How Many Stomachs Does a Cow Have? Rumen to Abomasum

One stomach, four compartments

Cows are ruminants, a group of hoofed mammals that digest plants by fermenting them before true digestion begins. Grass is full of cellulose, the tough material in plant cell walls. No mammal makes the enzymes needed to break cellulose down, so a cow relies on trillions of microbes living inside its stomach to do it.

The four compartments are connected in a chain:

The rumen: a giant fermentation vat

The rumen is by far the biggest compartment. Together with the reticulum, it makes up about 80% of the total stomach volume. In a large adult cow, the rumen is commonly said to hold roughly 100–200 liters (25–50 US gallons), enough to fill a bathtub.

Inside, the rumen is warm, wet and without oxygen, and it is packed with bacteria, single-celled protozoa and fungi. Bacteria and protozoa make up most of the microbial mass, fungi around 5–10%. Together they break cellulose into simple acids, mainly acetic, propionic and butyric acid. These volatile fatty acids pass through the rumen wall into the blood and supply most of the energy a cow uses.

The microbes also feed the cow in another way. As they flow onward and die, the cow digests them as protein. Microbes typically supply about 60–90% of the amino acids that a cow absorbs, so in a sense a grazing cow is farming microbes on grass.

Why cows chew the cud

A grazing cow swallows grass after only brief chewing, filling the rumen quickly. Later, often while lying down, she brings food back up into her mouth a mouthful at a time. This lump of partly digested food is the cud, and chewing it again is called rumination.

Re-chewing grinds the coarse plant material into fine particles that microbes can attack more easily. It also mixes in large amounts of saliva. A cow produces an estimated 100–150 liters (26–40 US gallons) of saliva a day, which acts as a buffer to keep the rumen at a weakly acidic pH of about 5.5–6.5 that suits the microbes. A dairy cow typically spends around 7–9 hours a day chewing the cud, often more than she spends eating.

Reticulum, omasum and abomasum

The reticulum sits next to the rumen and is lined with a pattern of ridges that looks like a honeycomb. It is the smallest compartment and in a mature cow holds around 20 liters (5 US gallons) of liquid. Its contractions sort the contents: fine, well-fermented particles move on, while coarse pieces go back to the rumen and up for more chewing. Heavy objects a cow swallows by accident, such as bits of wire or nails, tend to settle here.

The omasum is a round compartment packed with dozens of thin, leaf-like folds, which is why it is called the “book” or “manyplies.” The folds give it a large surface area for absorbing water, minerals and fatty acids, so the food that leaves it is drier and more concentrated.

The abomasum is the true, glandular stomach. It releases hydrochloric acid and enzymes, with a pH of about 2–4. This strong acid kills the rumen microbes as they arrive, and the cow then digests them along with the rest of the food in the small intestine. The abomasum of young calves is the traditional source of rennet, used to make cheese.

How a calf’s stomach is different

A newborn calf is a milk drinker, and its stomach works more like ours. In a newborn, the abomasum is the largest compartment, and the rumen is small and has no established microbes.

When the calf suckles, a reflex closes a muscular fold called the esophageal (or reticular) groove. This forms a channel that sends milk straight past the rumen and reticulum into the abomasum, so it is not fermented. As the calf begins nibbling grass and other solid food in its first weeks, microbes colonize the rumen, and over the following months it grows to become the dominant compartment.

Methane: a side effect of fermentation

Fermentation in the rumen produces gases, mainly carbon dioxide and methane. Methane is made by microbes called methanogenic archaea, which make up only about 3% of rumen microbes. Cows release most of the gas by belching, not from the other end. A single cow gives off roughly 100 kg (220 lb) of methane a year, and cattle farming as a whole is estimated to cause around 7% of global greenhouse gas emissions. Researchers are testing feed additives and breeding to reduce this.

Other animals with four-part stomachs

All true ruminants share the four-compartment design, including sheep, goats, deer, giraffes, antelopes, the American bison and the African buffalo. Camels, llamas and alpacas also chew the cud, but their stomach has three compartments and evolved separately. Horses and rabbits digest plant fiber with microbes too, but in the hindgut rather than the stomach.