In vitro screening of dihalomethanes as potential methane inhibitors in dairy cows

Volatile halogenated compounds are known to possess antimethanogenic properties with most in vivo studies focusing on trihalomethanes. However, concerns can be raised in relation to the use of these compounds as feed additives due to the transfer of halogenated metabolites into milk, which may pose a threat to consumer health. Hence, the objective of this in vitro study was to investigate the CH₄-mitigating potential of 5 dihalomethanes (DHM) to potentially reduce the transfer of halogenated compounds when used as feed additives in the future. The DHM, dibromomethane (BM), bromoiodomethane (BIM), dichloromethane (CM), chloroiodomethane (CIM), and diiodomethane (DIIM) were dissolved in 99% ethanol, reaching final solutions of 2 m<i>M</i>. In 2 separate runs, 0.1 mL of the solutions were incubated as triplicates in buffered rumen fluid together with 0.5 ± 0.02 g (± SD) of corn silage. Similarly, corn silage and buffered rumen fluid with the addition of 0.1 mL of 99% ethanol served as control and blank samples, respectively. Accumulated gas production was determined using the ANKOM^RF system equipped with airtight gasbags. Gas composition in collected gas was determined after 24 h, whereas the concentration of VFA in the fermented rumen fluid was determined after 48 h of incubation. All DHM except CM resulted in significant reductions and increases in CH₄ and H₂ production, respectively, compared with the standard feed incubated without DHM. Hence, BIM, BM, and DIIM reduced CH₄ production per gram of incubated DM by more than 90%, whereas CIM reduced CH₄ production by 58% per gram of incubated DM. Reductions in CH₄ production were accompanied by significant reductions in total VFA production, proportions of acetate in total VFA, and increases in proportions of propionate compared with the control treatment. In conclusion, BM, BIM, DIIM, and to a lesser extent CIM were efficient CH₄ inhibitors; however, the DHM also had negative effects on fermentation parameters.