of DM digested after a long period of
incubation in the rumen (48 or 72 h) was
taken as an index of organic matter
digestibility of forages. More recently, the
in saccotechnique has also been used to
forecast ruminal digestion of the cell walls
of forages. In an attempt to validate the use
of the in saccotechnique for estimation of
ruminal digestion of cell walls, a few
studies have been carried out where both
in sacco and in vivo degradation were
measured. Validations were based on the
comparison of the duodenal flows of struc-
tural carbohydrates measured in vivoand
the effective degradability calculated from
in sacco degradation and outflow rate.
Using in saccodigestion kinetic data leads
to an underestimation of ruminal cell
wall digestion (Archimède, 1992), but the
slope of the relationship between effective
degradability and the pre-duodenal
digestibility does not differ significantly
from 1 (Fig. 11.5). The underestimation of
ruminal digestion by in sacco measure-
ments can be due to an underestimation of
the rate of digestion by in situ incuba-
tion, as suggested by several authors
(Archimède, 1992; Stensig et al., 1994).
The lower number of cellulolytic bacteria
(Meyer and Mackie, 1986) and the lower
fibrolytic enzyme activities (Nozière and
Michalet-Doreau, 1996) within the bags
than in the surrounding digesta would
appear to be responsible for the under-
estimation of degradation rate by the in
sacco method. However, a threshold
activity value above which variations in
xylanase and cellulase microbial activity
no longer induce variations in the hemi-
celluloses and cellulose degradation rates
has been demonstrated (Nozière et al.,
1996). With forage-based diets, the fibro-
lytic activity of solid-associated micro-
organisms in the bags was above this
threshold activity, so the underestimation
of degradation rate by the in saccomethod
may be not related to the lower potential
microbial activity within the bags. Another
hypothesis to explain the underestimation
of the ruminal digestion of cell walls by the
in saccotechnique can be suggested. As
shown previously, the forage is ground
prior to incubation in order to reproduce
the effect of mastication, to reduce the size
of forage particles and to increase the
accessibility of cell walls to microbial
enzymes. In the bags, the accessibility of
digestible cell wall components to
microbial enzymes is lower than that of
forage particles in the rumen content
(Olubobokun et al., 1990). This lower
accessibility could be the main factor
responsible for the underestimation of the
degradation rate by the in saccomethod.Forage ingestibility and fill value
Ingestibility of forages is dependent on
their cell wall content and on the lignifica-
tion of these cell walls, i.e. the same
parameters as those conditioning their
digestibility. Thus ingestibility and
digestibility of forages are closely linked,
and methods used for prediction of diges-
tibility can also be used for prediction of
ingestibility. The nylon bag technique has
thus been used as a method for prediction
of the ingestibility of forages, with DM or
cell wall residues used in the estimations
(Michalet-Doreau, 1990). More recently, the
dynamic aspects of DM and cell wall rumi-
nal digestion, and its impact on rumen fill,
have been taken into account by the in
sacco degradation rate (Stensig et al.,
1994). The fill index, defined as the ratio
between the amount of rumen content (g
DM) and the amount of DM voluntary
intake (g day^1 ), can be estimated by the
ruminal retention time (RRT, day) of DM or
NDF calculated by the following equation:
RRT = (((1 ab)/k) + (b/(c+ k)))/24
(11.7)where a, b, c and k are as defined in
Equations 11.1 and 11.3. Such a definition
of RRT is based on the following hypo-
thesis: the rapidly degradable fraction (a)
does not fill the rumen; the potentially
degradable fraction (b) fills the rumen in
proportion to its degradation rate (c) and
the turnover rate (k); and the undegradable
fraction (1 ab) fills the rumen in pro-
portion to the turnover rate. RRT appears
to be a good estimate of both fill index and
ingestibility (see review by Faverdin et al.,In Sacco Methods 247