Major physiological factors affecting reproduction are those affecting the number of ova ovulated and fertilized, rate of embryonic survival, physical and biochemical components of uterine capacity, fetal/placental development and survival to term, as well as survival of the neonate as influenced by birthweight and maternal behavior of the dam. Ovulation rate can be increased nutritionally by increasing energy intake, by injection of exogenous gonadotropins during the preovulatory period and through genetic selection. Fertilization rates generally exceed 90 to 95% and are not considered as limiting to increasing reproductive efficiency. Embryonic death losses claim 25 to 50% of potential offspring and represent a major deterrent to improving reproductive efficiency in livestock. Interactions between the conceptus (embryo and its associated membranes) and maternal system are essential for establishment and maintenance of pregnancy. In livestock, these signals, in general, insure maintenance of the corpora lutea (CL) and production of progesterone, as well as stimulate uterine functions which insure delivery of adequate nutrients to the conceptus in the form of histotroph (epithelial secretions) and hematotroph (exchange of nutrients between the maternal and embryonic/fetal circulations). Autocrine, paracrine and endocrine factors affect growth and development of the embryo/fetus and placenta and these may be influenced by genotype of sire, dam and/or fetus. Hormones of the fetal/placental unit and maternal system interact to stimulate mammogenesis and lactogenesls to insure a source of nutrition necessary for survival of the neonatfe. These physiological limitations to increasing reproductive efficiency of livestock will be discussed and, where possible, differences between prolific and nonprolific breeds will be noted. 

F. W Bazer, M. Terqui, F. Martinat-Botte

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume XVI. Poultry, fish and horse genetics and breeding, growth and reproduction, immune response and disease resistance., , 292–298, 1990
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