When a cow consumes livestock feed or grazes, the microbes in its stomach ferment the food by utilizing carbon and hydrogen, thereby producing enteric methane. Enteric methane is intestinal and predominantly released through eructation (belching), yet a small percentage is also released through flatulence. Gerber et al. state that the emissions from the livestock sector are “estimated at 7.1 gigatonnes CO2 -eq per annum, representing 14.5 percent of human-induced GHG emissions,” noting especially that 41 percent of the total is from beef steers (cattle raised for meat). While this percentage is relatively low in the larger view of greenhouse gas emissions, any attempts to reduce emissions are beneficial.
A recent study supports the link between red macroalgae (Asparagopsis taxiformis) supplementation in beef steer diets and a decrease in methane emissions. A. taxiformis produces and contains the organic compound bromoform, which, when supplemented in steer diets as a freeze-dried, powdered additive, restricts the enteric carbon and hydrogen from producing methane. Roque et al. studied this relationship utilizing a control, and low and high taxiformis intake diets, and discovered that “supplementing low forage TMR [total mixed ration] reduced CH4 yield 69.8% (P <0.01) for Low and 80% (P <0.01) for High treatments.” Low forage TMR, specifically in this study, refers to 0 percent, 0.25 percent, and 0.5 percent algae additives for the control, low, and high treatments respectively. The substantial amount of methane reduction from a relatively small supplementation leads to optimism in the potential microalgae possess in combating methane emissions.
Despite the optimism spurred from this research, there are still reasons to not be entirely hopeful. As per the USDA cattle inventory, in 2020 there was approximately 94 million head (number of individual bovines) in total in the United States. Understandably, in order to reduce enteric methane emissions, there needs to be a sufficient supply of red algae for the livestock population. However, as this number accounts solely for the United States, the global total head of cattle is far greater, thereby increasing the demand exponentially for the algae.
Another potential issue might arise, as the results of these studies show that algal supplementation reduces enteric methane production in the short term, but the study does not have data for the long term. The potency of algal additives might not persist in the long run as bovine rumina (the first stomach of a cow) have historically adapted to other supplements, consequently diminishing the efficacy they once had.
Focusing on red algae specifically, there are some positive and negative aspects. For one, in order to support an algae-supplemented diet in steers, A. taxiformis must be sufficiently harvested and farmed to meet the demand. The industry for seaweed farming is still relatively new and developing, but there have been some large steps already. In general, producing seaweed as a crop is different from other aquaculture practices, as seaweed does not have a major water column restriction—the column being the vertical space between the surface and bottom of a body of water. Farmers, therefore, utilize long lines and vertical farming that extends their seaweed throughout the water column, which, in turn, allows for substantial production in a relatively small area.
Along with its methane reduction potential, seaweed is also recognized for its sequestration abilities as well. As NOAA acknowledges, “seaweeds pull more of the greenhouse gas from the water than all three [eelgrass, mangroves, salt marshes] combined based on biomass,” which can help decrease local ocean acidification. Likewise, seaweeds also collect phosphorus and nitrogen from the ocean and prevent eutrophication in the process. Thus, not only can red algae decrease enteric methane from steers, but it can also sequester carbon and other nutrients from the ocean during cultivation.
Yet, there are still some lingering concerns associated with red algae. Abbott et al. reviews many potential downsides of creating an industry for A. taxiformis. In order to combat enteric methane emissions, there will be a high demand for substantial amounts of seaweed. Although seaweed can be cultivated throughout the water column, there will still be a considerable amount of land and water required to meet the demand. Furthermore, A. taxiformis was formerly endemic to Southern Australia and is now an invasive species in the Northern Hemisphere. Cultivating an invasive species in the Northern Hemisphere may pose certain risks down the line of which scientists are currently unaware.
