Happy New Year! It is a privilege and a delight to experience this new beginning with all the prospects it brings, and in the words of Charles Lamb, “New Year's Day is every man's birthday.” Among my impractical new year resolutions of putting my clothes away after doing laundry and avoiding junk food, a more realistic one will be to learn and share more about enteric methane emissions and environmental sustainability of the dairy industry. With the growing interest in reducing greenhouse gases (GHGs) globally, and the evolving strategies aimed at mitigating enteric methane emissions from ruminant animals, I figured that would be a good place to start my Farm Report article this year. This will be the first part in a series, so stay tuned.
Greenhouse gases: GHGs are gases that trap heat in the atmosphere and increase the earth’s temperature. These gases are present in the atmosphere, and their heat-trapping effect (known as the greenhouse effect) helps to maintain a warm temperature (15°C) on the earth’s surface making it habitable. However, an upsurge in the amount of these gases in the atmosphere from human and agricultural sources has led to a continuous rise in the earth’s temperature, triggering the concerns of global warming and climate change. The impact of GHGs on global warming and climate change is dependent on the concentration of the gas in the atmosphere, the lifetime of the gas in the atmosphere, and the effectiveness of the gas at trapping heat in the atmosphere (estimated by the global warming potential of that gas). Examples of GHGs are natural compounds like carbon dioxide (CO2), methane, (CH4), nitrous oxide (N2O), water vapor (H2O), and synthetic fluorinated gases which include hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF6), and nitrogen trifluoride (NF3). (Sources: US Environmental Protection Agency, Natural Resources Defense Council, NASA Global Climate Change).
Methane (CH4): CH4 is a colorless odorless gas made up of one carbon atom and four hydrogen atoms. It is also known as marsh gas or methyl hydride. It is a major component of natural gas and is flammable, hence, its use as fuel. The combustion of CH4 in the presence of oxygen (O2) gives off CO2 and H2O. As a GHG, CH4 is the second largest source of GHG in the atmosphere after CO2 but it is more potent in absorbing heat in the atmosphere than CO2 which makes it a GHG of interest. Sources of CH4 include wetlands, oceans, landfills, natural gas production, livestock production, and manure management systems. In the US the largest sources of anthropogenic CH4 emissions (from humans) come from oil and gas systems, livestock enteric fermentation, and landfills. (Sources: US Environmental Protection Agency, National Library of Medicine, University Corporation for Atmospheric Research).
Methanogenesis: this is simply the process of CH4 formation in an anaerobic environment. In ruminants, methanogenesis occurs primarily in the rumen during the process of fermentation (enteric fermentation) of ingested feed, but this process can also take place in the lower gut. The process of carbohydrate degradation in the rumen involves the hydrolysis of cellulose (and other polysaccharides) to simple sugars which then undergo fermentation to yield volatile fatty acids (VFAs) such as acetate, propionate, and butyrate. Other end products of this fermentation process are formic acid, hydrogen (H2), and CO2, and specific microbes in the rumen known as methanogens (prokaryotic organisms that belong to the archaea domain) use H2 to reduce CO2 to produce methane (CO2 + 4H2 → CH4 + 2H2O). The removal of H2 from the rumen after fermentation is important to ensure normal metabolism of other rumen microorganisms, but the CH4 produced as a result is not useful to the animal as it causes a loss of 2-12% of gross energy. CH4 is released from the rumen via eructation (burping) into the atmosphere. (Danielsson et al., 2017; Hook et al., 2010; Russell & Hespell, 1981).
Metrics for enteric methane: various metrics are used to determine the efficacy of any strategy aimed at reducing enteric CH4 emissions. They include total/absolute CH4 production (g of CH4 produced per day), CH4 yield (g of CH4 produced per kg of dry matter intake), and CH4 intensity (g of CH4 per kg of milk or animal product). Other methods include CH4 yield (g of CH4 produced per kg of digested organic matter), and Ym, which is a variable that assesses CH4 energy loss as a proportion of gross energy intake. Sometimes, these metrics do not yield the same amount of CH4 when used simultaneously, hence, the amount determined would vary depending on the method adopted. (Beauchemin et al., 2022; Fouts et al., 2022).
I find it important to know what these different terms mean, so I hope we can use this as a baseline for further discussions on this topic.
— Gift Omoruyi