sized screen. The chemical composition of
different sized particles is different, and
fractions of different particle size poten-
tially have different rates and extents of
degradation. Therefore, it is necessary to
control the extent of particle losses that are
sometimes important, and which could
result in: (i) an overestimated extent of
degradation; (ii) an underestimated rate of
degradation if the lost particles have a
faster rate of degradation than those
remaining in the bag; and (iii) an over-
estimation of the immediately soluble
fraction. A more rational approach to the
use of milling would be to characterize the
processing of feeds not by the grinding
screen aperture, but rather by foodstuff
particle size (Michalet-Doreau and Cerneau,
1991), and to establish some degree of uni-
formity in particle size within foodstuff
categories.
Drying
For fresh forages, the lacerated form prior
to rumen incubation is probably the
sample preparation which best reproduces
chewing. However, the use of fresh sample
is difficult; it is necessary to have the
animal and the forage samples at the same
moment. Therefore, samples commonly are
conserved by freezing or drying. Several
drying methods have been proposed, and
they affect ruminal degradation in different
ways. The nature and content of various
constituents may be influenced by the
drying process, and long drying times are
required when a low temperature is used
in order to avoid thermochemical degrada-
tion of the sample. In comparison with
freeze-drying or fresh forage, oven-drying
between 45 and 60°C decreased the soluble
N content and in situ N degradability
(Lopez et al., 1995; Dulphy et al., 1999),
and this decrease was due essentially to a
smaller rapidly degradable fraction. The
freezing of chopped forage does not affect
the N degradability (Dulphy et al., 1999).
However, when samples are ground after
freeze-drying, N degradability increases
(Michalet-Doreau and Ould-Bah, 1992), the
freezing probably inducing a disruption of
plant structures.
Exchanges between bag and rumenBags must be placed in the rumen so as to
permit free movement within the rumen
liquor and so that the bags are squeezed
during muscular contractions, facilitating
fluid exchange between the internal
environment of the bag and the rumen. The
bacteria present in the more aqueous
regions of the rumen, the ventral sac, could
colonize and attack freshly exposed feed
surface more easily than the microflora in
the dorsal sac. In most studies cited by
Huntington and Givens (1995), the authors
incubated the bags in the ventral sac. How-
ever, the distribution of microorganisms in
the ruminal content depends on the physi-
cal and chemical conditions inside each
rumen compartment, and these conditions
themselves vary during the nycthemeral
period. Therefore, the site of bag incuba-
tion and the incubation sequence in rela-
tion to animal feeding can influence
digestion rates inside bags.
As it is necessary for microorganisms
to enter the bag in order for feed degrada-
tion to take place, it is also essential that
they be eliminated from the bag after
incubation to avoid underestimation of DM
and N degradation.
To begin with, we will show diurnal
fluctuations of digestive ruminal capacity
in relation to time after feeding and site of
sampling in the rumen, and the implica-
tions of these spatial variations within the
rumen on degradation rates in the bags.
Then we will study microbial colonization
of feed samples and exchanges between
bags and the surrounding digesta.Effect of bag procedure
Ruminal contents are very heterogeneous.
The DM content drops considerably from
the dorsal rumen to the ventral rumen,
whereas the bottom of the ventral sac and
the reticulum show comparable values.
These differences in DM between compart-
ments can be explained by the consider-
able differences in size and density of food
particles in them. A matted mass of large
particles occupies the dorsal sac, whereas
small, high-density particles accumulate236 P. Nozière and B. Michalet-Doreau