The compartments in the ruminant stomach and intestines combine to create a large factory that breaks down (digests) feed into substances that can be absorbed into the bloodstream. The entrance to this factory is the cow’s muzzle where the chewing movement grinds the plant material and moistens it with saliva. Waste and undigested feed components are excreted by the cow via the manure.

In between the entrance and exit are the rumen and the other compartments of a cow’s stomach and the intestines. The ruminant forestomachs (reticulum, rumen, and omasum), small intestine and large intestine each have their own, unique task in the digestive process. In order to assess the quality of the silage, knowledge of the functioning of the rumen is very important. The feed is predigested in the rumen. The majority of the feed is converted by microbial predigestion into substances that can be easily absorbed by the cow, so-called rumen degradable substances. The components that pass through the rumen in undigested state are referred to as rumen undegradable.

The rumen

The rumen is divided into roughly three layers. The lower part holds the ruminal fluid. A thick mat of coarse, fibrous forage floats on this fluid. The space above this layer is for rumen gas. Depending on the structure of the grass silage, lumps of this material are swallowed and travel to the middle layer of the rumen or the lower layer containing the ruminal fluid. In the middle layer, bacterial activity breaks down the plant cells into small particles, which dissolve or sink into the fluid in the lower layer. The lower layer breaks down the rumen degradable components (carbohydrates and proteins).

The role of bacteria

The microbes in the rumen utilise the protein and energy provided by the grass silage to grow and reproduce. Some of the plant-based protein is converted into bacterial cells, which are called microbial protein.

Two thirds of the protein consumption of a cow is supplied by this microbial protein. It is vital in this process to ensure that protein and energy are available in the right balance. This is expressed as the rumen degradable protein (RDP) balance of the ration.

The microbes in the rumen degrade the energy-rich substances . This process releases propionic acid, butyric acid and acetic acid. The volatile fatty acids serve as a source of energy for the cow and as building blocks for milk production. So strangely enough, it is not grass that sustains the cow, but actually bacteria and their by-products.

NDF and feed intake

The digestibility of a plant largely determines a cow’s feed intake. Forage enters the rumen where the microbes found there only have a limited period to access the cell content of the feed. These cells store components including sugars, proteins, fats and minerals. 

The cell content, and partly the cell wall, of a plant cell are used to produce milk. It is therefore essential that the rumen microbes can access the content of these cells as quickly as possible. Once the cell content is available, the components are absorbed and converted by the microbes. The better the digestibility of the cell wall, the more the forage cow can eat.

A good measure of this is the NDF digestibility. It indicates how quickly the cell contents are released, and also the proportion of cell walls used for rumination. This is important to ensure a good rumen buffer and to maintain a stable rumen pH.

The role of enzymes 

Enzymes are essential for the digestion of feed and the cell walls especially. Rumen microbes themselves produce enzymes that degrade the cell wall, but enzymes can also be added. This reduces the time needed by the rumen microbes to produce enzymes. The feed is only present in the rumen for a limited period, so the rumen microbes can use this time optimally to access the cell content.

Rumen acidosis

Acidity (pH) and fibre structure are factors that affect the ‘working environment’ of bacteria. If a high number of volatile fatty acids, for example, are produced due to a high supply of carbohydrates, the rumen will acidify.  The disadvantage of forage with high levels of carbohydrates but with a poor fibre structure is that the cow ruminates less. This reduced rumination activity also reduces how much saliva reaches the rumen. Buffers in the saliva neutralise the acids produced in the rumen. The high acid production and low buffering effect of the saliva cause a rapid drop in the pH and microbial activity will stop. The digestion process grinds to a halt. This is called rumen acidosis.


Forage with plenty of long, coarse fibre stimulates the rumen wall. As a result, the muscles in the rumen wall contract regularly and stimulate the processes in the rumen. Fibre-rich forage – in addition to rumination – is also necessary to ensure the permeability of the solid, fibrous mat in the rumen. This permeability allows the gases formed in the lower part of the rumen to escape. Feed that is low in fibre will turn this solid mat into a papery mass. The gases in the rumen accumulate underneath this layer, and may even obstruct and block the oesophagus.

Silage grass is retained in the rumen for a while before it travels through the omasum and abomasum to the small intestine. Substances (such as enzymes) secreted by the pancreas, gall bladder and the intestinal wall continue the digestive process in the small intestine. It is mainly the undegradable portion of the feed – the part not degraded by the rumen microbes – that is digested in the intestines. As soon as the substances have been digested sufficiently, they are absorbed into the bloodstream.

Read more about NDF digestibility