Nutritional and lactational regulation of fertility in sows

Two major conclusions were drawn from early studies on lactating sows. First, gonadotrophin secretion is suppressed during lactation and this constitutes the primary block to ovarian function and lactational oestrus. Second, in addition to the suckling stimulus per se, nutrition and changes in body condition during lactation have critical effects on post-weaning fertility of sows. Considerable progress has been made in defining the mechanisms that mediate the inhibitory effects of lactation on ovarian development. In sows, active secretion of luteinizing hormone (LH) constitutes an important luteotrophic signal throughout pregnancy. LH secretion may therefore increase in the immediate postpartum period in the suckled 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 exogenous gonadotrophins or luteinizing-hormone-releasing hormone (LHRH), 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. In two recently completed studies, however, it has not been possible to block the development of the suckling-induced suppression of LH by treatment with the opiate antagonist, naloxone, suggesting that other inhibitory mechanisms are involved. Differential regulation of LH and follicle-stimulating hormone (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 still very little direct evidence for the mechanisms by which the metabolic state of lactating sows affect fertility after weaning. Low plasma insulin, glucose and insulin-like growth factor 1 (IGF-1), and high growth hormone (GH) and cortisol concentrations in sows in a catabolic state are all potentially 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.