1. Why a Cow's Four Stomach Rooms Include Only One True Stomach
A cow’s stomach is divided into four rooms, called the first, second, third, and fourth stomach, in order from the mouth.[2] When you hear "four stomachs," it might seem like there are four places for digestion. However, only the last one, the fourth stomach, produces stomach acid.[2] Nutrition researcher Keiichi Anbo explains that the first through third rooms are originally parts of the esophagus that changed shape, and they do not produce stomach acid.[2] According to the Agricultural and Livestock Industry Promotion Organization, the first to third rooms work like a human’s esophagus, while the fourth room handles the final digestion just like a human stomach.[4] So, having four stomachs means one true stomach is connected to three rooms that changed from the esophagus.[4]
2. How the Cow's First Stomach Works as a Tank for Fermenting Grass
In a mature cow, the first stomach has a volume of about 100 L (about 26 gallons), which takes up about 80% of all four rooms.[1] Some documents say it is 120 L (about 32 gallons).[6] There is also an explanation that a cow’s stomach is so large it makes up about 70% of the entire digestive tract, and most of that is the first stomach.[2] The first stomach is a place where eaten grass is stored for a long time and changed into nutrients through fermentation.[6] There is almost no oxygen inside, so the microbes living there get energy through fermentation that does not use oxygen. Compared to breathing with oxygen, the energy gained is less than one-sixth.[1] This is an inefficient method, but the microbes live there in large numbers, constantly.[1]
3. How Microbes, Not the Cow, Break Down Grass Fiber
The hardness of grass comes from a fiber called cellulose. The cow’s body does not have a digestion system that can break this down on its own. Documents state that the first stomach does not make digestive enzymes; instead, countless microbes are the main actors in digestion.[3] The microbes in the stomach use their power to break down the cellulose in grass, which other animals cannot digest, making it useful for making milk and meat.[2] The microbes in the first stomach are made up of bacteria, archaea, protozoa, and fungi. It is said that there are about 1,000 types of bacteria alone, depending on how they are counted.[1] An old review article mentions numbers like 1 billion to 10 billion bacteria and more than 1 million protozoa per 1 mL of stomach contents. Since this is an old way of counting, it is good to view these as rough estimates.[2]
4. What the Cow Gets from Microbes: Fatty Acids and the Microbes Themselves
When microbes break down carbohydrates, they create volatile fatty acids like acetic acid, propionic acid, and butyric acid. These are absorbed and become the cow’s main energy source.[1] Just as humans get sugar from rice, cows get heat and power from grass through microbes.[1] Another thing the cow receives is the microbes themselves. Microbes keep multiplying in the first stomach, and their cells are sent to the lower digestive tract to be used as a main protein source. This is called microbial protein.[1] This explanation says that while eating grass, the cow is also using microbes for nutrition in a sense, but this is a way to make it easy to understand. The cow does not chew and eat the microbes directly.[1]
5. Why Cows Chew Their Food Again and Again (Rumination)
Rumination is bringing food back to the mouth once it has been eaten, chewing it again, and swallowing it.[8] The grass in the first stomach has hardly dissolved in water. Digestion cannot begin unless microbes stick to the surface of the grass and attack it. By repeatedly making the grass finer through rumination, the surface area increases, giving microbes more chances to attach.[1] It is also said that the cow damages the fibers by chewing, making them easier to break down.[3] According to a dairy industry group, the time spent ruminating in one day is 6 to 10 hours, and the number of chews during rumination is 40 to 60 times per minute.[7] If you simply multiply these, it is about 14,000 to 36,000 times a day. This is a rough estimate that changes depending on the individual cow and feed. Saliva is also produced at 90 to 180 L (about 24 to 48 gallons) per day, which is compared to one bathtub full.[5] In terms of evolution, this lifestyle is linked to quickly eating plants to escape enemies and then digesting them with the help of microbes in a safe place while ruminating.[1]
6. Why Newborn Calves Do Not Yet Use Their First Stomach for Grass
In newborn calves, the fourth stomach takes up about 70% of the whole stomach. When drinking milk, food goes through a groove in the esophagus, skipping the first stomach and going directly to the fourth stomach.[4] This means that while they are calves, they live with a body that digests with stomach acid, similar to a human baby.[4] As they grow and start eating grass, the first stomach develops.[4] However, some foods are good or bad for microbes. If starch and sugar are broken down too quickly, the first stomach becomes acidic, and the microbes’ work slows down. It is said that giving too much grain-based feed can damage the lining of the first stomach.[3] The function of the first stomach is related to what the cow eats.[3]
7. How to Observe and Count a Cow's Rumination
Try counting how many times you can move your mouth in 1 minute without putting anything in it, and write it in your notebook. It is said that cows move their mouths 40 to 60 times per minute.[7] Imagine how hard it would be to keep that up for 10 minutes. This might help you understand what it is like for a cow to keep it up for 6 to 10 hours a day.[7] When you see cows at a farm or zoo, look for cows that are lying down and moving their mouths. Please watch quietly from a distance and follow the instructions of the staff.[7] Information about livestock can be read on the pages of the Agricultural and Livestock Industry Promotion Organization and local dairy industry groups.[4]