The establishment of pregnancy in prolific Chinese Meishan and Large White x Landrace gilts : a comparative study
Chinese Meishan pigs exhibit exceptional prolificacy compared with common domestic breeds and therefore provide a model for investigating the mechanisms involved in a successful reproductive outcome. Previous studies have shown that the Meishan gains its reproductive advantage through reduced conceptus mortality at the time of conceptus attachment to the uterine endometrium and the establishment of pregnancy. The purpose of this thesis was to consider factors that might affect conceptus viability in the Meishan and a control breed (the Large White x Landrace), to determine where differences arise. Meishan and Large White x Landrace function was compared, in the period leading up to the establishment of pregnancy (days eight to 12 after the onset of oestrus, inclusive). Many factors that might impinge on conceptus survival were investigated, including maternal nutrition, metabolism, endocrinology and conceptus and endometrial morphology and biochemistry. Conceptus oestradiol-17β secretion, believed to be the signal for the establishment of pregnancy in the pig, was delayed and more synchronised in the Meishan than in Large White x Landrace. Also, Meishan plasma progesterone concentrations were elevated compared with the Large White x Landrace gilts. Other studies have positively correlated conceptus survival with raised plasma progesterone concentrations, which can be manipulated by altering the nutritional intake of the dam. The role of conceptus oestradiol-17β in the establishment of pregnancy in the Meishan was also investigated. A lack of aromatase enzyme activity, before day 12 after the onset of oestrus, prevents the conversion of testosterone to oestradiol-17β. Thereafter, substrate availability limits the reaction. Exogenous oestradiol benzoate extended the interoestrous interval of treated Meishan gilts, although differences in the physiology of pregnant and pseudopregnant gilts suggested that other conceptus-specific activity influences the establishment of pregnancy in this breed. These studies have highlighted specific aspects of pig reproduction, manipulation of which might improve the prolificacy of domestic breeds. Further investigations are required to establish the whether such techniques would be practical in the commercial setting.