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Role of fibre in rabbit diets. A reviewCarlos de Blas, Javier García, Rosa Carabaño

To cite this version:Carlos de Blas, Javier García, Rosa Carabaño. Role of fibre in rabbit diets. A review. Annales dezootechnie, INRA/EDP Sciences, 1999, 48 (1), pp.3-13. �hal-00889777�

Review

Role of fibre in rabbit diets. A review

Carlos De Blas Javier García, Rosa Carabaño

Departamento de Producción Animal, ETS Ingenieros Agrónomos,Universidad Politécníca, 28040 Madrid, Spain

(Received 27 January 1998; accepted 7 May 1998)

Abstract - The effect of fibre on rate of passage, control of gut flora, caecal fermentation, andperformance of rabbits has been reviewed. Both physical and chemical characteristics of fibre affectthese variables. An increase of the proportion of fine particles (< 0.315 mm) increases NDF digestibil-ity, acidity and weight of caccal contents, fermentation time and microbial protein recycled throughcaecotrophy, but decreases rate of passage and intake. A minimal proportion of large particles(> 1.25 mm) is also required to allow an adequate turnover rate of caecal contents, and then to maxi-mise microbial efficiency. The fraction of pectin components (arabinose, galactose and uronic acids)of cell walls accounts for most of the total fibre digestibility. An increase in the dietary concentrationof these constituents leads to an increase of acidity of caecal contents and microbial protein recycledthrough caecotrophy. Dietary lignin content is negatively related to energy digestibility and also tothe accumulation of digesta in the caecum. Both excessive and insufficient dietary fibre levels leadto an impairment of rabbit’s performance. Practical recommendations on optimal fibre concentrationsand minimal proportion of large particles are given for breeding does, fattening rabbits and mixed diets.&copy; Elsevier / Inra

fibre / rabbit / digestion / performance

Résumé &mdash; Rôle des fibres dans l’alimentation du lapin : revue. Cette revue concerne les effetsdes constituants pariétaux sur le transit digestif, le contrôle de la tlore intestinale, la fermentation excaleet les performances des lapins. Les caractéristiques, tant physiques que chimiques, des constituantspariétaux interviennent sur ces variables. Un accroissement de la proportion des particules de petitetaille (< 0,315 mm) augmente la digestibilité des constituants pariétaux, l’acidité et le poids ducontenu caecal, ainsi que la durée de fermentation et le recyclage des protéines microbiennes via lacaecotrophie, mais, en revanche, produit une diminution du transit digestif et de l’ingestion. Uneproportion minimale de particules de grande taille (> 1,25 mm) est nécessaire pour obtenir un tauxde renouvellement du contenu caecal convenable et augmenter, ainsi, l’efficacité de la synthèsemicrobienne. La digestibilité totale des parois cellulaires résulte en grande partie de celle des substancespectiques (arabinose, galactose et acides uroniques). Un accroissement de la teneur de ces constituantsdans l’alimentation augmente l’acidité du contenu caecal et le recyclage des protéines microbiennes

* Correspondence and reprintsTel.: (34) 91 549 7978; fax: (34) 91 549 9763; e-mail: cdblas@pan.etsia.upm.es

via la caecotrophie. La teneur en lignine du régime est négativement liée à l’énergie digestible et à laréplétion du caecum. Les niveaux tant excédentaires que déficitaires de constituants pariétaux dans lerégime donnent lieu à une diminution des performances des lapins. Des recommandations pratiquespour les teneurs optimales en constituants pariétaux et la proportion minimale de particules de grandetaille sont donnés pour les lapines allaitantes, les jeunes en croissance et les rations mixtes.&copy; Elsevier / Inra

fibre / lapin / digestion / performance

1. INTRODUCTION

Fibre is one of the main constituents ofcommercial diets for intensively reared rab-bits, which typically include around one-third of forages and fibrous by-products.The role of fibre in rabbit nutrition is notlimited to nutrient supply. Fibre also plays amajor role in the regulation of rate of pas-sage of digesta, the control of gut flora andthe maintenance of intestinal mucose

integrity. Chemical composition and phys-ical structure of plant cell walls vary widelyamong fibre sources, so that both type andlevel of fibre have important effects on rab-bit digestion.

The aim of this paper was to reviewrecent works on the feeding value of fibre inrabbits.

