Genetic parameters for carcass and ultrasound traits in Hereford and admixed Simmental beef cattle: Accuracy of evaluating carcass traits

Abstract

Genetic parameters are required to evaluate carcass merit using correlated real-time ultrasound (RTU) measurements. Many registered bulls and heifers are measured using RTU before consideration for selection as parents, whereas few animals are recorded for carcass traits and those are often crossbred steers. The objective of this study was to estimate genetic parameters required for evaluating carcass merit in the American Hereford Association (AHA) and the American Simmental Association (ASA) using multivariate models and to assess accuracy of carcass trait estimated breeding values (EBV) for selection candidates. All available carcass data including carcass weight (CWT), fat thickness (FAT), longissimus muscle area (LMA), and marbling score (MRB) were provided by the AHA and the ASA along with RTU data including fat thickness (UFAT), longissimus muscle area (ULMA), and percentage of intramuscular fat (UIMF). Carcass data comprised 6,054 AHA and 9,056 ASA cattle, while RTU data in comparable numbers from close relatives comprised 6,074 AHA and 7,753 ASA cattle. Pedigrees included 33,226 AHA and 37,665 ASA animals. Fixed effects for carcass and RTU data included contemporary group, age at scan/slaughter, and major breed percentages. Restricted maximum likelihood procedures were applied to all the carcass and RTU measurements, along with birth weight to account for selection, fitting 8-trait multivariate models separately for each breed association. Heritability estimates for AHA and ASA carcass traits were 0.41 ± 0.04 and 0.25 ± 0.03 for FAT, 0.47 ± 0.04 and 0.32 ± 0.03 for LMA, 0.48 ± 0.04 and 0.43 ± 0.04 for MRB, 0.51 ± 0.04 and 0.34 ± 0.03 for CWT, and for RTU traits were 0.29 ± 0.04 and 0.37 ± 0.03 for UFAT, 0.31 ± 0.04 and 0.44 ± 0.03 for ULMA, and 0.45 ± 0.04 and 0.42 ± 0.03 for UIMF. Genetic correlations for AHA and ASA analyses between FAT and UFAT were 0.74 ± 0.08 and 0.28 ± 0.13, between LMA and ULMA were 0.81 ± 0.07 and 0.57 ± 0.10, and between MRB and UIMF were 0.54 ± 0.08 and 0.73 ± 0.07. Predictions of carcass merit using RTU measurements in Hereford cattle would be more reliable for FAT and LMA than MRB, but the reverse would be true for admixed Simmental cattle. Genetic correlations for MRB in AHA and for FAT and LMA in ASA are less than currently assumed in their national evaluations. Collection of greater numbers of carcass measurements would improve the accuracy of genetic evaluations for carcass traits in both breeds.

Figure 1.

Distribution of theoretical prediction accuracy of EBV for carcass traits based on various sources of phenotypic information from the American Hereford Association. The boxes inside the violins showed the quartiles of the data set while the whiskers extended to show the rest of the distribution, and the orange dashed lines represent square roots of estimated heritabilities, which are the theoretical upper bounds of prediction accuracies. Abbreviations: AHA, American Hereford Association; FAT, back fat thickness; LMA, longissimus muscle area; MRB, marbling score; CWT, carcass weight; RTU, real-time ultrasound.

Figure 2.

Distribution of theoretical prediction accuracy of EBV for carcass traits based on various sources of phenotypic information from the American Simmental Association. The boxes inside the violins showed the quartiles of the data set while the whiskers extended to show the rest of the distribution, and the orange dashed lines represent square roots of estimated heritabilities, which are the theoretical upper bounds of prediction accuracies. Abbreviations: ASA, American Simmental Association; FAT, back fat thickness; LMA, longissimus muscle area; MRB, marbling score; CWT, carcass weight; RTU, real-time ultrasound.

Figure 3.

Pair-wise scatter plot of theoretical prediction accuracies of EBV from Scenario 1 to Scenario 3 for the American Hereford Association evaluation. Each point stands for an individual, and the color represents the number of its carcass phenotyped siblings. Dashed line is y = x. Abbreviations: FAT, back fat thickness; LMA, longissimus muscle area; MRB, marbling score; CWT, carcass weight.

Figure 4.

Pair-wise scatter plot of theoretical prediction accuracies of EBV from Scenario 1 to Scenario 3 for the American Simmental Association evaluation. Each point stands for an individual, and the color represents the number of its carcass phenotyped siblings. Dashed line is y = x. Abbreviations: FAT, back fat thickness; LMA, longissimus muscle area; MRB, marbling score; CWT, carcass weight.