Basics of the Ruminant Digestive System

Part 1

BY Dr. Stephen Blezinger | | Comments (0)

To be effective as cattle producers, it is necessary to understand, at least to some degree, the complexities of the ruminant digestive system.  The ability of these amazing animals to digest material that is mostly indigestible to other food animal species provides an important place for cattle, as well as sheep and goats, in world food production.  Their unique and fascinating digestive system allows them to extract nutrients needed for growth and production from high fiber materials that grow in great abundance.  This section of this series will provide insights into what a ruminant animal (as noted, all cows are ruminants but not every ruminant is a cow) is, how it develops and how this unique digestive system works.

What is a Ruminant Anyway?

What makes a ruminant animal unique from other food animal species, primarily pigs and chickens, is that the first part of the digestive tract after the mouth and the esophagus is its stomach system.  This system is comprised of four separate stomach compartments as compared to other farm animals such as pigs, chickens, dogs, cats, horses, etc., which only have one (monogastrics).  These compartments are known as the rumen (for which this system is named), reticulum, omasum and abomasum, respectively.  This system is found in cattle and other animals such as buffalo, deer, and camels. Some ruminant animals are slightly different and only have three compartments, but that’s another story. This four-stomach system allows animals such as those listed to subsist and thrive on high quantities of roughage/fiber – grass, hay, leaves, browse, etc.  Let’s briefly discuss the four components of this system:

The Rumen

The rumen is the first compartment of the rumen system.  It attaches to the esophagus and is the first area of digestive activity.  In its most basic form, the rumen serves as a fermentation vat where microorganisms (bacteria, protozoa, fungi, etc.) break down the forage and feed that the cow consumes.  In a mature cow, it is very large – in many cases capable of holding 40 to 50 gallons of liquid, semi-liquid and gaseous materials.  The feed consumed by the animal (hay, grass, silage, feed grain mix, water and saliva) initially serve as food for the microbial population.  These microbes break down the forages and feed and produce certain by-products.  The microorganisms convert carbohydrates (mainly starch, cellulose and hemicellulose) to materials known as volatile fatty acids (VFAs). Rumen VFAs are absorbed from the rumen and are the major source of energy for the cow. The bacteria break down protein to keto acids and ammonia.  The microbes then use the ammonia and acids to rebuild protein for themselves as they grow.  The animal digests this microbial protein.

Fiber, as well as minerals, salts and buffers (from saliva) are also important in that they help control rumen pH. This is important because the bacterial and protozoa responsible for rumen digestion are very sensitive to pH change and have very specific pH requirements.  Those which digest cellulose and hemicellulose, primary components of grass, hays and high roughage rations, require a pH in the 6.0 to 7.0 range. Bacteria that digest rations high in starch, such as high grain rations, prefer a pH in the range of 5.3 to 6.5. It is estimated that 60 to 90 percent of total digestion occurs in the rumen. Therefore, it is extremely important that cattle be fed properly in order for the rumen to function optimally and maintain good rumen health.

The Reticulum

The reticulum is basically an extension of the rumen. The reticulum, by means of regular contractions, aids in keeping the forages and feed in the rumen mixed with the water and saliva until it has a consistency that it can pass into the lower tract digestive areas.

The Omasum

The omasum serves largely as a dehydration area.  As the feed passes through the omasum it is squeezed and compressed by the contractions of the omasum. This removes 60 to 70 percent of the water from the partially digested feed (ingesta).

The Abomasum

The abomasum is often referred to as the “True Stomach” and is very similar to the human stomach and that of other monogastrics.  As the feed passes into the abomasum, gastric juices secreted by the abomasum are mixed with the ingesta, producing a material about the same consistency of that in the rumen. The high acid content of the gastric juices lowers the pH rapidly and kills the protozoa and many of the bacteria. The ingesta passes rapidly through the abomasum into the lower tract, where digestion is completed in a similar manner to simple stomached animals.

