escape rate for microbial protein was less
responsive to flux rate of UF than was
(^2) UF–
e (1.4 versus 2.1, Table 16.3).
Differential escape rates for different feed
residues and for microbial protein exempli-
fied in Table 16.4 will complicate more
mechanistic interpretations of physical and
metabolic regulation of flux of specific feed
and microbial entities.
These interpretations suggest that the
ruminal microbial ecosystem may be con-
sidered a continuous flow system whose
mean residence time may be regulated in
response to intake level and composition of
diet that is driven to provide for the
ruminant’s requirements at the tissue level.
Dietary–Digestive–Metabolic
Interactions
More extensive in vivodata are needed to
verify or refute and extend the above inter-
pretations of the nutrient acquisition
system as both a regulating and a regulated
open flow system. Such data were collated
from the literature. The following assump-
tions were utilized to standardize the avail-
able literature and acquire the maximal
number of observations relevant to kinetic
measurements.
1.NDF is the major feed entity contribut-
ing mass and volume of feed residues to
ruminal digesta and, until hydrolysed, the
rates of ruminal escape of potentially
hydrolysable fibre (HF),^2 HF–eand poten-
tially unhydrolysable fibre (UF),^2 UF–eare
equal.
2.The rate of ruminal escape of HF is
estimated equally either as the mean age-
dependent rate, HF–e, or as the mean effec-
tive escape rate from two sequential pools,
(^2) HF–. Specifically bound rare earths (Ellis
et al., 1994) and tris-(1,10-phenanthroline)-
ruthenium(II) chloride were considered
indelible markers of the ruminal flux of UF.
3.Where only age-independent escape
was estimated by a single-pool exponential
distributed residence time model (ke), the
mean residence time in the lag-rumination
pool was assumed to be 10 h and the mean
effective rate of escape from the two
sequential pools,^2 UF–e, was calculated as
1/(10 + 1/k 2 ).
4.In order to incorporate data involving
the lactating ruminant, data obtained via
the rumen evacuation procedure were
used. The mean ruminal residence time
calculated by this procedure assumes that
all ruminal residence time is represented
by mass action turnover and therefore does
not include mean residence due to the lag-
rumination pool. Therefore,^2 UF– was
calculated as for 3 above. Values were
recalculated from the raw data in one
report where an incorrect mathematical
expression was cited by the authors.
5.Rumen digesta load of undigested NDF
(UNDFL) was computed as ruminal efflux
of NDF/^2 UF–e. As calculated, UNDFL
represents indigestible NDF plus poten-
tially hydrolysable NDF that escapes the
rumen. The actual load of digesta NDF, as
estimated via rumen evacuation methods,
will exceed UNDFL due to the quantity of
potentially digestible NDF that will be
hydrolysed.
Animal Bymicrobial Metabolic Regulation of
Ruminal Residence Time
Output from an open flow mixing system
such as the rumen, may be conceived as
the product of UNDFL and^2 UF–eso that:
UNDF intake rate = UNDFL ^2 UF–e
= UNDFL output rate (16.4)
Flux through the open flow mixing
system may be altered via changes in either
UNDFL or^2 UF–e. If only passive, mass
action effects regulated UNDFL and^2 UF–e,
these two variables should be exponentially
related. Earlier investigations of the rela-
tionship between UNDFL and escape from
the mass action turnover pool indicated
that this relationship contained more curvi-
linearity than could be attributed to an
exponential relationship (Ellis et al., 1999).
The relationship between UNDFL and the
effective escape from the sequence of the
two pools,^2 UF–e, is shown in Fig. 16.6.
348 W.C. Ellis et al.