Farm Animal Metabolism and Nutrition

(Mertens, 1993). Thus, as feed particles
have the opportunity to pass out of the
rumen undegraded, the extent of degrada-
tion in the bag will not give a correct
estimate for the effective degradation under
normal rumen conditions. In this respect, it
is necessary to consider both the rate of
degradation and the turnover of particles in
the rumen. To take account of the turnover
rate in the estimation of feed ruminal
digestion, mathematical methods based on
interpretation of the in saccodegradation
profile including a fixed turnover rate were
proposed, by summing up step by step the
amount of feed degraded before leaving the
rumen during each incubation period
(Kristensen et al., 1982), or by integration
of the in saccodegradation profile in rela-
tion to the ruminal particulate outflow rate
(Ørskov and McDonald, 1979).
With the step by step method, the rate
of degradation in the rumen as well as the
outflow rate of the feed are combined to
express the effective degradability (ED) by
the following equation (Kristensen et al.,
1982):

ED = (D(ti+1)
D(ti)) f(ti, ti+1) (11.4)

where (D(ti+1)
D(ti)) is the amount of feed
degraded during the time interval from tito
ti+1, and f(ti, ti+1) is the average proportion
of feed remaining in the rumen from tito
ti+1. The amount of feed remaining in the
rumen (f(ti)) is estimated from the outflow
rate (k, h
^1 ) by the following equation:

f(ti) = exp(
k ti) (11.5)

The effective degradability can also be
calculated from the parameters of kinetics
obtained from exponential adjustment
without (Ørskov and McDonald, 1979) or
with (Dhanoa, 1988) lag time, both by the
following equation:

ED = a+ (bcexp(
k )/(c+ k))

(11.6)

where a, b, c and  are as defined in
Equations 11.1 and 11.3, and k is the
rumen small particle outflow rate (h
^1 ).
The adequacy of the calculation of
effective degradability between these two
methods obviously depends on how good

the fit is to the regression, and this is essen-

tially linked to the pertinence of the

incubation schedule.

As discussed previously, numerous

factors affect degradability data obtained

with the in sacco technique. In order to

allow comparisons between laboratories, a

standard procedure of measurement has

been proposed to measure N degradation

rate of feeds (Madsen and Hvelplund,

1994), but some points such as processing

of wet samples, taking account of microbial

contamination in bags, etc., have not yet

been the subject of specific recommenda-

tions. In spite of efforts to standardize the

methodology for measurements, intra- and

inter-laboratory variability remains high

(Michalet-Doreau and Ould-Bah, 1992;

Madsen and Hvelplund, 1994). Use of

reference samples as internal standards has

been proposed to reduce variability by

incubating a standard sample of known

degradation alongside test substrates

(Michalet-Doreau and Ould-Bah, 1992).

Increasing use of a single standard sample

in the various laboratories carrying out in

sacco degradation measurements should

help to reduce variability between labora-

tories, but also to understand the origin of

these variations when the degradation

results for this standard sample are

compared.

Utilization of the In SaccoTechnique

To predict rumen escape and

small intestine digestion

N escape

The most recent methods to evaluate the N

values of feeds are based on estimation of

the quantity of amino acids that can be

absorbed by the small intestine. These

amino acids have two possible origins:

dietary or microbial. The feed N which

escapes rumen degradation is one of the

major elements in the determination of

feed N value. For these systems to be

effective, reliable procedures must be

developed to provide rapid and realistic

estimates of ruminal protein degradation

244 P. Nozière and B. Michalet-Doreau