The effect of dietary saponin inclusion on in vivo methanogenesis, nutrient digestibility, and rumen fermentation of domesticated ruminants: A meta-analysis

Saponins have the potential to reduce methane emissions from ruminants. However, contrasting findings are often reported in in vivo studies. This study used a meta-analytical approach to evaluate the impact of dietary saponin inclusion on in vivo methane reduction, total tract digestibility, and rumen fermentation characteristics of three domesticated ruminant species, i.e., cattle, goats, and sheep. Following strict selection criteria, data from 18 articles were included in the database. Random-effects models were used to examine the mean differences (MD) between treatment (with saponins) and control (without saponins) groups. In case of significant effects and high heterogeneity ( p < 0.05, I 2 > 50%), the models were extended to mixed-effects (meta-regression) and subgroup analysis by including several explanatory variables; saponin-related data (i.e., dose, source, form), length of experiment, and experimental design. Results showed that saponin supplementation reduced methane emission per unit of dry matter (DM) intake (MD = -1.25 ± 0.35, p = 0.0004), metabolic body size (MD = -0.11 ± 0.027, p < 0.001), and digested organic matter (OM; MD = -1.91 ± 0.64, p = 0.003). Total tract digestibility of OM, crude protein, and fiber fractions was not affected by saponin inclusion. Ruminal parameters including ammonia, acetate to propionate ratio, and protozoa numbers were reduced, while saponin inclusion increased total short chain fatty acid concentrations and molar proportion of propionate. Additionally, subgroup analysis showed that saponin inclusion of up to 10 g/kg DM and of up to 25 days was effective in reducing methane without negatively affecting rumen fermentation.