*Sargassum fusiforme*, a brown algae, offers fiber and minerals for ruminant diets. In vitro tests at varying doses (1-10%) showed increased total volatile fatty acid (VFA) concentrations at 1% and 10%, alongside decreased gas, methane, and carbon dioxide emissions at 3 and 24 hours of incubation at 10%. In situ trials compared *S. fusiforme* degradation to timothy hay. ¹H NMR spectroscopy revealed mannitol, guanidoacetate, and ethylene glycol as major metabolites. While arsenic was present, it was within allowable limits. *S. fusiforme* shows potential as a renewable ruminant feed ingredient, offering nutritional and environmental benefits.

You Young Choi, Shin Ja Lee, Ye Jun Lee, Hyun Sang Kim, Jun Sik Eom, Sam Churl Kim, Eun Tae Kim, Sung Sill Lee