Nutrient utilisation and growth performance of broiler chickens fed standard or moderately reduced dietary protein diets with and without β-mannanase supplementation

Use of reduced protein diets in broiler chicken production provides potential benefits for performance and environmental footprint of production. The effectiveness of β-mannanase supplementation in wheat and soy based standard protein (SP) and reduced protein (RP) diets was tested for growth performance, nutrient utilisation and selected intestinal gene expression of broiler chickens. In a 2×2 factorial arrangement of treatments, two main factors included dietary protein (standard and reduced protein) and β-mannanase supplementation (with or without). All diets contained phytase and carbohydrases (xylanase and glucanase). A total of 480 Ross 308 male off-sex day-old chickens were assigned to the four experimental diets in a 35-d study. Each diet was replicated 12 times with 10 birds per replicate. Using an additional 160 birds, separate apparent metabolizable energy (AME) and nutrient digestibility assays were undertaken for the 4 experimental diets from d 21 to 24 of age. Selected gene involved in gut integrity, inflammation and immune response were quantified using quantitative PCR assays. There was no interaction between β-mannanase and dietary protein for any of the studied parameters except ileal viscosity. Enzyme had no effect on feed intake but tended to increase body weight gain (BWG) from d 0 to 35 of age (P=0.079). Birds fed RP diet consumed more feed when assessed from d 0 to 35 of age (P=0.029). At the same time, β-mannanase tended to reduce feed conversion ratio independent of dietary protein (P=0.069). β-mannanase reduced ileal viscosity of the birds fed RP diet (P<0.001). Reducing dietary protein increased nitrogen retention, nitrogen digestibility coefficient and digestibility coefficients of 11 amino acids (P<0.001). β-mannanase significantly improved digestibility coefficients of nitrogen and Arg, Gly, Thr, Lys, and Ile (P<0.05). Dietary treatments had no effect on AME or gene expression of selected tight junction proteins, interleukin-10, interleukin-1β, mucin-2 and nuclear factor-kappa B. In conclusion, supplementation of β-mannanase tended to improve feed efficiency and increased nutrient digestibility of broilers fed wheat-based diets independent of a moderate reduction in dietary protein. Complementary mode of actions of β-mannanase for intestinal health requires further investigation.