Effects of different feed additives alone or in combination on broiler performance, gut microflora and ileal histology.

1. The objectives of this experiment were to investigate the effects of different xylanases, alone or in combination with different organic acid and oligosaccharide sources, on bird performance, gut microflora and ileal histology. 2. Birds were given a diet based on a commercial formulation, which was split into 8 batches. Batch 1 contained the antibiotic growth promoter Avilamycin and acted as the positive control. To batch 2 the enzyme Allzyme PT was added and to batch 3 Allzyme PT was added with the organic acid and oligosaccharide mixture Avimos. To batch 4, Allzyme PT was added with the oligosaccharide mixture Biomos. To batch 5, yeast extract 2012 was added with the organic acid mixture Gustor and the enzyme xylanase XP20. To batch 6, yeast extract 2012 and feed acidifier Gustor were added as before, with the enzyme Avizyme 1300. Batches 7 and 8 both acted as negative experimental controls, with no added growth promoters. 3. A total of 64 birds were housed in individual wire cages in each of three consecutive experimental replicates (24 birds/treatment). Birds were fed ad libitum from 7 to 28 d and a 7-d excreta collection was carried out to determine apparent metabolisable energy (AME) content. 4. At 28 d, the birds were killed and viscosity of jejunal digesta supernatant and gizzard weight were determined. Samples were taken from the crop, ileum and caecum and analysed for viable presumptive lactic acid bacteria and coliforms. The overall microbial flora was determined using denaturing gradient gel electrophoresis of 16S ribosomal DNA followed by DNA sequence analysis in order to assign amplicons to a bacterial species. Ileal sections were also collected for histological analysis. 5. Total live weight gain (12%) and gain:feed (9%) were significantly improved for all diets containing additives, compared to the negative control diets. All diets containing xylanases gave significantly lower in vivo viscosity values than the positive and negative controls. Diet treatment significantly affected viable coliform numbers in the ileum and also viable lactobacilli in the ileum and caecum. A substantial proportion of the bacteria present in the GI tract (40%) belong to unknown species. No effects of diet treatment on histological measurements were observed in this study. 6. All the additive combinations studied were at least as effective as the antibiotic growth promoter and the results for Allzyme PT suggest that xylanase alone is as effective as any of the combinations studied.