plastic syringe with a wide nozzle, and
leaving it soaking overnight before
administering it directly into the rumen.
Administered Ru-phe is strongly adsorbed
by the particulate fraction of the digesta. It
is not absorbed to any significant extent in
the digestive tract, and 93–102% is
recovered in the faeces, while <0.4% is
excreted in the urine of sheep (Tan et al.,
1971).
Ruthenium is assayed by atomic
absorption spectroscopy or the isotope is
measured by autogamma spectrometry. As
(^103) Ru-phe and (^51) Cr-EDTA can be assayed
simultaneously, these markers conveniently
may be used together as particulate and
fluid marker, respectively.
Titanium
Titanium oxide is insoluble in water and
dilute acid and is not taken up into plants
to any extent. It has no detrimental effect
on sheep if ingested at a concentration of
2–3 g day^1 , and is completely excreted in
the faeces (98% recovery). Titanium is
assayed spectrophotometrically after oxida-
tion with hydrogen peroxide (Brandt et al.,
1983).
Hafnium
Hafnium binds strongly to forages and con-
centrates, and is resistant to displacement
in acidic media (pH 2) (Worley, 1987). It
may therefore be a useful marker in acidic
segments of the digestive tract. Because of
its strong binding capacity (up to 5% of the
dry matter), the concentration of hafnium
used for mordanting feeds must be
restricted in order to avoid an adverse
increase in the density of the marked feed
and an excessive reduction in digestibility.
Its strong binding properties should render
hafnium a suitable marker for concentrate
feeds.
Rare earth elements
The rare earth elements appear to be
indigestible and are not absorbed from the
gastrointestinal tract of mammals. Several
of these metals, such as lanthanum,
cerium, neodymium, samarium, europium,
dysprosium, erbium and ytterbium, have
been bound to plant material and used as
indigestible markers. Since the rare earths
give identical digesta kinetic estimates,
they can be used for marking different
particles (Quiroz et al., 1988) or different
ingredients (Moore et al., 1990) in a single
experiment. The strength of binding to
fibre particles depends on the method of
application. Spraying rare earths onto feed
particles results in the saturation of strong
as well as weak binding sites (Mader et al.,
1984) and can lead to substantial dissocia-
tion of the marker in the digestive tract
unless the loosely bound marker is
removed by thorough rinsing. The marker
is prepared by soaking feed samples
overnight in a solution of rare earth
nitrate, acetate or chloride, rinsing in tap
water and drying in a forced-air oven
(Beauchemin and Buchanan-Smith, 1989;
Remillard et al., 1990; Moore et al., 1992).
A method has been developed in which
rare earths are bound to feed particles in
the presence of a soluble ligand such as
citrate, ensuring that rare earths only attach
to the strongest binding sites, thus prevent-
ing exchange during passage through the
digestive tract (Faichney et al., 1989).
There are relatively large differences
among rare earth elements in their binding
affinities. Ytterbium tends to form the
strongest complexes with feed samples.
Trivalent cerium does not form stable com-
plexes with plant fibre. However, stable
complexes are formed with tetravalent
cerium. Dissociation losses appear to be
minimal during a 24 h incubation period if
concentrations of rare earths lower than the
binding capacity of the fibre particles
(<0.18%) are applied to feed samples
(Hartnell and Satter, 1979; Teeter et al.,
1984).
The marker also tends to dissociate
from the feed particle and migrate to
unlabelled particles and soluble components
in the digestive tract, especially in an acidic
environment such as the abomasum
(Crooker et al., 1982). Displacement is of
little consequence in digesta flow studies
involving ruminants since no variation in
Use of Markers 261