232 T
. Gidenne et al. Livestock Production Science 64 2000 225 –237
Fig. 2. Caecal total VFA concentration mmol l, mean6sd, according to age and dietary fibre level.
cording to age and fibre level in Fig. 2. In 6 weeks with a change in the VFA molar proportion: increase
old rabbits 2 wk after weaning, the VFA level in butyrate compensated by a decrease in acetate.
decreased linearly and significantly P ,0.01 from A20 to A12. For instance, VFA level reached 72
mM l for rabbits fed A20, compared to only 52
4. Discussion
mM l for those fed A12. Conversely at 10 weeks of age no significant effect of the dietary treatments was
4.1. Comprehensive approach of the rabbit observed. Besides the VFA molar proportion evolved
digestion similarly with age according to the diets no signifi-
cant interaction: the fermentation profile was char- Previous studies of digestion in the rabbit provide
acterised by a higher butyrate and a lower acetate little quantitative information on the partition of the
proportion for 10 weeks old rabbits compared to 6 digestion from enzymes of the host in the small
weeks old Table 8. In addition, the caecal pool of intestine or from bacteria in the caeco-colic seg-
VFA almost doubled between 6 and 10 wk old. With ment. Furthermore, such measurements were never
the reduction of the fibre supply, we observed a correlated with other parameters such as rate of
linear reduction of the VFA pool 215 associated passage or microbial activity. The present set of
Table 8 Volatile fatty acids molar proportions in the caecum, and starch concentration in the ileum of growing rabbits
a
mol 100 mol Fibre level
Age SEM
P level A20
A16 A12
6 wk 10 wk
Diet Age
Diet3Age n 520
n 520 n 520
n 530 n 530
Acetate 82.1b
81.3b 78.0a
82.5 78.4
0.56 ,0.001
,0.001 NS
Propionate 4.01
4.43 4.38
3.86 4.72
0.18 NS
,0.001 NS
Butyrate 13.6b
13.8b 16.9a
13.1 16.4
0.56 ,0.001
,0.001 NS
0.3b 0.34b
0.52a 0.36
0.41 0.04
,0.01 NS
NS Caecal VFA
pool mmol 5.66b
5.11ab 4.44a
3.84 6.38
0.32 0.041
,0.001 NS
Ileal Starch DM
0.9b 1.4b
2.9a 1.8
1.7 0.3
,0.01 NS
0.14
a
SEM, a,b: see Table 3.
T . Gidenne et al. Livestock Production Science 64 2000 225 –237
233
experiments aimed to gather these various digestive criteria and animal performances to obtain a com-
prehensive view of rabbit digestion in relation to dietary fibre level.
Compared to other monogastric animals, serious methodological problems were encountered in the
measurement of the digestibility of nutrients anterior to the terminal ileum, as suggested here by the
higher variability of these values compared to faecal digestibility results. Similar variability was previous-
ly reported for rabbits Gidenne, 1992; Merino and
˜ Carabano, 1992. This relatively high variability in
ileal digestibility could be attributed to several factors, but particularly to the following points: the
necessity of a supplementary measure compared to other monogastric that is a quantification of the soft
faeces production, the impossibility to obtain a complete collection of the ileal flow as for pigs, the
smallness of ileal digesta sample that limits chemical analysis, and particularly the feeding system of
rabbit ad libitum that implies circadian changes in feed intake and in composition of digesta and
differing feeding patterns between animals. How-
Fig. 3. Partition of the digestion between small intestine and caeco-colic segment in of digestible nutrients, organic matter
ever, we observed reliable results for the partition of
or crude protein, according to dietary fibre level.
the digestion between small intestine and caeco-colic segment, showing that about two thirds of organic
matter was digested before the caecum Fig. 3. Crude protein digestion in ranged from 80 to 90 in
be active during their passage through the stomach the small intestine, as found previously by Gidenne
and small intestine. This recycling may explain at 1992.
least in part that positive ileal digestibility was The estimation of the digestibility of fibre before
found for uronic acids from 20 to 38; Gidenne, the caecum is more questionable and seemed here
1992. However, further investigations into fibre slightly overestimated, as already observed by
digestion in the pre-ileal tract stomach, small intes- ˜
Merino and Carabano 1992. We measured in the tine are necessary to elucidate this phenomenon that
present study a high ileal recovery rate for ADF seemed specific to the rabbit among monogastric
mean 87, but if we consider that fibre degra- herbivores.
dation only occurred in the caeco-colic segment it The caecum is the major organ where microbial
should be close to 100, as previously observed by activity takes place in rabbits. The caecal microbial
Gidenne and Ruckebusch 1989 and Gidenne population secretes enzymes that can hydrolyse the
1992. Our results suggest that fibre digestion was main components of dietary fibre. In agreement with
as important before the caecum as after. However, two previous studies on rabbits Marounek et al.,
we cannot exclude that some fibre degradation could 1995; Jehl et al., 1996, we found greater enzymatic
occur before
the caeco-colic
segment, since
activity for degrading pectins and hemicelluloses Marounek et al. 1995 found relatively high pec-
than for degrading cellulose. These results reflect the tinolytic activities in the stomach of rabbits that
faecal digestibility of the corresponding dietary fibre would originate from soft faeces consumption. This
constituents in rabbits Gidenne et al., 1998, and are recycling of pectinolytic enzymes causes us to
also consistent with the smaller counts of cellulolytic inquire about the ability of pectinolytic enzymes to
bacteria in
the rabbit
caecum compared
to
234 T
. Gidenne et al. Livestock Production Science 64 2000 225 –237
xylanolytic or pectinolytic bacteria Boulahrouf et MJ kg for diets containing 20–16–12 ADF.
