It is possible to estimate diet composition from an analysis of
n-alkanes in the faeces of ruminant
animals. The method requires the estimation of the concentrations of n-alkanes
in the plants and
faeces and then the solving of a system of simultaneous equations. There
are at least three places in
which significant measurement error may be introduced. First, there may
be error in the determination
of the concentrations of the n-alkanes in the herbage. This error may be
the result of analytical error
in the chemical analysis, or in the gathering of the representative sample
of herbage. In either case,
error in this estimate may be particularly important, since this estimate
is not independently repeated
for each animal in the study, but is conducted once and used throughout
the study. Error may also
be introduced in the estimates of digestibility of the n-alkanes themselves.
The n-alkane method might
be ideal if in fact the n-alkanes were completely indigestible –
they are not and, furthermore, they are
differentially digestible. Lastly, there may be measurement error in the
estimate of the n-alkane
concentrations in the faeces, which utilize the same analytical procedures
that are used on the
herbage. That is, if measurement error exists in the herbage estimates,
it is quite possible that it also
exists in the faeces estimates. We address these issues through the use
of Monte Carlo simulation to
investigate the likely effects of measurement error on diet composition
and digestibility estimates
obtained using the n-alkane method. Our results suggest the following conclusions:
(1) in the face of
any sort of measurement error, estimates of digestibility are likely to
be unreliable; (2) when
measurement error exists, one of the diet components will usually be under-estimated
and the other
will usually be over-estimated; (3) any sort of progressive bias in the
n-alkane recovery estimates will
probably have large and very significant effects on the results; and (4)
if measurement error in the
estimates of the n-alkane concentrations in the herbage and in the faeces
are similar in expectation,
then their effects tend to cancel each other out.