Endocrine changes in sows weaned at two stages of lactation

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Abstract
Plasma concentrations of LH [luteinizing hormone], FSH, prolactin, estradiol-17.beta. and progesterone were determined in 18 multiparous sows at 4-h intervals for 15-18 days around weaning at 3 or 5 wk post partum. Sampling at 10-min intervals for 6 h occurred every 2 days throughout the same period. Shortening lactation significantly reduced the preovulatory LH surge and altered the pattern of FSH release. There was no significant effect on ovulation rate or interval from weaning to estrus between groups. Weaning was consistently associated with a significant rise in basal LH concentrations while FSH secretion remained unaffected. Lactation length did not appear to affect the characteristics of episodic LH secretion before weaning, nor were any consistent changes in LH secretion apparent until the preovulatory rise in LH. Plasma prolactin values declined rapidly at weaning and remained low thereafter. The trigger controlling the return to cyclic ovarian activity after weaning in the pig is complex, but it is suggested that lactational anestrus and anovulation result primarily from a lack of LH stimulation to the ovary.