However, possibly the most alarming risk is that of ozone depletion. Bromoform, the organic compound in the seaweed that reduces methane production, is also an element that can deplete ozone. Carpenter and Liss warn that “detailed reviews of algal halocarbon emissions and biomass estimates imply that macroalgae produce around 70% of the world’s bromoform, rather than only ~20% as previously thought.” This potential for ozone depletion is often overlooked when discussing the positives of steer algal supplementation but is a necessary inclusion into the discussion.
Connecting ozone depletion potentials with farming requirements, in order to farm A. taxiformis for supplementation without encouraging invasive species reliance in the Northern Hemisphere, the cultivation must be done in Southern Australia. However, under Australia’s Ozone Protection and Synthetic Greenhouse Gas Management Act of 1989, the export of A. taxiformis would be highly debated and potentially illegal. Under these circumstances, algal supplementation appears to be a moderately difficult goal to achieve.
Overall, while red algae supplementation in steers is an encouraging solution to enteric methane emissions, there is still a long way to go in order to prove that its benefits outweigh its potential harm.
Sources:
Abbott, DW., Aasen, IM., Beauchemin, KA., Grondahl, F., Gruninger, R., Hayes, M., Huws, S., Kenny, DA., Krizsan, SJ., Kirwan, SF.,.; Lind, V., Meyer, U., Ramin, M., Theodoridou, K., von Soosten, D., Walsh, PJ., Waters, S., Xing, X., (2020). Seaweed and Seaweed Bioactives for Mitigation of Enteric Methane: Challenges and Opportunities. Animals 10(12): 2432. https://doi.org/10.3390/ani10122432
Carpenter, LJ & Liss, PS. (2000). On temperate sources of bromoform and other reactive organic bromine gases. JGR Atmospheres 105(D16): 20539-20547. https://doi.org/10.1029/2000JD900242
Gerber, P.J., Steinfeld, H., Henderson, B., Mottet, A., Opio, C., Dijkman, J., Falcucci, A. & Tempio, G. (2013). Tackling climate change through livestock – A global assessment of emissions and mitigation opportunities. Food and Agriculture Organization of the United Nations (FAO), Rome. http://www.fao.org/publications/card/en/c/030a41a8-3e10-57d1-ae0c-86680a69ceea/
Mulhollem, J. (2019). Seaweed feed additive cuts livestock methane but poses questions. Penn State News. https://news.psu.edu/story/578123/2019/06/17/research/seaweed-feed-additive-cuts-livestock-methane-poses-questions
National Agricultural Statistics Service (NASS), Agricultural Statistics Board, United States Department of Agriculture (USDA). (2020). January 1 Cattle Inventory Down Slightly. USDA. https://www.nass.usda.gov/Publications/Todays_Reports/reports/catl0120.pdf
Office of Communications. (2020). Seaweed Aquaculture. National Oceanic and Atmospheric Administration. https://www.fisheries.noaa.gov/national/aquaculture/seaweed-aquaculture
Roque, BM., Venegas, M., Kinley, RD., de Nys, R., Duarte, TL., Yang, X. & Kebreab, E. (2021). Red seaweed (Asparagopsis taxiformis) supplementation reduces enteric methane by over 80 percent in beef steers. PLoS ONE 16(3): e0247820. https://doi.org/10.1371/journal.pone.0247820
Schlossberg, T. (2020). An Unusual snack for cows, a powerful fix for climate. The Washington Post. https://www.washingtonpost.com/climate-solutions/2020/11/27/climate-solutions-seaweed-methane/?arc404=true&utm_source=google&utm_medium=acq-nat&utm_campaign=eng-hub-search&utm_content=hub-climate_soultions&utm_keyword=&campaignid=8202785432&adgroupid=111150677210&adid=504274061467&gclid=CjwKCAjwu5CDBhB9EiwA0w6sLYkjc-hpXLBe2K3kccGo54CRFko4tnkQJ628XFXlM9w2eXqsP03vpxoCm2IQAvD_BwE