Rumen Development from Birth

The rumen in a newborn calf is very small and nonfunctional  At this time the calf is essentially a monogastric.  The rumen in the young calf begins growing at two to three weeks of age, depending on when hay and grain are consumed in addition to the milk it receives from the cow.  It develops the bacterial population in the small growing rumen by exposure to its environment and by the cow licking its head and muzzle.  The rumen grows and develops rapidly during the first five to six months. From this point on, it grows at a rate proportional to other parts of the animal. During the early period of growth, the specialized function of the rumen is established as the microbial population grows and develops. The rumen is considered to be fully functional at six to nine months of age.

In the early stages of the calf’s life, while it is nursing the cow and consuming primarily cow’s milk, the rumen does not develop very quickly. The milk, for the most part, is shuttled past the rumen and reticulum into the abomasum and on down the digestive system.  This shuttling continues even as the calf grows and the rumen develops.

As the calf suckles, the rumen system creates what is referred to as an esophageal groove which acts like an extension of the esophagus, helping the milk by-pass the first compartments and the digestive activities which are found there. This helps the milk to be digested in a more complete form farther down the digestive tract.

As the calf gets a bit older and starts nibbling on grass and hay, the rumen grows and develops and takes on a digestive role. The feeding of grass and hay actually helps stimulate the rumen’s development.

Many factors play a role in the ruminal system development, but there are five “ingredients” that are required to cause ruminal development. They are:

-Establishment of bacteria in the rumen

-Liquid in the rumen

-Outflow of material from the rumen (muscular action)

-Absorptive ability of the tissue of the ruminal wall

-Substrate (feeds and forages) available in the rumen

Let’s discuss these ingredients and a few others below:


Fresh, clean water is critical to rumen development. Without sufficient water, bacteria cannot grow, and ruminal development is slowed. Most water that enters the rumen comes from free water intake. In most cases, water is available to calves at an early age and is not a problem.

Milk does not constitute “free water.” As discussed before, when calves consume milk it bypasses the rumen and reticulum by the action of the esophageal groove. The esophageal groove is active in the calf until about 12 weeks of age. The groove closes in response to nervous stimulation, shunting milk past the reticulum and rumen and into the abomasum. Closure of the groove occurs whether calves are fed from buckets or bottles. Therefore, the feeding of milk or milk replacer should not be construed as providing “enough water.”  Calves need fresh, clean water from as early as 3 days of age in addition to milk.

Rumen Bacteria in Calves

Rumen bacteria in the pre-ruminant animal (before rumen development begins) are quite different from those in the mature ruminant.  When the calf is first born, the rumen is sterile – there are no bacteria present. However, by one day of age, a large concentration of bacteria can be found — mostly aerobic (or oxygen-using) bacteria. These bacteria appear to enter the rumen from swallowing items containing bacteria from the outside environment (bedding, saliva, etc). To a certain degree the calf is also “inoculated” by the cow as she licks it, especially around the muzzle area.  Aerobic bacteria aren’t the type of bacteria normally found in rumen of mature animals. You can think of these bacteria as temporary – they will be replaced by bacteria that can better compete in the rumen once the calf begins to eat dry hays and feed.

The number and types of bacteria change as hay and feed intake occurs and the substrate (feed and forage material) available for fermentation changes.  There is a dramatic decrease in the number of aerobic bacteria that occurs shortly after calves begin to consume forage and/or grain.  The change in bacterial number and type is almost always a function of intake of substrate. Prior to consumption of dry feeds, bacteria in the rumen exist by fermenting ingested hair, bedding, and from milk that flows from the abomasum into the rumen. The substrate ingested will also affect the types of ruminal bacteria that flourish in the young rumen. For example, calves fed mostly hay develop a different population of bacteria from those fed mostly grain, as is the case with young dairy calves.


This is simply the beginning of our discussion concerning the ruminant digestive system along with its development and function.  In subsequent parts to this series, we will provide more background information and expand on the cow’s digestive system and nutrition.

Discuss this article

Post a comment

Sign In or Register to use your BeefCattle ID