flow between solute and particulate matter
occurs in the acidic segment of the digestive
tract. During in vitro incubation of
ytterbium- or dysprosium-marked feeds,
6–23% of the marker dissociated from the
particles, but only 0.3–1.4% of the marker
re-associated with unmarked feeds, suggest-
ing that dissociated marker largely passes
with the liquid fraction of the digesta.
If rare earths (or any other marker) are
used for estimating particle mean retention
time, marker should only be applied to
particles with a narrow size range.
Faichney et al. (1989) described a wet-
sieving procedure for sieving rumen
digesta through a stack of vibrating sieves
and using only one particle size fraction for
labelling with marker. Marker can be
administered by means of gelatin capsules
or can be incorporated directly into the
feed. Rare earths are analysed by atomic
absorption spectrophotometry after wet
ashing (Quiroz et al., 1988; Luginbuhl et
al., 1994), by neutron activation analysis
(Kennelly et al., 1980) or by plasma
emission spectroscopy (Combs and Satter,
1992). Cerium was also estimated by
ashing the sample, oxidizing it to the
Ce(IV) form and titrating it against
ammonium iron(III) sulphate, using O-
phenanthrolene as indicator. Methods for
extracting rare earths to avoid ashing have
been developed (Hart and Polan, 1984).
Even-chain n-alkanesEven-chain alkanes occur in plants in low
concentrations only, but are available
commercially and can be administered
suspended on cellulose powder in gelatin
capsules (Dove and Mayes, 1991; Vulich et
al., 1991), as alkane-impregnated shredded
filter paper or paper stoppers (Giráldez et
al., 1997) or as an aqueous suspension of
alkane-impregnated grass particles (Marais
et al., 1996). In some instances, even-chain
alkane marker can be sprayed onto the
forage (Mayes et al., 1997). An intraruminal
controlled-release device has also been
developed and evaluated for sheep, and
gave a constant release of even-chain
alkane within 1.5–4.0% of the nominal
release value and a coefficient of variation
of release rate between animals of 4.07%
(Dove et al., 1991). Dotriacontane (C 32 ) is
the most commonly used external alkane
marker and has a recovery in the faeces of
about 87%. Even- and odd-chain alkanes
are quantified simultaneously by means of
capillary gas chromatography.Polyethylene glycol (PEG)PEG is used as a liquid marker, but it has
been shown to be excluded from a large
proportion of water in beet tissue when
used at a molecular weight of 4000 Da
(Czerkawski and Breckenridge, 1969), and
is precipitated by feeds rich in tannins
(Kay, 1969). PEG is infused continuously
into the rumen (De Smet et al., 1992), and
has a recovery in faeces and digesta of
sheep and rabbits of about 95%, suggesting
the partial absorption of PEG in the
digestive tract. PEG in the liquid phases of
undiluted digesta and faeces is assayed by
turbidimetry according to Malawer and
Powell (1967).Bacterial sporesBacillus stearothermophillusspores have
been used to measure the rate of passage of
both liquid and solid particles in a single
procedure (Mir et al., 1997). Bacterial
spores were isolated from commercially
available ampoules and cultured on agar
plates. Cells and spores were washed from
the plates and further propagated by
repeated rounds of shock treatment in
boiling water, and culturing on agar.
Following administration to heifers or
steers of the bacterial suspension contain-
ing 10^8 spores, rumen contents were
sampled at regular intervals and strained
through cheesecloth to give a solid and liq-
uid fraction. Both fractions were plated on
agar and incubated at 65°C. Microbial con-
centrations were expressed either as
colony-forming units (c.f.u.) ml^1 of rumen
fluid or c.f.u. g^1 dry matter.262 J.P. Marais