SUMMARYSheep from the Soay, Finnish Landrace, Southdown and Oxford Down breeds were serially slaughtered at about 40, 50, 60 and 70% of their estimated mature body weight. Breed and sex comparisons were made at these degrees of maturity. Large differences were found in the weight of muscle, fat and bone, total muscle ranging from a mean of 3·3 kg in the Soay to 13·0 kg in the Oxford Down; the corresponding range for dissected fat was 1·2 to 9·2 kg.Most of the breed and sex differences observed at the same degree of maturity disappeared when expressed as a percentage of body or carcass weight. Thus both sexes had almost the same body and carcass composition in terms of fat, muscle and bone percentage. There were breed differences in fat percentage, with the Soay significantly less fat than the other breeds. Total muscle as a percentage of body weight was remarkably close to 28·5% in all breeds in both sexes, and at all four stages of maturity.
Striking similarities are known to exist in the growth processes of different mammalian genotypes. Two genetic size-scaling rules formally summarize these well-established similarities. In consequence, experimental results in one breed or species can often provide an excellent quantitative guide to biological relationships in other species. Despite strong genetic similarity, much genetic diversity usually remains.
As part of a multibreed cattle project for studying genetic variation between breeds, four adult females from each of five breeds of cattle were kept for up to 2 years in a non-pregnant, non-lactating state on each of four feeding levels. The breeds, which were Hereford, Aberdeen Angus, Dexter, British Friesian and Jersey, provided a wide range of genetic potential for body size and milk yield. The feeding levels were arranged to make the weight of lipid in the whole body about 0·05, 0·15, 0·25 and 0·35 times body weight. After an animal had been assigned to a fixed daily intake, its body weight and condition score were monitored over a period varying from 6 to 18 months until an equilibrium body weight and body composition had been established.For standard adult body weight, Akg, and for an equilibrium body weight, Wkg, and an associated metabolizable energy intake, f/MJ, the maintenance efficiency of a breed adjusted for breed size was defined as Em = W/fA0·21. The ‘lactability’ of a breed, that is, its genetic potential lactation yield, Ykg, adjusted for breed size, was defined as Y = Y/A. The between-breed regression of Em on Y had a coefficient of -0·043 (s.e. 0·007). There was, thus, a highly significant dairy-beef gradient in the equilibrium maintenance efficiency of these non-pregnant, non-lactating adult females.An expected value for the equilibrium maintenance requirement, of a breed or individual as a function of its lactability was estimated to be:which implies that the maintenance requirement of dairy breeds is about 0·2 times greater than that of beef breeds, a result which was strongly supported by a literature survey, although mean estimates from feeding trials and fasting trials were anomalous.Combining this result for milk yield with published results on growth rate, leads to the hypothesis that the greater the maximum gross efficiency of a breed for meat or milk production, the less efficiently it maintains itself, not because of any difference in fasting metabolism, but because of a reduced efficiency of food utilization for maintenance.
SUMMARYThe larger the mature size of a mammal, the longer tends to be the time it takes to mature. In an attempt to quantify this relationship, Brody's (1945) studies on post-natal growth and Weinbach's (1941) study on pre-natal growth are re-examined. Both these authors described growth by attributing to each species a live-weight scaling parameter, an age scaling parameter, and a parameter for age origin. By examining the interrelations of these six pre- and post-natal growth parameters, a general empirical relationship between mature weight and time taken to mature is obtained. The time a species takes to reach any particular degree of maturity, that is any fraction of its mature weight, tends to be directly proportional to its mature weight raised to the 0·27th power. This result is in general agreement with previous theoretical work and with results on the time required to reach adult weight.The simplest way to make use of this general relation is considered. When age measured from an origin at or near conception is divided by the 0·27th power of mature weight, then on this modified scale, the age at which a species reaches a given degree of maturity has an expectation independent of mature weight. This age scale has been called metabolic age, since it amalgamates the properties of Brody's physiological age and Kleiber's metabolic turnover time. The most useful working form, however, is the logarithm of metabolic age which is approximately normally distributed with constant variance at all degrees of maturity. Equations are derived giving the expected metabolic age of a species at any degree of maturity in the range covered by Brody's and Weinbach's growth curves. Metabolic age thus appears to furnish a one-parameter representation of comparative mammalian growth.
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