fibrolytic activity induced by changes in
feeding level.
The in saccomethod has been used
extensively for quantification of the
associative effects between forage and
readily fermentable carbohydrates. How-
ever, results have to be interpreted with
caution. The amplitude of depression of
the fibre degradation rate depends on the
variations in microbial fibrolytic activity
but also on the nature of the forage in the
bag (reviewed by Huhtanen, 1991), and
more precisely on the amount and the
nature of the cell wall components, carbo-
hydrates and lignin. The effect of a
decrease in microbial activity on the cell
wall degradation rate depends largely on
the accessibility of cell wall components to
the enzymes. A relationship has been
observed between fibrolytic enzyme
activity of solid-associated microorganisms
and cell wall polysaccharide degradation
rate, but the impact of variations in
microbial activity on cell wall degradation
appeared to increase with greater accessi-
bility of cell wall components to microbial
enzymes (Nozière et al., 1996). In experi-
ments where forage incubated in bags was
the same as in the diet, some authors
focused on comparisons between in vivo
and in saccomethods, and reported that
the in sacco method overestimates the
decrease in cell wall ruminal digestion for
diets supplemented with rapidly or slowly
degradable starch or with digestible fibres
(Archimède, 1992; Stensig et al., 1994).
Nozière and Michalet-Doreau (unpublished
data) observed a decrease in fibrolytic
enzyme activities of the solid-associated
microorganisms in relation to supple-
mentation of the diet with up to 60%
barley, and this decrease was more marked
in incubated nylon bags containing the
dietary hay than in rumen content. Part of
these differences can be explained by a
more significant decrease in pH inside the
bags than in the digesta (Table 11.2). This
could be due to an accumulation of end-
fermentation products within the bags.
Quantification by the in saccomethod of
ruminal digestive interactions induced by
fat supplementation has been the subject of
less study. Variations in forage ruminal
degradation induced by fat supplements
have been shown to be underestimated by
the in saccomethod (Doreau et al., 1991).
The depressive effect of lipids on forage
degradation is not due to water-soluble
factors such as those observed when diets
are supplemented with cereals (increase in
readily fermentable substrates, drop in pH),
but rather to an adsorption of lipids on
forage particles which may reduce their
microbial colonization in the rumen. It can
be expected that lipids do not enter the bag
and that the colonization of feed particles
within the bags occurs in a normal way.
In conclusion, variations in the fibrolytic
activity of solid-associated micro-organisms
can be more marked (highly digestible carbo-
hydrates supplementation) or less marked
(increase of level of intake, fat supplementa-
tion) in the bags than in the rumen. This
could be attributed to problems of exchange
between the rumen digesta and the bag,
including both input of ruminal milieu and
output of end-fermentation products.Conclusions
With N rationing systems for ruminants,
the use of the in saccotechnique to predict
dietary N by-pass has become common. A
large number of methodological studies
have been carried out to pinpoint the
factors which vary and their respective
importance, and also to establish the limits
for use of this technique. At present, the
technique is used increasingly to predict
not only degradation of feed N in the
rumen, but also degradation of other feed
components and more especially that of
starch and the cell walls of forage.
The principle of the technique means
that in saccodegradation of feeds repre-
sents their degradation in vivo if the
exchanges between the ruminal milieu and
the bag, i.e. the influx of degradation agents
and the outflux of microorganisms and of
degradation products, are adequate and if
the feed components potentially degradable
by rumen microorganisms are accessible to
the microbial enzymes.In Sacco Methods 249