Digestive Physiology and Anatomy of Cows

Cattle, along with sheep, goats and deer, are known as ruminants. This term means that they have more than one stomach (or more correctly, digestive compartments) compared to mammals such as humans and horses, which only have one, and so are called monogastric animals.

Cattle being ruminants gives them the ability to feed and produce of feedstuffs that would otherwise be wasted. Plant matter, such as forages and roughage contain cellulose which in most animals cannot be digested/broken down for use in the body. Ruminants possess the necessary digestive processes to make it possible to use cellulose as an energy source.

The digestive system in cows consists of a 4 compartmental stomach system followed by the intestinal system. The oesophagus is where the food is first brought into the system and at the end of the process, faeces (what is left over from the process) is excreted from the anus.

Stage 1: Ingestion and the Oesophagus

Starting at the beginning of the process, the cow will ingest food. Cows are herbivores so this food will be plant matter and therefore contain high amounts of cellulose. The food will first be slightly chewed by the cow, and then swallowed down the oesophagus.

The oesophagus is a muscular tube that connects the mouth with the fore stomach. Food passes down the oesophagus by action of muscular contractions in the walls of the tube that pushes the food along; this process is known as peristalsis. Ruminants are also capable of ‘reversed’ or ‘antiperistalsis’ which brings food from the fore stomach, back to the mouth; a process known as regurgitation.

Stage 2: The Rumen

This is the first compartment in the 4 compartment stomach system. In this compartment, fermentation occurs; which allows for the digestion of fibre and other feeds. 

Fermentation overall takes place in the rumen and reticulum (which is the second compartment). Fermentation is the conversion of carbohydrates into volatile fatty acids and gases; in terms of the cow, it allows the conversion of cellulosic fibre (found in plants) into energy that can be used for growth, production or metabolism.

The rumen is the largest compartment and can hold up to 40-50 gallons/150-200 litres of digested food.

The first thing to note about the rumen is that it is also home to a wide range of other microbes – such as bacteria and protozoa. The number of bacteria and protozoa in the rumen is astronomical; in 1ml of rumen fluid there is approximately 10-50 billion microbes and 1 million protozoa. These microbes are the organisms which actually digest the feed taken in by the cow.

Because bacteria digest by working on the outside of the food particles first, increasing the surface area of the food particles overall will increase the bacterial digestion. In simple terms, instead of having one large clump of food to digest, if that clump is smashed into tiny little pieces, the bacteria are more efficient at digesting it.

The microbes in the rumen digest cellulose and hemicellulose (i.e.  Fibre). They also produce high quality protein, volatile fatty acids (VFAs) and B vitamins. As well as these actions they also detoxify toxic compounds.

One of the main by-products of fermentation is VFAs, which are absorbed through the rumen wall and used as energy by the animal.

Protein, which is digested in the rumen (digestible intake protein = DIP) is used to support microbial growth and metabolic functions.

Plant matter also contains lipids (fat compounds) which are broken down by the bacteria into glycerol which can be used to produce VFAs and fatty acids (both of which are needed by the cattle). However, the fatty acids are not absorbed in the rumen; they will be absorbed later on by the small intestine.

The rumen wall (or mucosa) is a major site of nutrient absorption. It is convoluted to give it tremendous surface area for absorption. Rumen papillae (thousands of finger-like projections on the inside surface of the rumen) are responsible for absorbing the volatile fatty acids from the rumen for use by the cow. Volatile fatty acids (VFA’s) are absorbed from the rumen and used as an energy source for the cow. Microbial protein (the actual bodies of the microbes) is not absorbed from the rumen. It absorbed from the cow’s intestine and is used as a source of quality protein for the cow.

Unsaturated fatty acids are toxic to the rumen bacteria and the rumen can only tolerate a certain level of these toxic substances. The unsaturated fatty acids can be converted to non-toxic fatty acids such as conjugated linoleic acid (CLA). However if too much fat is fed the bacteria will be overwhelmed and this will lead to a reduction in fibre digestibility.

Figure 1: Rumen

Of the gases produced within the rumen during fermentation (500-1500litres per day), around a quarter consists of methane and carbon dioxide. Production of fermentation gases represents a considerable energy loss. Certain modifiers, such as ionophores, improve energy efficiency of ruminants by reducing those gas energy losses. The gases produced are expelled as belching; when belching is not possible bloat occurs.

