devices. Controlled-release devices offer
the advantage of eliminating the need for
repeated daily dosing or the continuous
use of faecal bags during total faecal collec-
tion, which could interfere with normal
animal behaviour. However, problems
associated with these devices sometimes
result in a lack of consistency in faecal
output predictions (Buntinx et al., 1992).
Controlled-release devices often appear to
overestimate faecal output when the manu-
facturer’s prescribed release rates are used
for calculations. External markers are
administered either as a single pulse dose
or continuously or frequently over a period
of time in an attempt to reach steady-state
conditions under which the digesta is
labelled uniformly and the ratio of digesta
to marker is constant. The time required for
the excretion of a marker to reach a steady-
state depends on the diet and the type of
animal. An adaptation period usually of
between 6 and 8 days for sheep and cattle
is required before the experimental period.
Transition metalsOxides and salts of trivalent and tetra-
valent metals such as titanium, chromium,
cobalt, ruthenium and hafnium have
characteristics of inert markers and have
been used as such with varying degrees of
success. Of these, chromium has been most
widely employed.
Chromium
CHROMIUM SESQUIOXIDE (CR 2 O 3 ).Chromium
oxide is practically insoluble in water, but
slightly soluble in alkalis and acids, and is
one of the most commonly used markers
for estimating faecal output. It is adminis-
tered orally to ruminants, by means of
gelatin capsules, or mixed with the ration.
A controlled-release device for the con-
tinuous delivery of Cr 2 O 3 to the ruminant
is also available. The chromium release
rate in the rumen has been shown to
increase with decreasing dry matter
digestibility of the feed and also varies
from one animal to another, but not
between devices within the same animal
(Pond et al., 1990). These workers con-
cluded that controlled-release devices are
unsuitable for the accurate estimation of
the intake of individual animals, but are
suitable for mean animal intakes in group
situations.CHROMIUM MORDANTS.Trivalent and tetra-
valent metals, such as chromium, which
form strong ligands with plant cell wall
components, are commonly bonded onto
fibre to form mordants used as particulate
markers. Since the stable coordinate bonds
of trivalent chromium result in slow rates
of ligand exchange, chromium mordants
are prepared preferably by reducing hexa-
valent dichromate compounds with
ascorbic acid, rather than from the trivalent
salts. However, the hydroxides of most of
these elements, which are insoluble at
rumen pH, make it difficult to establish
whether the element is moving as a
complex with the food ingredients or
separately as the insoluble hydroxide. In
order to give reliable results, metal
mordants must be recovered on the plant
matrix to which they are bonded.
Prior to mordanting, soluble inter-
fering substances, which could reduce the
binding of chromium to the fibre, should
be removed by treating small batches of
plant material with neutral detergents such
as lauryl sulphate (Van Soest, 1963; Udén
et al., 1980). Larger batches could either be
sealed in a cloth bag and put through the
laundering cycle of a washing machine, or
could be boiled in a laundry detergent in a
large pressure cooker. The fibre prepara-
tions or neutral detergent residues (NDRs)
should be rinsed thoroughly in water and
acetone, and dried at 65°C.
Although chromium binds firmly to
partriculate matter, it may adversely affect
particulate passage due to the increased
density of the feed (Ehle et al., 1984) and
by rendering the cell wall virtually
indigestible (Faichney and Griffiths, 1978;
Udén et al., 1980). Due to limited fermenta-
tion, the passage rate of chromium-
mordanted neutral detergent fibre (Cr-NDF)
particles has been shown to be significantly
slower than more digestible tritium-labelledUse of Markers 259