Marine Macroalgae as Sustainable Feed Additives in Ruminants: A Systematic Review of Methane Mitigation and Animal Responses

This systematic review synthesized evidence on the effects of marine macroalgae supplementation on enteric methane (CH4) emissions, rumen fermentation, nutrient utilization, animal performance, animal health, and product quality in ruminants. Google Scholar, Scopus, Web of Science, and CAB Abstracts with Full Text were systematically searched for peer-reviewed in vivo studies published between January 2015 and 30 July 2026. Following PRISMA 2020 procedures, 25 studies were included in the qualitative synthesis, and risk of bias was assessed using the SYRCLE tool. Red macroalgae, particularly Asparagopsis spp., produced the greatest reductions in CH4 production, yield, and intensity, although responses varied with species, inclusion level, basal diet, animal species, and supplementation duration. Brown macroalgae produced less consistent CH4 responses, while evidence for green macroalgae was limited. Several studies reported reduced acetate and increased propionate, whereas effects on nutrient digestibility varied. Animal performance was generally maintained at lower inclusion levels, but higher inclusion levels of some red macroalgae reduced dry matter intake and, in some studies, milk production. Some seaweed treatments increased iodine and bromine concentrations in milk and other biological matrices, highlighting the importance of residue monitoring. The evidence was limited by methodological and biological heterogeneity, small sample sizes, short experimental periods, variation in macroalgal composition and CH4-measurement methods, geographical concentration, and limited long-term evidence. Marine macroalgae shows potential as functional feed additives for enteric CH4 mitigation, but the evidence does not support a universally optimal inclusion rate. Longer-term, adequately powered in vivo studies are needed to establish species-specific dose-response relationships, persistence of CH4 mitigation, food-safety requirements, and economic feasibility.