Enteric methane mitigation through Asparagopsis taxiformis supplementation and potential algal alternatives

Abstract

Research is increasingly directed towards decreasing the greenhouse gases contribution, specifically methane, from the livestock agriculture sector. Macroalgae supplementation has emerged as a promising tool to mitigate enteric methane emission in ruminants. The mode of action responsible for the mitigation effect centers around the content of volatile halogenated compounds, primarily bromoform. The sub-tropical red seaweed, Asparagopsis taxiformis, is the most well researched bromoform containing species. While several studies, both in vitro and in vivo, have demonstrated the effectiveness of A. taxiformis at reducing enteric methane emission (> 80% reduction), questions surrounding sustainability, animal productivity, animal product quality, and commercial practicality remain. These questions by no means disqualify the practice of feeding macroalgae to cattle to reduce methane emission, but they must be answered before implementing macroalgae as a feed additive commercially. Also, limiting scientific inquiry to a few species reduces the potential of discovering other compounds and modes of action that could produce the desired mitigation effect without the inherit drawbacks of the current options. Work conducted in both ruminant nutrition and human health fields have identified numerous bioactive compounds within plants that exhibit anti-microbial functions that could modify the rumen microbiome for beneficial outcomes. These compounds are also found in macroalgae. Phlorotannins, saponins, sulfonated glycans, other halocarbons and bacteriocins found within macroalgae have demonstrated antimicrobial activity in vitro. However, it is unclear what effect these compounds may have when used in vivo. Once identified, extracting these compounds for supplementation in lieu of feeding the entire plant may be a more practical solution. Dietary inclusion levels of macroalgae in ruminant diets can be limited by variation in active ingredient concentration, palatability to cattle, and excessive dietary mineral content. There are multiple in vitro studies that have demonstrated a methane reduction potential of non-bromoform containing species, but inclusion levels are often well above the effective levels of A. taxiformis (< 0.5% of dietary dry matter). In some animal studies, A. taxiformis supplementation has led to decreased dry matter intake and productivity and elevated mineral accumulation, such as iodine, in animal products. Therefore, methane mitigation by macroalgae will likely have to occur at low dietary concentrations to be practical. This review aims to highlight potential benefits and challenges that feeding macroalgae as a tool for methane reduction may have on animal production, the environment, animal and consumer health.