The rising interest in algae necessitates exploring their potential applications, including animal feed. In Korea, feed costs constitute a significant portion (up to 50%) of livestock production expenses. *Sargassum fusiforme* (*S. fusiforme*), a type of brown algae, is abundantly available in Korea. In 2018, production yielded approximately 54,624 tons, with 5% comprising residues that often end up polluting the marine environment. This study explores the potential of using *S. fusiforme* as an alternative feed supplement, addressing both cost-reduction and environmental concerns. Algae are fast-growing organisms rich in minerals, carbohydrates (60-70% of total content), and proteins. Previous research highlights the prebiotic and other beneficial effects of algae-derived molecules such as polysaccharides, phlorotannins, minerals, and omega-3 fatty acids. *S. fusiforme*, being edible and containing various polysaccharides (fucoidan, alginate, laminarin, mannitol), could enhance ruminant growth and rumen fermentation. Rumen fermentation efficiency is crucial, aiming to reduce methane production (which accounts for up to 15% energy loss) and improve overall productivity. This study aimed to investigate the physicochemical and nutritional profiles of *S. fusiforme*, determine its degradation parameters, and assess its impact on gas emissions in ruminants.

Several studies have explored the potential of seaweed as an alternative feed source for livestock. Makkar et al. (2016) provided a comprehensive review of seaweed for livestock diets, highlighting their nutritional value and potential benefits. Maia et al. (2016) discussed the role of seaweeds in manipulating rumen fermentation and methane production. Tayyab et al. (2016) investigated the ruminal and intestinal protein degradability of various seaweed species in dairy cows. Other research has focused on the nutrient content of macrophytes in relation to eutrophication and biogas production (Bucholc et al., 2014) and the use of seaweed in pig nutrition (Corino et al., 2019). Studies on *Sargassum* species as tropical alternatives for goat feeding (Casas-Valdez et al., 2006) and the impact of seaweed extracts on rumen fermentation, methanogenesis, and microbial populations (Lee et al., 2019) also exist. These studies provide a foundation for understanding the potential of *S. fusiforme* as a ruminant feed, highlighting the need to analyze its nutritional composition, digestibility, and effect on methane emissions.

The study involved *in vitro* and *in situ* experiments. *S. fusiforme* was collected, cleaned, and dried before being ground for analysis. Chemical composition (dry matter, crude protein, crude fiber, ether extract, crude ash, neutral detergent fiber, acid detergent fiber, lignin, non-fiber carbohydrates, nitrogen-free extract, minerals, and heavy metals) was determined using standard methods (AOAC, Van Soest et al.). For the *in vitro* trial, rumen fluid from two fistulated Hanwoo cows was used as inoculum. *S. fusiforme* was tested at four doses (0%, 1%, 3%, 5%, and 10%) in combination with timothy hay as the basal substrate. Samples were incubated for 72 hours, with measurements taken at various time points (3, 6, 9, 12, 24, 48, and 72 hours) for pH, dry matter digestibility, microbial growth rate, volatile fatty acid (VFA) concentrations, gas production (total gas, methane, carbon dioxide), and protozoa counts. The *in situ* trial assessed dry matter disappearance (DMD) of *S. fusiforme* and timothy hay using nylon bags incubated in the rumen for 6, 12, 24, 48, and 72 hours. ¹H nuclear magnetic resonance (¹H-NMR) spectroscopy identified and quantified metabolites in *S. fusiforme*. Statistical analysis involved one-way ANOVA, Tukey's multiple range test, and polynomial contrasts to analyze the data. Pearson's correlation coefficient assessed the relationship between methane and VFA concentrations. All experimental protocols were approved by the Animal Care and Use Committee of Gyeongsang National University.

*S. fusiforme* had high moisture content, minerals (crude ash), nitrogen-free extract, and non-fiber carbohydrates, but lower neutral detergent fiber and acid detergent fiber compared to timothy hay. Macro minerals (salinity, chloride, sodium) and micro minerals (iron, zinc, manganese) were abundant, while arsenic was present but within the allowable limit. *In vitro*, 1% and 10% *S. fusiforme* increased total VFA concentrations at 12 and 24 hours. At the same dose, gas emissions decreased significantly at 3 and 24 hours. Protozoa numbers decreased linearly with increasing *S. fusiforme* concentration at 24 hours. *In situ*, *S. fusiforme* showed faster initial dry matter disappearance compared to timothy hay, but the insoluble potentially degradable fraction was lower. ¹H-NMR revealed mannitol, guanidoacetate, and ethylene glycol as major metabolites. Correlation analysis showed that methane was positively correlated with total VFA and propionate concentrations. The reduction in gas emission was time- and dose-dependent, with minimal effects at 3 and 24 hours but a rapid increase after 48 hours of incubation.

The study's findings indicate that *S. fusiforme* could serve as a valuable alternative feed ingredient for ruminants. The high mineral content contributes to the overall nutritional value, potentially supplementing essential elements in animal diets. The increased VFA production, particularly propionate (which is less associated with methane production), and the reduction in methane emissions at certain time points suggest an improvement in fermentation efficiency. The observed reduction in protozoa numbers might be due to the presence of phlorotannins or other bioactive compounds in *S. fusiforme*. However, the time- and dose-dependent effect on gas emissions warrants further investigation. The higher initial digestibility of *S. fusiforme* compared to timothy hay might be attributed to its higher NFC content and smaller particle size post-processing, while the lower insoluble potentially degradable fraction could be due to the presence of lignin and polysaccharides. The presence of arsenic, although within safe limits, warrants careful monitoring and further research on its speciation and potential impact on animal health and the environment.

This study demonstrates the potential of *S. fusiforme* as a renewable and sustainable feed ingredient for ruminants, offering both nutritional and environmental benefits. The findings suggest that *S. fusiforme* could improve rumen fermentation efficiency and potentially reduce methane emissions, although its effects appear to be time- and dose-dependent. Further research should focus on *in vivo* trials to confirm these findings, investigate fatty acid composition and their effects on rumen microbiota, and assess the long-term impact of *S. fusiforme* on animal health and productivity. Speciation of arsenic and its potential impact should also be investigated.

The study primarily utilized *in vitro* and *in situ* methods, limiting the direct applicability to real-world ruminant feeding scenarios. The use of a sealed system in *in vitro* incubation might not fully reflect the dynamics of gas exchange in a living animal. Further research using *in vivo* studies with animals is essential to validate the findings and assess the effects under real-world conditions. Additionally, the particle size and hardness of the *S. fusiforme* were not analyzed, which might influence its degradation characteristics.