Farm Animal Metabolism and Nutrition

underestimation of N degradability varied
from 8 to 15 points when the crude protein
content of forages decreased from 18 to 8%
DM (Michalet-Doreau and Ould-Bah, 1992).
Different techniques are used to dis-
lodge the bacteria attached to the bag
residues: physical treatments such as the
freezing–rethawing technique associated
with stomaching (Michalet-Doreau and
Ould-Bah, 1992) or not associated with
stomaching (Kamel et al., 1995); or
chemical treatments using the properties of
surfactants, salts or detergents associated
with stomaching (Beckers et al., 1995) or
not associated with stomaching (Hof et al.,
1990). The efficiency of these techniques in
detaching associated bacteria from bag
residues is variable, but no procedure
completely removes microbial contamina-
tion. Moreover, these treatments not only
remove bacteria fixed to the residues, but
also partially remove N feed residues from
the bags (Beckers et al., 1995).

In summary, microbial contamination

of forage residues can result in a substan-

tial underestimation of degradability value,

and this effect is particularly important for

feeds in which the N content of the feed

constitutes only a small proportion of the

DM. To take the bacterial contamination

into account, a marker technique of the

microbial population can be used, but it is

laborious and expensive. In routine experi-

ments, treatments for decontamination can

be useful to dislodge bacterial DM fixed to

the bag residues, but they generally tend to

underestimate N degradability. Another

promising method consists of predicting

microbial contamination of the bag residue

by near infrared spectrometry (Lecomte et

al., 1994).

Modelling of kinetics of degradation

The ruminal degradation kinetics of dietary

DM or any component may be described by

a curvilinear regression according to

incubation time. The exponential model,

described by Ørskov and McDonald (1979)

for N degradation, is the most commonly

used. This model, the biological significa-

tion of its parameters and their determina-

tion will be presented and discussed in

this chapter. We will then briefly present

the main methods used to determine the

ruminal degradability of feeds from the in

saccodegradation profiles and the turnover

rate of particles in the rumen.

The exponential model

First-order degradation kinetics, reported

by Ørskov and McDonald (1979), have

been commonly used to describe ruminal

N degradation kinetics, and more recently

for cell walls and starch degradation

kinetics. This model supports the existence

of three dietary fractions (Fig. 11.3).

1.The undegradable fraction represents the

amount of residue remaining in the bag after

a long incubation time. For starch, it is

generally believed that there is no undegrad-

able fraction, as observed by several authors

(review of Sauvant et al., 1994).

240 P. Nozière and B. Michalet-Doreau

0 20 40 60 80 100 120

Underestimation of N degradability (points)

30

20

10

0

N (g kg
^1 DM)

Fig. 11.2.Relationship between underestimation
of N degradability due to bacterial contamination
and N content of forage (●) and concentrate ()
(Michalet-Doreau and Ould-Bah, 1992; Wanderley
et al., 1993; Beckers et al., 1995).