Rumen Conditions

In order for the bacteria to work efficiently in the rumen, there must be certain conditions met in the rumen. Most importantly there must be a pH (acidity value) of around 6-7. One main reason for this is because the bacteria digest cellulose and other molecules by producing substances called enzymes – enzymes are protein molecules which help to allow a reaction to take place, such as the breakdown of cellulose. These enzymes are very delicate in terms of what they can withstand. pH and temperature are the two main factors that affect enzyme efficiency.

If the pH of the rumen drops below 6.0 (becomes more acid) then the rumen is known as acidotic. Ruminal acidosis can be acute with rapid and severe drops in pH. If the cattle are fed frequently on small amounts of food there will be less fluctuation in rumen pH compared to two large feeding times…

The rumen is anaerobic. This means that there is little or no oxygen in it. The rumen microbes cannot grow in outside air. They will tolerate a small amount of oxygen so long as the fermentation is going strong enough so that they can get rid of the oxygen quickly. Some oxygen does, of course, get into the rumen through feed and water. Secondly, the rumen temperature is one degree above body temperature at 39°C (102.5°F).

Rumen pH ranges between 5.7 and 7.3. The high side of this pH range (> 7) will be seen on poor quality forage diets supplemented with urea. In high-producing dairy cows, acidosis (rumen pH<6.0) is a common problem. This occurs when the cow eats too much rapidly digestible starch or sugar that creates acid and overwhelms the rumen’s buffering system. Most of the buffer in the rumen comes in the form of saliva that is generated when the cow chews her cud. Inadequate intake of long fibre that promotes rumination (cud-chewing) can also result in acidosis because it provides less salivary buffer to counteract the acid produced by grain fermentation. The rumen microbes, especially those that primarily digest fibre, are acid intolerant. They do not grow well in acid and they don’t digest feed, especially forages, well under acid conditions.

Rumination

As the food passes through the first part of the digestive tract (the oesophagus and rumen), certain stages occur in order to process the food. Generally these processes are collectively called ‘Rumination’. 

At first the cow chews the food with almost any sorting. 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.

Rumination process:

1. Mastication – the process of grinding enlarges the surface area of feedstuff. This greater surface area helps the ruminal microorganisms and digestive juices to break down the feedstuff.

2. Saliva is added – during mastication, large amounts are added.
A cow produces between 40- 150 litres of saliva per day, depending on the type of food that is fed. Roughage has the effect of increasing rumination, whereas concentrates reduce it.

Saliva has two main functions:

A. Buffering – because saliva has a pH of roughly 8.2 and a high sodium bicarbonate level, it has a buffering effect in the rumen. A buffering effect acts to keep the acidity (pH level) of the rumen fluid at a constant level, and so avoiding very acidic or very alkaline conditions in the rumen. This is important because many feedstuffs, e.g. cereals, molasses, potatoes and fodder beets produce acidity when digested. Without the buffering effect, the acidity level of the rumen would lead to inefficient digestion of feed as the bacteria would be working in unfavourable conditions.

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

3. Chewing the cud – this process usually takes place when the animal is resting. When a cow initially ingests food, there is minimal mastication and the food is mixed with saliva to form a bolus and the bolus is swallowed into the rumen. When they turn in to rest they regurgitate the bolus, re-chew it and swallow it again. This process is known as “chewing the cud” or “rumination”.

Overview of the process of food through the rumen…

·        

Food enters the rumen through the oesophagus, at this stage is only slightly chewed. The bolus is layered onto the rumen mat, which floats upon top of the rumen contents.

·         Rhythmic contractions of the ruminal wall cause the freshest food to accumulate at the rear 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 the cud is grinded to increase the surface area.

·         The feed particles become smaller and the feed is swallowed again, the bacteria digest again and this process gradually digests all the food. The bacteria gradually absorb fluid and sink to the bottom of the rumen. At the bottom of the rumen there is a dry matter content of only 5%.

·         The ruminal contractions occur once every minute. These contractions allow mixing of fluid and solid contents in the rumen to stimulate fermentation and avoid stagnation. Contractions also serve to release trapped gas in either the mat or fluid portion of the ruminal contents. These gases are then released by belching.

·         Feed particles at the bottom of the rumen, where the rumen meets the reticulum (2^nd compartment) are sorted into correct size and density and segregated into the fluid in the reticulum. Subsequent contractions force these particles and some fluid out of the reticulo-rumen and into the omasum (3^rd compartment).

Rumen Summary…

·    

     Largest compartment – in the cow’s digestive system.

·        Stores and processes food – which is its primary function.

