Effect of breed body size and the muscular hypertrophy gene in the production and carcass traits of concentrate-finished yearling bulls

To examine the extent of the effect of muscular hypertrophy character in beef of northern-Spanish breeds, animal performance and carcass characteristics of 152 finishing steers from 5 genotypes were studied: 32 yearling bulls from a rustic Asturiana de la Monta a (AM) breed, 96 yearling bulls from Asturiana de los Valles (AV) breed, divided in 3 groups depending on the presence of the gene responsible for double-muscling (i. e., 32 AV mh/ mh, 32 AV mh/+, 32 AV +/+), and 24 yearling bulls from AM x AV cross were used. Each genotype was composed of 8 animals per year (4 animals per pen) for 4 yr, except for the AM x AV genotype, which was only evaluated in the last 3 yr of the experiment. All animals were fed indoors with concentrate meal and barley straw ad libitum. Average daily gains in AV animals (1.41 kg/ d) were greater P < 0.01) than in AM (1.12 kg/ d), whereas AM x AV were intermediate (1.29 kg/ d) to these. No significant differences (P = 0.604) in ADG were found among the 3 AV genotypes. Longer fattening periods (P < 0.001) were taken for AM animals to reach acceptable BW at slaughter. Double- muscled animals (AV mh/ mh) were found to have the best feed efficiencies when expressed as G: F (P < 0.001). However, residual feed intake calculated on a daily basis showed a greater efficiency in AV mh/ mh and AM than in other genotypes. Carcasses from double- muscled animals had greater BW, yield, conformation and compactness index, and less fat cover than the other genotypes (P < 0.001). Carcasses from AM breed were the lightest and had the worst conformation, whereas those from AM x AV generally presented intermediate characteristics between AV and AM. Double- muscled animals had the greatest LM weight and area. The sixth- rib dissection revealed a greater percentage of muscle (84.6%) and decreased percentages of subcutaneous fat (1.1%), intermuscular fat (4.7%), bone (8.5%), and other tissues (1.2%) in AV mh/ mh compared with other genotypes. Water- holding capacity was greatest in AM and least in AV mh/ mh. In conclusion, double- muscled AV bulls would provide the greatest economic returns at intensive feeding systems because of their greater efficiency, reduced feeding costs, greater carcass weight and conformation, and greater lean yields, though some LM characteristics ( reduced fat content and water- holding capacity) could negatively affect the sensory attributes of the meat compared with other genotypes with greater fatness.