methods of choice for the future. To fulfil
this objective, no effort should be spared to
resolve the uncertainty surrounding the
size of the EEL. At the present time, there
seems good grounds for suspecting that
many values quoted and used in the
derivation of TME values are overestimates
and that under the practical conditions of
ad libitumfeeding, EELNlies somewhere
between 0 and 20 kJ 24 h^1 for a mature
bird. With our 48 h + 48 h regime, giving
25 g of glucose at the start of the balance,
we most frequently find an EELNof around
20 kJ 24 h^1. Applying this value to birds
eating 100 g day^1 , it can be readily
calculated that the difference between
TMENand AMENis only 0.20 kJ g^1. In
other words, an ingredient with a TMEN
value of 15.0 kJ g^1 would have an AMEN
value of 14.8 kJ g^1 , only 1.3% lower. The
effect is, of course, greater with raw
materials of low ME content, but can still
be considered small. If the conditions
adopted for food presentation can be con-
firmed to affect EEL, as has been suggested
by a number of people, then the differences
between AMEN and TMENmay be even
smaller and it is doubtful if biological
assays would be capable of their detection.
With food intakes greater than 100 g the
differences would be reduced further and
this may explain why efforts to detect them
have largely been unsuccessful.Conclusions
Despite the enormous amount of research
devoted to the development of a universally
acceptable rapid bioassay to derive the ME
values of diets and feedstuffs, attempts have
foundered for two reasons. Firstly, rapid
assays almost invariably require birds to be
starved and this has led to the introduction
of the controversial concept of EELs.
Secondly, problems have been experienced
in some laboratories when tube feeding
birds, and this has led to the introduction of
many technique modifications. Although
the prospect of establishing a single
standard method seems remote, this chapter
reviews the advantages and disadvantages
associated with three general approaches to
the derivation of ME in poultry.314 J.M. McNab
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