Metagenomic characterization of rumen microbial communities in cattle fed tannin-rich browse diets

Introduction

The rumen is, by any reasonable measure, the most productive anaerobic fermenter on the planet a half-litre-to-150-litre bioreactor that breaks down structural carbohydrates no mammalian enzyme can touch and converts them into volatile fatty acids the host animal uses for energy [1] . Its efficiency depends entirely on the microbial consortia that inhabit it, and anything that alters those consortia alters the animal's capacity to extract nutrition from its feed [2] .In semi-arid rangelands of East Africa, cattle rely heavily on browse species shrubs and trees whose leaves are available even when grass has dried out [3] . Many of these browse plants are rich in condensed tannins, polyphenolic polymers that bind to dietary protein and fibre, slowing ruminal degradation [4] . The nutritional consequences of tannin intake are doubleedged: moderate levels can reduce ruminal protein degradation and increase the flow of bypass protein to the small intestine, improving nitrogen efficiency, while excessive levels depress overall dry-matter intake and fibre digestion [5] . What happens to the microbial Community itself when tannins enter the rumen has been explored mainly through culture-based methods and terminal restriction fragment length polymorphism, both of which capture only a fraction of the true diversity [6,7] . Amplicon sequencing of the 16S rRNA gene offers a far more complete picture, and its cost has fallen enough to make it feasible for livestockExperiments Conducted outside well-funded laboratories [8] . A handful of sequencing-based research projects have examined tannin effects in sheep and goats, but equivalent data for cattle especially tropical crossbreeds grazing African browse are almost nonexistent [9,10] . An additional motivation is methane. Enteric methane from ruminants contributes roughly 14.5% of global anthropogenic greenhouse-gas emissions, and the search for dietary interventions that reduce methanogenesis without harming animal performance is intense [11] . Condensed tannins have been shown to lower methane output in vitro, presumably by suppressing methanogenic archaea or by diverting hydrogen toward alternative sinks [12] . Whether these effects translate to measurable microbiome shifts under in vivo conditions in tropical cattle fed local browse is the question this research addressed [13,14] .

Material and Methods

Material

Forty-five crossbred Boran steers (mean initial body weight 247±18 kg, age 2.5-3.5 years) were sourced from the research herd of the Addis Ababa Research Institute of Livestock Sciences, Addis Ababa, Ethiopia. Animals were blocked by weight into 15 blocks of three, and within each block randomly assigned to one of three dietary treatments: (a) a basal total mixed ration with no added tannin (control, 0% CT on a dry-matter basis), (b) the same ration supplemented with dried Acacia tortilis leaf meal to provide 3% CT (moderate), or (c) supplemented to provide 6% CT (high). The basal ration contained 11.8% crude protein and 62.1% TDN; tannin inclusion replaced an equivalent weight of grass hay. The feeding trial ran from 1 June to 31 July 2023 after a 14-day adaptation period. Animals were housed in individual pens with ad libitum water. The protocol was approved by the Institutional Animal Research Ethics Committee (Protocol AALRI-AE-2023-009, approved 3 May 2023).

Extraction Protocol

Rumen fluid (approximately 200 mL per animal) was collected four hours after morning feeding on day 60 using a flexible orogastric tube connected to a vacuum pump. The first 50 mL was discarded to minimise saliva contamination. Samples were strained through four layers of cheesecloth, pH Was recorded immediately, and 2 mL aliquots were snapfrozen in liquid nitrogen within 10 minutes of collection and stored at -80 °C. Total metagenomic DNA was extracted using a bead-beating protocol with a commercial kit optimised for high-humic-acid samples. Cell lysis was performed with 0.1 mm zirconia beads at 5,500 rpm for 45 seconds, followed by silica-column purification. DNA concentration was measured by fluorometric assay, and purity was checked spectrophotometrically (260/280 ratio 1.8-2.0 accepted).

Methods

The V3-V4 hypervariable region of the 16S rRNA gene was amplified using primers 341F/806R with Illumina overhang adapters. PCR was run for 28 cycles at an annealing temperature of 55 °C. Amplicons were purified, indexed with Nextera XT adapters, normalised, and pooled for sequencing on an Illumina MiSeq instrument (2 × 300 bp paired-end mode) at a commercial facility in Addis Ababa during September 2023. Raw reads were processed through the DADA2 pipeline within QIIME2 v2023.5. Forward and reverse reads were truncated at 280 and 250 bp, respectively, chimeric sequences were removed, and taxonomy was assigned against the SILVA v138.1 database at 99% similarity. Alpha diversity metrics (Shannon index, observed ASVs, Faith's phylogenetic diversity) were compared across treatment groups using Kruskal-Wallis tests with Dunn's post hoc correction. Beta diversity was assessed by weighted and unweighted UniFrac distances, visualised by principal coordinates analysis, and tested with PERMANOVA (999 permutations). Differentially abundant taxa were identified by LEfSe (LDA score > 2.0) and confirmed with ANCOM-BC at an adjusted p-value threshold of 0.05.

