digestive physiology of the cow

8/2/2019 Digestive Physiology of the Cow

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Digestive physiology of the cow

Provides an overview of the digestive system of the cow. Describes each of thefour stomachs as well as the small and large intestines. Covers rumen function indetail. 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 150200 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 (5001500 litres per day) (150-400 gallons), 2040% 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?


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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 cows 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.


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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 cows stomach. It is characterized by the presence of alarge number of leaves, which provide a wide absorption surface

(about 45 m2). This surface absorbs water (3060% 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 23.

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.


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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.

Ruminant Anatomy and Physiology

Anatomy of the Adult


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The cow's digestive tract consists of the mouth, esophagus, a complex

four-compartment stomach, small intestine and large intestine(figure

1). The stomach includes the rumen or paunch, reticulum or

"honeycomb," the omasum or "manyplies," and the abomasum or "true

stomach."

The rumen. The rumen (on the left side of the animal) is the largest of

four compartments and is divided into several sacs. It can hold 25

gallons or more of material, depending on the size of the cow. Because

of its size, the rumen acts as a storage or holding vat for feed. It is also

a fermentation vat. A microbial population in the rumen digests or

ferments feed eaten by the animal. Conditions within the rumen favor

the growth of microbes. The rumen absorbs most of the volatile fatty

acids produced from fermentation of feedstuffs by rumen microbes.Absorption of volatile fatty acids and some other products of digestion

is enhanced by a good blood supply to the walls of the rumen. Tiny

projections called papillae increase the surface area and the absorption

capacity of the rumen.

The reticulum. The reticulum is a pouch-like structure in the forward

area of the body cavity. The tissues are arranged in a network

resembling a honeycomb. A small fold of tissue lies between the

reticulum and the rumen, but the two are not actually separate

compartments. Collectively they are called the rumino-reticulum. Heavy

or dense feed and metal objects eaten by the cow drop into this

compartment. The reticulum lies close to the heart. Nails and other

sharp objects may work into the tissue and cause "hardware disease."

If not prevented by a magnet or corrected by surgery, infection may

occur and the animal may die.

The omasum. This globe-shaped structure (also called the "manyplies") contains leaves of tissue (like pages in a book). The omasum absorbs water and other substances from digestive

contents. Feed material (ingesta) between the leaves will be drier than that found in the other compartments.

The abomasum. This is the only compartment (also called the true stomach) with a glandular lining. Hydrochloric acid and digestive enzymes, needed for the breakdown of feeds, are

secreted into the abomasum. The abomasum is comparable to the stomach of the non-ruminant.

The small intestine. The small intestine measures about 20 times the length of the animal. It is composed of three sections: the duodenum, jejunum, and ileum. The small intestine receives

the secretions of the pancreas and the gallbladder, which aid digestion. Most of the digestive process is completed here, and many nutrients are absorbed through the villi (small finger-like

projections) into the blood and lymphatic systems.

Cecum. The cecum is the large area located at the junction of the small and large intestine, where some previously undigested fiber may be broken down. The exact significance of the

cecum has not been established.

Large intestine. This is the last segment of the tract through which undigested feedstuffs pass. Some bacterial digestion of undigested feed occurs, but absorption of water is the primary

digestive activity occurring in the large intestine.

Figure 1. Anatomy of the adult digestive tract.
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digestive physiology of the cow

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