Live Animal, Carcass and End Point
Developing Carcass Trait EPDs for Commercial Bulls Using On-Ranch Genetic Evaluations
Abstract:
The purpose of this study was to determine on-ranch EPDs (rEPDs) for carcass traits derived from three seasons of steer performance data, sire-identified by DNA-testing, and to assess their economic value. Paternity assignments were determined for the first season based on microsatellite analysis and for the second and third season based on single nucleotide polymorphism (SNP) analysis.
Only registered Angus bulls were used — 30 herd bulls (21 used in multiple seasons) and two artificial insemination (AI) sires, producing 541 steers. Steers sired per bull ranged from 0 to 26 for a single breeding season and 0 to 41 for the three-season period. On-ranch EPDs and BIF accuracies were derived for each calving season and cumulative from three seasons of steer performance data for carcass weight (CWT), fat thickness (FAT), marbling score (MRB) and ribeye area (REA). We used an animal model, with no fixed effects except season in the multi-season analysis and with the covariate of days of age at harvest. BIF accuracies of natural service (NS) bull rEPDs derived from multi-season data were higher for all traits (+0.18, +0.13, +0.16, and +0.08 BIF accuracy units for CWT, FAT, MRB, and REA, respectively) compared with American Angus Association (AAA) expected progeny differences (EPDs). For NS bulls with AAA EPD accuracy of ≤0.15 (n=6), the average improvement in rEPD accuracy was 0.21, 0.17, and 0.20 for CWT, FAT, and MRB and 0.17 for REA (n=1).
For each trait, bulls with the lowest AAA EPD accuracies were improved with rEPD determinations, but bulls with higher AAA EPD accuracies were not substantially improved. Correlations, both simple and rank order, between AAA EPDs and rEPDs derived from multi-season data for bulls with nine or more phenotyped steer progeny were stronger for ribeye area (0.591; 0.470) and marbling (0.580; 0.609) than for backfat thickness (0.149; 0.230) and carcass weight (0.009; 0.047). Correlation for rEPDs from season-to-season was consistent and moderate for ribeye area (average 0.48) and carcass weight (average 0.32), but inconsistent for fat thickness and marbling (averages 0.18 and -0.07, respectively).
We simulated genetic selection based on single-season marbling rEPDs, and corresponding phenotype changes of progeny in the next season. Marbling and the related traits grid value and gross returns did not exhibit the change that would have been predicted based on the marbling rEPDs. The same was true for simulated selections based on AAA based marbling EPDs. This may have been due to relatively large CIs for low-accuracy AAA EPDs and rEPDs resulting from small numbers of progeny per sire (5.1 average) despite large herd sizes (n>300).
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