National cattle evaluation system for combined analysis of carcass characteristics and indicator traits recorded using ultrasound in Angus cattle

Objectives were to 1) evaluate genetic relationships of sex-specific indicators of carcass merit obtained using ultrasound with carcass traits of steers, 2) estimate genetic parameters needed to implement combined analyses of carcass and indicator traits to produce unified national cattle evaluations (NCE) for longissimus muscle area, subcutaneous fat depth, and marbling with the ultimate goal of publishing only EPD for the carcass traits; and 3) compare resulting evaluations with previous ones. Four data sets were extracted from the records of the American Angus Association (AAA). Records from 33,857 bulls, 33,737 heifers, and 1,805 steers that had measures of intramuscular fat content (IMF), longissimus muscle area (uLMA), and subcutaneous fat depth (SQF) derived from interpretation of ultrasonic imagery, and weight recorded at the time of scanning. Also used were 38,296 records from steers with marbling (MRB), fat depth at the 12-13 rib interface (FD), carcass weight, and longissimus muscle area (cLMA) recorded upon harvest. (Co)variance components were estimated with ASREML using the same models as used for NCE by AAA. Heritability estimates for carcass measures were 0.45±0.03, 0.34±0.02, 0.40±0.02, and 0.33±0.02 for MRB, FD, carcass weight, and cLMA, respectively. Genetic correlations of carcass measures from steers with ultrasonic measures from bulls and heifers indicated gender specific relationships for IMF (0.66±0.05 vs. 0.52±0.06) and uLMA (0.63±0.06 vs. 0.78±0.05), but not for weight at scanning (0.46±0.07 vs. 0.40±0.07) or SQF (0.53±0.06 vs. 0.55±0.06). For each trait, estimates of genetic correlations between bulls and heifers measured using ultrasound were greater than 0.8. Prototype national cattle evaluations were conducted using the estimated genetic parameters resulting in some re-ranking of sires relative to previous analyses. Rank correlations of high impact sires were 0.91 and 0.84 for the joint analysis of MRB and IMF with previous separate analyses of MRB and IMF, respectively. Corresponding results for FD and SQF were 0.90 and 0.90, and for cLMA and uLMA 0.79 and 0.89. Unified national cattle evaluation for carcass traits using measurements from harvested animals and ultrasonic imagery of seedstock in a combined analysis appropriately weights information from these sources and provides breeders estimates of genetic merit consistent with traits in their breeding objectives upon which to base selection decisions.