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J. agric. Sci., Gamb. (1978), 90, 47-68 4 7 With 6 text-figures Printed in Great Britain The estimation of the nutritive value of feeds as energy sources for ruminants and the derivation of feeding systems B Y K. L. BLAXTER AND A. W. BOYNE Rowett Research Institute, Bucksburn, Aberdeen AB2 9SB {Received 17 May 1977) SUMMARY The results of 80 calorimetric experiments with sheep and cattle, mostly conducted in Scotland, were analysed using a generalization of the Mitscherlich equation R = B(l-exp(-pG))-l, where R is daily energy retention and G daily gross energy intake, both scaled by dividing by the fasting metabolism. The relations between gross energy and metaboli- zable energy were also examined. Methods of fitting the Mitscherlich equation and the errors associated with it are presented. I t is shown that the gross energy of the organic matter of feed can be estimated from proximate principles with an error of ± 2-3 % (coefficient of variation) and that provided different classes of feed are distinguished, the metabolizable energy of organic matter can be estimated from gross energy and crude fibre content with an error of ±6-9%. Parameters of the primary equation made with cattle agreed with those made with sheep and there was no evidence of non-proportionality of responses on substitution of feeds in mixtures. The efficiency of utilization of gross energy for maintenance and for body gain of energy was related to the metabolizability of gross energy and, in addition, to fibre or to protein content. Prediction equations are presented which describe these relation- ships. It is shown that the primary equation can be manipulated to express a number of biological concepts and that its two parameters B and p can be simply derived from estimates of the two efficiency terms for maintenance and production. The results are discussed in relation to the design of feeding systems for ruminant animals and to the derivation of optima in their feeding. TNTTt ODTTPTTOW energy for maintenance, km, varied with the quality J.1M l H W U ^ l l U a o f ^ ^ d . e t from a b o u t Q 6 Q t Q Q ^g T h e g l o p e o f t h e The relationship between the rate of feed intake equation above maintenance, called the efficiency by a growing or fattening ruminant and the rate at of utilization of metabolizable energy for fattening, which it retains energy in its body is curvilinear. kf, varied more with quality of the diet than did Successive increments of daily intake result in km, ranging from O2 to 0-6. progressively smaller increments in daily energy In an attempt to devise a feeding system based on retention. Blaxter & Graham (1955) showed that these relationships Blaxter (1962) had to introduce this relationship could be described by a simple a component to accommodate the decline in pro- exponential equation and during the next few years portional retention of the gross energy of the feed it was shown that no great error was involved if the as the amount ingested each day was increased. This relationship between daily rate of energy retention term, the feeding level correction, in effect intro- and rate of feed intake expressed as metabolizable duced a curvilinearity to the system above main- energy was approximated by two straight lines tenance and it then approximated closely to the intersecting at zero energy retention, that is at underlying continuous curvilinear function. This energy maintenance (Blaxter & Wainman, 1961). system, usually called the metabolizable energy The slope ofthe linear equation below maintenance, system, was adopted by the Agricultural Research called the efficiency of utilization of metabolizable Council's Working Party on the Nutrient Require- 48 K. L. BLAXTER AND A. W. BOYNE ments of Ruminants and included in its 1965 publication (ARC, 1965). It was later adopted in principle in a slightly modified form by the Agricultural Departments of the United Kingdom (MAFF, DAFS & DANI, 1975) and replaced the older starch equivalent system which was shown to be less precise in estimating animal needs (Alderman, Morgan & Lessells, 1970). Even so, the system, although capable of accom- modating new findings, is cumbersome to use and probably leads to difficulty on extrapolation to high levels of production. Several attempts have been made to simplify the computation of rations by algebraic manipulation of the ARC System as exemplified in the Agricultural Departments' publication (MAFF et al. 1975), without taking into account that the system was an approximation to a continuous relationship. For these reasons, and in an attempt to provide a simple and firm basis for the development of feeding systems, we have under- taken a reassessment of the basic calorimetric data on feed evaluation for ruminants. A progress report has been published on this work based on fewer experiments than are now included (Blaxter & Boyne, 1970) and an account of some of the conclu- sions arising from preliminary calculations has been given (Blaxter, 1974). MATERIALS AND METHODS The Appendix Table lists details of the 80 experi- ments which were analysed. Most of these were carried out at the Hannah Dairy Research Institute, Ayr and at the Rowett Research Institute, Aberdeen and we are grateful to our colleagues for access to unpublished details. Fourteen experiments made in America, Australia or Japan were recorded in sufficient detail to permit their inclusion. The experiments comprise sets of determinations of energy retention when each animal was given various amounts of the same diet and also when it was starved to provide an estimate of its fasting metabolism. The methods used are described in the references to the Appendix Table. In 70 of the experiments the diets had been analysed chemically to give values for ash, N, crude fibre and ether extract contents and in 34 of these lignin content had been determined. The heat of combustion of each diet was also known. These diets were classified as shown in the Appendix Table into six classes: (1) Pelleted diets: mainly pelleted roughages including pelleted mixtures of roughages and cereals (13 diets). (2) First harvests of grasses: artificially dried herbages including not only very young spring grass but also more mature herbage which would normally have been made into hay rather than artificially dried (15 diets). (3) Begrowths of grasses: second and subsequent harvests of grasses which had all been artificially dried (11 diets). (4) Hays: both legumes and grasses distinguished from 2 above only because of the method of drying (10 diets). (5) Cereal mixtures: mixed diets of cereals and hay or dried herbage; the lowest cereal inclusion was 20% (24 diets). (6) Other mixtures: roughages together with oil- seed cakes and meals, animal products and some cereals (6 diets). This classification into six groups was found to be too fine; classes 5 and 6 were combined to give a group of 30 diets and classes 2 and 4 combined to give a group of 25 diets. One experiment, No. 61, was omitted from the analysis since it gave com- pletely anomalous results compared with the remainder. The results of analysing these data are presented in two parts. In the first a simple mathematical model is developed which describes the relationship between rate of energy retention and rate of energy intake; the estimation of the parameters of the model is described and aspects of the parameters themselves discussed. The algebraic derivation of efficiency terms is presented and their relation to the ARC (1965) system described. In the second part these efficiency terms are then related to attributes of the diet to provide prediction equa- tions. Finally further simplification of the approach is presented. RESULTS Development The basic descriptive equations relating energy reten- tion to feed energy intake The Mitscherlich equation, which describes a system to which the law of diminishing returns applies, was first used by Wiegner & Ghoneim (1930) to analyse the two calorimetric