The traditional model of litter size as the product of ovulation rate and embryo survival is reviewed for cattle, sheep and swine. Little is known about genetic parameters of component traits of cattle. The preeminent model for sheep assumes that genetic variation in litter size is due solely to ovulation rate; consequently, selection for ovulation rate is predicted to increase litter size more effectively than direct selection for litter size. Evidence for genetic variation in embryo survival in sheep is building. Genetic (co)variation in ovulation rate and embryo survival in swine has been considered so that selection pressure on components was shifted to favor ovulation rate, rather than embryo survival. Selection based on the resulting optimal index is expected to be more efficient than direct selection for litter size. An alternative model for swine allows uterine capacity to interact with ovulation rate and potential embryonic viability to determine embryo survival and litter size. Phenotypic and genetic parameters depend on distributional properties of components. Selection for litter size is equivalent to selection of components based on independent culling levels. Responses to several selection strategies were simulated and compared to experimental results where possible. 

K. A Leymaster, G. L Bennett

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., , 299–308, 1990
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