with flux of net protein estimated as the
chemical score of the mixture of amino
acids derived from rumen escape protein of
forage and supplemental protein and from
microbial true protein. TDOMI was posi-
tively related to estimated flux of duodenal
methionine and lysine (Figure 5 of Ellis et
al., 1999) but not to the flux of any other
amino acid. Equivalent intake responses to
flux of either supplemental lysine or
methionine are consistent with rumen
microbial protein being equally deficient in
lysine and methionine when used for the
net protein requirements of body weight
gain (Storm and Oskrov, 1985).
Response to the balance of amino acid
supply is as expected for a metabolism
having a fastidious amino acid requirement
such as that of mammalian tissue. Such
responses to balance of amino acids would
not be expected for a metabolism having a
relatively non-fastidious requirement such
as the rumen microbial ecosystem.
Collectively, these results suggest that
feed intake is regulated to provide the net
energy and net amino acid requirements of
the ruminant’s aerobic metabolism. The
major uncertainty involved in predicting the
level of feed intake by ruminants resides in
predicting the yield of metabolizable
nutrients derived from the ruminant’s
nutrient acquisition system.
The Ruminant’s Nutrient
Acquisition System
For simplicity, most models of ruminal
digestion assume steady-state flux, i.e.
perfect and instantaneous mixing of a
constant input rate with a constant mass of
resident feed residues from which efflux is
constant via escape and hydrolysis of
potentially hydrolysable entities. Obviously,
such constancy does not prevail in the
ruminant because of its consumption of
meals that are irregular in their frequency,
level and, for the grazing animal, chemical
and physical composition. Effects of meal
consumption have important implications
in short-term (hourly) regulation of feed
intake (Aitchson et al., 1986) and microbial
metabolism (Baker and Dijkstra, 1999).
However, this chapter will focus on the
regulation of feed intake over longer terms,
i.e. a daily mean of observations during
1–2 weeks. Therefore, emphasis is placed
on the mean daily flux of feed residues
over days and we make the assumption
that such longer term responses reflect the
corresponding mean for shorter term,
rapidly fluctuating influx, flux and efflux.Flux of unhydrolysable entitiesMatis (1972) proposed that flux through the
ruminant’s gastrointestinal tract may be
modelled either as flux through a single
age-dependent mixing pool or sequential
flux through two sequential pools having
age-dependent or sequential age-dependent
and age-independent residence time
distributions. Either model yields identical
estimates of total residence time and total
pool size (Ellis et al., 1994, 1999). Graphical
illustrations of age-dependent distributions
of residence time are given in Fig. 16.3.
Different processes constrain escape of
feed residues from each of the two sequen-
tial mixing pools. To be descriptive of
these processes, the two sequential pools
are proposed to be referred to as the lag-
rumination and mass action turnover pools,
respectively. Because of the imperfect
mixing of ruminal digesta, these two pools
are commingled and cannot be fully
resolved by sampling ruminal digesta.
Resolution of the two pools requires
sampling of ruminal efflux to reveal their
functional differentiation. Fitting either a
single age-dependent mixing pool or a
sequential age-dependent, age-independent
mixing pool model yields identical profiles
of marker efflux from the rumen, as illus-
trated in Fig. 16.3. The two-pool model is
preferred in order to identify separately the
two different causal biological processes.
However, equal statistical fit is possible for
either model provided with adequate
quality data (Figure 10 of Ellis et al., 1994).
Both the lag-rumination and mass
action turnover pools primarily (≥95%)
reside in ruminal digesta (Wylie, 1987;Feed Intake in Ruminants 341