GENETIC-IMPROVEMENT OF LITTER SIZE IN SHEEP - A COMPARISON OF SELECTION METHODS

The objectives of this work were to examine the usefulness of measurin g ovulation rate (OR) in order to improve genetic progress of litter s ize (LS) in sheep and to study different selection criteria combining OR and prenatal survival (ES) performance. Responses to selection for 5 generations within a population of 20 male and 600 female parents we re compared using Monte-Carlo simulation techniques with 50 replicates per selection method. Two breeds with low (Merino) and medium (Lacaun e) prolificacy were considered. Records were generated according to a bivariate threshold model for OR and ES. Heritabilities of OR and ES i n the underlying scale were assumed constant over breeds and equal to 0.35 and 0.11, respectively, with a genetic correlation of -0.40 betwe en these traits. Four methods of genetic evaluation were compared: uni variate best linear unbiased prediction (BLUP) using LS records only ( b-LS); univariate BLUP on OR records (b-OR); bivariate BLUP using OR a nd LS records (b-ORLS); and a maximum a posteriori predictor of a gene ralised linear model whereby OR was analysed as a continuous trait and ES as a binary threshold trait (t-ORES). Response in LS was very simi lar with b-LS, b-ORLS and t-ORES, whereas it was significantly lower w ith b-OR. Response in OR was maximum with b-OR and minimum with b-LS. In contrast, response in ES was maximum with b-LS. This study raised t he question as to why selection based on indices combining information from both OR and ES did not perform better than selection using LS on ly.