T. Gidenne

Fibres in rabbit feeding for digestive troubles prevention: respective role of low-digested and digestible fibre

A sufficient dietary fibre supply is essential to prevent digestive troubles in the growing rabbit. The most recent recommendations have implicated several classes of fibre, including low-digested fibre (lignocellulose) and digestible fibre (hemicelluloses and pectins). Moreover, agro-industrial by-products (brans and pulps), rich in digestible fibre, are often incorporated in rabbit feeding, although this fibre fraction is not analysed by the current routine method. Therefore, the present review focuses on the respective role of low-digested and digestible fibre fractions in the digestive health of the growing rabbit, after weaning. It gives levels of fibre classes, including pectins, in some feed ingredients classically used in rabbit feeding. A brief overview of some characteristics of dietary fibre and some routine methods to estimate fibre fractions in animal feed ingredients are presented.

Caeco-colic digestion in the growing rabbit: impact of nutritional factors and related disturbances

Abstract One of the main problem in the nutrition of the growing rabbit is to control the incidence of enteritis related to unbalanced nutrient supply (especially fibre) and to disorders in caecal microbial activity (CMA). The caeco-colic digestive processes need thus to be well understood in connection to nutritional and ontogenic factors. The present review first summarises the main traits of the caecal physiology and recent advances along with methodological approach used. Ontogenic and nutritional factors affecting CMA are then reviewed. Finally, caeco—colic disturbances arising from inadequate nutrient supply are described, including the related hypothesis explaining the occurrence of enteritis. New methodological approach has been recently developed in order to evaluate more precisely the CMA under several aspects including in vivo fermentation pattern, enzymatic fibrolytic bacterial activity or production of biomass. The studies measuring the needs of nutrients for the caecal flora have developed a new approach that considers the respective dietary fibre fractions and the interactions with the starch supply. But, further research is needed to improve our knowledge of the flow of nutrients entering the caecum, particularly for the growing rabbit.

Effect of fibre level, particle size and adaptation period on digestibility and rate of passage as measured at the ileum and in the faeces in the adult rabbit

The use of specific methods other than gravimetric ones to analyse fibre permitted a more precise study of the degradation of cell wall (CW) in the digestive tract. Digesta flow and rate of passage measurements have not been assessed previously in rabbits to investigate fibre digestion and fibre effects simultaneously in the proximal and in the distal segments of the tract. The effect of the level of dietary fibre on ileal and overall digestibility and rate of passage were studied by comparing semi-purified diets containing only one source of CW (dehydrated lucerne (Medicago sativa) meal) given to adult female rabbits. The effect of fibre particle size and adaptation period were also investigated. Measurements of fibre composition using both colorimetric and gas-liquid chromatographic techniques, showed that large amounts of (CW) were degraded in the caecocolic segments. Increasing dietary fibre level reduced the rate of passage but fibre degradation was increased, at the same time a lower digestive efficiency for energy in the small intestine was found. A small quantity of CW was apparently degraded before the caecum, assuming that the water-soluble fraction of CW was essentially implicated. Grinding lucerne meal through a 1 mm instead of a 3 mm screen did not improve CW digestibility in spite of a longer rate of passage in the caeco-colic segments. Adaptation to a high-fibre diet resulted in an higher digestive volume for colon and caecum, related to an improved degradation of CW. Furthermore, digestive efficiency in the small intestine appeared higher for rabbits adapted to a high-fibre diet than that for rabbits initially fed on a low-fibre diet.

A comprehensive approach of the rabbit digestion: consequences of a reduction in dietary fibre supply

The effects of dietary fibre level (20-16-12% acid detergent fibre (ADF)), without variations in the proportions of the fibre fractions (hemicellulose, cellulose, lignin . . . ), were studied with respect to many aspects of rabbit digestion (ileal digestibility, transit, microbial activity), growth and health. For the lowest level of ADF, the voluntary intake of animals was reduced by 25% and weight gain by 9%, during the two weeks postweaning (28-42 days), and there was a sharp increase (P , 0.01) of morbidity between 42 and 70 days old. A linear reduction of the whole tract digestibility for OM was observed according to ADF level (21.6 unit per % ADF), without significant changes in fibre digestibility. When ADF level passed from 20 to 12%, ileal digestibility of organic matter increased from 38 to 61%, while it decreased from 27 to 17% in the caeco-colic segment. Starch ileal digestibility was lowered by 5.7 units with the reduction of the ADF / starch ratio, but remained at a very high level ( . 93%). A sharp increase in mean retention time between ileum and rectum ( 1 38%) was noticed, reaching 19 h for the lowest ADF level. The bacterial fibrolytic activity was significantly reduced with the lowest ADF level (Fig. 1), and pectinolytic activity was more particularly affected (240%). Two weeks after weaning the VFA level decreased linearly and significantly (P , 0.01) with reduction of ADF level, while at 10 weeks no significant effect was observed.

