Genetic implications of a simulation model of litter size in swine based on ovulation rate, potential embryonic viability and uterine capacity: I. Genetic theory.

A simulation model of litter size in swine based on ovulation rate, uterine capacity and potential embryo viability was compared to three genetic models to clarify its genetic characteristics. The simulation model is equivalent to independent culling based on fixed levels of potentially viable embryos and uterine capacity. Litter size also can be described by a combination of additive, additive x additive, mean environment x additive, random environment and additive x random environment effects. A third genetic model that can describe the simulation model is the associative effects model, in which litter size is the result of grouping two genotypes. The fixed independent culling levels model predicts that genetic parameters will change as the component means change. This genetic model also predicts that selection on an index of ovulation rate and uterine capacity would improve selection response for litter size. This genetic model predicts asymmetry of correlated responses in ovulation rate and uterine capacity when selecting for high and low litter size. The nonadditive genetic model predicts covariances among relatives that are different from their additive relationships; however, simulated results did not detect any differences. The nonadditive genetic model also predicts that heterosis for litter size will differ among crosses based on the mean environment and on additive x additive genetic interaction. The associative effects model predicts that selection for litter size will always lead to a positive response in litter size.