| Summary: | Condensed tannins (CTs) are plant anti-herbivore compounds with antimicrobial activity that can be used in ruminant diets as ruminal microbiome manipulators. However, not all CTs from fodder legumes are bioactive due to their wide structural diversity. The aim of our study was to investigate the effect of 10 CT-containing plants (<i>Flemingia macrophylla</i>, <i>Leucaena</i><i> leucocephala</i>, <i>Stylosanthes guianensis</i>, <i>Gliricidia sepium</i>, <i>Cratylia argentea</i>, Cajanus cajan, Desmodium ovalifolium, <i>Macrotiloma axilare,</i><i> D. paniculatum</i>, and <i>Lespedeza procumbens</i>) on in vitro fermentation kinetics of Nelore beef cattle. Polyethylene glycol (PEG), a specific CT-binding agent, was added to neutralize condensed tannin. Tifton and alfalfa hay were used as controls lacking CT. The experimental layout included a randomized complete block with factorial design and four blocks. The data were subjected to analysis of variance followed by Duncan’s test to determine differences (<i>p </i>< 0.05) among treatment means. The addition of PEG in browse incubations resulted in increased gas production, fermentation rate, short-chain fatty acid (SCFA) and N-NH<sub>3</sub> release. Within our study, <i>Lespedeza procumbens</i>, <i>Desmodium paniculatum</i>, <i>Leucaena leucocephala</i>, <i>Desmodium ovalifolium</i>, and <i>Flemingia macrophylla</i> showed superior bioactivity compared to other species evaluated, suggesting a natural alternative for replacing ionophores to modify ruminal fermentation.<b> </b>Condensed tannins from <i>L. pocumbens</i>, <i>D. paniculatum</i>, <i>L. leucocephala</i>, <i>D. ovalifolium</i>, and <i>F. macrophylla </i>have the potential to modify rumen fermentation in beef cattle.
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