Published online by Cambridge University Press: 27 March 2009
Food utilization in relation to growth of body components, and fat mobilization and its hormonal control in vivo, were examined in 15 Hereford and 15 Friesian steers which were slaughtered at 20 months of age. Changes in body composition between 6 and 20 months were calculated from the body composition of these animals and from an additional four and two steers from each breed slaughtered at 6 and 13 months of age, respectively.
The Friesians consumed more food overall (14%) and grew more rapidly (14%), but their intake in relation to metabolic body weight was not different from that of the Herefords. Although the Friesians deposited more protein in relation to lipid there was no breed difference in food conversion ratio, and maintenance requirement relative to empty-body weight0·75 was estimated to be 7% greater in the Friesians than the Herefords. Friesians therefore had a lower efficiency of conversion of food energy to body energy. It is speculated that the higher maintenance requirement of the Friesians was due to a faster rate of protein deposition and a higher proportion of visceral organs with an associated higher rate of protein turnover.
Changes in plasma concentrations of free fatty acids (FFA), insulin, growth hormone (GH), adrenaline and noradrenaline were examined in response to fasting at 12 and 20 months of age. Inaddition plasma concentrations of glucose, thyroxine (T4) and triiodothyronine (T3) were measured at 20 months of age.
At both ages, FFA concentration increased almost linearly with duration of fasting and was not different between breeds. It was therefore unrelated to fat partitioning and a poor index of breed differences in metabolic and body type. Within breeds, the rate at which FFA concentration increased during fasting was correlated with estimated maintenance requirement (r = 0·53). This suggests a different relationship between FFA utilization and maintenance requirement in the two breeds.
During fasting at 12 months of age, Friesians had higher concentrations of plasma GH and noradrenaline. At 20 months of age they had higher concentiations of glucose, insulin and catecholamines. There was no obvious hormonal explanation for the observed differences in body composition or growth efficiency. Correlations between indices of fat partitioning and maintenance requirement were low, suggesting no direct link between the two traits.