·         Site of fermentation – which converts carbohydrates (e.g. cellulose) into volatile fatty acids.

·         Home to millions of microbes – which make fermentation possible.

Stage 3: The Reticulum

*Sometimes the reticulum is known as the reticulorumen as it is a small compartment almost directly connected to the rumen*

The rumen are reticulum are basically one compartment, but with different functions. As explained above, the rumen has a mostly fermentative action whereas the rumen serves as more of a staging area for passage into the omasum or regurgitation. The rumen and reticulum are partially separated by the reticular fold, which allows mixing between the two compartments.

The structure of the reticulum is similar to a honeycomb structure in that the walls are lined with membranous structures. The main function of this compartment is to trap large feed particles. This is important for two reasons – firstly so that they do not enter the next section of the stomach before they’re digested (as this could lead to blockages) and secondly to be returned for additional rumination (as to not lose any cellulose energy that can be gained by digestion). The particles that are large enough to be trapped are ruminated and those that are small enough pass through to the next compartment (the omasum).

The reticulorumen is also famous for being the site at which “hardware” (nails, screws, bits of fence) becomes lodged and often leads to serious problems.

Summary of the Reticulum…

·         Staging area – for the feed particles in the rumen.

·         Large particles – are re-ruminated.

·         Small particles – pass through to the next stage.

·         Hardware articles – often found in the reticulum.

Stage 4: Omasum

This is the 3^rd compartment of the digestive system of the cow. Once the food has been ‘broken down’ enough (digested by the bacteria in the previous stages) and is small enough to pass through the reticulum, it will enter the omasum through the reticulo-omasal orifice.

The omasum wall is highly folded containing ‘leaf like’ structures which give the omasum a large surface area (4-5m²). A large surface area increases the efficiency of absorption – which is the main function of the omasum.

The omasum surfaces absorb water and salts (potassium and sodium) which are necessary for normal bodily functions. The amount of water absorbed is around 30-60% of the water intake of the cow.

The many folds of the omasum wall are used to ‘squeeze’  the feed passing through the omasum, so that the majority of the water is absorbed here and does not pass into later stages of the digestive system and eventually lost through excretion. It is important that water is not lost because the rumen, which is 80% water, requires a lot of water intake to maintain mixing conditions (roughly 35-40 gallons of water per day).

Summary of the Omasum…

·         Main function – is to absorb water, salts and squeeze the partly digested food.

·         Leaf like structures – increase the surface area to increase absorption efficiency.

·        

Reticulo-omasal orifice – joins the omasum to the previous fore stomachs.

 

Stage 5 Abomasum:

This is the last stage of the 4-part stomach system. Unlike a ruminant’s 3 fore-stomachs, the abomasum is a ‘secretory stomach’. This means that cells in the abomasum wall produce enzymes and hydrochloric acid which hydrolyses (splits/breaks down) proteins in the food and also in the microbes mixed in the food.

The main function of the abomasum is to digest protein from both feed and ruminal microbes. The secretion of gastric juices (enzymes and acid), accomplish this. The pH value in this part of the digestive system is between 2-3 (very acidic compared to the previous stomachs).

This compartment is very similar to the stomachs that monogastric animals use; because it is used to breakdown the constituents of the feed stuff ready for absorption in the intestines.

Microbial protein plus indigestible intake protein from the feed is broken down and these amino acids (the constituents of proteins) are absorbed later on in the small intestine. The proteins present in the feed at this point are much different from those available in the rumen.

Summary of the Abomasum…

·         Similar to a normal stomach – of a monogastric animal.

·         Low pH – due to acidic conditions needed to hydrolyse the proteins and microbes.

·         Secretory organ – acid and enzymes are secreted by the abomasal cells.

The Small Intestine

When the feed has passed through the acidic abomasum it enters the small intestine. In the small intestine the pH is much higher, around 8. This is because pancreatic and liver secretions contain bicarbonate to counteract the acidic conditions from the stomach which would destroy the enzymes needed for the function of the small intestine. These enzymes are either on the surface of the intestinal cells or are secreted from ducts from the liver and pancreas.

The main function of the small intestine are:
> To enzymatically break down nutrients so that they can be absorbed;
Carbohydrates are broken down into simple sugars (monosaccharides), fats into fatty acids and monoglycerides, nucleic acids into nucleotides and proteins into amino acids. The products of the breakdowns are the simplest form of which that molecule exists and so are the building blocks for all metabolism and growth in cell; hence, are the building blocks of the animal.

