How rumen works

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We spend a lot of time figuring out what to feed our cows, we see feed go in, we see the results of this feed in production and we see what's left come out the back end. So what happens in between? Understanding how feed is utilised it makes it easier to see where problems can arise and how we can manage and prevent these problems.



Cows rely on the rumen to convert feed into useable sources of energy and protein

How quickly feed is digested depends on the size of particles and how digestible they are

Growth and increase of bugs in the rumen depend upon the pH of the rumen and the source of energy and protein

Rumen bugs break down carbohydrates to make volatile fatty acids (VFA's) and gases (methane)

VFAs are the main energy source of the cow

The amount of VFA's produced depends on the food source

Different VFAs produce fat or protein in milk

Rumen bugs break down protein and nitrogen to produce new bugs

Bugs are flushed out of the rumen into intestines where they are absorbed

This protein is the cows main source of protein

Protein not digested in the rumen can be digested in the intestine

Most fats are digested in the intestine



Three steps are involved in cows obtaining nutrients from their diet

1) Ingestion - taking food into the body]

2) Digestion - food is mechanically and chemically broken down

3) Absorption - nutrients pass from the digestive system into the cows blood stream


The digestive system of dairy cows is well adapted to a diet of plant material. As ruminants, cows have one true stomach (the abomasum) and three other compartments (the rumen, reticulum and omasum) that have specific roles in the breakdown of feed eaten. The rumen is the critical part and contains many bugs to help digestion.

It is the bugs (microbes) in the rumen that are essential for the cow to function. AS the saying goes "look after the rumen bugs, they will look after the cow"

The Rumen And Reticulum

Once food has been digested it is briefly chewed and mixed with saliva, swallowed and passed down the oesophagus into the rumen. It is the largest compartment of the adult ruminant stomach.

The rumen is like a big "fermentation vat" as its full of fluid containing millions of microbes that ferment feed to release its energy. The rumen and reticulum together have the capacity of 50-120 litres of food and fluid. The temperature inside the rumen remains stable at around 39oC, which is suitable for a range of microbes.

The microbes break down feed through a process of fermentation. Under normal conditions, the pH of the rumen and reticulum is maintained in the range of 6-7 this is sometimes a bit lower in grain fed cows.



Saliva has several roles, it makes chewing and swallowing easier, but primarily it contains sodium and potassium salts that act as buffering agents against acidity. A cow can produce 150L or more of saliva daily. The volume of saliva secreted depends on the time spent eating and ruminating.

Chewing and ruminating

Before food reaches the rumen its breakdown has already begun by the mechanical action of chewing. Chemical breakdown is initiated by the microbes in the rumen. The walls of the rumen and reticulum help the flow of finer food particles into the next chamber. 

The rumination, or chewing the cud, is the process whereby newly eaten food is returned to the mouth for further chewing. This extra chewing breaks down the feed into smaller pieces, making it easier to digest. The more fibre in the diet, the more time spent chewing.

There is a constant flow of food through the digestive tract. Because food larger than 1mm cannot leave the rumen until its length is reduced, the rumen is probably the major regulator of feed intake.

Microbes of the rumen and reticulum

Billions of bacteria, fungi and protozoa are found in the rumen. They digest about 70-80% of digestible dry matter in the rumen. Different species of bacteria and protozoa perform different functions. Some digest starch and sugar while others digest cellulose. The numbers and proportions of each type of microbe depends on the individual animals diet. Maintaining a healthy mixture of different microbes is essential for keeping the rumen functioning efficiently.

The major end products of microbial fermentation are:

Volatile fatty acids (VFA's)

Ammonia. Bacteria are 60% protein, making them the major source of protein for the cow as they leave then rumen and are digested in the abomasum and small intestine.

Gas - mainly methane and carbon dioxide - these are a source of energy loss as they are belched out regularly

Dietary upsets, such as feeding too much grain too quickly can cause a rapid change in the microbial population. This changes fermentation patterns and interferes with digestion. Adjusting the level of grain should therefore be done gradually so that populations of rumen microbes can adjust.



The omasum lies between the reticulum and the abomasum. The primary function of the omasum is to remove of some of the high percentage of water leaving the rumen and to further grind and break down feed.


The abomasum connects to omasum to the small intestine. The abomasum is like the human stomach and uses acid to break down food - including microbes from the rumen.

Small intestine

From the abomasum the digested food and digested microbes move into the small intestine. There enzymes continue the digestion of feeds and microbes. Most nutrient absorption occurs in the small intestine.

Large Intestine:

The large intestine, mainly the caecum and colon is the site of secondary fermentation, particularly of fibre. 10-15% of the energy used by the cow is absorbed from the large intestine. Absorption of water, minerals and ammonia also occurs here.


The components of feed not digested in the large intestine are passed through and expelled as faeces.