Brown seaweed supplementation in dairy and beef cattle: effects on methane mitigation, productivity, reproductive responses, and safety

Abstract

Brown seaweed supplementation has attracted increasing interest as a nutritional strategy for mitigating enteric methane (CH 4 ) emissions in cattle while potentially supporting animal productivity and health. This structured narrative review synthesizes current evidence on the effects of brown seaweed supplementation on CH 4 emissions, reproductive-related responses, productivity, and safety outcomes in dairy and beef cattle. A structured literature search identified 34 primary research studies included in the final narrative synthesis, comprising in vitro rumen fermentation studies, rumen simulation technique (RUSITEC) studies, and in vivo cattle trials. Available evidence indicates that several brown seaweed species, including Ascophyllum nodosum , Saccharina latissima , Fucus spp., and Laminaria spp., can influence rumen fermentation and reduce CH 4 production under specific experimental conditions. However, CH 4 -mitigation effects have generally been more consistent and pronounced in vitro than in vivo , and reductions observed in cattle have often been modest and variable. Evidence supporting improvements in milk production, feed efficiency, reproductive performance, or fertility remains limited and inconsistent, with many reported effects derived from short-term studies, small sample sizes, or combined feed-additive systems. In contrast, increased iodine transfer to milk is among the most consistently reported outcomes, with the magnitude of transfer depending on the brown seaweed species (e.g., Ascophyllum nodosum versus Saccharina latissima ) and dietary inclusion rate, highlighting important food-safety and regulatory considerations. Interpretation of the current literature is constrained by substantial heterogeneity in seaweed species, chemical composition, processing methods, inclusion levels, study duration, and outcome measurements. Overall, brown seaweed supplementation shows potential as a component of sustainable cattle production systems; however, current evidence remains insufficient to support broad recommendations for CH 4 mitigation or improvements in productivity and reproductive performance. Future research should prioritize well-designed, long-term, in vivo studies that integrate measurements of CH 4 emissions, animal performance, reproductive outcomes, safety assessments, and economic feasibility to better evaluate the practical applicability of brown seaweed supplementation under commercial production conditions.