Food intake, growth and body composition in Australian Merino sheep selected for high and low weaning weight. 3. Energy balance

Abstract

ABSTRACTUsing a simplified energy balance equation, changes in energetic efficiency and energy expenditure were examined as a function of stage of maturity in rams and ewes from flocks of Merino sheep selected for high (weight-plus) and low (weight-minus) weaning weight and from a randomly bred control flock.Specific combustion energy of the body (i.e. MJ/kg of body weight) was an increasing linear function of stage of maturity, which increased at a slightly faster rate in the weight-minus than in the weight-plus animals (15·1 v. 13·1 MJ/kg per unit of maturity), and at a faster rate in ewes than in rams (17·2 v. 11·3 MJ/kg per unit of maturity). At maturity, all strains had a similar mean specific combustion energy (18·3 MJ/kg), whereas the ewes were greater than the rams (20·1 v. 16·6 MJ/kg).Thermochemical efficiency (TCE, defined as the gain in body energy per unit of metabolizable energy intake) was a quadratic function of stage of maturity. In the early post-weaning period, the weight-plus had a greater TCE than the weight-minus animals and the maximum TCE occurred at an earlier stage of maturity in the weight-plus than in the weight-minus animals (0·40 v. 0·45 maturity). The ewes had a higher TCE than the rams at all stages of maturity, although the differences decreased as the animals matured. The maximum TCE occurred at a later stage of maturity in the ewes than in the rams (0·46 v. 0·38 maturity).After scaling for differences in mature size there was little difference between the strains in the rate of energy expenditure, with the exception that at the later stages of maturity the weight-minus animals had a slightly greater rate of energy expenditure. After scaling for differences in mature size, the rams had a greater rate of energy expenditure than the ewes from weaning to maturity.