Comparison of ruminal microbiome responses of temperate and tropically-adapted beef steers to dietary supplementation with monensin

Ross L Thorn¹Lauryn E Coffman¹Madeline M Rabalais Bass¹Charles R Long¹Ronald D Randel¹Thomas H Welsh Jr^2*Anil C Somenahally^1*

• ¹AgriLife Research and Extension Center at Overton, Texas A and M University, Overton, United States
• ²Texas A&M University, Department of Animal Science, College Station, Texas, United States

The interactive responses of cattle genetics and the rumen microbiome (G×M) govern variations in feed efficiency and methane emissions, which subsequently impact cattle productivity and their environmental footprint. Modulating the rumen microbiome can be done through dietary supplements like the antimicrobial ionophore monensin, which offer a pathway to favorably alter metabolic outcomes. However, limited data on breed-specific microbiome shift in response to dietary changes restricts understanding of G×M impacts on fermentation and nutrient utilization. The objective of the study was to determine the effect of a monensin fed diet on the ruminal microbiome and short-chain fatty acid profile of temperate and tropically-adapted cattle breeds. Ten steers each of Angus, Brahman, and F1 (Angus × Brahman) breed types were fed forage ± a monensin ionophore supplement. Ruminal fluid samples were collected during four 21-day periods (1 equilibrium, 3 treatment). At the conclusion of each period, short-chain fatty acids (SCFAs) were analyzed via gas chromatography. Microbiome profiles were analyzed through DNA extraction, qPCR assays, and sequencing to evaluate GxM interactions. Short-chain fatty acid analysis showed a decrease in the acetate:propionate ratio (P=0.001) across all breed types under monensin treatment. However, breed type variations were evident, as total SCFA concentrations were lower only in the Brahman steers that consumed monensin. qPCR assays indicated significantly lower ruminal methanogen content (mcrA gene; P<0.01) and a reduced methanogen:prokaryote ratio (MPR; P<0.001) in monensin-fed steers compared to the control. A treatment-by-breed interaction was observed for the fungi:prokaryote ratio (FBR; P=0.003), with only F1 steers on the monensin diet showing a lower FBR than those on the control diet. PERMANOVA and beta diversity analyses demonstrated significant differences in ruminal microbiome structure between control and monensin-treated groups for both prokaryotic and fungal communities. Several ASVs within genera Faecalimonas, Streptococcus, and Prevotella, showed variable abundance among breeds in response to monensin treatment, confirming the influence of G×M interactions on microbiome structure. This study established the potential of diet supplementation with an antimicrobial ionophore (monensin) to modulate rumen microbiome structure, alter metabolic profiles, and reduce methanogens while emphasizing the need for breed-specific dietary strategies due to the influence of (G×M) interactions.