The effect of fumaric acid on ruminant enteric methane emission and ruminal volatile fatty acids concentration: A meta-analysis

Methane emitted by ruminants represents an energy loss from feed intake and contributes to global warming. Fumaric acid (FA), a key intermediate in rumen metabolism, acts as an alternative electron acceptor and offers a potential strategy to reduce methane production. This meta-analysis systematically evaluated the effects of FA supplementation on enteric methane emissions and rumen fermentation in ruminants. Thirteen peer-reviewed studies met the inclusion criteria, contributing 22 effect sizes from 13 studies: six on cattle (dairy and beef cattle), seven on small ruminants (sheep and goats). Effect sizes were calculated as mean difference (MD) for methane yield (g/kg dry matter intake (DMI)), relative mean difference (RMD) for methane production (g/day) and DMI, (kg/day), and standardized mean difference (SMD) for volatile fatty acids. A multilevel meta-analysis model was used to account for study-level variation. Fumaric acid supplementation had no effect on DMI (P = 0.25; RMD = -2.75) but significantly reduced methane production (P = 0.005; RMD = -19.21). Meta-regression showed that increase in FA (g/kg DMI) decreases in methane production by 0.272% (P = 0.02). Methane production was significant in small ruminants (P = 0.002; RMD = -24.67) but not in cattle (P = 0.52; RMD = -7.03). The effectiveness of FA in reducing methane production was not (P > 0.05) affected by variations in dietary forage, concentrate, or neutral detergent fiber (NDF) content in FA-supplemented animals. Methane yield decreased (P = 0.001; MD = -1.954) in FA supplemented animals, while the efficacy of FA was not influenced (P > 0.05) by diet composition (forage %, concentrate %, NDF %). Fumaric acid had no effect on ruminal acetate (P = 0.49; SMD = -0.299), but increased propionate (P = 0.01; SMD = 0.970). In summary, FA supplementation did not affect DMI but reduced methane production and yield in small ruminants. While in cattle, FA supplementation may have limited impact on methane emission.