Cattle are ruminant animals. When eating fibrous plants, they repeatedly digest the food in their stomach, the first of four stomachs, and then bring the digested food back to their mouths to chew it again. During this process, each cow is said to emit 200 to 800 liters of methane per day in the form of burps.
In Japan, methane gas emitted from cow burps accounts for 27% of total methane emissions. It is also excreted not only through burping but also through feces, which accounts for about 8%. [1] Worldwide, methane emissions from cattle are estimated to be 2 billion tons per year in terms of CO2 emissions, accounting for about 5% of all greenhouse gas emissions. [2] In addition, methane has has 25 times the greenhouse effect of carbon dioxide.[3] Given these facts, it is crucial to reduce methane emissions.
In fact, in 2021, methane emissions was raised as a theme at COP26 in the United Kingdom, and the Global Methane Pledge on reducing methane emissions was launched [4], and research and development is progressing around the world. In Japan, research to reduce methane emissions from cow burps is being conducted under the initiative of the Ministry of Agriculture, Forestry and Fisheries' "Green Food System Strategy" [5] and the Cabinet Office's moon shot type R&D program [6].
However, progress has not been made in reducing methane emissions from cattle, with the largest increase in total atmospheric average concentrations of methane in 2021 confirmed since 2011. [7]
Thus, the disposal of methane emitted from cattle remains a major issue for a sustainable, decarbonized society that is friendly to the global environment.
Before we continue with a look at the measures that have been taken to control methane, let's consider why cattle emit methane in the first place.
[9] https://extension.umn.edu/dairy-nutrition/ruminant-digestive-system
As mentioned at the beginning of this section, cows ruminate. To understand cow digestion, including ruminating, let's look at the process by which foods taken by the cow are digested and expelled in the form of stool. See the image above. First, the foods chewed in the cow's mouth pass through the esophagus to the rumen, which is the largest of the cow's four stomachs, occupying 80% of the total stomach volume. It is home to a large number of microorganisms called rumen flora, which allow the cow to break down dietary fiber.
This microorganism is responsible for methane emissions, which will be discussed in detail later. Then there is the honeycomb stomach, which pushes food that is difficult for the rumen to digest back to the mouth. The food returned to the mouth by the honeycomb stomach is slowly chewed and then returned again. In the third stomach, the food is sorted according to digestibility, with the less digestible food being returned to the hive stomach and the more digestible food being taken to the fourth stomach. Digestion takes place in the fourth stomach, called the abomasum, just as in the human stomach. The food is then passed through the small and large intestines, where it is expelled in the form of feces. This is the process of how cattle digest their food and expelled them as feces.
[10] https://ruminantdigestivesystem.com/rumen-environment/rumen-fermentation/