with Co-EDTA, and could be due to the
difference in specific gravity between
bacterial spores and Co-EDTA (Mir et al.,
1997).
Faecal output can be estimated by the
dilution of an orally dosed external marker
using Equation 12.1.
Faecal output = Md/Mc (12.1)where Md= daily dose rate of the marker
and Mc= the marker concentration in the
digesta or faeces.
The flow of a nutrient component in
the digestive tract or part of it can be
calculated using Equation 12.2.
Component flow = Nd(Mh/Md) (12.2)where Nd= the concentration of the
nutrient component in the digestive tract,
Mh= the concentration of the marker in
herbage or diet and Md= the concentration
of the marker in the digestive tract.
Both internal and external alkane
markers appear to be strongly associated
with the particulate phase of abomasal
digesta, but the carrier matrix used for
external markers can affect the passage
rate. Alkane-coated cellulose powder gives
faster flow rates than alkane-impregnated
shredded paper or paper bungs. Dosed
alkanes appear to be as effective as
ytterbium acetate as a digesta passage rate
marker (Giráldez et al., 1997). The natural
alkanes, with their quantitative duodenal
recovery and strong affinity for the solid
phase of the digesta, appear to be suitable
markers for studying digesta kinetics,
particularly since they flow with undigested
feed residues and the rate of breakdown of
the marked particle in the gut will be similar
to that of the digesta particles. In order to
use natural alkanes as markers for this
application, the plant can be labelled with
(^14) C and fed as a pulse dose. Digesta kinetics
can be studied by following the^14 C label in
the alkanes of digesta or faeces.
Microbial protein synthesis
Estimation of microbial nitrogen flow to the
duodenum and the efficiency of microbial
nitrogen synthesis depend largely on the
marker used. By using^15 N as marker, the
bacterial nitrogen fraction of the digesta
nitrogen is calculated using Equation 12.3.
Bacterial N fraction = Ed/Eb (12.3)
where Ed=^15 N enrichment of the digesta
and Eb=^15 N enrichment of the bacterial
fraction.
For internal markers such as DAPA,
the bacterial nitrogen fraction is calculated
using Equation 12.4.
Bacterial N fraction = Md/NdNb/Mb
(12.4)
where Md= the marker concentration in
the digesta, Nd= total N in the digesta,
Nb= total N in the bacterial fraction and
Mb= the marker concentration in the
bacterial fraction.
Ideal protein markers should account
for both bacterial and protozoal synthesis
associated with the solid and liquid phases
of the digest. At present, no simple ideal
marker system exists. Bacterial plus
protozoal crude protein flow from the
rumen is computed from the marker to
bacterial plus protozoal crude protein ratio
of a sample of rumen microorganisms.
Reviewing protein markers, Broderick and
Merchen (1992) pointed out that because
none proved completely satisfactory, yield
estimates obtained are relative rather than
absolute. The^15 N procedure gives more
plausible and less variable results than the
DAPA procedure (Sadik et al., 1990).
During short-term dosing studies with^15 N
ammonium salts, only microbial nitrogen
is labelled, since it is not found in the
normal diet above natural enrichment. All
nitrogen pools will be labelled, but with
different enrichments since bacteria are
labelled directly by the incorporation of
(^15) NH
3 , while protozoal nitrogen is labelled
indirectly through predation of labelled
bacteria, thus rendering the basis for
calculating microbial yield in ruminants
uncertain.
Schönhusen et al. (1995) compared
D-alanine,^15 N, DAPA, RNA and amino acid
profiles as microbial markers and con-
cluded that D-alanine and^15 N gave the best
266 J.P. Marais