1. This study was conducted to investigate the effect of particle size of insoluble fibre on growth performance, apparent ileal digestibility (AID) and caecal microbial population in broiler chickens fed barley-containing diets. 2. The dietary treatments included: a barley-based diet (control, CTL) or test diets which contained high-fibre ingredients, either sunflower hulls (SFH), sugarcane bagasse (SB) or wheat bran (WB) ground through a 1.0 (fine) or 3.0 mm (coarse) screen that were added to the control diet at 30 g/kg. 3. For the entire experimental period, insoluble fibre inclusion improved ADG (P < 0.05) and FCR (P < 0.05) compared to the CTL group. Broilers fed SFH had higher (P < 0.05) ADG and better (P < 0.05) FCR than broilers fed SB. 4. Fibre inclusion increased the relative weight of breast and thigh and decreased relative weight of liver compared to the CTL group, but coarse grinding of the SB decreased relative weight of abdominal fat (P < 0.05). 5. The digestibility of nutrients increased with dietary inclusion of insoluble fibre compared to the CTL group. Coarse grinding of SFH increased AID of crude protein compared to the coarse grinding of WB or SB. Fine grinding of various types of fibre improved the AID of fat (as ether extract) and organic matter (P < 0.05). 6. Fine grinding of the WB decreased caecal populations of E. coli. The inclusion of SFH increased caecal populations of Lactobacillus spp. (P < 0.05). Coarse grinding of various types of fibre decreased the caecal population of coliforms (P < 0.05). 7. The inclusion of WB increased digesta viscosity in the ileum compared to samples from the SB and SFH groups (P < 0.05). Coarse grinding of various types of fibre decreased the digesta viscosity in the ileum (P < 0.05). 8. Overall, the data showed that dietary inclusion of insoluble fibre improved growth performance, increased AID of nutrients and decreased ileal viscosity in the birds fed diets containing barley.

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