Milne observed that prediction of MCPY as
a function of abomosal OM flux differed
between experiments but yielded a common
regression to ruminal hydrolysed OM
when molar proportions of propionate were
included in a multiple regression model.
Results with LY29 and DIC and those
of Ishaque et al.(1971) suggest considerable
potential for increasing the efficiency of
ruminal microbial protein synthesis and,
thereby, the level and efficiency of
ruminant productivity via favourably
modifying the rates of ruminal amino acid
deamination. Growth rate hysteresis
recently has been suggested as a possible
cause of variations in MCPE (Baker and
Dijkstra, 1999). Growth rate hysteresis
represents inefficiencies in microbial growth
as the consequence of asynchronous flux of
precursors for the energetically expensive
synthesis of macromolecules such as rRNA
and RNA polymerase. Protein synthesis per
se is preceded by the synthesis of poly-
nucleosides, and lack of synchrony of this
rate with the subsequent synthesis of
polypeptides will contribute energetic
inefficiency associated with the degradation
of excessive polynucleotides. Occurrence
of growth rate hysteresis as conceived by
Baker and Dijkstra (1999) could be accentu-
ated via the rapid fluctuations in the short-
term dynamics associated with infrequent
meals.
The increased ruminal efflux of
microbial protein associated with inhibitors
of amino acid deamination suggests that
the intermediates of ruminal protein
degradation are free amino acids rather
than peptides or products of amino acid
fermentation.
IonophoresIonophores such as monensin appear to
affect microbial utilization of protein and
inhibit microbial species having an affinity
for amino acid fermentation (Krause and
Russell, 1996). We have demonstrated con-
sistent responses to supplemental protein
and monensin in liveweight gain by
grazing calves which is attributed to
increased grazed intake (Hill, 1991). A
synergistic effect of supplemental protein
and monensin was demonstrated by
increases in liveweight gain from 207 g of
supplemental protein plus 200 mg of
monensin to a level otherwise obtained for
320 g of supplemental protein without
monensin.
We have interpreted these responses to
monensin in grazing ruminants to be
associated with the growth inhibition of
some but not all species of amino acid-
fermenting bacteria (Krause and Russell,
1996). Responses in calves grazing annual
ryegrass were similar whether supple-
mented with monensin plus grain or
protein. Thus, responses due to monensin
were interpreted as due to effects of
monensin on reducing amino acid
deamination of this high CP forage and
improved synchrony with energy genesis
from the more slowly hydrolysing supple-
mental starch. In contrast, semi-tropical
forage contained 1.0–1.3 g CP kg^1 and
benefited from supplemental protein whose
ruminally degraded amino acids were
fermented less rapidly due to the effects of
monensin. The relatively large liveweight
gain responses, 1.3 and 1.6 times the
mineral controls, from ryegrass and
bermudagrass respectively, are proposed to
reflect the extensive degradation of proteins
of growing foliage (0.85+) that contributed
to a deficiency of amino acids as first limit-
ing grazed forage intake and liveweight
gains. It is further proposed that the
productive potential of growing ruminants
commonly is limited by deficiencies of net
amino acids when grazing actively growing
forages. This generalization is illustrated
most dramatically by the responses in
empty body weight gain of early weaned
lambs to abomasally infused amino acids
when the lambs grazed white clover
pastures that contained 3.8 g CP kg^1
(Frazer et al., 1991).Voluntary Intake
Evidence reviewed to this point suggests
that the metabolic flux of indispensable358 W.C. Ellis et al.