still find favour in many laboratories today.
In the substitution method (Hill and
Anderson, 1958), the dietary ingredient to
be assayed is substituted for an ingredient
of known ME value (invariably glucose) in
a basal diet to yield a test diet. Simply by
comparing the energy balances (gross
energy eaten–gross energy excreted) of both
basal and test diets and with a knowledge
of the ME value of glucose, the ME of the
test diet can be calculated. In the replace-
ment method (Sibbald and Slinger, 1963),
the ingredient for which the ME is required
is exchanged for a known amount of basal
diet to form the test diet. By deriving the
ME values of basal and test diets, it is a
simple matter to calculate the ME of the
ingredient. Both these assays were assumed
to result in identical values for the test
ingredient, but this may not be correct.
Although these two approaches have
led to the derivation of values which have
long been referred to in terms of ME, they
should perhaps more correctly be described
as apparent ME (AME). The energy
metabolized is now considered ‘apparent’
because only part of the energy excreted
after the food is consumed has emanated
directly from the food, i.e. only part
consists of undigested and unmetabolized
residues. Part of what is excreted has come
directly from the bird and has come to be
known as the endogenous energy loss (EEL).
Part of the EEL is of faecal origin and is
generally considered to consist of gut tissue,
bile excretions and unabsorbed enzymes;
part is urinary and consists primarily of
products of nitrogen metabolism.
Throughout the years, the AME system
has generally served the scientific com-
munity well and played an invaluable role
in the development of poultry nutrition.
Occasionally, however, concern has been
expressed on reliability and, although the
reasons put forward generally centred
around interactions between dietary ingre-
dients, Sibbald (1982, 1986) has argued
persuasively that most of the anomalies
which had been associated with AME
derivations could be attributed to the
effects of EEL. He devised a novel bioassay
in which EEL was determined directly and
proposed that, in future, the energy status
of both diets and their ingredients should
be expressed in terms of their true ME
(TME).Nitrogen Correction
It would be misleading to present the
debate as simply one between AME and
TME. For many years, it has been common
practice to ‘correct’ AME values derived
from balance experiments to take into
account any changes in the nitrogen status
of the birds during the period of the trial.
The rationale for this adjustment arises
from the knowledge that the catabolism of
protein stored in the body results in the
need to dispose of the nitrogen it con-
tained. In poultry, this nitrogen is excreted
mainly as uric acid which contains energy;
for each gram of nitrogen excreted as uric
acid, 34.4 kJ of energy are lost from the
body and appear in the urine (droppings).
However, a bird which is storing protein
(e.g. a broiler) is spared the energy cost of
excreting so much nitrogen, and less uric
acid, and hence energy, appears in the
droppings. Thus the same diet (or
ingredient of a diet), when eaten by
different birds, may have a different AME
value because of differences in the
amounts of the ingested protein different
birds have retained. To make ME values
independent of the conditions under
which they were derived, it has become
usual to adjust them to what they might
have been under standard conditions. The
most frequently, although not exclusively,
used standard is one where the birds are in
nitrogen equilibrium (i.e. where nitrogen
retention is zero). The principle of nitrogen
correction has often been criticized
because a diet (and feedstuff) is penalized
as an energy source when it is promoting
the retention of protein, often the objective
of animal production. However, because
the function of any ME system is to
describe the energy status of diets and
ingredients and not their ability to retain
nitrogen, correction to nitrogen equilibrium
is justifiable. In any case, failure to make aRapid Metabolizable Energy Assays 309