Endocrine changes in sows weaned at two stages of lactation.

Plasma concentrations of LH, FSH, prolactin, oestradiol-17 beta and progesterone were determined in 18 multiparous sows at 4-h intervals for 15-18 days around weaning at 3 or 5 weeks 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. However, there was no significant effect on ovulation rate or interval from weaning to oestrus between groups. Weaning was consistently associated with a significant rise in basal LH concentrations whilst 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. These results indicate that the 'trigger' controlling the return to cyclic ovarian activity after weaning in the pig is complex, but it is suggested that lactational anoestrus and anovulation result primarily from a lack of LH stimulation to the ovary.