Quality Control

Library quality was monitored at three stages. First, amplicon size was verified on a microfluidic electrophoresis chip (expected range 450-550 bp including adapters). Second, library concentration was quantified by qPCR targeting adapter sequences, and equimolar pooling was adjusted accordingly. Third, sequencing run quality was evaluated by per-base Q-scores (median Q30 > 75% required) and PhiX spike-in error rate (< 1.5%). Post-sequencing, rarefaction curves were generated to confirm that sampling depth (20,000 reads per sample) captured ≥ 95% of observed ASV richness in all groups. Two samples from the moderate-tannin group that fell below the rarefaction threshold were excluded from downstream analysis. The phylum-level shifts were driven mainly by contrasting responses at the genus level. Fibre-degrading specialists like Prevotella and Fibrobacter lost ground with rising tannin levels, while tannin-tolerant groups Ruminococcus and several Lachnospiraceae expanded to fill the vacated niche.

Results

Methanobrevibacter, the dominant rumen archaeon, showed a 50% decline from control to the high-tannin group.

Comprehensive interpretation

The dose-dependent decline in alpha diversity and the clear treatment clustering in beta diversity space together confirm that condensed tannins from Acacia tortilis restructure the rumen microbiome in a predictable, concentration-linked manner. The suppression of Fibrobacter a genus specialised in cellulose degradation helps explain the reduced fibre digestibility often reported in tannin-feeding trials, while the expansion of Ruminococcus and Lachnospiraceae members may represent a compensatory shift toward organisms better equipped to metabolise tannin-protein complexes. The 28.4% drop in Methanobrevibacter abundance in the high-tannin group has direct implications for methane mitigation strategies, though whether this translates into proportionate reductions in enteric methane emission would require respiration chamber measurements.

Discussion

The linear decline in Shannon diversity across tannin levels mirrors results from sheep fed quebracho tannin extract reported by Vasta and colleagues [6] , though the magnitude here was steeper a drop of 1.57 index units from control to high-tannin versus roughly 0.9 in their ovine trial. The difference may reflect the higher tannin dose (6% vs 4% of DM) or the fact that tropical Boran crosses harbour a microbial baseline adapted to low-tannin grass diets rather than browse [3] . Prevotella's sensitivity to condensed tannins is consistent with in vitro evidence that tannin-protein complexes coat bacterial cell surfaces and impair substrate adhesion [4] . Since Prevotella contributes substantially to hemicellulose fermentation and propionate production, its decline could shift the volatile fatty acid profile toward acetate dominance a nutritional outcome worth measuring in future work.

Fibrobacter's reduction is similarly expected, given its obligate requirement for direct fibre attachment [7] .

The rise in Lachnospiraceae is less straightforward. Some members of this family possess tannin-acyl-hydrolase genes that enable them to cleave gallotannin ester bonds [10] . Whether the Lachnospiraceae ASVs enriched here carry equivalent hydrolytic capacity for condensed tannins which have carbon-carbon interflavan bonds rather than ester linkages remains to be confirmed by shotgun metagenomics. The archaeal findings are encouraging from a greenhouse-gas perspective. A 28.4% reduction in Methanobrevibacter relative abundance, if it tracks with proportional methane output reductions, would represent a practically relevant mitigation effect achievable through a locally available feed ingredient [11,12] .

Conclusion

Cattle in semi-arid East African systems inevitably consume tannin-containing browse, yet until now the consequences for their rumen microbial communities had been characterised only with low-resolution techniques. This research provides the first 16S amplicon-sequencing dataset linking dietary condensed tannin concentration to rumen microbiome composition in tropical crossbred Boran cattle under controlled conditions. Three findings stand out. Microbial diversity dropped in a dose-dependent manner, with the high-tannin group harbouring roughly 35% fewer amplicon sequence variants than controls. Fibre-degrading genera Prevotella and Fibrobacter were the main losers, while tannin-tolerant Ruminococcus and Lachnospiraceae expanded. And methanogenic archaea declined meaningfully at the 6% tannin level, opening an avenue for enteric methane reduction using locally sourced browse.

For livestock advisors working in tannin-rich rangeland systems, the practical recommendation is to target tannin intake at or below 3% of dietary dry matter to preserve fibredegrading capacity while still capturing some bypass-protein benefits. At 6%, the microbial disruption is substantial enough to risk reduced feed digestibility, and the methane-mitigation advantage may be offset by lower overall animal productivity. Blending high-tannin browse with tannin-free forages or supplementing polyethylene glycol to deactivate excess tannin are both strategies that could fine-tune the balance. Looking ahead, shotgun metagenomic sequencing would clarify the functional gene repertoire of the enriched taxa particularly whether the Lachnospiraceae that expand under tannin pressure carry genuine tannin-degrading enzymes.

Linking microbiome shifts to measured volatile fatty acid profiles, fibre digestibility coefficients, and real-time methane emissions via portable head-chamber systems would bridge the gap between community composition and animal-level outcomes. Expanding the work to include multiple browse species Acacia nilotica, Dichrostachys cinerea, Faidherbia albida would test whether the microbiome response is specific to the tannin source or generalisable across the condensed tannin class.

Acknowledgement

Acknowledgements Funding SourcesThis research was funded through an internal competitive grant from the Addis Ababa Research Institute of Livestock Sciences allocated to rumen ecology projects. No external commercial funding was received. Institutional SupportThe authors thank the animal nutrition unit of the Addis Ababa Research Institute of Livestock Sciences for feed preparation and animal management during the trial. Contributions not qualifying for authorshipMr. Dawit Assefa assisted with rumen fluid collection, and Ms. Meron Tadesse performed DNA extractions. Their contributions are gratefully acknowledged.

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