Fiber in Animal Nutrition

55 Inpact of fibers on digestion and performances also corresponds to a “ballast” value that limits the intake. Thus fiber deficient diets (<12% ADF) led to low nutrient intake and to low and variable growth performances. The replacement of starch by digestible fibers fractions (hemicelluloses or pectins), without changing the ADF level, did not greatly affect the intake. First results of the effects of SF on the intake behaviour suggest a 5 to 10% impairment (Jacquier et al., 2014). In return, when the dietary fibers level is very high (>25% ADF), the animal cannot increase its intake sufficiently to meet its energetic needs, thus leading to a lower growth rate, but without digestive problems. 4.4.2 Fiber digestion in the rabbit and hind gut microbial activity. Cell wall polysaccharides are hydrolysed and then fermented only by bacterial enzymes, whereas lignins are considered totally un- degradable. The fibers are an energy source derived from the activity of the microbiota. This takes place mainly in the caecum and proximal colon of the rabbit.The extent of the fiber digestion is rather different according to the fraction (table 26), and is ultimately de- termined by the time necessary for microbiota to hydrolyse and ferment the polysaccharides. Because the retention time in the caeco- colic segment of the rabbit is relatively short (8–12h), only the most rapidly fermentable polysaccharides are highly digested: such as uronic acids (pectins) or soluble fibers (table 26). Lignocellulose is degraded at a smaller extent ranging from5 to 25%. Obviously, the fiber digestion is lower than that of protein or starch, and high fiber levels correspond to a lower digestive efficiency. However, adding DF in place of starch moderately reduces the digestive efficiency of the rabbit. For the adult rabbit, fed a high fiber diet, the energy provided by the caecal VFA absorption could represent up to 50%of themaintenance energy. But, increasing the fiber intake (and lowering that of starch) either increases or has no effect on the fibrolytic activity and caecal VFA concentration (ranging from 80 to 100 mM), while a lower butyrate molar proportion is generally observed. The quality of fibers, particularly their fermentability, modulates the microbial activity. For instance, increasing the levels of pectins through the incorporation of beet pulps in a diet increases the VFA concentration in the caecum. Reversely, increasing the lignin level reduces the VFA concentration (Krieg et al., 2012). Besides, the morphology of the caecal mucosa seems modified by the quality of the fibers. For example, increasing the ratio lignin/cellulose reduces the crypt depth and width (Krieg et al., 2012). 4.4.3 Impact of fiber on performance and health of the growing rabbit Digestive pathology of the growing rabbit is the major cause of losses after weaning. Moreover, digestive disorders are responsible Table 26: Digestibility of some fiber fractions by the growing rabbit Dietary fiber criteria Mean range, % Neutral detergent fiber (aNDFom) 10 to 60 Cellulose (ADFom-ADL) 5 to 25 Hemicelluloses (aNDFom-ADF) 20 to 60 Water insoluble pectins 30 to 80 Uronic acids 30 to 85 Soluble fibre (TDF-aNDFom) 70 to 90 Lignin (ADL) -15 to 15 © ERLING Verlag