Enteric methane emissions from ruminants constitute a large proportion of agricultural greenhouse gas emissions, particularly in New Zealand. The recent increase in enteric methane emissions has driven the development of innovative strategies for mitigating these emissions. Red seaweed from the genus Asparagopsis has demonstrated elimination of enteric methane due to the presence of the active anti-methanogenic component, bromoform. Spatial variation in bromoform content for Asparagopsis armata throughout the North Island, New Zealand, was quantified to determine the region that produces the highest concentration of bromoform. Alongside Asparagopsis, the New Zealand red seaweed species Bonnemaisonia hamifera, Delisea compressa, Plocamium sp., Vidalia colensoi, and identified aquaculture-target seaweed species, Ecklonia radiata, and Ulva sp. B, were investigated as ruminant feed additives to reduce enteric methane emissions. Polyphenol quantification and compositional analyses were carried out for these seaweed species to provide a baseline for interpreting anti-methanogenic effects. Seaweed species were included at 0 %, 2 %, 6 %, and 10 % of feed organic matter (ryegrass hay) during in vitro fermentation assays using rumen inoculant from non-lactating cows. Total gas, methane, hydrogen, volatile fatty acid (VFA), and ammonia production were measured during the incubations. Bromoform concentration was highest in A. armata sampled from Matheson’s Bay at 1 % of the biomass dry weight. Species of red seaweed had a high halogen content, while E. radiata and Ulva. sp. B had a high iodine and crude protein content, respectively. Inclusions of A. armata and B. hamifera demonstrated near elimination of enteric methane production at doses of 2 and 6 % organic matter, respectively, while the remaining species (except for Ulva sp. B) caused moderate reductions at doses of 6 and 10 % organic matter in comparison to these two species. The anti-methanogenic effects of A. armata and B. hamifera resulted in a 22 % reduction in total VFA production, accompanied by changes in the relative proportions of individual VFAs, and had little or no effect on organic matter degradation. The effectiveness of A. armata and B. hamifera demonstrates the potential of these species for mitigating ruminant methane emissions at low inclusion rates, dependent on the concentration of their active components, while E. radiata and Ulva sp. B could be used as feed additives for nutritional benefit. The undertaking of larger scale sampling of A. armata throughout New Zealand, the identification of the active component(s) in B. hamifera, and the development of methods and infrastructure required for successful large-scale aquaculture and application of these seaweed species to livestock management systems are key areas of future research highlighted by this thesis.