Live weight growth curves
The live weight growth curves for large and small mature size breeds differ mainly in magnitude, with little difference in shape of the growth curve. Large breeds mature to a greater mature weight and take longer to reach maturity than small mature size breeds. The time taken for an animal to mature is proportional to mature weight raised to the power 0.27.
Mature composition
Butterfield et al., (1983) conducted an experiment where animals from large and small mature size Merino strains were fed from weaning to maturity (2 years of age). Animals were slaughtered at regular intervals from weaning to maturity.
The results showed that at maturity, large and small mature size strains had different weights of tissues, although when expressed as proportions of live weight there was no difference between these strains (Table 4.1). Also there was no difference between the strains in the rate at which tissues matured towards this end-point (i.e. both strains showed the same pattern as in Figure 4.2).
Similar results were obtained for Angus steers, which were selected for high and low yearling weight gain (Perry and Arthur 2000) shown in Table 4-2. These cattle lines differed by 80 kg in mature weight, but as in the sheep lines had similar proportions of tissues in the body at maturity.
Therefore large and small breeds (or strains) had different mature weights, the same proportional composition at maturity and a similar pattern of maturity for the different tissues relative to carcase weight.
Given the same compositional end-point and similar patterns of maturity for tissues in the large and small breeds, the changes in composition for all breeds can be described by the one graph (Figure 4.4). This graph shows that as the animal progresses from birth to its mature weight there is a decrease in the percentage of bone, a slight decrease (or no change) in the percentage of muscle and a large increase in the percentage of fat.
In effect, Figure 4.4 provides a model by which we can compare body composition between all breeds, regardless of their mature size.
