The main factors which govern ex-
changes between the rumen contents and
the bag are the bag characteristics, pore size
and open area. Recommended procedures
are generally agreed between users and
relatively well standardized. The problems
of exchanges between the bags and the
ruminal milieu have become better under-
stood over the past few years due to studies
of the microbial ecosystem involving
techniques of numeration and enzymology.
The main questions which remain today
concerning the exchanges between the bag
and the ruminal contents are the chemical
composition and degradation characteristics
of the fraction lost through the pores of the
bag, which naturally vary according to the
nature of the feed and how to account for
microbial contamination relative to dietary
N degradability measurements, because this
contamination may introduce a non-
negligible bias for feedstuffs with little N.
Different degrees of accessibility of
components to microbial enzymes may
also be at the origin of a bias in classifica-
tion of feedstuffs according to their degrad-
ability in the rumen. Accessibility of
components depends on how the sample is
presented. Whereas dry feedstuffs are
presented in more or less standard fashion
and therefore do not pose any major
problems, the same cannot be said for moist
feedstuffs and in particular for forage. Lack
of knowledge of the dynamic processes
involved in accessibility of digestible com-
ponents to microbial enzymes during and
after the lag time is a major hurdle yet to be
overcome for more accurate modelling of
the degradation processes.
Validation of the in saccotechnique to
predict degradability of dietary N in the
rumen is now well established thanks to
the concordant studies aimed at establish-
ing the relationship between in vivoand
in sacco measurements. The in sacco
technique is now also used to predict
ruminal degradability of starch and cell
walls. However, comparisons between in
vivoand in saccodegradation measurements
are few as yet, and this aspect needs to be
studied more closely. Finally, in the light of
knowledge today, the in sacco technique
would seem capable of revealing variations
in the fibrolytic activity of ruminal micro-
organisms in relation to changes in the diet,
the level of intake, etc., but not of quantify-
ing them. Indeed, the variations in in sacco
degradation amplify or minimize the varia-
tions in fibrolytic activity according to
changes in dietary conditions.250 P. Nozière and B. Michalet-Doreau
Table 11.2.Differences in pH and fibrolytic enzyme activities in solid-associated microorganisms (μmol
reducing sugars released g^1 DM h^1 ) between rumen content and bags, with cows fed various levels of
concentrate (Nozière and Michalet-Doreau, unpublished data).
Barley content of diet (g kg^1 )
0 300 600pH
Rumen 7.16 6.99 7.09
Bag 6.64 6.19 5.78
Difference 0.52 0.80 1.31
Avicelase
Rumen 17.7 23.3 17.7
Bag 15.7 14.3 5.9
Difference 2.0 9.0 11.8
Xylanase
Rumen 263 318 248
Bag 251 231 69
Difference 12 87 179