> To absorb nutrients (e.g. fatty acids, sugars and amino acids) and water via the intestinal villi.

These stages of the small intestine can be explained by breaking down the small intestine into its constituent components, of which there are 3:  Duodenum, Jejunum and Ileum. (Listed in ascending order in terms of food passage). Together these 3 components make up what is known as the small intestine. The length of the small intestine is around 20 times the length of the animal (if a cow is 2m long, the intestines are 40m in length!).

Food passes along the intestine in the ascending direction by actions of contracting muscles which push the food along - this overall action is called peristalsis (similar to the way the food moves down the oesophagus). The contractions start at the abomasum and work their way down the duodenum first, carrying the food along with it; eventually passing down the jejunum and then the ileum.

Stage 1 The Duodenum:

This part of the small intestine is supplied with secretions produced by the liver and pancreas.  These secretions are dripped through ducts into the duodenum cavity.

The common bile duct carries bile salts, a greenish fluid that is manufactured in the liver, stored in the gall bladder. This bile secreted into the duodenum in order to help digest fats.

The main pancreatic duct carries digestive secretions, which are rich in enzymes and bicarbonate.  The bicarbonate neutralises acid from the stomach. The acidity from the stomach would otherwise denature (inactivate) the enzymes needed for digestion.

Stage 2 The Jejunum:

The lining of the jejunum is specialised for the absorption of carbohydrates and proteins.

Its inner surface is covered in finger-like projections called villi; the function of these villi is to increase the surface area available to absorb nutrients from the gut contents. The villi in the jejunum are much longer than in the duodenum or ileum. The epithelial cells (cells on the outer most layer) of the villi possess even larger numbers of microvilli; known collectively with the villi as the brush border. The combination of villi and microvilli increases the surface area of the small intestine, increasing the chance of a food particle encountering a digestive enzyme (which are located on the surface of the microvilli) and so being absorbed across the epithelial cells and into the blood stream (and so completing the absorption of that food particle).

Nutrients can cross the intestine wall by either passive transport or active transport.
In passive transport molecules diffuse into the intestinal cells down a concentration gradient (i.e. from an area with lots of the molecule to an area where there is a lesser amount of the molecules). An example of this is the sugar, xylose.
Active transport requires energy. Amino acids, small peptides, vitamins and most glucose are moved across the intestine lining by active transport. Once nutrients have moved through the epithelial cells, they are taken up by either capillaries or lacteals and then transported around the body.

Stage 3 The Ileum:

The main function of the ileum is to absorb vitamin B12, bile salts and whatever nutrients were not absorbed by the jejunum.

At the point where the ileum joins the large intestine there is a valve, called the ileocaecal valve, which prevents materials flowing back into the small intestine.

The Caecum

This part of the intestinal system is a connection between the end of the small intestine (ileum) and the start of the large intestine. The ileocaecal valve separates the caecum and the ileum. The caecum is considered to be the start of the large intestine.

In herbivores, such as cattle, the caecum is greatly enlarged and serves as a storage organ that permits bacteria and other microbes time to further digest cellulose. In carnivores, including humans, the caecum is relatively unimportant in function.

The Large Intestine

The main functions of the large intestine are to absorb, re-circulate and conserve water; as well as being a major site of mineral absorption and an area of microbial development.

The large intestine is made up of the caecum, ascending colon, transverse colon, sigmoid colon, rectum and anus. Much of the large intestine comprises the colon, which is shorter in length but larger in diameter than the small intestine.

The colon is involved in the active transport of sodium and the absorption of water by osmosis from the digest material. It also provides an environment for bacteria to grow and reproduce. These symbiotic bacteria produce important vitamins such as Vitamin K, thiamine and riboflavin. Undigested and unabsorbed food leaves the body, as well as other wastes, in the form of faeces which exits via the rectum and anus.

The large intestine, especially the caecum and colon, supports an active fermentation that is quite similar to that of the rumen. The fermentation in the large intestine supplies roughly 10-15% of the gross energy available to a dairy cow. However, most microbial protein is lost via excretion.

An excess of fermentable carbohydrates reaching the large intestine can result in digestive problems. These problems can range from diarrhoea to caecal torsion.
The diarrhoea that frequently characterises subclinical acidosis occurs when suppressed digestion in the rumen, results in greater flow of fermentable carbs to the intestines. Because there is little buffering in the large intestine, an active fermentation can drastically reduce pH leading to very acidic conditions - which may result in excess fermentation and gas production.