at estrus. None of the sows treated with saline plus corn oil had consistent changes in circulatory concentrations of 17β-estradiol or lh and none exhibited estrus or ovulated. Seemingly, the components of the hypothalamo-hypophyseal-ovarian axis were functional in the persistently anestrous sow. In addition, pmsg appeared to be the only hormone effective in inducing a fertile estrus.