In this review, the methods used to measure fibre and soluble fibre fractions are briefly presented and the effects of the soluble fibre content in diets for growing rabbits reviewed by a meta-analysis of studies available in literature, with the aim of elucidating the relationships with other dietary nutrients. Soluble fibre was assumed as the difference between total dietary fibre (TDF) and neutral detergent fibre (NDF), as it is simple to obtain and has been measured in numerous studies. Dietary soluble fibre content affects the digestive utilisation of soluble and insoluble fibre fractions and its increase is associated with reduced mortality in growing rabbits affected by epizootic rabbit enteropathy. This effect could be attributed to the high fermentability of soluble fibre, the consequent changes in the intestinal microbiota and an enhanced gut barrier function just after weaning. A supply of 12-14% of soluble fibre (as-fed) is recommended in diets for post-weaning and growing rabbits containing around 30% NDF and 18% acid detergent fibre. The positive effects of increasing dietary soluble fibre are linked to the use of sugar beet pulp as primary source of soluble fibre and should be confirmed with other sources of soluble fibre.

The effect of neutral detergent-soluble fiber level on gut barrier function and intestinal microbiota was examined in weaned rabbits. A control diet (AH) containing 103 g of neutral detergent-soluble fiber/ kg of DM included alfalfa hay as main source of fiber. Another diet (B-AP) was formulated by replacing half of the alfalfa hay with a mixture of beet and apple pulp resulting in 131 g of soluble fiber/kg of DM. A third diet (OH) was obtained by substituting half of the alfalfa hay with a mix of oat hulls and a soybean protein concentrate and contained 79 g of soluble fiber/kg of DM. Rabbits weaned at 25 d and slaughtered at 35 d were used to determine ileal digestibility, jejunal morphology, sucrase activity, lamina propria lymphocytes, and intestinal microbiota. Suckling 35-d-old rabbits were used to assess mucosa morphology. Mortality (from weaning to 63 d of age) was also determined. Villous height of the jejunal mucosa increased with soluble fiber (P = 0.001). Rabbits fed with the greatest level of soluble fiber (BA-P diet) showed the highest villous height/ crypt depth ratio (8.14; P = 0.001), sucrase specific activity (8,671 mumol of glucose/g of protein; P = 0.019), and the greatest ileal starch digestibility (96.8%; P = 0.002). The opposite effects were observed in rabbits fed decreased levels of soluble fiber (AH and OH diets; 4.70, 5,848 mumol of glucose/g of protein, as average, respectively). The lowest ileal starch digestibility was detected for animals fed OH diet (93.2%). Suckling rabbits of the same age showed a lower villous height/crypt depth ratio (6.70) compared with the B-AP diet group, but this ratio was higher than the AH or OH diet groups. Lower levels of soluble fiber tended (P = 0.074) to increase the cellular immune response (CD8+ lymphocytes). Diet affected IL-2 production (CD25+, P = 0.029; CD5+CD25+, P = 0.057), with no clear relationship between soluble fiber and IL-2. The intestinal microbiota biodiversity was not affected by diets (P >/= 0.38). Rabbits fed the B-AP and AH diets had a reduced cecal frequency of detection compatible with Campylobacter spp. (20.3 vs. 37.8, P = 0.074), and Clostridium perfringens (4.3 vs. 17.6%, P = 0.047), compared with the OH diet group. Moreover, the mortality rates decreased from 14.4 (OH diet) to 5.1% (B-AP diet) with the increased presence of soluble fiber in the diet. In conclusion, increased levels of dietary soluble fiber improve mucosal integrity and functionality.

This chapter gives a general and brief description of the morphological and functional characteristics of the digestive system of the rabbit that may be important for understanding several digestive processes. The changes in these characteristics from the time of weaning until attainment of maturity are discussed.

