The Nutritive Value of Feeds as Sources of Energy: A Review

Abstract

The purpose of this review was to discuss the determination of nutritive value in terms of energy. The conclusion which emerges is that there is an urgent need for new investigations to confirm or refute the limited experi- mental evidence that forms the basis of present practice. Editor. It is obvious that in the feeding of the individual animal a primary considera- tion must be the adequacy of the energy supply. Shortages of dietary energy are usually far more important causes of low productivity in farm livestock than are dietary deficiencies of vitamins, minerals, or amino acids. Furthermore, although deficiencies of most dietary essentials normally give rise to spectacular syndromes that can be easily recognized, shortages of dietary energy result in no obvious abnormality other than an insidious reduction in the animal's pro- duction. A knowledge of the nutritive value of foo~ls as sources of energy is essential to ensure that each individual can attain an optimal level of produc- tivity. Information on the nutritive value of feeds has, however, a wider utility than merely catering for the needs of the individual. Nutritive values as tabulated in the many manuals dealing with livestock feeding in use throughout the world are used to plan the cropping of farms and to plan the importation of concen- trated feeds. Also, they form the basis of livestock improvement through crop development. It is impossible to assess the value of new varieties of forage plants, new cropping techniques, new managerial or manurial methods or new conserva- tion processes without a consideration of the nutritive value of the fodder pro- duced. Probably these wider uses of the information we have on the nutritive value of feeds are of far greater importance in determining the over-all level of farm productivity than is the more restricted application of nutritive values in feeding the individual (17). It is in relation to the assessment of the relative nutritive value of foods that the limitations of our present knowledge are most apparent. If a comparison is made between two methods of conserving a crop in terms of the nutritive worth of the fodder produced, different methods of assessment give different--widely different--results. In Table 1 a comparison has been made between two feeds, one an artificially dried grass, one a mature hay, showing the different methods employed in Europe and America to express nutritive value. The chemical com-