to account for the major proportion of
amino acids fermented in vivo(Krause and
Russell, 1996).
The relatively high levels of RHP
apparently required for MCP synthesis may
be a reflection of limiting concentrations of
the precursor(s) relative to its utilization
affinity. The concentration of such indis-
pensable precursors could differ markedly
due to slight changes in the dynamics of
their rates of genesis and utilization
(Wallace, 1991). It is therefore not reason-
able to expect a consistent relationship
between the ruminal concentration of the
first limiting molecular intermediate and
the quantity of protein hydrolysed.
Consistent with this proposal is the fact
that the significance of a relationship
involving RHP was only evident when data
from forage legume diets were included,
i.e. diets that provided extremely large
concentrations of dietary protein (0.20–0.36)
that were hydrolysed extensively (0.8–1)
(Figs 16.9 and 16.10).
These considerations suggest the prob-
ability that low concentrations of peptides,
amino acids or fatty acids derived from
amino acids first limit rumen efflux yield
of microbial protein from the usual levels
of dietary protein. Assuming that the first
limiting nutrient is a peptide or amino
acid, an obvious approach for improving
MCPE would be to limit the rate of removal
of this indispensable intermediate.
Manipulating protein hydrolysis
and fermentationThe feasibility of specifically inhibiting the
rate of fermentation of amino acids as a
method for improving efficiency of
ruminant protein utilization has been
demonstrated. Chalupa et al.(1983) demon-
strated that the diaryliodonium compounds,
and specifically diphenyldiodonium chloride
(DIC), reduced accumulation of ammonia
from dietary protein in vitro, increased
ruminal efflux of CP and reduced the
dietary requirements of the ruminant for
CP. Although DIC specifically inhibits
oxidative deamination of isoleucine, the
rate of hydrolysis of protein is also
slowed (Ramirez Piñeres et al., 1997).
Considerations unrelated to its efficacy
limited further development of DIC.
Another compound (LY292253, LY29)
has been identified that is equally as
effective in limiting the accumulation of
ammonia from dietary proteins when
incubated in vitro with rumen micro-
organisms. In vivo, both LY29 and DIC
increased flux of trichloroacetic acid-
soluble and insoluble, Lowry-reactive
protein to the duodenum of steers grazing
pastures or receiving a diet containing
grain at 80 g kg^1 (Ysunda et al., 1995).
Common effects of both LY29 and DIC
(Table 16.5) were to increase ruminal efflux
of OM and protein by approximately 140%
of that of the control animals. Unfortunately,
efflux specifically of microbial protein and
free amino acids were not measured in
these studies. However, subsequent studies
have established that LY29 does not inhibit
initial hydrolysis of proteins or peptides
but inhibits deamination by mixed rumen
bacteria in vitro(Floret et al., 1999). Also,
LY29 was without effect on rumen
protozoa but inhibited growth of several
ruminal bacterial species including all
species of bacteria identified as having a
high affinity for amino acid fermentation
(Krause and Russell, 1996).
The in vivoresponses to LY29 and DIC
summarized in Table 16.5 are similar to
those reported by Ishaque et al.(1971) for
lambs that, as evidenced by rumen VFA
profiles, appeared to adapt different
microbial populations during transition to
grain-based diets. Lambs exhibiting rumen
VFA profiles characterized by large pro-
portions of propionate also exhibited
greater ruminal efflux of OM, CP and
diaminopimelic acid. Ruminal efflux was
increased in the order of 150–180% for
animals fed the same diet but having an
adapted microbial ecosystem characterized
by smaller proportions of propionate (Table
16.6).
Although not replicated by subsequent
experimentation (J.A.F. Rook, personal
communication), the results of Ishaque et
al. (1971) are suggested to represent an356 W.C. Ellis et al.