Effect of fumaric acid in combination with Asparagopsis taxiformis or nitrate on in vitro gas production, pH, and redox potential

[Display omitted] •Fumaric acid had no effect on hydrogen production when methane was inhibited with A. taxiformis.•Hydrogen was not detectable when methane was inhibited with nitrate.•Only minor effects were observed on redox potential.•pH was lowest for treatments including A. taxiformis. Reduction in enteric methane (CH4) emissions from cattle can be achieved through use of feed additives, which often results in increased emission of hydrogen (H2). The objective of this study was to investigate in vitro effects of a known hydrogen sink, fumaric acid, in combination with either of 2 methane inhibitors, the macroalga Asparagopsis taxiformis or nitrate, on CH4 and H2 production, feed degradability, pH, and redox potential. A corn silage (0.5 g; control) was incubated in buffered rumen fluid with the addition of 0.025 g of nitrate (Nit), 0.025 g of dried A. taxiformis (Asp), 0.025 g of nitrate + 0.025 g of fumaric acid (Nit+Fum), or 0.025 g of dried A. taxiformis + 0.025 g of fumaric acid (Asp+Fum). Accumulated gas production was determined using the AnkomRF system equipped with airtight gasbags. There were 9 replicates per treatment with 3 replicates per treatment stopped after 24, 36, and 48 h of incubation. The amount of undegraded feed was determined by filtration. Gas composition was determined by gas chromatography. Degradable dry matter, degradable organic matter, pH, redox potential, and gas production data were analyzed using a mixed model. Asp and Asp+Fum reduced CH4 production by 98% or greater at all incubation times, whereas Nit and Nit+Fum reduced CH4 production (mL of CH4/g of dry matter) by 52% to 63% compared with the control. Hydrogen was only detectable in gas from Asp and Asp+Fum treatments, with no difference in H2 production between the 2 treatments. The treatments had only minor effects on redox potential in the fermented rumen fluid, and pH was lowest for treatments including A. taxiformis. In conclusion, both A. taxiformis and nitrate reduced CH4 production. Fumaric acid in combination with A. taxiformis did not reduce H2 production, and treatments including nitrate did not result in any detectable levels of H2. Future dose-response in vitro studies will contribute to investigating the potential of fumaric acid as a hydrogen sink during CH4 mitigation.