Making grass silage is a method of preserving forage. It is a preserving method, just like pickling, freezing, drying, salting or canning. The process of silaging is based on creating anaerobic conditions and sufficient acidity. Harmful micro-organisms like mould have little chance of developing in anaerobic conditions and acidification stops the decomposition process. This enables long term storage of the grass without compromising on the quality.

Grass is naturally rich in bacteria, moulds and yeasts. As soon as the grass has been cut and starts to wilt, all these micro-organisms do their best to decompose the grass. The different types of bacteria compete and develop at the expense of the others.

Two important bacteria in this process are lactic acid bacteria (the right kind of beneficial bacteria) and butyric acid bacteria (less desirable bacteria). For a successful silaging process, the lactic acid bacteria must outcompete their rivals. Farmers can encourage a good lactic acid bacteria population by choosing the right moment to harvest the grass, the field drying period, applying silage additives and the method of silaging.

Lactic acid bacteria

Lactic acid bacteria do not like moisture and oxygen. They grow by producing  lactate from sugar. Allowing grass to wilt and dry and rapidly removing the air by compacting the grass and sealing it airtight creates the ideal conditions for lactic acid bacteria to thrive. Once there are high enough numbers of these bacteria under the plastic cover, the silage will start to acidify. Depending on the dry matter content, it stabilises at a pH value of between 4 and 5.5. At this stage, the lactic acid bacteria will also start to suffer from the acidic environment and their activity will stagnate. Good, airtight silage will now stabilise and the fermentation process stops. Bacterial activity comes to a halt and the grass has been preserved. 


Sugar is necessary for lactic acid bacteria to grow. This sugar is contained in the grass. Grass that is rich in sugar is easy to preserve. Farmers can influence the sugar content in the silage grass by choosing the right moment to cut the grassland. The art is ensuring the highest possible sugar content. This moment is usually just before the ears or panicles emerge. At this stage the grass can still be easily digested by the cow and it is high in sugar and protein.

Field drying period

As soon as the grass has been cut, its feed value starts to decline. It is therefore not advisable to leave the cut grass in the field too long before starting the silaging process. The drier the grass becomes, the more difficult it is to digest the cell walls. Ultimately the cows will extract fewer nutrients from the silage as the rumen bacteria have to work too hard to degrade these cell walls. Grass left to wilt and dry for a shorter period retains a higher proportion of sugar. This is beneficial for the preservation process.


Ensiling is a method of storing cut grass in an airtight state. Firmly compacting, or consolidating, the silage expels all of the air from the grass. Plastic sheeting or wrapping applied afterwards creates an airtight cover.

It is quite difficult to effectively compact dry grass (more than 45% dry matter). The grass is too resilient and allows air to enter the gaps between the grass. This increases the risk of mould formation and spoilage caused by heating. The silage will become less palatable and the cows will eat less of it.

Silage with a dry matter percentage of between 30-35% is easier to compact by rolling. If possible, a second compaction vehicle can drive over the silage in the clamp to help maximise the compaction. This will have a positive effect on preservation, especially when there is a high feed-out rate.

Covering the clamp

When the silage has been rolled and compacted, the clamp can be covered. It is important to do this using airtight barrier film. Suction will quickly attach this type of film to the clamp so that oxygen rich air is excluded from reaching the grass. More importantly, the barrier film stops more air from entering the clamp. It is advisable to apply a barrier film with the highest oxygen impermeability.

After the barrier film has been applied, plastic sheeting can be laid over the clamp. Plastic sheeting protects the barrier film from UV radiation. Plastic sheeting with a white side reflects heat, and keeps the clamp cooler, than black sheeting. If the silage clamp is covered with a layer of sand, the tear resistance of the sheeting is important. High forces are exerted on the sheeting, so it must have a high tensile strength. If there is no layer of sand on the clamp, standard plastic sheeting with protection nets will do the job. These protection nets protect the clamp from external influences such as weather conditions and vermin.

Preserving the silage

The process of preserving the silage starts when the clamp is sealed and airtight. The silage in the clamp must stabilise. This occurs as the pH decreases. The less oxygen there is in the silage, the sooner the desired pH is reached to encourage lactic acid bacteria activity. The lactic acid bacteria will continue to lower the pH. A large population of lactic acid bacteria will speed up this process and prevent the activity of undesirable bacteria (such as butyric acid bacteria). When the desired pH value is reached, the silage will stabilise so preservation can proceed properly. The grass can be stored well for a longer period.

Even if all the conditions are right at the start, silage that does not contain enough sugar in relation to the dry matter percentage will not preserve well. The lactic acid bacteria will run out of fuel for activity before the desired pH is reached. They will be outcompeted by the butyric acid bacteria and the lactic acid will be converted into butyric acid. This causes the pH to increase, activates putrefactive bacteria and will eventually cause the entire silage stock to deteriorate.


When dairy farmers use the multi-layer (‘lasagne’) technique and ensilage several cuts of grass in layers, the clamp is exposed to oxygen after the barrier film has been removed. It is therefore advisable to remove the film at the last minute, just before a new batch of cut grass is supplied.

Sometimes the clamp is driven over a few times before the first cut of new grass arrives. This is not good practice. Driving over the clamp loosens the upper layer of silage and allows the ingress of oxygen.

After the new cut of grass has been added, the entire preservation process must take place again. Not just of the new layer, but also the existing layers underneath. Ensure the silage in the clamp also contains as little oxygen as possible before adding the new grass so the lactic acid bacteria can get to work as quickly as possible.

Opening the clamp

After a few months the clamp will be opened to feed out the forage to the cows. Opening the clamp also exposes the silage to oxygen. The more compacted the clamp is, the less chance oxygen has to enter. This is the theory, of course - in reality there will always be a ‘weak’ spot. To minimise the impact of these weak spots on the silage quality, the feed-out rate - and therefore the length of time the plastic is removed - must be as short as possible. Never open the cover of the clamp too far.

If the silage looks poor and there are signs of potential spoilage caused by heating at the silage face, adding acids can be a solution. The silage will heat up quickly, especially in warm weather. The pH will increase and harmful bacteria will predominate.  Spraying acids against the silage face inhibits mould and yeast formation and will maintain the desired pH value.

Read more about Digestion by the Cow