for a wide variety of dietary feed ingre-
dients. In vivomeasurements of ruminal
degradation of dietary protein require that
animals be surgically prepared with
cannulas in the rumen and duodenum. This
method also requires suitable markers for
calculating the flow rate of digesta and a
reliable microbial marker for differentiation
between microbial and dietary protein flow-
ing to the small intestine. In vivoestimates
of protein degradation are expensive, labour
intensive, time consuming and subject to
error associated with the use of markers of
digesta flow rate, microbial markers and
animal variation. Therefore, alternative
procedures for measuring ruminal digestion
of dietary protein are needed. Among the
many proposed methods, the bag technique
has been used widely.
The closest relationships between in
vivo (measured by difference between
N intake and non-microbial non-NH 3
duodenal N divided by N intake) and in
sacco degradation were reported by
Madsen and Hvelplund (1985) with 11
feeds (R^2 = 0.79) and by Vérité et al.(1987)
with eight feeds (R^2 = 0.90). More recently,
Poncet et al. (1995) compared in vivo
measurements with in situestimates from
five experiments pooling 21 comparisons.
The intercept of the linear relationship
differs between experiments but, in all
trials, the slope is the same and does not
differ from 1.
In the in saccotechnique, N which
disappears from the bag is considered to
have been degraded in the rumen, and the
term ‘disappearance’ is synonymous with
‘degradation’. As discussed previously, an
overestimation can be due to undegraded
particle losses through the bag pores, intro-
ducing a bias in the range of feeds accord-
ing to their N degradability. Numerous
methodological reviews recently have dealt
with these factors, and it is now possible to
appreciate this bias in relation to feed in
the bag. In situdegradability measurement
can also underestimate dietary N supply
with leguminous seeds. A significant
amount of soluble dietary proteins and
small peptides from dietary protein
degradation are found and can accumulate
in the rumen during the first hours after
feeding (Williams and Cockburn, 1991).
With these feeds, Poncet et al.(1998) also
reported a discrepancy between in sacco
and in vivodata because the peptides and
soluble proteins are included in the in vivo
N by-pass and considered as degraded
proteins in the in saccotechnique.
Another more recent application of
the in sacco technique is the mobile
bag technique. This is developed for esti-
mation of intestinal availability of rumen-
undegraded dietary protein (Hvelplund,
1985; De Boer et al., 1987). After incuba-
tion in the rumen for a variable period, the
food is placed in a bag and introduced via
a duodenal cannula into the intestine
where it transits freely, then is recovered at
the end of the small intestine or in the
faeces, when the quantity of residual N is
determined. A relationship between digesti-
bility of N in the bags and digestibility
in the small intestine was reported by
Hvelplund (1985). However, the intestinal
N digestibility of rumen-degradable N
depends on many factors: the duration of
the preceding rumen incubation time; the
retention time in the intestine; the location
of bag recovery (ileum or faeces); etc.
Therefore, this technique can give only an
indication of the intestinal availability of
rumen-undegradable protein.Starch escape
Research in starch utilization by ruminants
was reviewed recently. Although starch in
cereal grains is almost completely digested
in the whole digestive tract, the rate and
extent of ruminal fermentation vary widely
with grain source and cereal processing
(reviewed by Huntington, 1997). The site of
starch digestion has implications for the
nature and amount of nutrients delivered
to the animal. The absorbed nutrients,
volatile fatty acids or glucose, are used
with different efficiencies for energy pro-
duction (ATP synthesis) during catabolic
reactions and for anabolic processes such
as fat and protein synthesis. The rapid
fermentation of starch in the rumen
increases the acidity in this compartment
and increases the incidence of acidosis andIn Sacco Methods 245