The influence of fiber source on fiber digestion and mean retention time was investigated. Six fibrous feedstuffs with wide differences in chemical composition and particle size were selected: paprika meal, olive leaves, alfalfa hay, soybean hulls, sodium hydroxide-treated barley straw, and sunflower hulls. Six diets were formulated to contain one of these ingredients as the sole source of fiber. To avoid nutrient imbalances, fiber sources were supplemented with different proportions of a concentrate free of fiber based on soy protein isolate, wheat flour, lard, and a vitamin and mineral mix to obtain diets containing at least 18.5% CP and 5% starch. Fecal apparent digestibility of nonstarch polysaccharides (NSPd) and its monomers, NDF, NDF-ADL, and ADF-ADL, were determined using four New Zealand White x California growing rabbits per diet. Total, ileorectal, and cecal mean retention times (tMRT, i-rMRT, and cMRT, respectively) were determined for diets based on paprika meal, olive leaves, soybean hulls, and sunflower hulls in 16 does (four per diet) fitted with T-cannulas at the terminal ileum. In both trials, DMI was negatively correlated with the proportion of fine particles (FP: < .315 mm) and positively correlated with the proportion of large particles (LP: > 1.25 mm) (P < .01). Stepwise regression analysis showed that FP was the dietary characteristic best related to digestibilities of NSP, uronic acids, glucose and NDF, tMRT, and cMRT (P < .001), showing a positive correlation with these variables. In all these cases, this procedure selected the proportion of large particles as a second variable in the model. Degree of lignification of NDF, considering lignin as the difference between ADL and acid detergent cutin, was only included as the third variable for the model of NDF digestibility. Digestibility of NSP was positively correlated with those of NDF, NDF-ADL, and ADF-ADL (r = .82, .87 and .85, respectively, P < .001); the latter was also highly correlated with the digestibility of the glucose included in the NSP fraction (r = .86; P < .001). Cecal mean retention time accounted for 63% of average tMRT, for most of the variability in tMRT (r = .99; P < .001), and was positively related to NSPd (r = .89; P < .001). From these results, we conclude that particle size is a major factor affecting fiber digestion efficiency, rate of passage, and feed intake in rabbits.

The effect of source of carbohydrate on gut histology, digestion efficiency, and growth performance in early-weaned (25 d) rabbits at the starter period (25 to 39 d) was investigated. Six diets were factorially arranged to study the effect of partial substitution of starch (0, 25, or 50%) by lactose at two levels of fiber (30 or 36% NDF). Diets were formulated to meet or exceed essential nutrient requirements of growing rabbits. A feeding trial was conducted to measure the effect of treatments on growth performance in 252 rabbits that were fed the experimental diets in the starter period and thereafter received a common feed until 60 d of age. Fecal apparent digestibility was determined at 35 d of age in nine animals per diet. The four diets with extreme lactose content were used to determine ileal apparent digestibility of starch and lactose (nine replicates per diet), weights of stomach and cecum, stomach pH, cecal fermentation traits, amylase and disaccharidase activities (10 animals per diet), and jejunal morphology (six animals per diet). Weaning increased (P < 0.001) amylase activity by 59% but decreased (P < 0.001) maltase, sucrase, and lactase activities by 30, 48, and 72%, in parallel with a reduction of villus height by 19%. Dietary NDF level did not affect either jejunal morphology or sucrase and lactase activities but increased amylase (P = 0.05) and maltase (P < 0.001) activities by 22 and 92%, respectively. Substitution of starch by lactose had no effect on jejunal morphology or enzymatic activity. Ileal lactose and starch digestibility were not affected by dietary NDF or lactose level and averaged 73.8 and 90.8%, respectively. Substitution of starch by fiber and lactose affected ileal flux of starch plus lactose (by -0.5 and +1.7 g/d) and cecal pH (by +2.1 and -2.8%, respectively). Fecal NDF digestibility was relatively low (23.1% on average) and was not affected by treatments, whereas that of lactose and starch was almost complete. An increase of dietary NDF level led to an impairment of ADG and feed efficiency in the starter (P < 0.002) and in the overall (P < 0.03) fattening period. Substitution of starch by lactose linearly decreased (P < 0.001) feed efficiency in the starter period and linearly increased (P < 0.001) diarrhea incidence in the fattening period. The results indicate that digestive capability of early-weaned rabbits is limited and should be taken into account to establish optimal levels and sources of carbohydrates in the starter diet.

The influence of fiber source on fiber digestion in rabbits was investigated. Six fibrous feedstuffs with wide differences in chemical composition and particle size were selected: paprika meal, olive leaves, alfalfa hay, soybean hulls, sodium hydroxide-treated barley straw, and sunflower hulls. Six diets were formulated to contain one of these ingredients as the sole source of fiber. To avoid nutrient imbalances, fiber sources were supplemented with different proportions of a fiber-free concentrate, based on soy protein isolate, wheat flour, lard, and a vitamin and mineral mix, to obtain diets containing at least 3% nitrogen and 5% starch. Daily soft feces excretion, and its NDF, and total and microbial nitrogen content were determined in 60 fattening rabbits (10 per diet). Seven days after the last cecotrophy control, the same animals were used to determine weight of stomach, cecum and their contents, and cecal fermentation traits (pH, VFA and ammonia concentrations, and buffer properties of cecal contents). Stepwise regression analysis showed a positive effect (P < .001) on soft feces excretion, total and microbial nitrogen concentrations in soft feces, cecal acidity, and total VFA in the cecum of dietary pectic constituents (2.9, 3.5, 2.5, .9, and 6.6%) and proportion of fine particles (< .315 mm) (1.8, .9, 1.3, .15, and .9%) per each increment of one percentage unit of the independent variables. Proportion of fine particles also increased weight of cecal contents (P < .001). Soft feces excretion and weight of stomach and of its contents increased (P < .001) by 5.2, 2.8, and 10.2% per each percentage unit increment of proportion of large particles (> 1.25 mm). Degree of lignification of NDF decreased total nitrogen concentration in soft feces and cecal VFA concentration (P < .001). Source of fiber affected cecal pH not only by its influence on the cecal concentrations of the final products of fermentation, but also through its effect on the pH of dry cecal contents (P < .001). The latter was negatively correlated with dietary proportion of fine particles, degree of lignification of NDF, and base-buffering capacity of dry cecal contents (r = -.52, -.37, and -.49, respectively). From these results, we conclude that pectic constituent concentration, degree of lignification of NDF, and particle size are the variables that best characterize the influence of the source of fiber on soft feces excretion and cecal fermentation traits in rabbits.

