Antibiotic resistance is regarded as one of the most serious threats to human health worldwide. The rapid increase in resistance rates has been attributed to the extensive use of antibiotics since they became commercially available. The use of antibiotics as growth promotors has been banned in numerous regions for this reason. Mannan-rich fraction (MRF) has been reported to show similar growth-promoting effects to antibiotics. We investigated the effect of MRF on the microbial community, resistome and metabolic pathways within the caecum of commercial broilers at two different timepoints within the growth of the broiler, day 27 and day 34. The data indicated an overall increase in health and economic gain for the producer with the addition of MRF to the diet of the broilers. The only significant difference across the microbial composition of the samples was in the richness of the microbial communities across all samples. While all samples harboured resistance genes conferring resistance to the same classes of antibiotics, there was significant variation in the antimicrobial resistance gene richness across time and treatment and across combinations of time and treatment. The taxa with positive correlation comprised Bacilli and Clostridia. The negative correlation taxa were also dominated by Bacilli, specifically the Streptococcus genera. The KEGG-pathway analysis identified an age-related change in the metabolism pathway abundances of the caecal microflora. We suggest that the MRF-related increases in health and weight gain in the broilers may be associated with changes in the metabolism of the microbiomes rather than the microbial composition. The resistome variations across samples were correlated with specific genera. These data may be used to further enhance the development of feed supplements to reduce the presence of antibiotic resistance genes (ARGs) within poultry. While the ARGs of greatest concern to human or animal health were not detected in this study, it has identified the potential to reduce the presence of ARGs by the increase in specific genera.