Genetic evaluation of carcass traits in Simmental-sired cattle at different slaughter end points

Our objectives were to estimate genetic parameters for carcass traits and e valuate the influence of slaughter end point on estimated breeding values ( BV). Data provided by the American Simmental Association were divided into three sets: 1) 9,604 records of hot carcass weight (CW) and percentage reta il cuts (PRC), 2) 6,429 records of CW, PRC, and marbling score (MS), and 3) 1,780 records of CW, PRC, MS, fat thickness (FT), and longissimus muscle a rea (LMA). Weaning weights (WW) from animals with carcass data and from the ir weaning contemporaries were used. Data were analyzed with a multiple-tra it animal model and REML procedures to estimate genetic parameters and BV o n an age-, CW-, MS-, or PT-constant basis. The model for carcass traits inc luded fixed contemporary group and covariates for breed, heterozygosity, an d slaughter end point and random additive direct genetic and residual effec ts. Weaning weight was preadjusted for founder effects, direct and maternal heterosis, age of dam, and age of calf. The model for WW included fixed co ntemporary group and random additive direct genetic, maternal genetic, mate rnal permanent environment, and residual effects. Heritabilities from data set 1 were 0.34 for CW and 0.25 for PRC on an age-constant basis and 0.25 f or PRC on a CW end point. Heritabilities for data set 2 were 0.35, 0.24, an d 0.36 for CW, PRC, and MS, respectively, on an age-constant basis. Data se t 2 heritabilities were 0.25 for PRC and 0.34 for MS on a CW-constant basis and 0.33 for CW and 0.25 for PRC at a constant MS end point. Heritabilitie s on an age-constant basis for data set 3 were as follows: CW, 0.32; PRC, 0 .09; MS, 0.12; FT, 0.10; and LMA, 0.26. Heritability estimates for data set 3 on a CW-, MS-, and PT-constant basis were similar to those on an age-con stant basis. Heritabilities were 0.12 for PRC, 0.12 for MS, 0.14 for FT, an d 0.22 for LMA on a CW-constant basis; 0.30 for CW, 0.09 for PRC, 0.10 for FT, and 0.28 for LMA at a constant MS end point; and 0.33, 0.17, 0.13, and 0.29 for CW, PRC, MS, LMA on a PT-constant basis. Genetic correlations amon g traits varied across groups and end points but suggested that it should b e possible to select for improved lean yield without sacrificing quality gr ade. Correlations were calculated among BV computed at different end points . Adjustment to various end points resulted in some changes in BV and reran king of sires, especially for PRC; however, the number of records available had a larger influence than slaughter end point.