Farm Animal Metabolism and Nutrition

feed intake problems related to digestive
upsets, hence the need to evaluate starch
escaping rumen digestion.
Sauvant et al. (1994) list rumen-
degradable starch for various feedstuffs,
and propose the use of the in sacco
technique to characterize starch degrad-
ability. However, in sacco degradability
tends to underestimate the degradability of
slowly degraded starches (Fig. 11.4). This
discrepancy could be the result of the
failure of this technique to account for a
reduction in the particle size of grains
through mastication. The differences be-
tween cereals or between cultivars in rumi-
nal starch degradation rate depend on the
starch accessibility to microorganisms, i.e.
starch which is protected by the protein
matrix surrounding the starch granules. As
an increase in cereal particle size involves
a decrease in starch ruminal digestion, its
effect is more important for slowly degrad-
able cereal than for rapidly degradable
cereal (Huntington, 1997). Therefore, when
cereals are coarsely cracked, in sacco
measurements give the same values as in
vivoresults (Philippeau et al., unpublished
data). In the bag technique, the cereals are

ground, and it is not possible to take

account of this effect of particle size.

To predict cell wall digestion

Organic matter digestibility of forages

depends mainly on their cell wall digesti-

bility, and thus on the amount of cell walls

undigested within the digestive tract.

Because ruminal cell wall digestion repre-

sents on average 80% of their digestibility in

the total gastrointestinal tract, the amount of

cell walls escaping ruminal digestion is a

good estimate of the digestibility of forages.

In the same way, the ingestibility of forages

depends on the amount undegraded in the

rumen which induces the ruminal fill. As

the in saccomethod makes it possible to

assess the quantity of cell walls undegraded

in the rumen, it appears an interesting

method for the prediction of forage

digestibility and ingestibility.

Digestibility and cell wall ruminal digestion

The in saccotechnique has been used to

estimate digestibility of forages (reviewed

by Michalet-Doreau, 1990). The percentage

246 P. Nozière and B. Michalet-Doreau

Fig. 11.4.Relationship between in saccoand in vivomethods for determining rumen-degradable starch for
various feedstuffs (Sauvant et al., 1994).