Milled and cryogenic micronised wheat bran inclusions stimulate fibre degradation and fermentation differently throughout the broiler’s gastrointestinal tract

Inclusion of fibre fractions in broiler feed can increase microbial short-chain fatty acid (SCFA) production upon fermentation, enhancing gut health and performance. The main site of fermentation is the caeca, and extensive particle size reduction of fibre fractions (< 100 µm) may increase their fermentability and influx into the caeca, extending the associated benefits. Coarser fibre fractions (> 400 µm) improve fermentation parameters as well, but are expected to act in the foregut (precaecal) as they seem too large to pass through the restricted caecal entrance. The mechanism behind the effect of different fibre particle sizes and their inclusion level on fibre degradation and fermentation is not yet fully understood. The role of fibre particle size along the gastrointestinal tract (GIT) and the possible increase of their effect with increasing inclusion level was studied by comparing 0.2% (L02) and 2% (L2) inclusion of coarse milled (452 µm, WB452) and cryogenically milled (27 µm, WB27) wheat bran in male Ross 308 broilers (570, d 1 to d 37) using a 2 × 2 factorial design. GIT parts, digesta and plasma samples were collected to measure GIT development, fibre degradation and fermentation, and gut health at d 10 and d 37. WB452 increased precaecal fibre degradation and fermentation through gizzard stimulation based on the decrease in gizzard pH and increase in ileal arabinoxylan digestion and ileal butyric acid content at d 37. Both bran sizes tended to increase the relative abundance of different SCFA-producing bacterial groups in the caeca at d 10, although this was not related to differences in caecal SCFA content. Inclusion level L02 had a stronger positive effect on caecal functioning and gut health through improving caecal lobe weight, rhamnose content and microbial composition and decreasing plasma d-lactate levels compared to L2. These results suggest that both fine and coarse fibre inclusions, particularly at a low inclusion level, can stimulate fibre fermentation through different mechanisms, highly depending on the GIT section and broiler age.