2. FIBRE DIGESTION

As in other animal species, fibre con-stituents can only be digested in rabbitsthrough microbial fermentation. The mainfermentative area is the caecum, which

accounts, including its contents, for about7 % of the whole body weight. Caecalmicroorganisms (10’°-10’2/g of caecal con-tents) ferment preferentially the fraction ofdigesta longer retained in the caecum: theshortest sized (< 0.3 mm) particles ofdigesta, the soluble fibre and some endoge-nous materials [1, 24]. As caecal contentsare emptied daily to produce soft faeces,caecal retention time is limited to around10 h [26]. Because of this short fermentationtime, the caecal cellulolytic activity is quitelow [29] (figure 1). However, the microfloraresident in the caecum is able to degradesignificant amounts of the more soluble non-

starch polysaccharides (pectins, pentosans,(3-glucans, oligosaccharides) [29]. Someenzymatic activity on these soluble con-stituents has also been observed in the smallintestine and in the stomach. The latter hasbeen related to the presence of intact softfaeces at the fundic area during several hoursafter meal [4, 13, 22].

Recent determinations of ileal and fae-cal digestion of pectin components (uronicacids, arabinose and galactose) of differentfeeds are presented in table I. According tothese results, average faecal digestibility ofpectin components (estimated as the sum ofuronic acids, arabinose and galactose) was72 %, ranging among feeds from 61 to 84 %.This variation was related to its proportionof fine particles, rather than to the chemi-cal composition of the cell walls. The effectmight be explained by differences in timeof fermentation among the ingredients stud-ied. In this way, Garcia [17] using severalfibrous sources observed a highly significant(P < 0.001) positive correlation (r = 0.89)between fermentation time and proportion offine particles (< 0.315 mm). Data in table Ialso showed that at least half of digestionof the main pectin components occurred

before digesta reached the caecum, asobserved previously by Gidenne [23] withdehydrated alfalfa.

Concentration of pectin components ofthese ingredients was 43 % (as % NSP) onaverage, but accounted for 68 % of the totalnon-starch polysaccharides (NSP) digested.Non-starch polysaccharides digestibility offibrous feedstuffs is then mainly related totheir content of pectin components, and alsoto their proportion of fine particles (r = 0.82;unpublished observations). However, dietarylevel of fibre has little effect on fibre

digestibility [26].

3. CONTRIBUTIONOF CAECOTROPHY TO PROTEINAND AMINO ACID INTAKE

Caecotrophy allows rabbits to utilisemicrobial cells contained in the soft faeces.The main nutrient recycled is protein, whichaccounts for around 16 % of total dailyintake [4--6, 13, 32]. Average contribution ofsoft faeces is slightly higher for some essen-tial amino acids as threonine, lysine andmethionine (25.4, 19.2 and 22.5 %, respec-

tively) [40]. These results agree with recentdata observed in our laboratory.

Fibre constituents are a main energysource for caecal microorganisms, so thatan effect both of level and type of dietaryfibre on the amount of microbial proteinrecycled daily throughout soft faeces, mightbe expected.

In a recent study, Garcia [ 17] comparedsemisynthetic diets, containing six sourcesof fibre varying widely in chemical com-position of cell walls and physical charac-teristics. Type of fibre affected significantly y(P < 0.001) recycling of microbial protein(MPr), which varied among sources from2.1 to 5.2 g/d. A stepwise regression analy-sis on these data, selected dietary concen-tration of pectin components (PC, %) andproportion of fine particles (< 0.315 mm,FP) as the best independent variables in thepredictive model. As described above, theseare also the variables that better explain theextension of NSP digestion. In a third stepthe model included a positive effect(P = 0.026) of the proportion of large parti-cles (> 1.25 mm, LP). A balance betweenfine and large particles increases caecalmotility and would lead to an adequateturnover rate of caecal contents and thereforeto a higher microbial efficiency.

Furthermore, an increase of dietary fibrelevel in balanced diets leads to an increase offeed intake and soft faeces production [4].However, both protein and amino acid con-centrations in soft faeces decrease parallely(unpublished observations), so that theamount of crude protein and amino acidsdaily recycled does not vary. As reviewed byGidenne et al. [26], caecal retention timedecreases when level of fibre in balanceddiets increases, which would impair rate ofmicrobial protein synthesis.

