Investigations into muscularity as a characteristic of sheep carcasses at various stages of growth : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science at Massey University, Palmerston North, New Zealand
Muscularity is a meat animal characteristic defined as the depth of muscle relative to skeletal dimensions. It is usually assessed subjectively, but a possible objective measure involves obtaining an average muscle depth by taking the square root of the weight per unit length of muscles around the femur, and expressing this relative to femur length. A series of experiments was conducted to assess this objective measure of muscularity (MUSC), and the value of muscularity as a meat production trait. These involved evaluation of first, the pattern of change in MUSC with growth of sheep from birth to near maturity, secondly, relationships between MUSC in different parts of the carcass, thirdly, relationships between MUSC and muscle fibre size and number, fourthly, breed differences in MUSC, fifthly, relationships between MUSC measured objectively and subjectively, and finally, indirect predictors of MUSC based on simple measurements. Southdown rams from lines selected for high- or low-backfat depths (n=40 per line) were studied at birth, 10, 20, 40, 60, and 80 kg liveweight and at near maturity. Muscularity and M:B ratios from different groups of muscles and bones, together with other indexes of carcass shape, including the depth to width ratio of a transverse section of M.longissimus and a carcass weight to length ratio (CWT:L3), increased at a decreasing rate with increasing carcass weight. For most ratios this increase was parallel for both lines with the high-backfat line having higher values, but for muscularity in the femur region the differences between the lines increased with growth. Muscularity based on the muscles around the femur showed line differences most clearly. Line differences in muscularity did not appear to be associated with consistent differences in bone shape. Proportions of muscle fibre types in the M.semitendinosus were generally similar for the two selection lines. Data from 211 carcasses from 4 trials were evaluated to study differences between breed and sex groups of sheep in the pattern of change in muscularity with increasing carcass weight. Leg muscularity increased for all groups with increasing carcass weight, and the rate of increase was similar at carcass weights above 10 kg. The Southdown breed had higher muscularity values and M:B ratios than Texel crosses, which in turn had higher values than all other groups. For some comparisons, there were important sex effects. At a similar carcass weight, Coopworth rams had slightly higher muscularity values (+1.7%; P<0.10), but lower M:B values (-8.8%; P<0.001) than Poll Dorset-cross cryptorchids. Relationships between objective measures of muscling and subjective scores of muscularity or conformation were studied using data from 95 lambs and 90 bulls. Muscularity calculated from the leg cut rather than whole side or eye-muscle dimensions had the closest relationships with subjective scores of muscularity or conformation (R2% = 69 to 80% for lambs and 56% for bulls), with leg M:B being only slightly inferior (R2% = 62% for lambs and 52% for bulls). Muscularity and M:B ratio calculated from the side were the next best as predictors, but variables based on the eye muscle were poor. Data from 5 trials were used to examine indirect objective methods to predict leg muscularity for sheep carcasses. Muscularities based on M.semimembranosus or M.biceps femoris were accurate predictors when compared with indexes based on other individual muscles. Muscularities based on the topside and outside commercial boneless cuts were also good predictors. Indexes of muscularity calculated from carcass linear and eye-muscle dimensions were poor as predictors. Leg width to length (W/L) ratios obtained from lateral leg photographs proved useful as predictors. Individual W/L values or groups of W/L values combined as bands were moderately effective as predictors for some trials. However, the regression prediction equations varied between trials. It is concluded that the objective measure of carcass muscularity investigated here is a carcass characteristic that reflects important differences in carcass shape, and that differences in this characteristic between carcasses are not necessarily accompanied by corresponding changes in muscle to bone ratio.