Consequences of reduced fibre intake on digestion, rate of passage and caecal microbial activity in the young rabbit

The present work was undertaken to study in vivo fibre degradation, rate of passage and caecal fermentation activity (CFA) in the young rabbit (7 weeks old) receiving ad lib. a control (C) or low-fibre (LF) diet (400 and 220 g neutral-detergent fibre (NDF)/kg respectively). As a consequence of the 50% reduction in the dietary fibre level, the voluntary food intake of the rabbits decreased by 25%, and the daily fibre intake was reduced by 60% (26.7 and 10.8 g NDF/d for groups C and LF, respectively). In spite of a longer mean retention time of the fibre particles, the quantity of fibre digested daily was significantly lower (P < 0.01) for the LF than for the C group (4.0 and 7.8 g NDF/d respectively). The circadian distribution of the faecal excretion (as a percentage of the total DM output) did not differ between diets (P = 0.52) and no interaction was found (P = 0.96) between diet and time of excretion. Also, variables describing the CFA showed no interactions between diet (C or LF) and time of sampling (during caecotrophy or during hard faeces excretion). Our results indicated no direct relationship between the quantity of fibre digested and the total short-chain fatty acid concentration in the caecum, but the fermentation pattern indicated lower proportions of acetate for the LF diet. Higher levels of diaminopimelic acid (DAPA) and ATP were found for the LF diet associated with an improved dietary fibre digestibility, suggesting a higher microbial activity. However, this effect was balanced by a lower caecal digesta turnover rate and the microbial biomass output estimated through the faecal DAPA output did not vary significantly.

Feed restriction strategy in the growing rabbit. 2. Impact on digestive health, growth and carcass characteristics

A field enquiry mentioned the potential positive impact of a feed restriction on the health of young rabbits, but no objective information relates the intake to digestive health. The effects of a post-weaning feed restriction strategy were thus studied on digestive health and growth and carcass parameters of the growing rabbit, using a monofactorial design that produces a quantitative linear reduction of the intake, from ad libitum (AL group) to 80%, 70% and 60% of AL. The study was performed simultaneously in six experimental sites, on 1984 growing rabbits (496 per treatment) collectively caged from weaning (34 to 38 days of age, depending on the site) to slaughter (68 to 72 days). The feeding programme was applied as followed: restriction during 21 days after weaning, and then ad libitum till slaughter. During the feed restriction period the growth rate was linearly reduced with the restriction level, by 0.5 g/day for each percent of intake reduction. When returning to ad libitum intake (after 54 days old) a compensatory growth and a higher feed efficiency occurred. Therefore, the impact of the feeding programme on the slaughter weight (SW) was significant (-4.5 g/% of restriction), but relatively moderate: the weight loss of the more-restricted rabbits (60%) reached 7.7% (-200 g) compared to the AL group. Over the whole fattening period, the feed restriction reduced linearly and significantly the feed conversion (FC) (-0.0077 unit/% of restriction). Carcass traits were little affected by the feeding programme, except for a slightly lower decrease of the dressing percentage (mean: 1.2 units between AL and the three restricted groups). On the six experimental sites, mortality and morbidity were always caused by acute digestive disorders, namely diarrhoea and/or caecal impaction. Independent of the treatment, the mortality rate strongly varied according to the site (between 7% and 18% from weaning to 54 days and for the AL group). During feed restriction, the mortality was significantly lower from a restriction threshold of 80% (meanly: -9% compared to AL). The morbidity was also significantly reduced (-6%) for the two most restricted groups (70% and 60%). The favourable effect of a lower intake on health did not persist after returning to ad libitum intake (54 days to slaughter), since mortality and morbidity were not significantly different among the treatments. Such a feeding strategy thus represents a double benefit in terms of feed costs and lower losses of young rabbits.

Potential core species and satellite species in the bacterial community within the rabbit caecum

A bacteria library was constructed from the caecum of a rabbit maintained under standard conditions. The complete gene 16S rRNA gene was sequenced. The 228 clones obtained were distributed in 70 operational taxonomic units (OTUs). The large majority of the OTUs were composed of one or two clones and seven OTUs contained half of the sequences. Fourteen sequences had high similarity to the sequence already registered in databases (threshold of 97%). Only one of these sequences has been identified as Variovorax sp. (99% identity). Units were distributed mainly (94%) in the Firmicutes phylum. Three sequences were related to Bacteroidetes. Nine clusters were defined in the phylogenic tree. A great diversity of caecal bacteria of the rabbit was shown. Half of the sequences generated in this library were distributed in the phylogenetic tree near the sequences characterized previously in rabbit caecum (potential core species), and the other half of the sequences were well separated (satellite species).

