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Digestion in Ruminants

Ruminant animals are set apart from other mammals by their complex digestive systems. The way they process food, absorb nutrients and gain energy is different from other herbivores.

The main distinction in a cow’s digestive system, or a ruminant digestive system is that the stomach has four separate compartments, each with a unique function, whereas most other animals only have a single compartment with a unified functionality.

The four compartments allow ruminant animals to digest grass or vegetation without completely chewing it first. Instead, they only partially chew the vegetation, then microorganisms in the rumen section of the stomach break down the rest. Animals with singular stomach compartments — known as a monogastric digestive system — do not have the same capability.

These animals convert plant matter and vegetation into useable energy more efficiently than other herbivores.

In cattle and dairy cows, the development, pH balance, functionality and bacteria levels of the digestive system are crucial to maintaining overall health and high yield.

While some parts of the ruminant digestive system are similar to those of non-ruminant systems, several essential components perform the necessary functions for digestion.

Components of the Ruminant Digestive System

While the ruminant digestive tract operates differently from the monogastric system, it is composed of the same six basic components:

1. Mouth

The mouth is where the process begins. Cattle will graze by wrapping their tongues around plants and tearing, pulling them into their mouth for mastication. They chew first with the lower jaw incisors, working against a hard dental pad on the front part upper palate, then second with the molars, grinding plant material down further. Chewing stimulates saliva production and the saliva mixes with plant matter before the animal swallows. Saliva contains enzymes capable of breaking down fats and starches and helps to buffer the pH levels in the reticulum and rumen segments of the stomach. Mature cattle will swallow from 50 to 80 quarts daily to aid in digestion, but the amount varies based on how much time they spend chewing.

2. Esophagus

When the cattle swallows the plant material and saliva mix, it will travel down the esophagus to the rumen. The esophagus performs the swallowing action through waves of muscle contractions, moving the feed down. It has a bidirectional function, meaning it can move feed from the mouth to the stomach or from the stomach to the mouth. Cattle need the latter to regurgitate “cud,” or the under-chewed plant matter and grain, back up to the mouth for further grinding. Once the cow is finished chewing the cud, it again swallows the matter back down to the stomach.

2. Stomach

Generally, the stomach functions to further break down plant matter and grain. More specifically, there are four sections of the stomach — rumen, reticulum, omasum and abomasum — each with a particular job to do. These sections store chewed plant material and grain, absorb nutrients and vitamins, break down proteins, aid in beginning digestion and dissolve material into processable pieces. The next section will focus more closely on the responsibilities and functions of each stomach compartment.

3. Small Intestine

The small intestine has three main sections — the duodenum, jejunum and ileum — that work together to complete most of the actual digestive process. In the duodenum, the section connected to the stomach, secretions from the gallbladder and pancreas mix with the partially digested matter. This process balances the pH in the intestine, ensuring the digestive enzymes work correctly. The jejunum section is lined with small, finger-like projections known as villi, which increase the intestinal surface area and absorb nutrients. The ileum absorbs vitamin B12, bile salts and any nutrients that passed through the jejunum. At the end of the ileum is a valve, preventing any backward flow of materials. Throughout the small intestine, muscular contractions move the matter forward. In a fully mature cow, the entire organ may be up to 150 feet long and has a 20-gallon capacity.

4. Cecum

Sitting between the small and large intestines is a three-foot-long pouch called the cecum. It has little function besides providing storage and a transition between the two intestines, but it does aid in the continual breaking down of material. The cecum has about a two-gallon holding capacity.

5. Large Intestine

Smaller in length but larger in diameter than the small intestine, the large intestine is the final step of the digestive process. It absorbs remaining water and contains bacteria microbes that finish digestion and produce vitamins the animal needs to grow and remain healthy. Its last job is to eliminate any undigested and unabsorbed food from the system in the form of waste.

When the cow is properly handled and fed, this process continually occurs, keeping the animal healthy and at the right weight. The entire digestion process should take anywhere from one to three days.

If something interrupts this process or the cattle is unhealthy, the sections will no longer be able to function as well as they should, causing diseases and complications.

The Four Components of a Cattle’s Stomach

Of the six components in the cattle’s digestive system, the most important part is the stomach. A ruminant animal’s stomach has four distinct compartments, each with its specific function. These compartments are:

1. Rumen

The rumen, also known as the “paunch,” is the first area of the cow’s stomach, connected to the cattle’s esophagus. This compartment acts as storage for chewed vegetation and forms balls of cud.

