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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
Rowett Research Institute, Bucksburn, Aberdeen AB2 9SB
{Received 17 May 1977)
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-
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-
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).
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
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.
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