Divergent residual feed intake is associated with distinct fecal microbiota in early fattening Hanwoo steers

This study compared the fecal microbiota of Hanwoo steers exhibiting divergent residual feed intake (RFI) during the early fattening stage. In total, 63 Hanwoo steers, aged 16 months and maintained under identical dietary and environmental conditions, were evaluated for RFI based on individual feed intake and growth performance. From this cohort, high-RFI (inefficient; n = 5) and low-RFI (efficient; n = 6) steers representing the extreme 10% of the RFI distribution were selected for fecal microbiota analysis. Fresh fecal samples were collected and subjected to metataxonomic analysis using the Illumina MiSeq platform and QIIME2. At the phylum level, no significant differences were observed between the two steer groups. However, at the genus level, the cellulolytic <italic>Cellulosilyticum </italic>exhibited a significantly higher abundance in the low-RFI group, suggesting enhanced hindgut cellulose degradation in more efficient animals. Alpha-diversity metrics displayed no significant differences between the two steer groups. Beta-diversity analysis based on unweighted UniFrac distance revealed significant differences in fecal microbiota structure between the two groups, whereas weighted UniFrac analysis showed no significant differences, suggesting that while the two steer groups shared dominant microbiota members, they had distinct low-abundance, phylogenetically distinct taxa. Functional predictions using PICRUSt2 revealed enrichment of the ATP-binding cassette (ABC)-2 type transport system in the low-RFI group, implying greater microbial potential for nutrient uptake and utilization. Collectively, these findings suggest that feed efficiency in Hanwoo steers during early fattening is associated with specific fecal microbial taxa and functional traits rather than differences in overall diversity or dominant taxa.