1995). This approach seems to be promis-
ing for predicting the fill effect of a feed-
stuff. However, this equation does not take
into account the particle size distribution
of the feeds, which is well known to affect
the rate of passage, rumen fill and, in turn,
ingestibility. The fill effect of large particles
can be assumed to be constant, as they
cannot flow out of the rumen and their
microbial degradation modifies their
volume only slightly. The increase in the
retention time due to comminution time
(CT) of large particles could be estimated
by the index of fibrousness, and/or by
particle size measurement. The RRT could
thus be calculated by the following
equation (Faverdin et al., 1995):
RRT = (((1 ab)/ksp) + (b/(c+ ksp))
+ CT)/24 (11.8)where a, band care as defined in Equations
11.1 and 11.3 and kspis the rate of passage
of small particles (h^1 ).
Fibrolytic activity of ruminal microorganisms
The in sacco method has been used in
research in order to follow the variations in
digestive activity of ruminal micro-
organisms, by measuring variations in
ruminal degradability and/or the degrada-
tion rate of a given feedstuff in different
conditions varying according to charac-
terics of the animals (species, physiological
stage, feeding level, etc.), or to charac-
teristics of the diet (additives, N content,
level and nature of concentrate, etc.). In
this chapter, only results related to feeding
level and energetic supplementation with
carbohydrates or lipids are reported as an
illustration of the limits of the in sacco
method for quantifying variations in fibro-
lytic activity of rumen microorganisms.
The depressive effect of an increasing
level of intake on forage fibre ruminal
degradation is attributed classically to the
decrease in ruminal retention time of
particles, without significant variation in
microbial fibrolytic activity, as estimated
by in saccodegradation. However, with an
increasing feeding level for sheep, the
xylanase and cellulase enzyme activities of
solid-associated microorganisms have been
shown to decrease in the rumen, without
significant variations in the forage fibre
degradation rate in sacco (Kabré et al.,
1994). This result illustrates the lack of
sensitivity of the in saccomethod for the
assessment of variations in microbial248 P. Nozière and B. Michalet-Doreau
Fig. 11.5.Relationship between in saccoand in vivomethods for determining forage cell wall digestion in
the rumen (Archimède, 1992).