correction is tantamount to correcting to
the nitrogen retention prevailing in the
assay. Just as AME values are corrected to
nitrogen equilibrium (AMEN), so too
should TME values (TMEN).
McNab and Fisher (1982) suggested
that the following three observations were
required from a bioassay designed to derive
ME values: (i) a knowledge of energy
balance at (ii) a known food intake and (iii)
an appropriate measure of EEL. For correc-
tion to nitrogen equilibrium, nitrogen
balance must also be known. The choice of
bioassay should then be determined largely
by how well it provides these essential
pieces of information; other factors which
may influence choice will be speed, cost
and, perhaps, convenience. Three general
types of energy balance assays have been
identified as follows.
1.Traditional assays which involve
preliminary feeding periods to establish a
state of ‘digestive equilibrium’. Differences
between the amounts of dry matter in the
digestive tract at the beginning and end of
the assay period (‘end effects’) are
controlled by taking precautions to ensure
they are the same. In most cases, complete
diets must be fed, and substitution or
replacement methods, which have been
described earlier, must be used to study
ingredients.
2.Rapid assays, using starvation both
before and after allowing the birds free
access to the diet to control any end
effects. Again, complete diets must be fed
and substitution/replacement methods
used to derive values for ingredients in
most cases.
3.Rapid assays, as above, but using tube
feeding to place the test material directly
into the birds’ crops. These methods
almost invariably avoid any need for
replacement/substitution, most diets or
ingredients being fed as received.
Whilst many individual variations are
found within these three general
approaches to deriving ME values, the
classification provides a convenient frame-
work within which to debate the many
procedural details.
Energy Balance and Food Intake
Food presentation and the accurate
measurement of energy intake are arguably
the most demanding aspects of ME resolu-
tion. When birds are given access to food
ad libitum, a practice which still seems to
be the most widely accepted and free from
criticism, great care needs to be taken to
prevent food spillage (loss), to minimize
any separation of the dietary components,
to correct for any changes in the moisture
content of the food during the course of the
assay and to take (analyse) representative
samples of both food and droppings. These
are all difficult to control in a consistent
way, but specially designed systems have
been described and used with apparent
success (Terpstra and Janssen, 1975).
Such free-feeding methods are used in
type 1 assays which, overall, probably
account for the most frequent approach to
ME derivation in poultry. Farrell (1978)
proposed that the advantage of a rapid
assay of the type 2 variety could be gained
by training birds to consume sufficiently
large intakes of food in the 1 h immediately
following a 23 h starvation period. In this
assay, it is recommended that equal
quantities of a basal diet and test ingredient
are combined, and pelleting was also
advocated in order to ensure that high food
intakes were maintained across a range of
raw materials. Soon after its introduction, a
number of laboratories reported serious
difficulties in achieving satisfactory food
intakes but, notwithstanding this, the assay
has had its adherents, although its popular-
ity does seem to have waned over the last
10 years. Apart from speed (which has
implications for cost) and a reduction in
the amount of raw materials required, the
technique offers few, if any, advantages
over type 1 assays in terms of the precise
measurement of food intake or energy
balance. Furthermore, the need to pellet
precludes the application of this approach
to studies concerned with the effects of
pelleting on ME values.
It should be beyond debate that the
introduction of the food into the crops of
birds by tube, as is effected in type 3310 J.M. McNab