Factors effecting reproduction in the pig: seasonal effects and restricted feeding of the pregnant gilt and sow.

Recent advances in research on seasonal infertility are discussed with a special focus on implications of the generally recommended restricted post-mating feeding strategy of the early pregnant gilt and sow for the physiology of seasonal infertility. The endocrinological basis of seasonal breeding of the wild and domestic pig is being clarified: as in other seasonal breeders, melatonin is relaying photoperiodic information about season to the pituitary-gonadal axis. Earlier confusion on this matter appears to have been caused by a lack of specificity of the melatonin assays employed. Group housing of the pregnant sow is becoming a common practice and, as an important environmental risk factor for seasonal infertility, may lead to an increase in the incidence of seasonal infertility in the future. After an initial progesterone-mediated beneficial effect on embryonic survival, a restricted post-mating feeding strategy may have a negative effect on maintenance of early pregnancy in the gilt and sow in the summer-autumn period. The endocrinological mechanism of seasonal disruption of pregnancy is yet to be determined. However, it is proposed that LH is reduced in the summer-autumn period and this reduction is amplified by the commonly applied restricted post-mating feeding strategy. These changes in LH secretion, although not as such inducing CL regression, may exert a progesterone-mediated detrimental effect on the capability of embryos to produce adequate embryonic signaling. This may lead to a seasonal disruption of pregnancy and a return to oestrus 25-30 days after mating.