Cud consists of large, non-digestible pieces of plant matter that must be regurgitated, chewed a second time and swallowed before continuing through the process. The rumen absorbs nutrients through papillae of the rumen wall and facilitates fermentation, creating the rumen bacteria and rumen microbes necessary to break down and digest the proteins in feed. Microorganisms in the rumen are responsible for digesting cellulose and complex starches, as well as synthesizing protein, B vitamins and vitamin K. As a storage area, it can hold up to 40 gallons of material. The rumen, combined with the reticulum, makes up 84% of the volume of the entire stomach. A few common health issues with the rumen include bloat, which occurs when a cow can’t eradicate a buildup of gas, acidosis and rumenitis, which occur when low pH balance allows for high acid production. These can be prevented by managing and paying attention to cattle’s food and water intake.

2. Reticulum

The reticulum is frequently referred to as the “honeycomb,” because the inner lining appears like and is structured similar to a honeycomb. While it does have its independent functionality, the reticulum is attached to the rumen with only a thin tissue divider. This component holds heavy or dense objects — such as metal pieces and rocks — and trap large feed particles that are not small enough to be digested. The reticulum facilitates regurgitation. Both the rumen and reticulum contain digestive bacteria, so no acid is included in the regurgitation of materials. The reticulum holds about 5 gallons of material. One common health issue involving the reticulum is hardware disease, which occurs when cattle ingest heavy or sharp objects — like nails, screws or wire. They are swept into the reticulum and may puncture the stomach wall. This disease is preventable by putting magnets on feeding equipment to catch any metal, or cured by the placement of an intraruminal magnet that traps already swallowed objects.

3. Omasum

It is lined with large leaves and folds of tissue that resemble the pages of a book. These folds absorb water and nutrients from feed that passes through after its second round of chewing. The omasum is smaller than the rumen and reticulum, making up about 12% of the stomach’s total volume. It can hold up to about 15 gallons of material.

4. Abomasum

The abomasum is the last component of the stomach and is often known as the “true stomach,” because it operates the most similar to a non-ruminant stomach. This true stomach is the only compartment of the stomach lined with glands. These glands release hydrochloric acid and digestive enzymes to help the abomasum further break down feed and plant material. In comparison to the other chambers, the abomasum is on the smaller side, representing about 4% of the total stomach volume and only holding about 7 gallons of material.

Each of these components is vital in maintaining a healthy digestive process. They must cooperate quickly and efficiently to turn grain and plant matter into energy for the cattle. If one section becomes incapable of performing or ceases to work correctly, it will affect all of the other functions in the digestive system.

Because the rumen is the largest area of the stomach and the section that focuses on reducing feed to be passed through the digestive process, it is crucial that it is properly developed and remains healthy.

The Development of the Rumen Compartment of the Stomach

The ruminant system relies very heavily on the rumen segment of the stomach. For cattle to convert food into energy, their rumen must be healthy at all times and properly developed. All cattle handlers, including both beef cattle and dairy cows, need to know how to ensure the success of a calf’s stomach growth.

When a calf is born, it begins its life as a functionally non-ruminant animal. It has the ruminant anatomy, but only the abomasum is fully developed at the time of birth. This is the compartment that has a similar processing ability to the human stomach.

While the other three chambers are present, they remain undeveloped and out of use as long as a calf continues feeding solely on milk. As the calf begins to consume starter grain and forage, bacteria microbes start to develop in the rumen and reticulum. The further fermentation of these bacteria is what causes the rumen to begin development.

Milk and liquid substitutes bypass the rumen and reticulum, but dry feed collects in these areas, beginning the chemical changes necessary for development. Dry feed absorbs water already ingested by the cattle, providing the right conditions for bacteria growth.

That bacteria then helps to metabolize nutrients and produce volatile fatty acids, effectively lowering the pH of the rumen by way of neutralizing acids and improving bacteria growth.

The acids produced by bacteria provide energy for the rumen wall to grow. Butyric acid does not absorb through the wall, so all the energy it produces goes straight to the development of the organ. Other acids provide energy for the entire calf to grow, which contributes to the digestive system organs, as well.

Weaning is one of the most significant key factors in the development of the rumen. Timing the weaning process correctly is crucial. The calf’s rumen should be allowed time to develop before weaning the calf off of liquid feeds entirely. It takes about three weeks of significant starter grain intake daily for any calf to develop its rumen to the point where the weaning process can begin.

This time period allows for the establishment of a sufficient microbial population and absorptive capacity for continued normal growth without the help of liquid feed. If the calf is weaned before this stage, the calf may lose weight or not grow for the three weeks it takes the rumen to develop.

To encourage proper rumen development, handlers need to maintain a certain level of care for all calves, keeping them well fed, housed and managed.Calves need to feed to gain the nutrients and energy that supplement growth. But, if it is stressed or sick, a calf may refuse to eat. For this reason, it is crucial that their environment is consistently low-stress and that they remain healthy. They should also have a free choice of clean, accessible water.