al., 1991. Rabbits vary their energy intake according to dietary
The relationship between BFA and caecal fermen- digestible energy DE content Lebas et al. 1982;
tation pattern has not previously been studied in the Partridge et al. 1989, but within moderate limits.
rabbit. Our results suggest that BFA is related to For instance, the DE intake of the growing rabbits
caecal VFA pool in finishing growing rabbits 10 slightly decreased for the lowest dietary fibre level
week old, since it decreased according to fibre compared to the two others respectively 1280 vs.
supply. Conversely no clear relation between the 1210 kJ d over the period 28–70 days, thus explain-
fermentation pattern VFA level or molar proportion ing the lower weight gain during the postweaning
and BFA was found. period.
It is particularly of interest to evaluate the flow of The health status of our growing rabbits was
starch to the terminal ileum in growing rabbits, as an moderately affected by the reduction of the dietary
overflow of starch in the caecum may be responsible fibre supply, except an increase of morbidity during
for microbial imbalance and thus for digestive the finishing fattening period. However, the animals
troubles Cheeke and Patton, 1980. However, direct were kept in individual cages thus reducing the
measurement of ileal flow is only possible in an possibility of interindividual contamination and
ileum-cannulated adult animal. Nevertheless, the therefore the risk of digestive troubles. In addition, to
combination of a quantitative approach to ileal be convincing, studies of rabbit nutritional pathology
digestion in the adult with a qualitative study of ileal should involve several experimental locations and
content in growing animals allowed us to estimate over 300 rabbits per dietary treatment, as achieved
the ileal flow of nutrients in growing animals. We recently by Perez et al. 1996.
hypothesised that the ileal flow of dry matter in Digestion of OM or energy before the caecum was
finishing growing rabbits 6–10 week old can be more affected by the reduction of the fibre than
estimated from that measured in the adult, according digestion in the whole tract. Consequently, there was
to the relevant feed intake. Thus, we calculated that an important change in partition of the OM digestion
ileal flow of starch in finishing growing rabbits between enzymatic and fermentative processes Fig.
would be very low, ranging from 0.7 to 1.8 g d 3. Conversely, the partition of the digestion of
respectively for the highest and lowest ADF starch protein was not greatly affected, although the level
ratio in our experimental diets. Therefore it seems of digestibility varied among diets. We reported a
unlikely that overflow of starch into the caecum large increase in NNCC ileal digestibility with
caused the digestive trouble observed here for grow- decreasing level of fibre, while NNCC digestibility
ing rabbits. Besides, we found here a low level of decreased in the caeco-colic segment. Since starch
ileal starch 0.9–2.9 DM compared to results of was almost totally degraded before the caecum
Blas et al. 1994 ranging from 3 to 7 at the same whatever the starch content, most of this effect was
age, but using maize starch that is less fully digested probably related to differences in pectin digestion.
in the rabbit Blas and Gidenne, 1998. Conse- Fibre digestibility was not greatly affected by the
quently, a possible effect of an overflow of starch reduction of level of DF, while the quantity of fibre
into the caecum should not be excluded when low- degraded decreased combination of a lower feed
quality starch is given, or in rabbits around weaning intake and of a lower dietary fibre level. Contrary to
which have a low secretion of pancreatic amylase. Bellier and Gidenne 1996, two weeks after wean-
ing we registered a linear decrease in caecal VFA level according to the decrease in fibre supply,
4.2. Impact of dietary fibre level whereas, no changes in the caecal VFA were found
for finishing
rabbits, as
previously reported
˜ A reduction of the dietary fibre level obviously
Carabano et al., 1988; Garcia et al., 1995. On the results in an increase in the digestible energy DE
other hand, the VFA molar proportion was affected: content of the diets respectively 10.46–11.75–12.55
the proportion of butyrate rose when the fibre starch
T . Gidenne et al. Livestock Production Science 64 2000 225 –237
235
ratio decreased Gidenne et al., 1991b; Bellier and rabbit remains difficult, particularly to quantify pre-
Gidenne, 1996. cisely the flow of nutrients entering the caecum.
We can suppose that the quantity of low digestible Further studies are necessary in this area, particularly
fibre such as cellulose entering the caecum is not a to reduce inter-individual variability of the measure-
limiting factor for the fermentation processes, proba- ments. Similarly, the quantitative approach to micro-
bly because the digesta retention time in the caecum bial activity needs to be improved. The present study
is too short to produce a significant hydrolyses. In confirmed the validity of the bacterial fibrolytic
return, caecal retention time seems sufficiently long activity measurement that should be further studied,
to allow substantial degradation of easily digestible particularly in relation to the development of the
fibre fractions such as pectins or hemicelluloses. caeco-colic segment.
Furthermore, the BFA decreased with the reduction The nutritional impact of dietary fibre level for the
of the fibre supply particularly for pectinolytic growing rabbit was now better understood Fig. 4,
activities 240, although the retention time in the but it should be completed with studies around the
caecum increased. Therefore, a reduction of the flow weaning period when digestive ability is developing.
of easily digestible fibre would limit the caecal Other comprehensive studies should be performed to
fermentation, which would explain the reduction in determine the effects of the quality of cell wall, as
NNCC digestibility in the caeco-colic segment this will vary greatly across the wide range of agro-
mainly due to pectins. industrial by-products used in rabbit feeding.
5. Conclusion Acknowledgements
