Hypothalamic-Pituitary-Ovarian Responses to Weaning — Retrospective Evidence for the Cause of Lactational Anoestrus?

Abstract

Evidence from the study of endogenous hormone changes at the time of weaning in the sow suggest that even during lactation, active secretion of both LH and FSH occurs. The extent to which this escape from the inhibitory effects of suckling and lactation occurs varies between sows and is related to both the response of the endocrine system to weaning and to the weaning to oestrus interval. The length of lactation also affects the endocrine status of the sow. A gradual recovery of tonic gonadotrophin secretion and in the responsiveness of the positive feedback system occurs, and the magnitude of the preovulatory LH surge after weaning increases, with time of weaning post-partum. Sows returning to oestrus immediately after weaning have a characteristic increase in basal LH secretion; in sows returning to oestrus later, a significant increase in episodic LH secretion occurs but not a sustained rise in baseline. Plasma FSH levels are related to the development of steroidogenic activity and to ovulation rate after weaning. The mechanisms controlling the inhibition of reproduction during lactation are uncertain, but a primary block at the hypothalamic level is evident. Both prolactin and oxytocin may mediate such effects and steroids may also exert important modulatory effects as lactation progresses.