Patterns of change with growth for muscularity and other composition characteristics of Southdown rams selected for high and low backfat depth

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Male Southdown sheep ranging in age from birth to near maturity (c. 28 mo) from genetic lines selected for about five generations for increased (n = 40) and decreased (n = 40) weight‐adjusted ultrasonic backfat depth were used to investigate patterns of change in muscularity and other composition characteristics. Measures of muscularity in terms of muscle depth relative to bone length were greater for the high‐backfat line in anatomical areas around or beside the femur, tibia, total pelvic limb, humerus, and scapula, but not the radius/ulna. Levels of muscularity increased at a decreasing rate with increasing carcass weight from about 1 to 36 kg in all areas. Where there was a genetic line effect on muscularity, this tended to be greater at higher carcass weights, with minimal differences at birth. Muscle to bone ratios corresponding to these measures of muscularity showed similar patterns of change and line differences. Leg bone weight was higher for the low‐backfat line; for the same femur length, a number of femur dimensions were greater for that line. Carcass length at a set weight was greater for the low‐backfat line, and the allometric growth ratio (AGR) for length declined with increasing carcass weight. Greater measures of fatness for the high‐backfat line only became apparent at carcass weights above about 10 kg and AGRs for measures of fatness increased with increasing carcass weight. Some non‐carcass offal components (liver, spleen, heart) were heavier in rams of the low‐backfat line at the same carcass weight, and AGRs for liver and spleen decreased with increasing carcass weight. It is concluded that, in addition to decreases in fatness, selection for lower backfat depths in these Southdown rams has led to a number of other undesirable composition changes including lower levels of muscularity and muscle to bone ratios, changes in the shape of the femur, and greater weights of some non‐carcass components. Generally these changes were more apparent at higher liveweights.