Rumen and cecum microbial dynamics following narasin inclusion in Nellore cattle diets

This study investigated the effects of narasin supplementation on the ruminal and cecal bacterial communities of feedlot Nellore cattle. We hypothesized that narasin would selectively modulate microbial populations in distinct gastrointestinal compartments without causing broad-scale disruption of overall community diversity. Sixty-four Nellore bulls (393 ± 24 kg) were assigned to a completely randomized block design and fed finishing diets containing either 0 or 20 ppm of narasin for 112 days. Rumen and cecal contents were collected at slaughter and analyzed using 16S rRNA gene sequencing to characterize bacterial community structure and composition. Overall, the rumen exhibited greater bacterial diversity and richness than the cecum, regardless of dietary treatment. Narasin supplementation did not affect Shannon diversity in either the rumen (<i>p</i> = 0.182) or the cecum (<i>p</i> = 0.298); however, Chao richness was reduced in the rumen of narasin-fed cattle (<i>p</i> = 0.028). Beta-diversity analyses based on Bray-Curtis and Jaccard dissimilarities revealed no significant differences in overall community structure between treatments in either compartment (<i>p</i> > 0.198). At the phylum level, narasin supplementation was associated with a reduction in <i>Firmicutes</i> and a concomitant increase in <i>Bacteroidetes</i> in the rumen. In contrast, <i>Firmicutes</i> predominated in the cecum, and narasin significantly increased the relative abundance of this phylum, particularly members of the order <i>Clostridiales</i> (<i>p</i> = 0.05). In conclusion, narasin exerts selective effects on specific bacterial populations rather than inducing widespread shifts in microbial diversity. These results provide novel insights into how narasin modulates microbial ecology in both the rumen and the understudied cecum, highlighting compartment-specific responses that may contribute to improved feed efficiency in beef cattle.