Replacement of starch by digestible fibre in feed for the growing rabbit. 2. Consequences for microbial activity in the caecum and on incidence of digestive disorders

Caecal fermentative activity and daily bacterial biomass production were compared for two groups of nine rabbits, fed a diet rich in starch (ST, 266 g starch kg−1 dry matter (DM)) or a diet rich in digestible fibre (DF, 271 g hemicellulose + pectin kg−1 DM), but which were similar in acid detergent fibre (ADF) content (205 g kg−1 DM). Caecal volatile fatty acid (VFA) concentrations, measured in vivo in healthy rabbits fitted with a caecal cannula, were significantly higher in the DF group (85.1 mmol l−1) than in the ST group (67.7 mmol l−1). Between 44 and 72 days of age, no change in in-vivo VFA level was recorded for the two diets. Acetate molar proportion decreased from 86 to 72% mostly between 44 and 53 days of age (P = 0.05), and was balanced by a slight increase of both propionate and butyrate proportions. The concentration of diaminopimelic acid (DAPA) was two-fold higher in soft than in hard faeces, and was not significantly affected by diet composition. The mean level of DAPA in bacteria was similar for the two groups of rabbits (19.8 μ mol g−1 DM). The mean bacterial biomass production was two-fold higher in the DF group (13.3 g DM day−1) and accounted for 13% of the feed intake. Although ADF intake was similar for the two groups of rabbits, the animals fed the starch-rich diet showed a higher incidence of digestive disorders (diarrhoea). The partial replacement of starch by digestible fibre improved the health status without major impairment of growth performances.

Molecular analysis of the bacterial community in digestive tract of rabbit.

This work aimed to study the stability over time of the bacterial community in caecum and faeces of the rabbit (diversity index and structure) without experimental disturbance and to evaluate its relationships with environmental parameters. Soft and hard faeces of 14 rabbits were sampled for 5 weeks while caecal content was sampled on the 3rd week (by surgery) and the 5th week (at slaughter). Bacterial communities were assessed by studying CE-SSCP profiles of 16S rRNA genes fragments. Redox potential, pH, NH3-N concentration and volatile fatty acid concentrations were measured in the caecum. Data showed that bacterial communities of soft and hard faeces barely differed from that of the caecum (ANOSIM-R<0.25; p<0.05). Without disturbance, the bacterial communities of faeces were stable over time (ANOSIM-R<0.25; p<0.001). However, the bacterial communities of caecum and faeces were affected by the surgery (ANOSIM-R=0.22-0.33; p<0.001). The caecal content was an acidic (pH=6.03+/-0.33) and an anaerobic environment (redox potential=-160+/-43 mV). Only the redox potential was correlated with the diversity index of the bacterial community of the caecum (R(2)=0.35; p<0.05) and no environmental parameters were correlated to its structure.

Inter-relationship of microbial activity, digestion and gut health in the rabbit: effect of substituting fibre by starch in diets having a high proportion of rapidly fermentable polysaccharides

Caecal microbial activity, digestion and gut health were analysed in the young rabbit, in response to fibre substitution by starch, in diets with high proportions of rapidly fermentable polysaccharides (pectins+hemicelluloses:acid-detergent fibre (ADF) ratio of 1.7). A range of five diets corresponding to a 60 % linear reduction of the ADF level (230 to 92 g ADF/kg) without changes in the fibre quality, and to a corresponding linear increase in dietary starch, was given ad libitum to young rabbits from 18 d until 70 d of age. A one half reduction of the ADF level resulted in a sharp increase in energy digestibility (+25 units), associated with a lower feed intake (-35 %) and to an increase of the mean retention time (+6 h) in the whole digestive tract. Despite large variations in the fibre intake (20 to 59 g ADF/d), the fibre digestive efficiency remained similar among the five diets. Starch ileal concentrations were low after 4 weeks of age (<5 %), and variations with age were significant when the dietary starch level was over 19 %. A 65 % lower biomass production was measured when the ADF level progressed from 230 to 165 g/kg, and no precise relationship was found with fermentative activity. Reducing the fibre intake led to a linear decrease of caecal volatile fatty acids concentrations, and to higher pH and NH3 levels. An increased occurrence of mortality by diarrhoea was registered with the lowest fibre intake. It can be concluded that a sufficient supply of fibre, with high proportions in rapidly fermentable polysaccharides, stimulates the maturation of microbial activity and reduces the occurrence of diarrhoea.