One hundred thirty New Zealand White x California rabbits were used to study the influence of the source of dietary fiber on several digestive criteria. Five diets were formulated to provide 10% crude fiber (CF) on a DM basis. Sixty percent of this dietary fiber was supplied by alfalfa hay, citrus pulp, beet pulp, grape marc, or rice hulls in the diets. Weights of cecal contents, cecal ammonia concentration, and molar proportion of acetic acid in diets including pulps were higher and cecal levels of DM, CP, and molar proportion of butyric acid were lower than those of the alfalfa diet. Cecal ammonia, VFA, and CF concentrations of the grape marc diet were the lowest of all the diets studied; cecal CF level and molar proportion of acetic acid were significantly higher and CP level and molar proportion of butyric acid were lower in rice hull than in alfalfa diets. Diet had no influence on daily soft feces excretion (10 g DM/d), but N contribution of soft feces to N intake was higher in the diet with citrus pulp (18.7%) than in the other diets (12%). Mean retention time (R) was measured using fuchsin-stained feed and was found to be higher (21.3 h) in the rice hull diet and lower (9.3 h) in the grape marc diet than in the other diets. When coprophagy was prevented, R decreased by 0 to 7 h. In conclusion, the use of byproducts to substitute for traditional sources of fiber in rabbit diets influenced the retention time of the digesta in some segments of the gut and thus altered several digestive criteria.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of dietary supplementation with 1% l-glutamine and a combination of 1% l-glutamine and 0.5% l-arginine on intestinal health was examined in weaned rabbits. A basal diet was formulated to meet nutrient recommendations. Another 2 diets were formulated by adding 1% (as-fed basis) Gln or a mixture of 1% (as-fed basis) Gln + 0.5% (as-fed basis) Arg (Gln-Arg) to the basal diet. In Exp. 1, a total of 357 rabbits were blocked by litter and assigned at random to the experimental diet to determine mortality (119 per diet) and growth performance (35 per diet; from weaning at 25 to 56 d of age). Rabbits were fed the experimental diets for a 2-wk period and thereafter received a commercial diet. Rabbits weaned at 25 d (blocked by litter and assigned at random to diets) were slaughtered at 35 d and used to determine apparent ileal digestibility of DM, CP, and AA (Exp. 2, a total of 60 rabbits), intestinal morphology, N-aminopeptidase and myeloperoxidase intestinal activity, the expression of PPARgamma at the ileum and kidney, serum immunoglobulin in healthy and sick rabbits (Exp. 3, a total of 24 rabbits), and ileal and cecal microbial composition by PCR-RFLP (Exp. 4, a total of 45 rabbits). Dietary treatment did not affect ADG, ADFI, or G:F, during the entire fattening period. Supplementation with Gln reduced mortality during the first 2 wk and the whole fattening period from 18.5 to 8.4% (P = 0.023) and from 31.9 to 20.2% (P = 0.039), respectively, whereas no effect was detected for Arg supplementation. Among all the variables studied, the reduction on mortality due to Gln was related to a reduced intestinal colonization (Eimeria lesions) and changes on microbial ecosystem in the ileum and cecum, reducing the frequency of detection of Clostridium spp. (from 86.7 to 33.3%, P = 0.003) at the ileum, and Helicobacter spp. at the ileum (from 86.7 to 46.7%, P = 0.003) and at the cecum (from 86.7 to 46.7, P = 0.028), whereas no effect was detected for Arg supplementation. In conclusion, 1% l-Gln supplementation to postweaned rabbit diets decreased fattening mortality and modified the intestinal microbiota (although no consistent effects were observed on mucosal histology or inflammatory and systemic immune response). Diets containing a combination of 1% Gln and 0.5% Arg were of little additional benefit.