ß-Mannanase Supplementation Enhances Fiber Digestion and Apparent Metabolizable Energy of Broilers Fed High Cassava Diets

A shortage and rising cost of major feed ingredients triggers more pressure on local poultry industries to maximize the use of alternative feed ingredients. Cassava is a potential alternative to corn. It is a good source of energy, with 60-70% of starch & 2-4 % of crude fiber (Staack et al, 2019). However, it has low protein (2%) with low methionine content (0.03%) (Morgan and Choct, 2016). When cassava meal is used at a high level, more SBM is needed to meet protein requirements. Soybean products and co-products are the major sources of β-mannans in the poultry diet. β-mannans in SBM are mainly associated with the hull (~5% β-mannans) and heat-resistant compounds that remain after the drying-toasting phase of processing soybeans (Hsiao et al., 2006). β-mannan content in SBM ranges from 0.7% DM in dehulled SBM (~48% CP) to 2.1% DM in 44% CP hulled SBM (Knudsen 1997; Knudsen, 2011). Exogenous β-mannanase enzyme can hydrolyze β-mannans and potentially eliminate the negative effect of these anti-nutritional factors (Vangroenweghe et al., 2021; Yaqoob et al., 2022). The present study was conducted to investigate the effects of β-mannanase supplementation on the nutrient digestibility of broilers fed cassavacorn-based diets with hulled and dehulled SBM.

A total of 256 Ross 308 broiler chicks were allocated to 4 treatments with 64 birds per treatment. All birds of each treatment were be raised together in one floor pen and fed the experimental diet until 16 day of age (DOA) then divided into 8 replications(8 birds/replicate) and raised in metabolic cages until 24 DOA. The experimental design was a 2 x 2 factorial in a completely randomized design with 2 SBM sources (hulled, 44% CP and dehulled, 48% CP) and 2 levels of β-mannanase supplementation (0 and 250 g/ton). Cassava meal was incorporated at the level of 30% in all starter and grower diets. The trial was run from 1 to 24 day of age (DOA) using 2-phase feeding. Starter diet was offered during 1-16 DOA, then changed to grower diets during 17-24 DOA. Titanium dioxide (0.3%) was added to all grower diets as an indigestible marker for nutrient digestibility study. All birds were kept in an environmentally controlled house. Feed in mash form and water were provided ad-libitum throughout the experimental period. Diet compositions and proximate analysis are shown in Table 1. The excreta were collected daily during the last three days of the balanced period (from day 21 to day 23). At 24 DOA, 2 birds per cage were selected for ileal digesta collection. Statistical analysis of all data was conducted using Analysis of Variance. Duncan’s new multiple range test of SAS University Edition (2018) was applied to compare treatment mean comparisons. Differences where P < 0.05 were considered significant, while differences where P < 0.01 were considered highly significant.

The effect of SBM source and β-mannanase supplementation in broilers fed a high level of cassava diets on growth performance is presented in Table 2. During 17-24 DOA, there was no significant interaction between SBM source and β-mannanase supplementation on final weight, body weight gain, feed intake, feed conversion ratio, mortality and culling of broilers (P > 0.05). There was no significance effect of either main effect (SBM source and β-mannanase supplementation) on overall growth performance of broilers (P > 0.05). This may be due to the period of enzyme supplementation not being long enough to elucidate the positive effect on the growth performance of broilers. Although no significant difference was observed, the lowest FCR was found in broilers fed the diet containing dehulled SBM supplemented with β-mannanase which was 4 points lower than birds fed dehulled SBM with no β-mannanase and hulled SBM with βmannanase, and 8 points lower than that of hulled SBM with no β-mannanase.

There was no significant interaction of SBM source and β-mannanase supplementation on ileal protein digestibility (P > 0.05). However, there was a significant interaction of SBM source and β-mannanase supplementation on ileal crude fiber digestibility and AME (P < 0.05). The supplementation of β-mannanase improved ileal crude fiber digestibility of birds fed the diet containing dehulled SBM but not of birds fed hulled SBM (P < 0.01). The supplementation of βmannanase significantly improved AME of broilers fed dehulled SBM when compared to that of hulled SBM with no β-mannanase, but not the other treatments (P > 0.05).

The supplementation of β-mannanase improved ileal crude fiber digestibility of birds fed the diet containing dehulled SBM which led to an increase in AME thus improving FCR. It is likely that β-mannanase increased the nutrient digestibility of broilers due to enzymes breaking down mannan, thereby sparing energy (Kiarie et al., 2022) and releasing the nutrient contents (Bedford, 2000; Chang et al 2020). β-mannanase also can reduce intestinal viscosity, increasing digesta flow, ameliorating the mixing of the substrate with the digestive enzymes, enhancing nutrient contact with the intestinal wall, and thereby improving nutrient utilization and contributing to high energy values (Shastak et al., 2014; Jiang et al., 2022). Dehulled SBM has lower β-mannans than hulled SBM which creates less of an antinutritional effect for broilers. The supplementation of β-mannanase, therefore, improved nutrient utilisation and feed conversion ratio of broilers.