Chromium oxide has been used to
determine the digestibility of amino acids
in pigs by ileal and faecal methods of
analysis (Li et al., 1993; see also Chapter
13). Animals receiving a chromium oxide
marker in their ration were fitted with a
single T-cannula at the terminal ileum.
Digesta were sampled through the cannula
and digestibilities of amino acids calcu-
lated on the basis of the concentration of
indigestible marker in the feed and digesta.
The ileal method gave more reliable results
than the faecal method. Comparison of the
chromium oxide method with the direct
method of measuring whole tract
digestibility in pigs showed that the
chromium oxide method gave lower results
(digestibility three percentage units lower
and ileal digestibility seven units lower),
probably as a result of low recoveries
(82–85%) of chromium in the faeces (Mroz
et al., 1996). Nevertheless, preference was
given to the marker technique over direct
measurement, since calculations based on
markers were independent of losses of
feed, digesta and faeces. Digestibility has
also been estimated successfully in horses
with chromium oxide as marker (Todd et
al., 1995b). Dysprosium is not satisfactory
as a digestibility marker for horses but can
be used to estimate relative rates of pas-
sage. The recovery of dysprosium in the
faeces of horses was found to be 82.9%.
Due to the incomplete recovery, the dry
matter digestibility was six to seven
digestibility units lower than that based on
total collection or chromium oxide as
marker (Todd et al., 1995a).
Feed digestibilities based on the rare
earth radionuclide^144 Ce as marker were
similar to those based on total collection
(Young et al., 1991).
Herbage and supplement intakeThe most commonly used method for
estimating herbage intake by animals is
based on the ratio between faecal output
and herbage digestibility as represented in
Equation 12.8.
Herbage intake = F/(1 D) (12.8)where: F = faecal output and D = herbage
digestibility.
Faecal output can be measured
directly by using faecal collection bags but,
apart from affecting the grazing behaviour
of the animal, losses from the bags may
lead to the underestimation of intake.
When animals cannot be restrained in pens
or when it is undesirable to burden them
with faecal collecting harnesses, faecal
output can be estimated from the concen-
tration in faeces of orally administered
external markers (Equation 12.1), while
digestibility is estimated either in vitroor
by means of internal markers (Equation
12.6).
Before the advent of the alkane
technique, dry matter intake was estimated
mainly using Cr 2 O 3 as marker to estimate
faecal output and measuring digestibility
by means of the in vitro technique. The
main disadvantage of this approach is that
the in vitrodigestibility technique does not
account for changes in digestibility of a
forage due to the level of intake, supple-
mentary feeding or parasite burden of the
animal. Alkanes are ideally suited for
estimating herbage intake in grazing
animals from the daily dose rate, and the
dietary and faecal concentrations of the
dosed even-chain alkane and an odd-chain,
natural alkane adjacent in chain length.
Both internal and external markers are
extracted and quantified simultaneously
and the results are calculated using
Equation 12.9.
Dry matter intake
= (Fo/Fe)De/(Ho(Fo/Fe)He) (12.9)where: De= daily dose of even-chain
alkane, Fo= faecal concentrations of odd-
chain natural alkane, Ho= herbage concen-
tration of odd-chain natural alkane, Fe=
faecal concentration of even-chain dosed
alkane and He= herbage concentrations of
even-chain natural alkane.
Alkanes of adjacent chain lengths
appear to have similar recoveries. The
adjacent pair, C 32 and C 33 , which are most
often used for intake predictions, have
recoveries of 0.868 ± 0.0175 and 0.872 ±
0.0125, respectively (Dove and Mayes,Use of Markers 269