The major nutritional limitations to cattle production in tropical and subtropical areas of Australia are deficiencies of energy, protein and phosphorus, which are often very severe. A dietary deficiency of any one of these nutrients results in suboptimal feed intake, and hence reduced rates of growth and reproduction.
The study of energy, protein and phosphorus nutrition involves consideration of both nutrient supply and requirements. In the case of supply, the nutrients may come from either the feed or from body tissue stores of energy (fat and protein), protein and phosphorus. The magnitude of these stores governs the extent of an animal's ability to withstand dietary deficiency, and thus assessment of these stores is essential for efficient diagnosis and research on these problems.
Published methods for determining body composition in vivo appeared to be invalid for very thin cattle, frequently encountered in the tropics; the major problem was one of gross overestimation of body fat. There was also no objective criterion of body phosphorus status. Level of circulating inorganic phosphorus is subject to many sources of variation, and its measurement is unsuitable for the evaluation of body phosphorus storage. The skeleton is the major body store of phosphorus, but does not appear to have been studied quantitatively with a view to deriving an objective definition of body phosphorus status.
In addition, recommended levels of dietary phosphorus requirements appeared to be excessive in relation to rates of growth of cattle grazing tropical pastures. This thesis describes three studies undertaken to resolve these problems.
(i) Estimation of the gross chemical composition of the body
The slaughter and analysis of 31 cattle of varied nutritional history provided data for the derivation of equations for estimating total body water, fat, protein and ash from measurements of fasted live weight and tritiated water (TOH) space. When these equations were applied to independent data from a further group of eight cattle, the mean quantities of water, fat, protein and ash were estimated with an error of less than 2%.
In these eight cattle, total body water of individual animals was estimated from TOH space with a standard error of ±2.03%, and total body protein from fasted live weight with a standard error of ±3.87%. The most accurate method of estimating total body fat was by subtracting fat-free weight from fasted live weight, after using the close relationship between total body water and fat-free weight (r = 0.996) to estimate fat-free weight. It was shown that total body water plus total body fat was virtually 80% of the total body tissues over a wide range of body fatness; recalculations of data in the literature indicated that this applies to a number of mammalian species.
(ii) Body phosphorus status and its assessment in vivo
Sixteen cattle from different levels of phosphorus nutrition were used to measure the quantities of phosphorus in the various tissues, and the relationships between these quantities, total body phosphorus and live weight. Concentrations of phosphorus in the tissues were also examined in order to produce an objective criterion of body phosphorus status. Phosphorus stress produced a significant decrease in the concentration of phosphorus in the fresh skeleton (w/w) and in its appendicular, axial and rib fractions. The fresh skeleton of animals fed a high phosphorus diet contained in excess of 5 g P/100 g, and that of phosphorus-stressed animals 4.5 g/100 g or less. Body phosphorus status may therefore be objectively defined in terms of the concentration of phosphorus in the fresh skeleton. ......................................