EFFICIENCY OF AN APPROXIMATE ANIMAL-MODEL FOR MATERNAL AND DIRECT GENETIC-EFFECTS OF LITTER SIZE IN SWINE

A simulated population of 120 sows and 24 boars in service per year wa s used to compare genetic evaluations for first-parity litter size and selection response when approximate and complete animal models were u sed. The complete animal model included maternal and direct effects. T he approximate model did not account for covariances between maternal breeding values, or between maternal and direct breeding values, but i ncluded a random litter effect of birth of sow. After 10 yr of selecti on, the overall response (maternal plus direct) was 2.94, 1.61, and .7 7 pigs using the approximate model and 3.11, 1.65, and .67 pigs using the complete model with equal weighting of maternal and direct genetic effects in the aggregated breeding value for genetic correlations bet ween maternal and direct effects of 0, -.5, and -.9, respectively. Hig her reduction was obtained in direct genetic response that was 5, 8, a nd 19% lower using the approximate model than the complete model for g enetic correlations of 0, -.5, and -.9, respectively. Use of the appro ximate model over 10 yr of selection resulted in a bias of estimation of direct genetic trend of .24, .54, and .48 pigs and in bias in estim ated overall response (maternal plus direct) of -.49, .67, and 1.28 pi gs for genetic correlations between maternal and direct effects of 0, -.5, and -.9, respectively. Bias in environmental trend estimated from year-season effects was of the same magnitude as the bias in estimate d overall response but with an opposite sign. With the approximate mod el, maternal genetic response was due to a correlation between materna l genetic effects and direct EBV, which for an embedded trait such as maternal genetic effects occurred even when there was no correlation b etween their true genetic effects. Findings suggested that over the sh ort term the approximate model can be used without substantial loss of overall response but the use of the complete model was recommended be cause of unbiased estimates in genetic and environmental trend and hig her direct genetic response.