Metabolizable energy intake is the key determining factor for the expression of an animal's genetic potential for growth, and current predictive growth models are not capable of accounting for all the nutritional variation that is commonly observed. The current study was designed to investigate energy transactions as lambs grow using CT scanning to assess body compositional changes at two levels of intake and two stages of maturity, and compare results to predictive equations. A pelleted diet was provided to cross-bred lambs (n = 108) at approximately 2.5 and 3.5% of liveweight (LW) in dry matter when the lambs were approximately four (31.8 ± 0.3 kg LW) and eight (40.5 ± 0.3 kg LW) months of age. A digestibility trial was run sequentially using 10 lambs of the same genetic and nutritional history fed at the same feeding levels to determine the digestibility of the diet. In the first feeding period, metabolizable energy intake was 15.3 ± 0.03 and 9.5 ± 0.03 MJ ME/d for high and low feeding levels respectively, resulting in higher rates of empty body gain for high feeding level lambs (197.7 ± 7.8 vs. 72.8 ± 8.2 g/d; P < 0.001). In the second feeding period, metabolizable energy intake was 15.2 ± 0.01 and 12.0 ± 0.01 MJ ME/d for high and low feeding levels respectively, resulting in higher rates of empty body gain for high feeding level lambs (176.3 ± 5.4 vs. 73.9 ± 5.3; P < 0.001). Lambs at later stages of maturity retained proportionately more energy as fat for every unit of retained energy compared to younger lambs (95.4 ± 0.40 vs. 90.0 ± 0.42%; P < 0.001). Lambs fed the lower feeding level in period two also retained proportionately more energy as fat for every unit of retained energy than lambs at the higher feeding level (97.1 ± 0.36 vs. 94.0 ± 0.37%; P < 0.001) which is hypothesized to be because of the rapid response of visceral lean tissue to changes in nutrition. There were no significant interactions between treatments in the first and second feeding periods, indicating an absence of a compensatory gain response to a nutritional restriction in the first feeding period. This experiment highlights the significance of a changing feed supply and the subsequent effects on body composition and the partitioning of energy to lean and fat tissue deposition. For improvements in the accuracy of predictive ruminant growth models it is necessary to gain a greater understanding of the different tissue responses over time to changes in nutrition.
Keywords: body composition; compensatory growth; digestibility; feed intake; lamb growth; nutrition.
Understanding animal responses to changes in nutrition will assist to create more efficient red meat production systems. Complex interactions exist due to the quality, physical characteristics and level of feeding of the diet, and the nutritional history of the animal. The resultant changes to body composition and the partitioning of energy to tissues has important implications for producers. A replicated trial used CT scans of live animals to investigate body compositional changes at different stages of animal maturity and different feeding levels. Given that fat deposition has been shown to increase at greater energy intake, it was expected that the contribution of fat to empty body gain would be smaller at lower feeding levels; however, lambs at later stages of maturity and lower levels of intake deposited greater amounts of fat as a proportion of empty body gain. Results are likely confounded by the response of visceral lean tissue mass to changes in nutrient supply. Irrespective of this, nutritional history had no effect on the rate or composition of gain at different levels of feeding, potentially demonstrating an inexistence of compensatory growth. A greater understanding of lean tissue responses to dietary changes is necessary for the increased precision of predictive growth models.
© The Author(s) 2023. Published by Oxford University Press on behalf of the American Society of Animal Science.