Kebreab, E.; Pressman, E. M.; Ramirez-Agudelo, J. F.; Bannink, A.; van Gastelen, S.; Dijkstra, J.

Methane (CH4) emissions from ruminants contribute significantly to global GHG emissions. Bromoform (CHBr3)-containing feed ingredients, such as Asparagopsis seaweed, have emerged as promising tools to reduce enteric CH4 emissions. This meta-analysis quantitatively assessed the effects of CHBr3-containing seaweeds and synthetic CHBr3-based additives on CH4 production (g/day), CH4 yield (g/kg DMI), and DMI, as well as CH4 intensity (g/kg product) and production (milk or daily gain in dairy and beef cattle, respectively). Data were collected from 14 studies, with 39 treatment mean differences for CH4 production, comprising both beef and dairy cattle fed CHBr3-containing ingredients, while accounting for dose, DMI, and diet composition. The random-effects model\'s estimates revealed that at an average CHBr3 dose of {approx}28.3 mg/kg DM, CH4 production was reduced by 47.3%, CH4 yield by 43.3%, and CH4 intensity by 39.0%, with increases in CHBr3 dose resulting in larger efficacy in mitigating CH4 emissions. The efficacy of CHBr3 was influenced by cattle type, with greater mitigation effects in beef cattle than dairy catle, and by dietary composition, with greater reductions observed in diets higher in starch, whereas higher NDF levels attenuate its effect. At the average CHBr3 dose, estimated DMI was significantly reduced by 6.45% and 3.26% in dairy and beef cattle, respectively. The significant reduction in DMI did translate into a significant effect on milk yield (-4.60%) at average CHBr3 dose. Carrier type (oil vs. biomass), measurement technique and cattle type influenced the results. Oil carrier potentially leading to more pronounced reductions, particularly for CH4 intensity, respiration chambers yielded significantly greater CH4 reductions compared to other methods, and beef cattle showed stronger mitigation effects than dairy cattle. This study highlights the CH4 mitigating potential of CHBr3-containing feed ingredients, providing predictive models to optimize CH4 reduction strategies under diverse production conditions. Future research should address long-term effects, dietary optimization, and practical implementation.