Rumen and Cecum Microbial Dynamics Following Narasin Inclusion in Nellore Cattle Diets

Johnny Souza¹Leandro A F Silva²Daniel M Casali²Janaina C S M Souza³Luke A Wolfe⁴Joseph H Skarlupka⁴Ibrahim Zuniga-Chaves⁴Andrew Steinberger⁴Courtney L Deblois⁴Andrew J Scheftgen⁴Ana Laura Januário Lelis²Tiago Leiva⁵Murilo Chuba Rodrigues⁵José Paulo Romano Barroso⁵Pedro Lasmar⁵Garret Suen⁴Danilo Domingues Millen^1*

• ¹Universidade Estadual Paulista Julio de Mesquita Filho - Campus de Jaboticabal, Jaboticabal, Brazil
• ²Universidade Estadual Paulista Julio de Mesquita Filho - Campus de Botucatu, Botucatu, Brazil
• ³Universidade de Sao Paulo Campus Fernando Costa, Pirassununga, Brazil
• ⁴University of Wisconsin-Madison, Madison, United States
• ⁵Elanco Animal Health, Sao Paulo, Brazil

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 (P = 0.182) or the cecum (P = 0.298); however, Chao richness was reduced in the rumen of narasin-fed cattle (P = 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 (P > 0.198). At the phylum level, narasin supplementation was associated with a reduction in Firmicutes and a concomitant increase in Bacteroidetes in the rumen. In contrast, Firmicutes predominated in the cecum, and narasin significantly increased the relative abundance of this phylum, particularly members of the order Clostridiales (P = 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.