Provides an overview of the digestive system of the cow. Describes each of the four stomachs as well as the small and large intestines. Covers rumen function in detail. Contains good basic diagrams of the ruminant digestive system.

The Stomachs

The cow is a ruminant with four stomachs:

rumen;

reticulum;

omasum; and

abomasum.

The Rumen

The rumen is the largest, with a volume of 150–200 litres (40-50 gallons).

In the digestion system there are billions of microorganisms. They help the cow to digest and utilize nutrients in the feed. To achieve efficient feed utilization and high milk yield, the bacteria must have optimal conditions. It is the bacteria that digest the feed. Feeding a cow, in fact, involves feeding the micro-organisms in her rumen.

The process of fermentation takes place in the rumen and the reticulum. Fermentation is when microorganisms convert carbohydrates into volatile fatty acids and gases. This process allows the cow to convert cellulosic fiber into energy.

Of gases produced within the rumen during fermentation (500–1500 litres per day) (150-400 gallons), 20–40% consist of methane and carbon dioxide. Production of fermentation gases represents a considerable energy loss. Certain fermentation modifiers, such as ionophores, improve energy efficiency of ruminants by reducing those gas energy losses.

The fermentation gases are expelled by belching. When belching is impossible or ineffective, cows can suffer from bloat.

Passage of feedstuffs through the digestive tract

Rumination and saliva

The cow chews feed almost without any sorting, which makes her different from other animals such as pigs. After a short period of mastication, when saliva is added, the feed is swallowed in the shape of a bolus. When the cow ruminates, feed returns back to the mouth and is masticated again. Most of the reduction of feed particles occurs during the rumination process.

Why is rumination so important?

Mastication. The process of grinding enlarges the surface area of the feedstuff. This greater surface area helps the ruminal microorganisms and digestive juices to break the feedstuff down. Saliva is added. During mastication, large amounts of saliva are added.

A cow produces between 40 and 150 liters (10-45 gallons) of saliva per day, depending on the feed she receives. Roughage has the effect of increasing rumination activity, where as concentrates reduce it.

Saliva has two functions:

A. Buffering. Saliva, with a pH value of approximately 8.2 and a high sodium bicarbonate level, has a buffering effect in the rumen. This means that the saliva helps to counteract the effects of acid-producing feedstuffs, such as cereals, molasses, potatoes and fodder beets, on the ruminal pH.

B. Suppressing foam. Saliva can reduce the risk of bloat as it also has a foam suppressing effect in the rumen.

Rumen & Reticulum

The cow’s rumen is like a large fermentation vat. More than 200 different bacteria and 20 types of protozoa help the cow to utilize fibrous feedstuffs and non-protein nitrogen sources.

When feed enters the rumen it is layered upon the rumen mat which floats upon the top of the rumen contents. Through rhythmic contractions of the ruminal wall, the freshly eaten material accumulates at the rear area of the mat. The rumen mat consists of non-digested material with a 15% dry matter content. Bacteria adhere to the feed and gradually digest the fermentable material. When the cow ruminates, cuds from the front layer are eructed. Saliva is added in the mouth and through the grinding action of the teeth, the surfaces exposed to micro-organisms become larger.

The feed particles become smaller as the bacteria work and the rumination process continues. They gradually absorb fluid and sink to the bottom of the rumen. The rumen contents in the bottom of the reticulo-rumen have a dry matter content of 5%.

The rumen contracts once every minute. The contractions allow mixing of fluid and solid contents in the rumen to stimulate fermentation and avoid stagnation. Contractions also serve to release gases trapped in either the mat or fluid portion of the ruminal contents. The fermentation gases are then released by belching. Disruption of this process can result in bloat. Feed particles of the correct size and density are segregated into the fluid in the reticulum by the ruminal contractions. Subsequent contractions force these particles and some of the fluid contents out of the reticulo-rumen and into the omasum.

The rumen and reticulum are basically one compartment, but with different functions. While much of fermentative action occurs in the rumen, the reticulum serves as a staging area for passage into the omasum or regurgitation.

Rumen pH

The ideal rumen pH value is between 6 and 7. The ruminal microorganisms are healthiest within this range. If the pH value varies too much, some types of micro-organisms are eliminated, and there is reduced utilization of the feed. Micro-organisms that digest cellulose (hay, silage, etc.) are unable to grow or ferment cellulose with a pH value below 6.0. When ruminal pH drops below 6, the rumen is considered to be acidotic. Ruminal acidosis can be acute with a rapid, severe drop in pH. More common in high producing herds is sub-clinical acidosis which is characterized by chronic, intermittent periods of low ruminal pH.

If the cow is fed large amounts of concentrates, her ration should be spread over the day. When the ration is fed only once or twice a day, the result is a large variation in the pH value in the rumen.

The figure shows a schematic description of what happens when concentrates are fed to the cow twice a day, 12 times a day, or in a Total Mixed Ration (TMR).

The Omasum

The omasum is the third compartment of the cow’s stomach. It is characterized by the presence of alarge number of leaves, which provide a wide absorption surface (about 4–5 m2). This surface absorbs water (30–60% of the water intake) and nutrients such as potassium and sodium. The omasum also prevents the passage of large particles through the digestive system, and may well have functions not yet discovered.

The Abomasum

The main function of the abomasum is to digest protein from both feed and ruminal microbes. Gastric juices, produced in abomasum, accomplish this. The pH value in this part of the digestive system is 2–3.

The Small intestine

When the feed has passed through the acid abomasum it enters the small intestine. Here, the pH value increases because the feed is mixed with pancreatic secretions, with a pH value of 8.

The main functions of the small intestine are:

to enzymatically break down nutrients so that they can be absorbed; and

to absorb nutrients (ie; fatty acids, sugars, and amino acids) and water via the intestinal villi.

The Large intestine

The large intestine absorbs, re-circulates and conserves water. The large intestine is also a major site of mineral absorption.

The large intestine, especially the caecum and colon, supports an active fermentation that is quite similar to that in the reticulo-rumen. The caecal-colonic fermentation may supply 10-15% of the gross energy available to a dairy cow. However, most microbial protein generated by this fermentation is lost via the manure.

An excess of fermentable carbohydrates reaching the large intestine can result in digestive problems. These problems can range from diarrhea to caecal torsion. The diarrhea that frequently characterizes subclinical acidosis occurs when suppressed digestion in the rumen results in greater flow of fermentable carbohydrates to the intestines. Because there is little buffering in the large intestine, an active fermentation can drastically reduce pH of the contents and subsequently result in detrimental conditions. One example is caecal torsion, a condition similar to an abomasal torsion that occurs under conditions of excessive fermentation and gas production.

In high producing dairy herds, a more recent condition called hemorragic jejunal syndrome (HJS) appears to be due to an overgrowth of pathogenic bacteria in the intestines. One of the predisposing factors to this disease is an increased flow of fermentable carbohydrates into the intestines, frequently due to high levels of intake and rapid passage of digesta through the entire digestive tract.