Estimation of Genetic Parameters for Carcass Traits and Their Corresponding Ultrasound Measurements in Crossbred Beef Cattle

and Implications Variance parameters including heritabilities, genetic and residual correlations are required for national cattle evaluation. There are huge amounts of data available for estimating such variance parameters for growth traits, but much less data is available for carcass traits. In this study, heritabilities and genetic correlations were estimated using restricted maximum likelihood on carcass weight (CWT), fat thickness (FAT), longissimus muscle area (LMA), marbling score (MRB), birth weight (BW), and ultrasound measurements of fat thickness (UFAT), longissimus muscle area (ULMA) and estimated percentage of intramuscular fat (UIMF) for crossbred cattle with carcass data recorded by the American Simmental Association. A multivariate animal model was fitted using ASREML4 software. The results demonstrate that UIMF measurements provide some useful information for carcass MRB (rg=0.73), but genetic correlations were only moderate between ULMA and LMA (0.56) and were weak between UFAT and FAT (0.38). The implications are that carcass measurements on progeny are the most reliable approach to evaluate carcass traits. Introduction American Simmental Association (ASA) like other breed associations has long been using real time ultrasound data in addition to carcass measurements to enhance national cattle evaluations on carcass traits. Studies revealed that evaluations combining ultrasound and carcass data outperform the ones that are based on carcass data alone. To date, most reports of parameters for use in national cattle evaluation were based on bivariate animal model analyses and some have fitted contemporary groups of fixed breed fractions rather than accounting for breed percentages. The objective of this study was to estimate genetic parameters required for evaluating carcass merit in the multi-breed analyses undertaken by International Genetic Solutions, using a single multivariate model fitting major breed percentages as fixed effects.