Should Angus Breeders Live-Animal Ultrasound Scan For Intramuscular Fat In The Genomics Era?

Accuracy of selection and response to selection for the breeding objective traits of carcase IMF, AUS-MEAT marbling score and MSA marbling score were generated by modelling seventeen different phenotyping and genotyping scenarios using selection index theory. The data used in the study was produced from the Angus Sire Benchmarking Program (ASBP). The study showed that the highest rates of selection accuracy and response (ΔG) were achieved through using a combina-tion of ultrasound scan phenotyping for intramuscular fat (IMF) and genotyping with a reference population including carcase IMF and marbling score phenotypes. For phenotype only scenarios, Central Ultrasound Processing (CUP) showed substantially higher response to selection and accuracy of selection to the Esaote Aquila ultrasound system (PIE) for IMF resulting from the higher herita-bility and stronger genetic correlations to the direct carcase breeding objective traits. The value of ultrasound scan phenotyping diminishes as the prediction accuracy of the genomic breeding value (GBV) increases. The GBV only scenario surpassed phenotype only scenarios once GBV prediction accuracy was greater than 59% when compared to CUP IMF phenotyping and 46% when compared to the PIE IMF phenotyping.