due to inefficiencies in ruminal amino acid
digestion reviewed here. Also shown in
Fig. 16.11 is the voluntary intake by early
weaned lambs of highly digestible, grazed
grass and legume pastures (Cruickshank et
al., 1992). In the case of these early weaned
lambs, intake responses were linear
throughout the range of 224–366 g CP kg^1
diet. Indicative of their growth rate poten-
tial for lean muscle growth, these early
weaned lambs exhibited very rapid
liveweight gains, and this consequential
large requirement for MP is proposed as
the cause of the linear responses in volun-
tary intake over the range in dietary CP.
Relationships between voluntary intake
and ruminal efflux of TDOM and MP were
also linear (Fig. 16.12). The ruminal efflux
proportions of TDOM:MP increased
slightly with level of dietary CP and ranged
from 5 to 6.4. This proportion is within the
range (4–10) of proportions of intake of
ruminal efflux of TDOM/MP observed in
Fig. 16.2. The observed range in propor-
tions of ruminal efflux yield of TDOM and
MP was proposed to be the consequence of
differences in feed compositional attributes
determining ruminal efflux of TDOM and
MP and of an MP requirement driving feed
intake of such variable TDOMI:MP as illus-
trated in Table 16.2.
Uncertainties about the amino acid
requirements, or even the MP requirements,
of the experimental ruminants in the
current data set do not permit testing of the
hypothesis of an MP requirement-driven
intake. However, it should be biologically
evident that voluntary intake, or the
processes constraining intake, cannot be
estimated reliably without quantitative
knowledge of the feed’s ruminal efflux
yield of the metabolizable nutrient that first
limits the ruminant’s genetic potential
for maintenance and production. Further,
the disparate proportions of TDOMI:MP
supplied versus those required all support
the suggestion that flux of indispensable
molecules, i.e. amino acids, rather than
energy may severely constrain feed intake
by ruminants.
Lippke (1980) investigated various
ruminant responses to a variety of forages
and reported that liveweight performance
could be best described by a multiple
regression equation having negative co-
efficients for indigestible NDF and
positive coefficients for CP. The observed
liveweight gain could be described by360 W.C. Ellis et al.
Fig. 16.12.Relationships between voluntary intake of grazed pasture by early weaned lambs and their
ruminal efflux yield of MP () and TDOMI ().