sources. The DAPA to N ratio in common
feedstuffs such as soybean meal could be as
high as 37% of that in isolated rumen bacte-
ria (Rahnema and Theurer, 1986). Abomasal
DAPA flows should therefore be corrected
for dietary DAPA intake. Due to catabolic
activity, a varying and often large proportion
(>50%) of DAPA leaving the rumen may
not be associated with the bacterial fraction
and may be free or in a non-cell-bound
peptidal form (Masson et al., 1991). Dispro-
portionately high DAPA outflow from the
rumen may lead to overestimates of bacter-
ial protein yields. High values obtained may
also be due to the analytical procedure
used.
It is difficult to resolve DAPA from
other amino acids by means of the conven-
tional amino acid analyser technique using a
ninhydrin reagent at pH 5.5. However, by
manipulating the pH of the eluting buffer, it
can be separated from most interfering
amino acids (Rahnema and Theurer, 1986;
Sadik et al., 1990). DAPA can also be
quantified by means of an amino acid
analyser after converting interfering methio-
nine to methionine sulphone with performic
acid (Ibrahim et al., 1970).
D-Alanine
D-Alanine is more widely distributed
among bacterial species than DAPA and
forms part of the oligopeptides cross-linking
bacterial cell wall peptidoglycans. It is
often not present in feed samples and has
been used successfully to estimate bacterial
protein synthesis and flow (Garrett et al.,
1987). However, reports of bacterial protein
flow values exceeding total protein flow
when estimates were based on D-alanine
question its usefulness as a bacterial
protein marker (Quigley and Schwab,
1988). It can be assayed by colorimetric
procedures (Quigley and Schwab, 1988), or
by means of an amino acid analyser.Nucleic acids
High concentrations of nucleic acids,
particularly RNAs in microorganisms, led
to their recognition as potential rumen
bacterial protein markers. Nucleic acids in
digesta samples are often analysed as the
purine bases, adenine and guanine, or their
derivatives (mainly allantoin) by means of
high-performance liquid chromatography
(Balcells et al., 1992), or after perchloric
acid hydrolysis of the sample (Martin Orue
et al., 1995). Invasive techniques requiring
experimental animals fitted with cannulas
in the rumen and proximal duodenum are
normally used for measuring protein flow.
Recently, non-invasive methods based on
the urinary excretion of purine derivatives
such as allantoin have been developed
(Perez et al., 1996).External Markers
External markers are indigestible sub-
stances which are added or bonded to the
feed or digesta. These markers commonly
are administered by mouth, through
fistulas or by means of controlled-release258 J.P. Marais
Table 12.1.Alkane profiles of a selection of forage species.
Alkane content (mg kg^1 dry matter)Species C 27 C 29 C 30 C 31 C 32 C 33 C 34 C 35 Total
Digitaria eriantha 28320374241 4130
Festuca arundinacea 48 278 0 594 12 114 0 0 1046
Lolium multiflorum 34 248 0 350 12 100 0 0 744
Lolium perenne 15 63 0 114 10 101 4 16 323
Paspalum dilatatum 3 70363161 268
Pennisetum clandestinum 17 29 7 124 9 300 13 241 740
Plantago lanceolata 47340240 90 0114
Setaria sphacelata 401166681 70 0238
Trifolium repens 34 168 40 443 46 134 6 15 886