Tannins in ruminant feeding: effects of tannin type and dosage on rumen fermentation, production performance, health, and sustainability.

Tannins are plant-derived polyphenolic compounds of variable molecular weights that have attracted considerable attention as natural feed additives in ruminant nutrition. Their ability to modulate rumen microbial ecology, improve nutrient utilization, mitigate enteric methane (CH4) emissions, and promote animal health has positioned them as potential tools for improved, sustainable ruminant production systems. This review critically evaluates and synthesizes published evidence on the effects of tannins on rumen fermentation dynamics, nutrient digestibility, hematological and biochemical parameters, production performance, and overall health status of ruminants. A systematic literature search was conducted using Scopus, Web of Science, ScienceDirect, Google Scholar, and PubMed, targeting peer-reviewed English-language studies published between January 2000 and September 2025. Search terms were organized using Boolean operators across tannin chemistry and type, ruminant species, and production-, environmental-, or health-related outcomes. Studies were included if they reported in vitro or in vivo responses of ruminants to tannins, while non-ruminant studies, conference abstracts, and non-English publications were excluded to ensure comprehensive comparability. Collectively, available evidence indicates that hydrolyzable tannins generally exert more consistent positive effects on feed intake, nutrient digestibility, and CH4 mitigation than condensed tannins, although responses remain highly dependent on tannin source, chemical structure, diet composition, dietary inclusion level, and the extent of animal adaptation. Low to moderate inclusion levels (generally <3% of dietary dry matter [DM]) were associated with neutral to beneficial effects on feed intake, rumen fermentation characteristics, nitrogen utilization, and animal performance. In contrast, higher rates (>5% of dietary DM) frequently impair digestibility, depress feed intake, compromise nitrogen efficiency, and reduce productive performance, indicating dose-dependent responses. Besides nutritional modulation, other ancillary benefits of tannins include reduced internal parasite burden and decreased incidence of ruminal bloat. Positive changes in hematological and biochemical indices further suggest potential improvements in physiological response and health status at moderate levels, although high rates may induce adverse metabolic alterations. In summary, tannins represent a promising nutritional strategy for improving the sustainability of ruminant production systems; however, their successful application depends on careful selection of tannin type, strategic dosage optimization, and feeding strategy. Future research should prioritize defining optimal inclusion thresholds, elucidating rumen microbiome adaptation mechanisms at the molecular level, and assessing long-term production and health outcomes under practical production conditions.