Heritability of muscle score and genetic and phenotypic relationships with weight, fatness and eye muscle area in beef cattle

To assess the potential for genetic improvement to help meet the increasing demand for high-yielding beef carcasses, the heritability of muscle score (MS) plus genetic and phenotypic correlations with weight and fatness traits were estimated on 1856 yearlings and 2596 weaners born from 1992 to 2012 in a predominantly Angus herd divergently selected for High/Low MS. In 2005, after noting that some cattle were positive for the 821_del11 myostatin mutation that causes muscle hypertrophy, procedures were modified to create a third group (HighHet) of High animals with one copy of this major gene. This allowed the additive genetic effects of MS to be assessed, and also the effect of the 821_del11 mutation. MS traits were found to be highly heritable (h2 = 56–63%), with an extremely high estimated genetic correlation of 99% between weaning and yearling MS. Estimated genetic correlations of MS with rib eye muscle area (EMA) in weaners and yearlings adjusted for either age or weight were 53–56%. Genetic correlations of MS with other traits were relatively low: liveweight (yearlings 5%, weaners 20%), rump fat (yearlings –7%, weaners 11%), rib fat (yearlings –17%, weaners –3%). Apart from weaning liveweight, the estimated genetic correlations were not significantly different from zero. MS had smaller estimated genetic correlations with fatness and weight traits than EMA adjusted for age. For yearlings and weaners born from 2010 to 2012, large significant differences were evident in MS of High and Low calves (4.5-point difference for yearlings; 3.4 points for weaners, on a 15-point scale) and significant differences in EMA (yearlings 2.5, weaners 1.5 cm2). In addition, the 821_del11 mutation reduced fatness, increased MS by 1.5–1.9 points and increased EMA by 2.8–3.6 cm2. With high heritability, low correlations with weight and fatness in weaners/yearlings, plus other research (Cafe et al. 2012, 2014a) showing no detrimental effect on maternal productivity or meat quality, but improvements in dressing percent, retail meat yield, meat : bone ratio and feedlot feed efficiency, the development of an estimated breeding value for MS could help improve the efficiency and profitability of beef production.