EFFECT OF SELECTION FOR MATERNAL AND DIRECT GENETIC-EFFECTS ON GENETIC-IMPROVEMENT OF LITTER SIZE IN SWINE

Optimum weighting ratios of maternal: direct EBV for litter size using an animal model were examined to achieve maximum genetic improvement (direct plus maternal response). Stochastic simulation of a 120-sow he rd over a 10-yr period of selection was used (20 replicates). Directio nal selection was based on a merit function of maternal and direct EBV for first-parity litter size. Optimum weighting ratios for maternal t o direct EBV in pure-and crossbreeding schemes with different genetic correlations between maternal and direct effects were obtained. Geneti c gain in maternal and direct effects was more sensitive to change in weighting ratios of maternal to direct estimates of breeding values un der an animal model than earlier theoretical studies showed for select ion index. In the purebreeding scheme, the weighting ratios of materna l:direct effects of 1:1, 1.25:1, and -.5:1 resulted in the highest ove rall response of 3.11, 1.73, and .69 pigs after 10 yr of selection wit h genetic correlations between maternal and direct effects of 0, -.5, and -.9, respectively. In the crossbreeding scheme with a male dam lin e selected for direct effects only and a female dam line selected for an optimum weighting ratio of maternal and direct effects the overall response was always higher with 3.19, 1.89, and 1.31 pigs/10 yr for th e genetic correlations 0, -.5, and -.9, respectively, than in the pure breeding scheme. With a large negative correlation between maternal an d direct effects a meaningful overall response of litter size was achi eved only in the crossbreeding scheme. The effect of negative weightin g of maternal effects to increase direct response with a large negativ e correlation between maternal and direct effects was also examined as well as the influence of weighting ratios on accuracy of evaluation, additive genetic variance, prediction error variance, and correlation between maternal and direct EBV.