Apparent metabolizable energy, ileal digestibility and bone quality of broiler chickens fed wheat-based diets supplemented with carbohydrases

I. INTRODUCTION

Microbial enzymes are now routinely used in broiler chicken diets to reduce the effects of antinutritive factors (ANF) in feed and/or improve the digestion of nutrients (Bedford, 2011). There is a suite of enzymes that target ANF such as non-starch polysaccharides (NSP) and phytic acid as well as products which improve the digestion of nutrients such as protein and minerals. The major cereals used in poultry diets are wheat, maize, sorghum and barley. The objective of this study was to investigate the effectiveness of two NSPases and a phytase in wheat-based diets fed to broiler chickens. The impact of the products was assessed with respect to digestibility of energy and nutrients such as starch and protein (amino acids) and parameters related to bone quality.

II. MATERIALS AND METHODS

A total of 648 male Ross 308 broiler chickens was randomly assigned to a 3 (none, low (30 mg/kg) and superdose (300 mg/kg) phytase levels) × 2 (none and optimum (100 mg/kg) xylanase levels) × 2 (none and optimum (100 mg/kg) β-glucanase levels) full factorial study in a completely randomized design. Each of the 12 treatments was replicated 6 times, with 9 birds per replicate. The diets were fed ad libitum from 0 to 35 days in 3 phases – starter (1-10 d), grower (11-24 d) and finisher (25-35 d). AME was determined using an indigestible marker (TiO2) and collection of excreta samples from trays underneath each replicate between 20 and 23 days of age. AME was then calculated as: AME = GEi - [GEo× (Ti/To)], where GEi is gross energy (MJ/kg) in feed; GEo is the gross energy (MJ/kg) in excreta, Ti is the concentration of titanium in the diets and To is the concentration of titanium in the excreta. At 35 days, two birds per pen were randomly selected, slaughtered by cervical dislocation; the right drumstick was collected and frozen for later assessment of bone breaking strength and concentrations of minerals in the tibia. Following defrosting of the tibias, adherent muscle, tissue, cartilage caps and fibula were removed manually. Breaking strength is the force required to break the bone and was measured in the range of 0 to 500 N using a Lloyd Tensile Testing machine. The entire bones were then dried and ashed, and these samples then analysed for mineral contents. A general linear model procedure was used to analyze the collected data (Minitab, 2013).

III. RESULTS AND DISCUSSION

Interactions (P < 0.01) between phytase, xylanase and β-glucanase with regards to gross energy (GE) and apparent metabolizable energy contents are shown in (Table 1).

Table 1 - Apparent metabolizable energy and ileal digestibility of protein, gross energy and starch of birds fed diets supplemented with different levels of Phytase, Xylanase and β-glucanase at 24 d of age.¹

Ileal amino acid digestibility was improved (P < 0.01) markedly and in a dosedependent manner by phytase and by β-glucanase to a much lesser extent (Table 2).

Table 2 - Amino acid digestibility of birds on diets supplemented with different levels of Phytase, Xylanase and β-glucanase at 24 d of age.¹

Table 2 - Amino acid digestibility of birds on diets supplemented with different levels of Phytase, Xylanase and β-glucanase at 24 d of age.1

Bone breaking strength for tibia increased (P < 0.001) with the low dose of supplemental phytase but there was no further increase with the higher dose (Table 3). Supplementation with β-glucanase also improved (P < 0.04) bone breaking strength.

Table 3 - Breaking strength and mineral contents (%) of tibia bone of chicks at 35d of age when fed wheat-based diets supplemented with Phytase, Xylanase and β-glucanase¹