rumen that invokes physical competition
among feed residues for escape from the
mass action turnover pool. Physical regula-
tion also occurs via interactions of these
mixing forces with the un-mixing forces of
buoyancy. Buoyancy is derived from the
innate buoyancy of feed residues on initial
ingestion and fermentation-derived buoy-
ancy that large feed residues acquire as the
result of age-dependent processes of
colonization and metabolism of feed
residues by the rumen microbial ecosystem
(Sutherland, 1986; Ellis et al., 1991). It is
the physical buoyancy that sorts ‘recent’
residues into flow paths for ruminative
mastication and ‘aged’ feed residues into
the mass action turnover pool. These flow
paths for the lag-rumination pool are easier
to conceive than to measure explicitly.
Such flow paths may not be mimicked via
insertion of feed fragments into the
ruminal digesta accessible via rumen
cannulae (Vega and Poppi, 1997).
The rather invariant nature of resident
time observed for the lag-rumination pool
(Fig. 16.4) suggests that variation in resist-
ance to mastication is small relative to
factors affecting their escape (Figure 13 of
Ellis et al., 1999). Diets of stem versus leaf
fragments (Poppi et al., 1981) may be an
extreme exception.
Regulation of residence time by
physical forces is in part self-regulating in
that increasing levels of NDF result in
increased load of NDF and proportional
increases in competition for escape from
the mass action turnover pool. This self-
regulating mechanism is proposed as a
mechanism for maximizing the extent of
digestion of HF. For example, fermentation-
based buoyancy would tend to constrain
escape until potentially digestible entities
were largely expended. Thus, the combina-
tion of physical factors in the lag-rumination
pool combined with the mass action
turnover pool constrain HF–e of forages
until HF is digested to the order of 0.85 in
forage-fed ruminants (Figure 18 of Ellis et
al., 1999).Metabolic regulation of residence timeMoir and Harris (1962) clearly demonstrated
that the level of dietary protein (30–110 g
kg^1 DM) was positively and curvilinearly
related both to concentrations of rumen
bacteria and to the rate of intraruminalFeed Intake in Ruminants 345Table 16.3.Ruminal dynamics of unhydrolysable fibre (UF) and crude protein (CP) in sheep consuming
cottonseed hull (CSH) or CSH and cottonseed meal (CSM) diets (adapted from Wylie, 1987).
Diet CSH + CSMItem CSH CSH + CSM CSH
Dietary composition: g kg^1 DM
CP 52 123 24
UF 518 362 7
Daily ruminal influx, g kg^1 BW
DM 8.6 33.7 3.9
UF 16.2 22.3 1.4
Ruminal digesta load, g kg^1 BW
UF 14.5 23.4 1.6
Daily mean ruminal escape rate, –e
UF 0.34 0.54 1.6
CSH-rare earth 0.31 0.50 1.6
CoEDTA-Co 0.46 1.06 2.3
Ruminal efflux composition, g g^1 DM
Efflux CP/efflux DM 0.08 0.14 1.8
Ruminal influx/efflux ratios, g g^1
Influx of CP/efflux of CP 0.577 1.20 2.1
Influx of DM/efflux of CP 9.77 9.78 1.0