1. Why Cows Make Methane: It Is a Byproduct of Their Digestion
Cows have complex stomachs with multiple chambers. The first chamber, called the rumen, is home to many microorganisms [1]. Grass contains cellulose, which is tough and cannot be broken down by the cow’s own strength alone. Instead, microbes in the low-oxygen environment of the stomach ferment the grass to break it down [1].
This fermentation process produces carbon dioxide, hydrogen, and methane together. The methane is mostly released from the body as a burp. The Food and Agriculture Organization (FAO) explains that methane is a byproduct of digestion in ruminant animals [2]. Cows are not doing something "bad"; this gas is simply an extra output of the system that turns grass, which humans cannot eat, into nutrients. The question remains: how much does this gas affect global warming?
2. How Much Methane One Cow Produces in a Day: Hundreds of Liters
How much methane does one cow produce? The Japanese Ministry of the Environment has formulas to estimate methane volume based on the weight of dry matter (feed without water) that a cow eats. For cows that eat large amounts, estimates suggest they can produce several hundred liters per day [1]. To visualize this, 300 liters is equal to 150 two-liter plastic bottles.
However, these numbers are rough estimates. They change significantly depending on the breed of cow, milk production levels, and diet. It is better to think of it as "hundreds of liters scale" rather than a fixed number for every single cow. This amount matters because methane has a strong greenhouse effect. Over a 100-year period, methane is said to trap about 28 times more heat than carbon dioxide (the IPCC shows values between 27 and 30 times) [5]. If the time frame is shortened to 20 years, it is considered about 80 times stronger. The multiplier changes depending on how far into the future you look.
So, it is true that cow burps relate to global warming. But the scale of this relationship differs greatly between the world and Japan.
3. Livestock Make About 30% of Human-Caused Methane Worldwide
According to the FAO, about 32% of methane from human activities worldwide comes from livestock [2]. The main sources are enteric fermentation (burps and digestion) and manure management. Another famous statistic states that the entire food system related to livestock accounts for about 12% of greenhouse gases from human activity (as of 2015, approximately 6.2 billion tons in CO2 equivalent). Within this, cattle account for about 62% [3].
It is important to note that the 12% figure includes feed production and land use; it is not just burps. A closer number to burps alone shows that enteric methane from ruminants accounts for about 5.5% of global greenhouse gases (about 2.7 billion tons in CO2 equivalent), with 77% of that coming from cattle [4]. However, because these statistics come from different years and scopes, you cannot simply add them together or compare them directly to judge which is "worse." Cattle make up a large portion of global livestock emissions. But in Japan, the numbers look very different.
4. In Japan, Cow Burps Are Less Than 1% of Total Greenhouse Gas Emissions
Let’s look at Japanese data. According to the Ministry of Agriculture, Forestry and Fisheries (MAFF), greenhouse gas emissions from the agriculture, forestry, and fisheries sector in fiscal year 2023 were about 51.03 million tons (CO2 equivalent). Of this, about 8.64 million tons came from enteric fermentation in livestock, which is 16.9% of that sector [6].
The Ministry of the Environment and the National Institute for Environmental Studies calculate methane from enteric fermentation (cows, sheep, pigs, etc.) separately from manure and rice paddies in their greenhouse gas inventory. However, the size of this category changes slightly due to revisions in calculation methods [7].
The 16.9% figure is only within the agriculture sector. Japan’s total national emissions are about 1.1 billion tons. If you divide 8.64 million by 1.1 billion, you get approximately 0.8% (an estimate). This is less than 1%. The statement "livestock is big" when looking at the world and "less than 1%" when looking at Japan are not contradictory; they just look at different scopes. However, being small does not mean it should be zero. So, how advanced are efforts to reduce it?
5. A Study Found Feeding Cows Seaweed Cut Methane by 45%
In a study by the University of California, Davis (published in PLOS ONE in 2021), researchers added a type of seaweed from the genus *Asparagopsis* to the feed of 21 steers. They tested two ratios: 0.25% and 0.5% of the feed weight, over 147 days [8]. Methane emissions (per amount eaten) decreased by 45% and 68%, respectively. There was no difference in how much weight the cattle gained.
0.25% of feed weight is about 2.5 grams per kilogram. A tiny amount reduced methane by nearly half. You may see reports claiming an "80% reduction," but that result came from specific conditions with low fiber feed. The overall results for this study were 45% and 68%. Also, because this was a small-scale test with only 21 animals, it cannot be automatically applied to all cows in the world [8]. What other methods are being tried in Japan?
6. How Japan Tests Feed Additives and Breeds Cows That Make Less Methane
In Japan, the MAFF lists two feed additives that help reduce greenhouse gases: 3-nitrooxypropanol (3-NOP) and cashew nut shell liquid [9]. These are recognized as having methane reduction effects and safety.
Another approach is "choosing cows." The National Agriculture and Food Research Organization (NARO) developed simple methods to measure methane between fiscal years 2017 and 2021 [10]. If measuring becomes easier, farmers can identify cows that naturally produce less methane and use this trait in breeding. NARO expects that combining feed improvements with selective breeding could reduce livestock-related greenhouse gases by about 20% without lowering productivity (based on Life Cycle Assessment). This is a stage of "expected results," not a guaranteed outcome [10].
SAFETY NOTE: When using feed additives, it is important to use feed additives recognized for their safety and methane-reduction effects, such as 3-NOP and cashew nut shell liquid [9].
So, reducing methane from burps is not just about "reducing the number of cows." It involves improvements in feed, breeding, and measurement.
7. Check the Numbers Yourself With a Calculator
If you have a calculator, you can check the numbers in this article. First, if a cow produces 300 liters a day, dividing by 2 liters gives you 150 bottles. Next, divide 8.64 million by 1.1 billion to get approximately 0.008, which is 0.8%. Solving the equations yourself helps you understand how "hundreds of liters per day" and "less than 1% of Japan's total" can both be true at the same time.
Another key question is: "What is the denominator?" The 12% figure refers to the entire food system related to livestock. The 16.9% figure refers to the agriculture, forestry, and fisheries sector. The 0.8% figure refers to Japan’s total national emissions. Even though we are talking about "cow burps," the impression of how big or small it is changes completely depending on what you are comparing it to.
You can find the original sources on the websites of the Ministry of the Environment and the Ministry of Agriculture, Forestry and Fisheries, where they publish data on Japan’s greenhouse gas calculations and livestock initiatives.