Factors effecting reproduction in the pig: seasonal effects and restrictedfeeding of the pregnant gilt and sow

Recent advances in research on seasonal infertility are discussed with a sp ecial focus on implications of the generally recommended restricted post-ma ting feeding strategy of the early pregnant gilt and sow for the physiology of seasonal infertility. The endocrinological basis of seasonal breeding o f the wild and domestic pig is being clarified: as in other seasonal breede rs, melatonin is relaying photoperiodic information about season to the pit uitary-gonadal axis. Earlier confusion on this matter appears to have been caused by a lack of specificity of the melatonin assays employed. Group hou sing of the pregnant sow is becoming a common practice and, as an important environmental risk factor for seasonal infertility, may lead to an increas e 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 endocr inological mechanism of seasonal disruption of pregnancy is yet to be deter mined. However, it is proposed that LH is reduced in the summer-autumn peri od and this reduction is amplified by the commonly applied restricted post- mating feeding strategy. These changes in LH secretion, although not as suc h inducing CL regression, may exert a progesterone-mediated detrimental eff ect on the capability of embryos to produce adequate embryonic signaling. T his may lead to a seasonal disruption of pregnancy and a return to oestrus 25-30 days after mating. (C) 2000 Elsevier Science B.V. All rights reserved .