0.65 CP (including that of nucleic acids)
and 0.5 of ruminally synthesized MCP is
recycled in the rumen (Wells and Russell,
1996), the maximal value for MCPE
reported in Fig. 16.9 (0.56) would be
equivalent to 1.7 g of intraruminally syn-
thesized DM g^1 of HCHO. Clearly, either
systematic errors are involved with estima-
tion of ruminal efflux yield of MCP or
indispensible molecular precursors for
microbial growth must be recycled very
efficiently in the rumen. Regardless of
these apparent discrepancies, it is clear
that efficiency of ruminal efflux yield of
intraruminal microbial growth is first
limited severely by some molecular
precursors associated with ruminal hydro-
lysis of CP.
Combining the expression of efficiency
of ruminal efflux yield of microbial growth,
MCPE, with the observed intraruminal flux
of hydrolysed entities yields the ruminal
efflux yield of MCP, MCPY (kg day^1 ).
Figure 16.10 displays relationships between
MCPY and flux of candidate independent
variables. Intraruminal flux of HCHO, NSC,
CP and RHP were all highly correlated
with MCPY. All of these variables are
positively and highly correlated with RHP,
so it seems probable that the nutritional
variable first limiting MCPY is RHP.
The significance of these relationships
involving RHP is not so much because they
are a reliable predictor of MCPE or MCPY
but rather because of their statistical sup-
port of the biological inference that some
product of ruminal hydrolysis of protein
and/or amino acid metabolism first limits
efficiency of ruminal efflux yield of
microbial protein. The results can be inter-
preted further in terms of current know-
ledge of ruminal proteolysis, peptidolysis
and amino acid utilization. Candidates for
indispensable precursor include degrada-
tion products of proteins and the accom-
panying nucleic acids. Microbial synthesis
presumably involves de novosynthesis of
the nucleosides and nucleotides rather
than utilizing any expected degradation
products of nucleic acids.
A nutritional requirement by a number
of species of rumen bacteria has been
demonstrated for molecular precursors
such as peptides, amino acids, branched-
chained and aromatic fatty acids. However,
it is generally assumed that requirements
for such molecular precursors is relatively
simple, relatively small and adequately
provided for in vivofrom degraded intake
protein (Hume et al., 1970) and the exten-
sive lysis and metabolism of a large propor-
tion of the microbial population (Wells and
Russell, 1996). Because of the kinetics of
hydrolysis and metabolism of proteins, it is
suggested that ruminal hydrolysis and
degradation of low levels of dietary protein
may not provide even the small require-
ments for a limiting precursor.
The hydrolysis of dietary proteins by
the rumen microbial ecosystem (Wallace,
1991) is characterized as a multistep
process consisting of: (i) initial hydrolysis
of dietary proteins to relatively large
peptides by a wide spectrum of endo-
peptidases; (ii) hydrolysis of large peptides
to tri- and dipeptides by a single, mole-
cularly specific dipeptidyl dipeptidase; (iii)
hydrolysis of tri- and dipeptides to amino
acids by a single, molecularly specific
dipeptidase; (iv) microbial uptake of amino
acids; or (v) their rapid oxidative deamina-
tion to carbon dioxide, ammonia and fatty
acids that are transported into the microbial
cell and used for synthesis of amino acids
and lipids. The rate-limiting step in this
process is thought to be hydrolysis by the
endopeptidases, enzymes that differ widely
in their mode of classification based on
cleavage site and which are produced by
the majority of feed-adherent bacterial
species.
In comparison with the initial cleavage
to peptides, rates of peptidylosis and
amino acid deamination are more rapid so
that extremely low concentrations of these
intermediate products exist in the rumen.
Many species of rumen bacteria are capable
of deaminating amino acids but most do so
at relatively slow rates, possibly reflecting
the energetic inefficiency for these as a
source of energy for microbial growth.
However, several bacterial species have
been identified that have high affinities for
fermenting amino acids and are suggested354 W.C. Ellis et al.