Role of tannin-rich plants and brown algae in reducing enteric methane emissions in ruminant livestock

Abstract

Cattle production is a cornerstone of global food security, but it represents a significant source of greenhouse gas emissions, particularly methane (CH₄) resulting from enteric fermentation. This process, mediated by the ruminal microbiota, enables the conversion of fibrous compounds into usable energy; however, it involves considerable energy losses in the form of CH₄. In this context, plant secondary metabolites, particularly tannins, have been proposed as an effective strategy to mitigate these emissions without compromising productivity.This review assesses the effect of hydrolysable tannins (HT), condensed tannins (CT) and phlorotannins (PT) on ruminal fermentation and CH₄ production. The evidence shows that CTs and PTs exhibit more consistent responses, due to their ability to selectively modulate the microbiota, reduce protein degradation and improve nitrogen use efficiency. In contrast, HTs exhibit greater variability and, in some cases, negative effects on digestibility. Of particular note is the potential of phlorotannins, found in brown algae, which have demonstrated significant reductions in CH₄ in in vitro studies, in some cases outperforming terrestrial sources. However, the efficacy of these compounds depends on factors such as dose, source, diet and experimental conditions, which explains the inconsistencies between studies. However, the efficacy of these compounds depends on factors such as dose, source, diet and experimental conditions, which explains the inconsistencies between studies. At moderate levels, tannins can reduce CH₄ without affecting digestibility, whilst high doses tend to impair fermentation. In conclusion, tannins represent a promising tool for more sustainable livestock systems. However, the high variability of  data and the limited quantitative evidence under in vivo conditions highlight the need to standardise their use and further investigate their productive evaluation.