4. CAECAL pH

Acidity of caecal contents is frequentlydetermined in digestion studies. Both a highcaecal pH and low VFA concentrations havebeen related in in vitro studies to a higherproliferation of E. coli [39, 43].A general relationship among caecal pH,

basic buffering capacity of caecal contents(BBC, peq) and caecal VFA and ammonia(N-NH3) concentrations (mmol/L), has beenestablished by Garcia [20] using semisyn-thetic diets containing six different sourcesof fibre:

Volatile fatty acids are a main productof microbial fibre fermentation. Accordingto the review of Gidenne et al. [26], caecalVFA concentration averages 57.2 mmol/L

ranging from 31.8 to 88.5. As could beexpected, the lowest values have beenobtained using highly lignified sources offibre (e.g., grape byproducts, sunflowerhulls), whereas the highest correspond tofeeds with a higher pectin content (e.g., beetand citrus pulps, alfalfa hay). The sameauthors indicate that fermentation of endoge-nous materials and other dietary compo-nents might contribute significantly to VFAproduction. Some authors [21, 41 haverelated an increase of hindgut VFA pro-duction to an improvement of mucosalintegrity in human and other non-ruminantspecies. This effect might be explained sinceVFA, preferentially butyric acid, are usedby the colonocytes as the main energysource [42]. According to this, the inclusionof purified lignin in rabbit diets compared tocellulose or pectins, produced a reductionin both caecal VFA concentration and mus-cle layer thickness of caecum and colon [7].

Ammonia concentration is slight posi-tively related to caecal pH. This concentra-

tion increases when dietary protein is inexcess, but also when using both highly lig-nified byproducts (wheat straw, sunflowerhulls) [ 19] and pectin enriched byproducts(beet and citrus pulps) [ 13.]

The relation between caecal pH anddietary fibre characteristics has been estab-lished by Garcia [ 17] using stepwise regres-sion analysis. The model included again asindependent variables the proportions ofpectin components and fine particles(< 0.315 mm).

The effect of dietary level of fibre on cae-cal pH can be analysed from the results pre-sented in figure 2, obtained using a com-mon methodology. As a whole, dietary NDFcontent had no effect on this trait, whichagrees with the above mentioned lack ofinfluence on fibre digestion. However, cae-cal pH decreased in those works where the

increase of fibre was associated to a parallelincrease of sugar-beet pulp content [6, 15,16]. The contrary occurred when fibre wasmainly supplied with wheat straw [9].

5. RATE OF PASSAGE TIMEOF FEED

Retention time of digesta in the diges-tive tract is an important characteristic offeeds, as it is related to voluntary intake.This trait has been measured in a few workswith rabbits [20, 23], which showed that itcan vary from 16 to 54 h depending on thedietary source of fibre, averaging 18 h fornormal diets [12, 25]. According to Gidenne[25] and Garcia et al. [20] caecal retentiontime accounts for more than 60 % of thetotal retention time, so that a negative cor-relation exists between rate of passage of

digesta and weight of caecal contents (r =- 0.56, P < 0.001; n = 40) [20]. The latter ismuch easier to determine, and there is alarge number of studies where it has beenrelated to type of diet. As it depends on sam-pling time, results presented in figure 3 are

restricted to those using the same method-ology. From a stepwise regression analysis,the following equation was derived:

where CC is the weight of caecal contents(% body weight), NDF is on dry matterbasis, and ADLNpF is the degree of lignifi-cation of NDF (%).

From this equation it can be calculatedthat CC takes a minimal value for a 39.5 %of dietary NDF content on DM. An increaseof the weight of digestive tract contents withhigh levels of fibre is common to other ani-mal species. However, accumulation ofdigesta in the caecum in low fibre diets ischaracteristic of rabbits and it is related tothe positive effect of fibre on ileocaecalmotility [25]. Furthermore, the regressionanalysis shows a negative influence of lignin

concentration on CC at a given dietary fibrelevel.

Recently, an effect of both particle sizeand lignin concentration on CC has alsobeen reported by Nicodemus et al. [33, 35].Accumulation of digesta in the caecum offattening rabbits increased significantly inthe first study when the proportion of fineparticles (< 0.315 mm) increased above78 %, and in the second for dietary ADLconcentrations lower than 4.1 % DM.

6. ENERGY EFFICIENCY

As reviewed above, rabbits do not fer-ment the largest feed particles, and time offermentation of the shortest is restrictedbecause of the daily caecum emptying. As aconsequence, rate of passage of digesta isfaster than in other species, as ruminants,pigs or horses [44], and extension of fibredigestion is lower.

The effect of dietary fibre content onenergy digestibility (Ed) is shown in figure 4.The regression equation was [10]:

This equation underestimated Ed of dietscontaining low-lignified sources of fibre,where an increase of total mean retentionand fermentation time would be expected[13] and dietary fibre was potentially moredegradable. On the other hand, the equationoverestimated diets with a high proportion oflarge sized particles, as those containingwheat straw. These diets would lead to afaster rate of passage, and therefore to adecrease of the digestibility of other dietarycomponents.

