Digesta characteristiscs and performance of African catfish (Clarias gariepinus) fed cereal grains that differ in viscosity

Inclusion of cereal grains in fish diets may induce changes in digesta characteristics with possible consequences for fish performance. Especially viscosity characteristics of the dietary ingredients may induce these changes. The aim of this study was to investigate effects of dietary cereal grains (maize, wheat, barley, rye) differing in viscosity on digesta characteristics (viscosity, dry matter and volatile fatty acids), nutrient digestibility and performance (feed intake and growth) in African catfish. We hypothesized that increased dietary viscosity will induce changes in digesta characteristics that are associated with reduced nutrient digestibility and growth performance. Five experimental diets were formulated by adding grains to a basal diet to obtain a range of dietary viscosities, increasing in the order of: 1) 400 g kg-1 maize (M-40); 2) 400 g kg-1 wheat (W-40); 3) 400 g kg-1 barley (B-40); 4) 200 g kg-1 maize and 200 g kg-1 rye (M-20/R-20); 5) 400 g kg-1 rye (R-40). The diets were assigned to 12 tanks with 50 fish each. Feed intake during satiation feeding did not differ between dietary groups (P = 0.46). Growth performance differed between dietary treatments (P = 0.04), but showed no decreasing trend with increasing dietary viscosity. Digesta viscosity in the proximal intestine differed between dietary treatments (P < 0.001) and increased with increasing dietary viscosity. Dry matter content of the distal digesta differed significantly between groups (P < 0.001) and was lowest for fish fed the most viscous diets (M-20/R-20 and R-40). Total volatile fatty acid concentrations in the distal intestine did not significantly differ between dietary groups (P = 0.48), but were numerically highest for fish fed the R-40 diet. Digestibility of protein and fat differed significantly between dietary groups (protein: P = 0.05; fat: P = 0.003). Fish fed the R-40 diet had the lowest digestibility for protein and fat and also the lowest plasma cholesterol concentrations. Digestibility of total non-starch polysaccharides (NSP) varied considerably between dietary groups (P = 0.03). Fish fed the W-40 diet had the lowest NSP digestibility (4.0%), which suggests that the NSP fraction of wheat was hardly utilized by the intestinal microbiota. Moderately positive NSP digestibilities for the M-40 (48.4%) and M-20/R-20 (55.5%) groups indicated that African catfish could utilize NSP derived from maize reasonably well. In conclusion, dietary inclusion of viscous grains increases digesta viscosity in African catfish. Intestinal fermentation activity, digesta dry matter content and nutrient digestibility seem to be affected beyond a certain threshold for viscosity. Changes in digesta characteristics are not accompanied by reduced fish performance.