An investigation was made of published data on the carcass composition of cattle, based on dissection of carcasses into bone, muscle, and fat. The data included females and castrate males, without regard to breed, age, or nutritional history. It was found that the relation between each carcass component and empty body weight could be described by a linear regression equation by using logarithmic values for the variables. The differential growth ratios given by the regression equations indicated, as empty body weight increased, that: (a) the weight of each of the dissected carcass components (i.e. bone, muscle, and fat) also increased; (b) the proportion of carcass bone fell, that of fat increased, and that of muscle remained almost constant. The relations between dissected bone, muscle, and fat and carcass weight were similar to those obtained between dissected carcass components and empty body weight. To obtain evidence on whether the differential growth ratios between dissected carcass components and empty body weight or carcass weight showed any change throughout post-natal life, quadratic equations were computed by using logarithmic values for the variables. These ratios fell for all carcass components, but in only three out of six equations were the quadratic terms statistically significant. This re-examination of published data indicates that any comparisons of the carcass composition of cattle may be invalid unless they are made at the same body (or carcass) weights. In addition, a comparison made by using regression equations, with the variables expressed as percentages, is confusing because it may not reveal abnormal composition in animals of particular weights. A satisfactory type of analysis can be made by using regression techniques with the original data. The above principles of analysis were applied in a breed comparison study of the carcass composition of 28 Hereford, 25 Angus, and 18 Shorthorn steers. These cattle comprised two age groups, born in 1957 and 1958 respectively. Carcass composition was estimated by dissecting, into bone, muscle and fat, the left and right 11th ribcuts from the carcasses of the 1957 steers, and the 9th–10th–11th rib-cuts from the left sides of the carcasses of the 1958 steers. When the rib-cut data were plotted, the relations appeared linear; the data were therefore analysed by using linear regressions with arithmetical values for the variables. Results showed that the fat content was greater and the muscle content smaller in the rib-cuts of the Shorthorns in both years than in those of either Hereford or Angus steers. Differences between Herefords and Angus were small. In view of the high correlations found by other workers between the results of rib-cut dissections and carcass composition, it is assumed that the breed differences reported here in rib-cut composition were reflections of breed differences in carcass composition. The carcass compositions of the cattle used in the breed comparison study were also estimated from hot carcass weight by using regression equations derived from the literature. A comparison of the two methods of estimating carcass composition suggests that, if hot carcass weight is to be used, regression equations will need to be developed for each breed in various environments.
