The Biology of the Rumen

Rumen Digestion Explained

The Biology of the Rumen

By John Hibma – Ruminants have a special function in our world’s complex ecosystem. They consume grasses grown in pastures and on rangeland along with fibrous shrubs and weeds, leaves on trees, and crop byproducts that humans cannot use directly in their diets. Animal agriculture is also critical to sustaining habitat that supports other wildlife as well as regulating weather systems and sequestering carbon dioxide. Without ruminants, many millions of acres of arable land unfit for crop farming and cultivation would lie fallow and unproductive.

All ruminants, which includes cattle, goats, sheep, deer, elk, moose, bison, alpacas, and llamas to name a few, have multi-compartment digestive systems that digest plant material in several stages. For cattle, goats, and sheep, the compartments are the rumen, reticulum, omasum, and abomasum. Camelid species such as alpacas and llamas have a modified system with only three chambers. In all ruminants, it’s the largest compartment — the rumen — that begins the digestion of organic material through a process called fermentation. The digestion process in ruminants is significantly different from monogastric species (e.g. humans, swine, dogs, cats, and poultry) which digest only non-cellulolytic or low-cellulolytic feedstuffs in a single stomach through the action of enzymes and acids.


Note: Other forage-consuming species such as horses, elephants, rodents, and rabbits are not ruminants. They are known as hind-gut fermenters. They have a single stomach that digests only a portion of the feed consumed with the remainder being digested in the lower colon.

At the heart of ruminant digestion is the fermentation process that takes place in the rumen. Fermentation in the rumen is the biological and chemical conversion of complex carbohydrates such as cellulose and starches into smaller molecular units known as volatile fatty acids (VFA) as well as proteins being disassembled into various peptides and amino acids. This bio-conversion is accomplished by literally billions of microbes consisting of bacteria, protozoa, fungi, and yeasts that reside in the rumen.

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One of the key elements of a healthy rumen is a having a diversified population of microbes. Cellulose digesters are different from starch and sugar digesters which are different from those that break down proteins. Still other microbes help regulate acid levels while others scavenge hydrogen in order to regulate methane production.

Working in conjunction with the rumen is the reticulum, a compartment located at the end of the esophagus and attached to the side of the rumen. As the rumen is fermenting feeds, muscular actions in both the rumen and reticulum constantly mix the digesta particles. The reticulum helps segregate feed particles by directing the larger particles back to the rumen where they continue to be fermented and the smaller particles are directed to the third compartment, the omasum, for further digestion. In neo-natal ruminants, the reticulum prevents milk and other feeds from entering the immature rumen and routes food directly to the omasum. As the rumen develops and is populated with microbes, the reticulum allows feedstuffs to enter the rumen.

Another function of the reticulum is to facilitate the chewing of the cud. A ruminant is well-known to regurgitate small portions of digesta from the rumen and spend time re-chewing after they have finished their main meal. The addition of more saliva aids the digestive process. This usually occurs while the animal is at rest. The action of cud-chewing is a positive indication that the rumen is functioning well and the animal is healthy.


Once the feed particles are reduced in size, the reticulum allows portions of the digesta to move to the omasum. The role of the omasum is to continue with some fermentation as well as the absorption of some volatile fatty acids (VFA) and soluble vitamins and minerals directly into the bloodstream. The size of the omasum varies among different ruminant species. Selective foragers such as goats and deer do not require large omasums. Non-selective ruminants such as sheep and cattle require relatively larger omasums so they can process a greater diversity of food.

As the size of digesta particles continue to be reduced, they move into the fourth compartment, the abomasum, which functions similarly to the single stomach of monogastrics. Here is where final digestion takes place with enzymes and acids, further reducing the size of food particles. From the abomasum, digesta flows through the duodenum and into the small intestine where it’s absorbed into the bloodstream.

Ruminants are unique in that nearly all their caloric nutrition comes from the fermentation process occurring in the rumen. Ironically, the volatile fatty acids that ultimately become the glucose required for the ruminant’s metabolizable energy are actually a byproduct of the life cycle of the rumen microbes as they consume cellulose and other carbohydrates. The interaction between the host ruminant and the rumen microbes is essentially a symbiotic relationship — one cannot exist without the other.


Ruminants have evolved to consume, and are the most efficient in fermenting cellulolytic and fibrous plant material. They are less efficient at fermenting sugars and starches even though most ruminant diets will contain some level of sugars and starches. They do a poor job of breaking down unsaturated fats. Therefore, adding vegetable fats to the diet as a source of calories should be avoided.

The foundation of all ruminant diets is forage and fibrous plant material. The molecular complexity of the different types of cellulose — how coarse and mature the plants are — determines how quickly or completely a forage can be fermented. Leafy forages with small stems will ferment more quickly and provide more energy. Even though the ruminant requires forage for nutrition, the more fibrous a forage is, the longer it takes the rumen microbes to break it down and the less nutrition the forage can provide.

Ruminants are also unique in that over half of their amino acid requirements come from the expiration of the rumen microbes. As the microbes — which are primarily protein — complete their life-cycle in the rumen, they flow out of the rumen and become part of the digesta that’s eventually absorbed in the small intestine. The other half of the metabolizable proteins and amino acids come from the proteins found in the dietary feedstuffs. Those proteins survive rumen fermentation in various forms, exit the rumen, and are also absorbed in the small intestine.

Even though consumption of forage is essential to the health of the rumen, ruminants can also be fed limited amounts of grains and other commodity byproducts. High producing dairy cow breeds such as Holsteins and Jerseys require high levels of both calories and amino acids coming from both highly fermentable forages as well as commodity byproducts such as soybean meal that delivers optimal levels of amino acids. However, many grains, such as corn, barley, and wheat, are fermented by a different group of bacteria other than fiber digesters. The fermentation of grains results in stronger acids that, in high enough levels, are toxic to the fiber-digesting bacteria. Rumen acidosis is a common ailment for high-producing dairy cows and if not held in check, will be fatal. Therefore, all diets fed to domesticated ruminants must be monitored so that grains and other byproducts are not overfed.

Different species of ruminants have varying preferences in what they eat. While goats seldom turn down high-quality hay and commercial grains, they never pass up a chance to browse bushes, trees, and other coarse vegetation. They are talented in selecting just the right combination of feedstuffs for a healthy diet. Just be careful of poisonous plants for goats to avoid disasters in your herd.


Making pasture and forages work for sheep is important as sheep prefer to graze. It’s also important to have high-quality pasture grasses in the early vegetative stage available for them. Sheep also like high-quality forages such as alfalfa and clover mixes along with immature grasses.

Broadly speaking, all ruminants will be more productive and grow faster when their diets are highly fermentative. The key to high-quality nutrition in all ruminant diets is for the rumen microbes to break down and ferment as much fiber as they can, all day, every day. For all ruminants, large and small, the two most important things to manage in their diets are to never deprive them of forages and to never overfeed feedstuffs that are high in grains and simple sugars. In order for a ruminant to be healthy, the rumen microbes must also be healthy.

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