The use of naturally occurring and artificially applied
n-alkanes as markers for estimation of short-term diet
composition and intake in sheep
Authors: DUNCAN A. J. , MAYES R. W. , LAMB C. S. , YOUNG S. A. , CASTILLO I. 
Published: July 2002

The potential use of faecal n-alkanes for estimation of intake
and diet composition over periods of
1–2 days was assessed in two experiments. The aim was to determine
the accuracy with which intake
and diet composition could be estimated by characterizing faecal excretion
of n-alkanes following a
discrete dose as opposed to steady state kinetics used in previous work.

In the first experiment, 16 sheep were fed mixtures of spinach (rich
in C31-alkane) and cabbage (rich
in C29-alkane) in known proportions and amounts for two days.
Artificial n-alkanes (C28- and C32-alkane)
were dosed on four occasions during this time. Total intakes were controlled
at 0·2, 0·3, 0·4
or 0·5 kg dry matter (DM) per day and nominal amounts of spinach
offered (as a proportion of the
total diet) were 0·00, 0·15, 0·30 and 0·45.
Each sheep received a unique combination of intake and
dietary proportions (four intake rates×four proportions). Sheep were
fed fresh grass (timothy,
Phleum pratense) before and after feeding spinach and cabbage.
Sequential rectal grab samples of
faeces were collected at regular intervals and total faecal collections
were carried out over 144 h from
the start of the spinach/cabbage feeding period to obtain samples for
n-alkane analysis. In the second
experiment, fresh grass was sprayed with two combinations of artificial
n-alkanes (C24- and C32- or
C28- and C36-alkane) and fed to 16 sheep over a 24 h period.
Each sheep received one of four intake
rates (0·8, 1·0, 1·2 or 1·4 kg DM/day)
and, within intake rates, each sheep received one of four different
proportions of the herbage sprayed with the combinations of n-alkanes (0·2,
0·4, 0·6 or 0·8), in a
similar fashion to the first experiment. In order to estimate intake, C26-
and C34-alkanes were dosed at
the start of the feeding period. Faecal sampling procedures were the same
as those in the first experiment.

Different parameters of faecal excretion curves of dosed and natural
n-alkanes were used to
estimate dietary proportions and intake. Parameters tested included area
under the excretion curve
and curve maximum. Dietary proportions were calculated using an iterative
minimization procedure
employing faecal and herbage n-alkane concentrations. Intakes were estimated
using ratios of
dosed:natural faecal n-alkanes.

The best estimates of dietary proportions were obtained using faecal
concentrations at a single
point in time in both experiments (>80% variation explained for regressions
of estimated v. actual
proportions). Intake estimates required the calculation of the area under
the excretion curve to obtain
acceptable estimates (70–90% variance explained for regressions of
estimated v. actual intakes in
Expt 2). The experiments demonstrate that precise estimates of diet composition
can be obtained
using single faecal samples following consumption of simple herbage mixtures
over 24–48 h. Intake
may also be estimated using this technique provided that a series of faecal
samples are collected over
4–5 days following the period of ingestion.