摘要A promising strategy to support broiler health and performance in a sustainable way is the enhancement of microbial fibre fermentation in broilers.This fermentation mainly occurs in the caeca,but the actual particle size range that allows caecal influx has not yet been described.This study aimed to understand the physical limitations of caecal influx as a function of broiler age by using both solid and soluble markers.In the first trial,the caecal filter mechanism was studied by microscopically visualising the caecal entrance and measuring caecal lobe development and digesta particle size as a function of age(d 8-36)for 44 broilers(Ross 308)receiving a conventional wheat-based diet.In two consecutive trials,microcrystalline cellulose beads(100-700 μm)and a combination of fluorescent polystyrene beads(5-30 μm)and chromium-ethylenediamine tetraacetic acid(Cr-EDTA)were administered to 176 and 189 broilers,respectively,at different ages(d 8-36).Results showed that the actual caecal entrance diameter is significantly reduced due to a dense villi network acting as a filter for digesta inflow.This explains the size gap between the average digesta particle size(D50)of the ileum(451-322 μm)and caeca(5-19 μm),and the outer diameter of the caecal entrance(2000-4000 μm)on d 8 to 36.In contrast to the caecal D50,cellulose beads of 700 μm already entered the caeca at 8 d of age,even though the general caecal influx of digesta particles larger than 100 μm seemed very limited.The caecal influx of the markers further exhibited large individual variation among birds.A maximum of 13.2％(d 9)and 4.3％(d 29)of the total administered soluble marker(Cr-EDTA)was detected in the caeca,5 h after bolus administration.Both solid and soluble markers showed a larger concentration in the caeca at a young age compared to older ages(P＜0.01),possibly related to the limited caecal functioning early in life.These findings highlight the importance of carefully selecting the physical properties of fibres to be added as a function of age to further improve caecal fibre fermentation in broilers.