Basic Ruminant Anatomy

Before we delve too deeply into the massive topic of ruminant nutrition.  Let’s start by covering the basics.  What is a ruminant animal and what is the role of their digestive system?  The primary difference between ruminants and non-ruminants is that ruminants’ stomachs have “four” compartments and they are known for their cud chewing behaviour.  There are roughly 150 different species of ruminants and they include both wild and domesticated animals.  These include animals such as deer, buffalo, elk, giraffes, camels and those that we are most concerned about in Australian agriculture sheep, cattle and goats.  

The food that ruminants eat are mainly forages and fibrous roughages that consist largely of cellulose, which cannot be broken down by mammalian digestive enzymes.  To handle this diet ruminants evolved with a special system of digestion that involves microbial fermentation of food before its exposure to their own digestive enzymes.  The stomach of a ruminant is divided into four compartments with each doing a different job.  The rumen changes as the animal grows and balancing the nutrient requirements of both the rumen microorganisms and the animal is essential for good animal performance.

What are the four stomachs and what do they do?

The four compartments of the stomach consist of the reticulum, rumen, omasum and abomasum.  The first two compartments are really one big sac called the reticulo-rumen.  

Rumen – 60-70% of all digestion occurs in the rumen.  Sometimes called the ‘paunch’ and it is lined with papillae for nutrient absorption and divided by muscular pillars into sacs.

Reticulum – The reticulum is sometimes called the ‘honeycomb’ due to the honeycomb appearance of its lining and is found directly in front of the rumen and acts as a ‘screening device’ and prevents foreign objects such as nails interfering with the digestive process.  Heavy, sharp objects caught in the reticulum can sometimes penetrate the reticulum wall and make their way to the heart which can lead to hardware disease.

At birth the animal’s rumen and its continuation the reticulum (reticulo-rumen), are relatively undeveloped.  Milk bypasses the reticulo-rumen by a tube-like fold of tissue called the oesophageal groove and is delivered directly to the third and fourth compartments, the omasum and abomasum.  As the calf or lamb begins to eat solid food, the reticulo-rumen grows in size, until in the adult they comprise 85% of the total capacity of the stomach.  The consumption of fibrous foods such as straw and hay stimulates the enlargement of the reticulum.  In the rumen fermentation of concentrates such as cereals, encourage the formation of papillae.  Papillae are small finger-like projections that increase the surface area for the absorption of nutrients.  A combination of fibrous and starchy foods assists in the development of the rumen and encourages the weaning process (McDonald, 2011).  In adult animals the oesophageal groove does not function,  allowing food and water to be deposited directly into the reticulo-rumen.

The breakdown of food is a combination of physical and chemical means.  Food is mixed with saliva and is transferred from the mouth to the rumen via the oesophagus.  The food is separated into layers of solid and liquid materials, the solids clump together to form the cud or bolus.  The cud is regurgitated and chewed to completely mix with saliva and break down the particle size.

The rumen and reticulum act as a fermentation vat where plant material is broken down by millions of microorganism (bacteria, fungi and protozoa).  Rumen conditions need to stay within a specific limited range to function properly.  The ideal environment for rumen microbes are:

6.5-7 – slightly acidic


No oxygen

Large quantity of water

2-3 contractions/minute

A plentiful supply of ammonia and carbohydrates

Obviously the type of feed available to the animal will influence these conditions.  Mature dry grasses are low in protein and carbohydrate and limit microbial growth.  High grain diets can lead to high acidity (low pH) that is toxic to many rumen microorganisms and will compromise microbial growth and digestion.

This inter-dependence between the ruminant animal and ruminal microbes is known as a symbiotic relationship.  The animal provides the home and forage; the microbes digest the forage to supply nutrients for their own growth and reproduction.  The nutrients not utilised by the microbes and the microbes themselves, supply the animal the nutrients it requires for growth and reproduction.  Increasing the production of microbes in the rumen is the key to lifting production.

The types of microbes and their role:

Different types of anaerobic fungi depending on the animal’s diet.  Fungi make up around 8% of the total microbial mass in the rumen.  Fungi play an initial role in digesting plant fibre and starches ingested by the animal.  They are particularly important when the animal is being fed a fibrous diet.

Different types according to diet.  There are 25-200 billion bacteria per gram of rumen fluid.  Bacteria undertake biochemical processes to break down and digest plant material.

Different types according to the diet.  In one gram of rumen fluid there would typically be around 90,000-100,000 protozoans present.  Protozoa mainly digest starch and sugar and also predate on bacteria.

There are two main groups of rumen microbes:  the slow-working fibre-digesters located on the fibre mat in the rumen and the fast-working microbes that float around in the rumen fluid, looking for easily digested nutrients like sugars and starches.  Sudden changes in diet from fibrous to grain will upset the balance of these microbe populations and can lead to conditions such as acidosis.

Omasum –  The omasum is round in shape and is connected to the reticulum by a short tunnel.  It is sometimes called the ‘butcher’s bible’ in reference to the many folds or leaves which resemble pages of a book.  These folds increase the surface area where nutrients are absorbed from feed and water.  Water absorption occurs in the omasum.

Abomasum –  The abomasum is known as the ‘true stomach’ and is the compartment that is most similar to a stomach of a non-ruminant.  The abomasum produces hydrochloric acid and digestive enzymes (pepsin to break down proteins) as well as receiving digestive enzymes (lipase to break down fats) from the pancreas.  These secretions help prepare proteins for absorption in the intestines.  The pH value of this part of the digestive system is between 2-3.

What role do the intestines have?

The small intestine is the main site for the digestion and absorption of amino acids, fats and the limited amount of glucose that may be available.  The intestinal wall contains numerous ‘finger-like’ projections called villi that increase the intestinal surface area to aid in nutrient absorption.  Muscular contractions aid in mixing digest and moving it to the next section.

The large intestine consists of the caecum, colon, rectum, anus and it functions to absorb water from the material passing through it and then to excrete the waste material as faeces.  Microbial digestion continues with volatile fatty acids and microbial protein being produced.  The protein cannot be subsequently digested and absorbed by the host animal.


The digestive process for ruminant animals is a complex system that relies on a symbiotic relationship with the animal and rumen microbes.  The maintenance of ideal rumen conditions is imperative in maintaining a healthy rumen and consequently a healthy animal.  Increasing the production of microbes in the rumen is the key to lifting production. 

Please contact us to find out how AgSolutions can help with your ruminant nutrition inquiries.

By Shannon Godwin (BAppSc GDTL)

McDonald, P. E. (2011). Animal Nutrition (Vol. Seventh Edition). Essex: Pearson Education Limited.

Meat & Livestock Australia. (2006). Beef Cattle Nutrition an Introduction to the Essentials. Sydney: Meat & Livestock Australia Lminited.

Parish, J. (2011). Ruminant Digestive Anatomy and Function. Mississippi State University.