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

The 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 × 2factorial 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 genes 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.