An increase of dietary fibre content alsodecreased the efficiency of retention ofdigestible energy in growing animals (fig-ure 5). This effect is explained by a parallel

increase of fermentation losses (methaneand heat of fermentation). Results presentedin figure 5 also showed that this efficiencydecreased more when dietary fibre was sup-plied by enriched pectin feeds (as beet pulp),than when using less digestible sources offibre (as a mixture of wheat straw and alfalfahay).

7. INTAKE AND PERFORMANCE

According to previous paragraphs, bothexcessive and insufficient dietary fibre lev-els might affect rabbit’s performance. Highconcentration of dietary fibre is associated toa low energy efficiency and net energyintake. On the other hand, diets with lowfibre level are associated with higher totalmean retention time, and then lower intakecapability and higher risk of digestive dis-orders. Furthermore, diets with low diges-tible fibre level lead to a lower VFA con-centration and to a higher caecal pH, so thatmucosal integrity and control of hindgut

pathogens would be decreased. Low levelsof fibre could also imply a high concentra-tion of dietary starch, which has been relatedto a higher diarrhoea incidence, especially inearly weaned rabbits [2, 28].

The effect of substitution of fibre forstarch in rabbit doe diets has been studied byDe Blas et al. [11]. Five diets were formu-lated with increasing levels of NDF (from 28to 37 %, as fed) and decreasing levels of

starch (from 24 to 12 %, as fed). Fat wasadded in parallel to fibre to obtain isoener-getic diets (10.6 MJ DE/kg). Results are pre-sented in figure 6, which shows a curvilin-ear effect of dietary fibre level on severalproductive traits, as litter weight at wean-ing, number of rabbits weaned per cage andyear and feed efficiency. Optimal values forthese traits were reached for about 35.5 %NDF on DM (equivalent to 20 % of starchon DM), which is close to the level of dietaryNDF that minimises weight of caecal con-tents. The impairment of performanceobserved for the high fibrous diets might berelated to higher fermentation losses and toan insufficient uptake of glucose to meetpregnancy and lactation requirements. Thenegative effect of high starch diets wasmainly related to an increase in diarrhoeaincidence.

A curvilinear effect of substitution offibre for starch on fattening performancehas also been observed by De Bias et al. [9],Partridge et al. [38] and Garcia et al. [15,16]. Growth traits were optimised for adietary fibre concentration slightly higherthan for rabbit does (37.2 % NDF on DM).An increase of digestive disorders wasobserved below this level, whereas high fibrediets were related to a decrease of growthrate, feed efficiency and carcass yield.

Beside an optimal level of fibre, an ade-quate balance of digestible fibre and longsized particles is required to obtain a maxi-mal performance. Recent work by Nicode-mus et al. [34] has determined the effect ofparticle size on growth and lactation traits inisofibrous diets (36 % NDF on DM). Accord-ing to this study, a minimal proportion oflarge sized particles (> 0.315 mm) of around22 %, would be needed to get maximal

weight gain, carcass yield, milk production,and litter weight at weaning. These effectswere parallel to an increase of weight ofcaecal contents and a decrease of feedintake. A similar negative effect of an excessof short sized particles, has been reportedby Laplace and Lebas [27] and Morisse [31] ]when reducing particle size by fine grinding.Another work [36] has measured the effectof a variation of ADL concentration (from3.3 to 5.9 % on DM) on productive traits inisofibrous diets (43 % NDF on DM) con-taining a sufficient proportion of large par-ticles. Results showed that a minimal ADLcontent of 4.1 % on DM would be needed in

fattening rabbits to maximise feed intakeand growth rate. This work also indicatedthat lignin requirement for maximal intakeand performance of rabbit does might behigher (5.9 % ADL on DM) than in growingrabbits.

8. PRACTICAL RECOMMENDATIONS

Optimal fibre concentrations for breedingdoes, fattening rabbits and mixed feeds aregiven in table II. Values in parentheses indi-cate the range of diets for which only a slightdecrease with respect to the maximal per-formance can be expected. Acid detergentand crude fibre recommendations have beencalculated from NDF levels, for diets con-taining common sources of fibre (alfalfahay, wheat straw, wheat bran), and wouldnot be adequate for highly lignified byprod-ucts. Optimal fibre balance is ensured byestablishing a minimal concentration of largeparticles and lignin. Both restrictions shouldbe followed simultaneously, as some highlylignified byproducts (e.g., rice hulls orpaprika meal) can have an insufficient con-tent of large particles.

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