The influence of maternal effects on accuracy of evaluation of litter size in swine.

The influence of maternal genetic effects on response to selection was examined by stochastic simulation. The selection process of a closed herd of 120 sows, with 24 boars entering the breeding herd each year, was simulated over 10 yr. Effects of different magnitudes of maternal heritability (0, .025, .05), genetic correlations between maternal and direct effects (0, -.5, -.9) and evaluation models (with or without maternal effects, referred to as complete or incomplete model) on response to selection, accuracy of evaluation, prediction error variance (PEV), bias, and mean squared error (MSE) were analyzed for litter size with a direct heritability of .10. Directional selection of replacement animals was on EBV of direct effects for first-parity litter size under an animal model. Using a complete animal model with maternal effects, response in direct genetic effects increased with magnitude of maternal heritability (0 to .05) from 2.22 to 2.32 pigs after 10 yr, when there was no correlation between direct and maternal effects. Additionally, a positive maternal response was achieved (with maternal heritability > 0), although no selection was on maternal EBV. Reduction in direct response due to negative genetic correlations between direct and maternal effects was up to 18% after 10 yr of selection. More important was the negative maternal response, which was up to -1.27 pigs after 10 yr for a genetic correlation of -.9. Consequently, the overall genetic merit (maternal plus direct) was reduced up to 77% compared with when maternal and direct effects were genetically independent.(ABSTRACT TRUNCATED AT 250 WORDS)