Metabolic costs related to protein turnover, fat turnover, and rate of gain determine the energetically optimal size of growing sheep

Context An adequate understanding of the energy requirements of growing animals is critical to developing efficient livestock production systems. Previously, we often found an empirically cubic relationship between fasting heat production rates (R) of growing animals and their body mass (M). The cubic allowed estimation of their energetically optimal size, i.e. the M at which R/M was minimal. Aims Our objective was to determine whether causal physiological factors could be identified that explained the cubic relationship between R and M. Our hypothesis was that these energy costs related to fat turnover, protein turnover, and prior rate of gain. Methods First, we developed equations to describe how fat, protein, and growth changed as crossbred wethers grew in a study from the literature. Fractional turnover rates of fat and protein were then estimated using these equations and additional literature data. This information and body composition, growth, and R data for intact males from another study were used to parameterise a factorial model of R as a function of protein turnover, fat turnover, and rate of gain. Finally, we used the factorial model to predict R in the crossbred wether experiment. Key results A modified exponential equation, an allometric equation, and a monomolecular function described how fat, protein, and growth changed in crossbred wethers. The best factorial equation to describe R for growing intact males was as follows: R (MJ/day) = 0.038 (fractional turnover rate, per day) × 12.74 (MJ/kg of protein) × Protein (kg) + 0.012 (fractional turnover rate, per day) × 8.60 (MJ/kg of fat) × Fat (kg) + 4.64 (MJ/kg of gain) × Rate of gain (kg/day), with an observed versus predicted slope of one and an intercept of zero. This equation allowed prediction of R in the crossbred wethers. Predicted R versus M was best explained by a cubic function. Conclusions Quantifying energy costs related to fat turnover, protein turnover, and prior rate of gain explained the cubic relationship found previously between R and M in growing animals. Implications The cubic equation allows identification of the energetically optimal M of a growing animal, provided suitable R versus M data are available.