assays, is the most precise means of
measuring energy intake, because both
food spillage and changes in dietary dry
matter content are avoided. However,
because the dose sizes are reduced and
much lower than those seen in ad libitum
feeding regimes, there may be difficulties
in achieving and feeding representative
samples. The only legitimate objections to
the technique are the obvious limits on the
amount of food consumed and, perhaps,
attitudes to a procedure frequently referred
to as ‘force feeding’. Long experience in
our laboratory is that, with practice, the
entire activity is very rapid (15–20 s per
bird to feed 50 g of most feedstuffs) and
there is little evidence of more stress
beyond that induced by simple handling.
Skill is required, however, and experience
is essential, although this is readily
achieved by most operators. The use of
slurry feeding as a means of reducing stress
has been proposed and apparently has
been reported successfully from a few
laboratories, but it is our experience that
slurry feeding prolongs the processes,
reduces the accuracy of determining food
intake and invariably further restricts the
quantity of dry matter introduced to the
crop. Finely divided, hygroscopic or very
bulky ingredients can present problems
but, with experience and patience, these
can all generally be overcome. In our
laboratory, glucose monohydrate is fed
routinely; this can cause complications,
and granulation is carried out to reduce the
difficulty in feeding a hygroscopic powder.
The collection of droppings is another
simple task which can be difficult to do
well in routine experiments. When trays
placed under the cages are used to collect
droppings, far and away the most common
practice, problems can be caused by the
adherence of the droppings to the birds’
plummage, contamination with scurf and
feathers, changes resulting from fermenta-
tion and perhaps losses during removal
and transfer from the trays to containers.
Losses may also result from the birds
excreting away from the tray, and there is
the possibility of droppings being
contaminated with regurgitated food. This
can be surprisingly difficult to detect and,
even when observed, almost impossible to
compensate for in a meaningful way. It is
relatively easy to draw up a list of sensible
precautions to take to ensure good
laboratory practices; frequent collections
(say, 8 or 12 hourly) are the sorts of devices
that are often judged beneficial but, as most
tend to be labour intensive, they often
erode the advantages of a low-cost
bioassay. Alternatives to collection trays
(e.g. plastic bags attached to O-rings
sutured round the anus) have been
promoted from time to time but have never
proved popular, and it has to be concluded
that the use of trays is unavoidable in
routine experiments.Minimizing End Effects
In assessing the reliability from type 3
assays, it is important to keep in mind that,
because inputs are small, any imprecision
or uncertainty in the measurement of the
intakes or outputs will have a potentially
greater effect on the ME value derived. In
the ideal system, only droppings stemming
from the recorded intake should be debited
against that intake. In type 1 assays, where
it is customary to carry out the balance
over several days and food intakes are
often several hundred grams, discrepancies
or differences in gut-fills at the beginning
and end of the experiment are considered
to cancel each other out. Although this is
widely accepted, it is not entirely satisfac-
tory because changes in the intakes of
rapidly growing birds or responses to
unpalatable ingredients are more likely to
result in systematic bias than random
error. With the much lower intakes found
in types 2 and 3 assays, great care must be
taken to ensure that the digestive tract is
empty of residues at both the beginning
and the end of the balance period. Factors
which are likely to influence the amount of
digesta remaining in the gastrointestinal
tract are the nature of the previous diet, the
period for which it was removed, the
nature of the test feedstuff and the amount
given, the length of the collection period,Rapid Metabolizable Energy Assays 311