response to small differences in animal
attributes (requirement for MP) in combina-
tion with different feed attributes (propor-
tions of DOM:MP). These considerations
clearly demonstrate that feed intake cannot
be estimated reliably without knowledge of
the productive potential of the ruminant
(requirement) and the ruminal efflux
proportions of TDOMI to the nutritional
entity that first limits feed intake.
Plots of intake of observed TDOMI kg^1
BW versus observed ruminal efflux of
MP kg^1 BW (TDOMI:MP) indicated a
conspicuous positive relationship with
considerable scatter. Nine reports contained
three or more treatment means having
values for TDOMI or MP that varied by at
least 15%. Data from these reports are
plotted in Fig. 16.2 together with the linear
regressions via report. Regression coeffi-
cients varied from 2.4 to 40 g TDOMI g^1 of
MP. With two exceptions, the regression
lines converged in the order of 1.5 g MP
kg^1 BW. The two exceptions were data
reported by Cruickshank et al. (1987) for
rapidly growing, early weaned lambs, and
Hannah et al.(1990) for cattle fed forages of
low OM digestibility (0.48) and CP content
(24 g kg^1 ). The linear regression for the
data of Hannah et al.(1990) encompassed
values reported by Gunter et al.(1990) for
diets with similar nutritive attributes to
those reported by Hannah et al.(1990).
Based on nitrogen balance data given
in these reports, the apparent convergence
point of 1.5 g MP slightly exceeded zero
nitrogen balance for the ruminants
involved. The large responses in TDOMI:MP
(regression coefficient in the order of 40 g
TDOMI g^1 of MP) observed in the report
of Hannah et al.(1990) were responses to
supplemental undegraded protein being
used to spare tissue losses of protein (MP
intake of 0.8 g kg^1 BW). The large response
for lambs was probably due to simple body
weight being an inadequate scaling para-
meter to correct for differences in body
weight across species. Consistency was not
improved via use of BW0.75. Other than for
these two exceptions, the TDOMI versus
MP response ranged from 4 to 10 TDOMI
g^1 MP. Thus, variations in intake responses
of TDOMI to MP largely reflect differences
in TDOMI:MP supply versus TDOMI:MP
requirements by the different experimental
animals (see example calculations in Table
16.2).
Positive responses in TDOMI intake to
flux of MP such as those in Fig. 16.1 are
similar to numerous reports of associations
between feed intake and various expres-
sions of CP concentrations and intake. Like
all such empirical relationships, it is diffi-
cult to interpret whether the intake rela-
tionship to CP was due to the nutritional
effects on the rumen microbial ecosystem
or to that of the ruminant’s tissues. Egan
(1977) observed linear responses in feed
intake in response to a wide range of levels
of casein infused post-ruminally. The range
of casein infused (2–9 g of digested amino
acids per MJ of digestible energy) spanned
that expected to be required by both the
microbial and mammalian ecosystems.
Thus the linear responses observed by
Egan (1977), and those in Fig. 16.2, appear
due specifically to enhanced flux of amino
acids to the ruminant’s tissues.Feed Intake in Ruminants 339Table 16.2.Estimation of a nutrient requirement-driven feed intake requires knowledge of the ruminant’s
tissue requirement for and the flux rate of the specific nutrient that first limits intake. Truly digestible OM is
not a specific nutrient so does not drive intake. Example calculations below assume that MP is the first
limiting nutrient so that expected intake of TDOM = MP requirement (TDOM/MP).
Assumed MP requirement Flux of TDOM:flux of MP Expected TDOM intake
Daily MP, g kg^1 BW g g^1 Daily g TDOM kg^1 BW
4 7 28
3721
4624
3618