of fibre digestion has been emphasized by
Huhtanen and Vanhatalo (1997). This addi-
tional residence time due to the lag-
rumination pool also imposes an additional
load of digesta mass.
Digestion and flux in sequential mixing poolsEstimating flux from a mixing pool requires
an integrating of the rates of escape and
hydrolysis of individual entities. This is
illustrated for potentially hydrolysable
neutral detergent fibre (HF), which is
assumed hydrolysed via an age-indepen-
dent rate (HFkh). The fractional extent of
hydrolysis of HF (HFH), from an age-inde-
pendent mixing pool is simply the propor-
tion HFkh/(HFkh+ HFk 2 ). However, this
expression is not applicable to an age-
dependent mixing pool. If the objective of
the integration is relatively long, e.g. daily
means, the age-dependent escape rate 1 or
e, (Fig. 16.3) can be converted to a mean
age-dependent rate (HF– 1 for two compart-
ment and HF– 1 for two pool models; Ellis
et al., 1994, 1999). Extent of hydrolysis of
HF in single age-dependent mixing pool is
computed as:
HFH = HFkh/(HF–e+ HFkh) (16.1)HF unhydrolysed via flux through a
sequence of two mixing pools,^2 HFU, is the
product of their fractional escape from
each pool, e.g.
(^2) HFU = fraction of HFU escaping
pool 1 fraction of HFU escaping
pool 2 (16.2)
The extent of hydrolysis of HF via flux
through two sequential pools,^2 HFH, is
then computed as:
(^2) HFH = 1((HF–
1 /(HF
1 + HFkh))
(HFke/(HFke+ HFkh))) (16.3)
Estimates of HFH or^2 HFH via one age-
dependent or two sequential pools are sim-
ilar. The two pool model is preferred
because of its delineation of the two pools
that contribute residence time. Resolving
and accurately estimating model parame-
ters of rumen flux requires adequate num-
ber and critical timing in sampling ruminal
efflux, timing that differs for one and two
pool models (Fig. 10 of Ellis et al., 1994).
Because potentially hydrolysable and
potentially unhydrolysed entities are con-
stituents of the same insoluble feed frag-
ment, HF– 1 and UE– 1 are equal until HF is
hydrolysed and no longer chemically asso-
ciated with UF. Escape rate for UF from a
specific meal must be estimated by markers
specifically applied (Ellis et al., 1994) to a
single meal of UF and indelibly associated
with UF during its residence time in the
rumen. Indelibility of marker and UF dur-
ing postruminal flux is not required to esti-
mate ruminal residence time. The validity
of rare earths as markers of UF flux in the
rumen has been established (Table 16.3 of
Faichney et al., 1989 and Fig. 7 of Ellis et
al., 1994).
Because UF can disappear from the
rumen only via escape, level of dietary UF
has the major impact on ruminal load. The
contribution of potentially hydrolysable
entities to ruminal digesta load is related
to the ratio of their HFkh/ HF– 1 (or HF–e
for a single age-dependent mixing pool).
Thus entities such as HF that is hydrolysed
at rates less than that of HF–eare major
contributors to ruminal digesta load.Regulation of Ruminal
Residence Time
Metabolically, the rumen microbial eco-
system should be viewed as an open flow
fermentation system whose kinetics may be
regulated by physical or metabolic
mechanisms. Physical mechanisms involve
entities that either are intrinsically
indigestible or are potentially digestible
but are undigested due to insufficient mean
residence time, i.e. ruminal escape.Physical regulation of residence timeMechanisms for physical regulation involve
the coordinated motility of the reticulo-344 W.C. Ellis et al.