Genetic Analysis of Weight and Carcase Traits in Australian Meat Sheep

Abstract

<p>Genetic evaluation of Australian sheep is conducted by Sheep Genetics Australia (SGA) for millions of animals for more than 100 traits. Sheep Genetics uses the Australian sheep genetic analysis software (OVIS), which applies a pre-adjustment of phenotypes for systematic environmental effects rather than fitting all environmental effects (fixed effects) and genetic effects (random effects) jointly in a linear mixed model to estimate breeding values. This thesis aims to compare different methods of genetic evaluation for weight and carcase traits for the Australian sheep industry with a specific emphasis on different methods of accounting for systematic environmental effects. In the first experimental chapter (Chapter 3), various methods of correcting systematic environmental effects were investigated using early body weight phenotypes in White Suffolk and Poll Dorset sheep breed. This was comprised of comparing currently available OVIS pre-adjustment factors for fixed effects with updated pre-adjustment factors, and testing a range of additional interactions between fixed effects. Correlations between EBVs obtained from different models and the regression slopes from forward prediction were used as best model selection criteria. Results showed that the updated pre-adjustment factors produced a slightly better regression slope of progeny performance on sire estimated breeding values (EBVs) than current OVIS pre-adjustment factors: 0.40 and 0.38 versus 0.37 and 0.35 for weaning weight and post-weaning weight, respectively, when averaged over two breeds. Analysis with a linear mixed model produced a significant improvement (P < 0.05) in the regression slopes (0.47 and 0.44) compared to analysis based on pre-adjusted phenotypes. A linear mixed model with flock by sex by age interaction produced better regression slopes (0.48 and 0.46) than the current linear mixed model without this interaction term. Results indicated that flock by sex by age interaction should be included in the linear mixed model for sheep genetic evaluation of early body weight traits in Australia.</p>