Nutritional and lactational regulation of fertility in sows
Foxcroft, G.R.
Journal of Reproduction and Fertility. Supplement 45: 113-125
1992
ISSN/ISBN: 0449-3087 PMID: 1304026 Document Number: 403729
This review considers 2 major conclusions drawn from early studies on lactating sows. Firstly, that gonadotropin secretion is suppressed during lactation and that this constitutes the primary block to ovarian function and lactational oestrus. Secondly, in addition to the sucking stimulus per se, nutrition and changes in body condition during lactation have critical effects on postweaning fertility. Considerable progress has been made in defining mechanisms that mediate the inhibitory effects of lactation on ovarian development. In sows, secretion of LH is an important luteotrophic signal throughout pregnancy. LH secretion may therefore increase in the immediate postpartum period in the suckling sow. In the absence of suckling, this high rate of LH secretion can result in the development of highly oestrogenic, but often cystic, follicles in the immediate postpartum period. Together with data from studies involving treatment with gonadotropins, this indicates that ovarian sensitivity is not compromised during lactation in sows. The inhibitory effects of suckling on LH secretion, typical of established lactation, are imposed during the first 72 h post partum, and from this time the inhibitory mechanism is at least partly opioid-dependent. Two studies, however, have shown that it is possible to block the development of the suckling-induced suppression of LH by treatment with the opioid antagonist naloxone, suggesting that other inhibitory mechanisms are involved. Differential regulation of LH and FSH secretion is evident during lactation, probably as a consequence of ovarian inhibitory, inhibin-mediated control of FSH but not of LH secretion. There is little direct information on the mechanisms by which the metabolic state of lactating sows affect fertility after weaning. Low plasma insulin, glucose and insulin-like growth factor-I, and high growth hormone and cortisol concentrations in sows in a catabolic state may all be involved. In lactating sows with a reasonable energy balance, the inhibitory neural effects of suckling are probably more potent inhibitors of LH secretion than are the metabolic demands of milk production.