Seaweed as a feed additive to mitigate enteric methane emissions in ruminants: Opportunities and challenges

Cutting farming-related methane emissions from ruminants is critical in the battle against climate change. Since scientists initially investigated the potential of marine macroalgae to reduce methane emissions, using seaweeds as an anti-methanogenic feed additive has become prevailing in recent years. Asparagopsis taxiformis is the preferred species because it contains a relatively higher concentration of bromoform. As a type of halogenated methane analogue, bromoform contained in A. taxiformis can specifically inhibit the activity of coenzyme M methyltransferase, thereby blocking the ruminal methanogenesis. However, bromoform is a potential toxin and ozone-depleting substance. In response, current research focuses on the effects of bromoform-enriched seaweed supplementation on ruminant productivity and safety, as well as the impact of large-scale cultivation of seaweeds on the atmospheric environment. The current research on seaweed still needs to be improved, especially in developing more species with low bromoform content, such as Bonnemaisonia hamifera, Dictyota bartayresii, and Cystoseira trinodis. Otherwise, seaweed is rich in bioactive substances and exhibits antibacterial, anti-inflammatory, and other physiological properties, but research on the role of these bioactive compounds in methane emissions is lacking. It is worthy of deeper investigation to identify more potential bioactive compounds. As a new focus of attention, seaweed has attracted the interest of many scientists. Nevertheless, seaweed still faces some challenges as a feed additive to ruminants, such as the residues of heavy metals (iodine and bromine) and bromoform in milk or meat, as well as the establishment of a supply chain for seaweed cultivation, preservation, and processing. We have concluded that the methane-reducing efficacy of seaweed is indisputable. However, its application as a commercial feed additive is still influenced by factors such as safety, costs, policy incentives, and regulations.