Rumen

The rumen, also known as a paunch, is the largest stomach compartment in ruminants. It works with the reticulum to form the reticulorumen.

Inside the rumen live many different microbial communities. These help break down the food that ruminants eat.

Ruminants often eat rough, fiber-rich plants that are hard to digest. The rumen helps change this tough plant material into energy and fatty acids they can use. This system helps ruminants survive where other animals might struggle to find enough food.

Brief anatomy

The rumen has five muscular parts. Each part has special tiny living things that help with digestion. The outer layer, called the epithelium, protects the rumen and helps with important substances.

Inside the rumen, there are tiny bumps called papillae that help take in nutrients. The reticulum works with the rumen. It has a pattern that looks like a honeycomb. This helps it take in nutrients and catch small pieces of food. The rumen and reticulum make up about 80% of a ruminant’s stomach.

Digestion

The rumen helps animals break down food through a process called fermentation. Many tiny living things, such as bacteria, archaea, viruses, fungi, and protozoa, live in the rumen. These microorganisms help digest tough plant materials and other foods that animals cannot break down on their own.

These tiny living things change the food into useful substances, including energy sources called volatile fatty acids. These energy sources pass into the animal's body and help it stay healthy. The rumen also helps with digesting proteins and other nutrients, creating a helpful partnership between the animal and the microorganisms.

Stratification and mixing of digesta

In the rumen, the first stomach part of animals like cows, the food they eat separates into layers. These layers are gas, liquid, and bits of different sizes. This mixing and moving of food is very important for digestion.

When an animal eats, the food goes down a tube called the oesophagus and ends up in a part called the dorsal reticulum. The rumen mixes this food into a thick mass. Tiny organisms break down the food, making gases. As the food breaks down more, it becomes smaller and moves to a lower part of the rumen. Some of this food may be brought back up to be chewed again in a process called rumination.

Liquids like water and saliva mix with the food but move through the rumen faster. Gases such as methane, carbon dioxide, and hydrogen build up and are released when the animal burps, a process known as eructation.

Microbes in reticulorumen

Further information: Methanogens in digestive tract of ruminants

The rumen has many tiny living things, such as bacteria, protozoa, fungi, archaea, and viruses. These microbes help break down food that animals cannot usually digest, like tough plant material.

Bacteria and protozoa are the most common microbes in the rumen. They help break down parts of the food, such as fibers and proteins. Fungi help break down some plant materials, and archaea make a gas called methane. Viruses also help by breaking down other microbes, which helps recycle nutrients. These microbes move through the animal's digestive system and give important nutrients when they are digested later.

Human uses

The food in the rumen, called "paunch waste," might be used as a fertilizer to help grow crops in a way that is good for the environment, called sustainable agriculture.

Development

When a baby ruminant is born, its rumen—the big first part of its stomach—is small and does not work yet. The rumen grows a lot before the young animal starts eating solid food, called weaning.

As the young ruminant eats solid food, tiny organisms called microbes enter the rumen. These microbes help break down tough plant material. This process creates important substances that help the rumen grow and work well. The inside of the rumen also grows tiny growths called papillae, which help absorb nutrients.

Rumen microbiome genetics

The tiny living things in a ruminant’s stomach, called the rumen microbiome, are very important. They help the animal stay healthy and grow while saving money on food. Scientists have studied the DNA of these tiny living things. They found genes that help the animal grow bigger and eat more. Some of these genes are linked to the animal’s weight gain and how much food it eats each day.