ENVIRONMENTAL AND SEASONAL INFLUENCES ON THE RETURN-TO-ESTRUS AFTER WEANING IN PRIMIPAROUS SOWS - A REVIEW

The domestic pig is not a true seasonal breeder but manifests variatio ns in reproductive performance throughout the year. For instance, the weaning-to-oestrus interval is longer in sows weaned in summer and ear ly fall. These variations are more pronounced in primiparous than in m ultiparous sows. Long light duration and more obviously high ambient t emperature play a role in this prolonged anoestrus. The neuroendocrine mechanisms mediating the effects of photoperiod and temperature on th e hypothalamo-pituitary-ovarian axis are poorly understood in the pig. In seasonal breeders, photoperiod regulates reproduction through diur nal variations in melatonin secretion by the pineal gland and subseque nt modifications in gonadotrophin secretion. In domestic pigs, diurnal patterns of melatonin are confusing and a nocturnal increase is not a lways observed. Light duration does not seem to influence gonadotrophi n secretion in primiparous sows. Under high ambient temperature, pulsa tility of the luteinizing hormone and circulating concentrations of co rtisol are decreased whereas those of growth hormone are increased. Si nce lactating sows and particularly primiparous sows reduce their food intake under high ambient temperature, it can be suggested that delay ed oestrus in summer is, at least in part, due to nutritional deficien cy.