Studies on the energy metabolism of ruminants

Published evidence suggests that feeding standards currently in use, that are based on the total digestible nutrient, Scandinavian feed-unit or net energy (starch equivalent) systems of evaluating feed energy, are not a sound guide for the feeding of cattle or the organization of feed supplies. Errors inherent in total digestible nutrient and feed-unit standards are largely a reflection of the limitations of the two systems. In contrast the net energy system gives a thermodynamically sound measure of the efficiency of utilization of feed energy but determined net energy values for individual feeds show wide variation and at the present time there are insufficient data to provide a reliable standard. In Section I of the thesis, an analysis has been made of sources of error in the determination of net energy values. Major errors are shown to arise in the determination of the net utilization of metabolizable energy, from variations in the efficiency with which cattle utilize feed energy and also from a magnification of technical and interpretational errors by the design of experiment used. The conclusion is reached that, to overcome variations due to the individuality of cattle used as test animals, net energy values should be determined on a much wider scale than hitherto. This would be possible if a simple method of determining energy exchange in cattle could be developed to replace the laborious and time-consuming calorimetric method. To this end, it has been proposed that fat storage should be estimated indirectly, from the gain in body weight less the protein and ash retentions determined by conventional balance methods and the water retention determined by some indirect means, a problem that has formed the basis of much of the experimental work reported in the thesis. Analytical methods used in the course of the work are described in the first part of Section II. In the second part, the details of construction and of operation of a closed-circuit respiration chamber for the determination of the energy exchange of the calf are given. Indirect methods of estimating water retention in cattle are discussed in Section III, and it is suggested that a suitable method would be to predict water retention from body retentions of sodium and potassium. Analyses of muscle, liver, fat, brain tissue, serum, erythrocytes, posterior chamber fluid and pericardial fluid from cattle of widely differing ages, for water, sodium and potassium are reported, and it is shown that with the exception of brain tissue and serum, the water content of these tissues can be predicted from the equation:- Water (g./100 g.) = 0.2922 Na (mg./100 g.) + 0.1471 K (mg./100 g.) This equation is shown to apply also to cartilage but to underestimate the water content of skin and to overestimate grossly the water content of bone. Part of the sodium of bone is not associated with water but a means of correcting for sodium stored in this way is given. The prediction equation is shown to apply also to gut contents and to the foetus and uterine fluids, with the exception of abomasal contents and allantoic fluid. In Section IV the empirical equations developed in Section III have been used to predict water retention in experiments with milk-fed calves and the results obtained have been compared with water retention determined simultaneously by methods based on respiration calorimetry. Though the overall agreement was good, over short periods marked discrepancies arose due to day-to-day variations in the excretion of water, sodium and potassium. The errors of prediction of water retention in these experiments suggested that for the determination of the energy exchange of the calf, the indirect method would be less satisfactory than the classical calorimetric method. Similar experiments with sheep are described briefly in Section V but in many of them the indirect method grossly underestimated energy storage, due to a secretion of potassium through the skin of sheep. Though the results could not, therefore, be used to assess the probable accuracy of the indirect method for the determination of energy exchange in cattle, they showed that difficulties would arise from the high potassium content of many cattle feeds if the method was applied to cattle. Under most circumstances, the intake of potassium by cattle would be so large that errors in the determination of potassium contents of feeds would in themselves be sufficient to prevent the accurate prediction of water retention, and therefore of energy storage, in cattle according to the scheme outlined