Making Silage – Keeping the Air Out

Tuesday 28 June 2016

By Dr. Tom Chamberlain, Silostop Tech Team member 

Silage is made when crops are stored in covered silos or bales, and air, particularly oxygen, is exhausted by the fermentation process. Sugars in the crop are converted by bacteria to simple organic acids, predominantly lactic acid. Acid production lowers the pH and once this drops low enough, further microbial activity is inhibited and the crop is preserved, enabling it to keep for months and years. If there is oxygen present at any stage, then bacteria and fungi break down the forage to produce compost, heat, and water. A large amount of the original crop material is lost and palatability and production potential plummets. There are several critical points where oxygen must be kept out.

Ensiling the Crop

A freshly harvested crop will contain a large amount of air, both between the strands of the forage and also inside hollow plant stems. Silage making requires consolidation to drive the air out. Over the past decades, forage harvester capacity has increased and the speed of harvest has risen. Silo sizes on farms have not increased to match this growth, with the result that many are filled too fast with insufficient machine weight available to compact the crop. Forage is resilient to compression and irrespective of the size of tractors being used, it is only compressed to a depth of about 300mm below the wheels. Silos should be filled in a shallow wedge and each incoming trailer load spread in thin layers over a large area so that it is all compacted.

Covering the Crop

Once the crop is ensiled it must be covered immediately. A layer of plastic film is commonly used but plastic films are permeable to oxygen. This can be measured in a laboratory as the oxygen transmission rate (OTR) being the amount of oxygen that will pass through a square meter of film per day under controlled conditions (cm3/m2/day) – see table. 

Oxygen transmission rates of different plastic films

Type of film


45 µm orange Silostop film


40 µm polyethylene ‘cling film’


125 µm polyethylene film


25 µm Silostop bale wrap film


25 µm polyethylene bale wrap film


OTR: Rates at 50% RH. 21% O2 atm cm3/m2/day DIN-53380-3

Simple polyethylene films have high OTR figures due to their inherent chemical structure. Silostop is a unique, patented film with a layer of very impermeable plastic (ethyl vinyl alcohol, EVOH) sandwiched between layers of polyethylene. The impermeable plastic gives the film a much lower OTR due to its more complex chemical structure.

The impact of the different OTR figures can be best seen on farm when silage is covered with a single layer of 125µm polyethylene film. Over time, oxygen continually permeates through the plastic and encourages aerobic rotting or spoilage. When the silo is opened after several months of ensiling, a black and spoiled layer of ‘compost’ is often found which has to be discarded before the silage can be fed. Using a specialist film with a lower OTR reduces the spoilage in the top layer, which minimises organic matter losses and effects on palatability.

The top surface of a clamp/bunker is the main surface exposed to the air and susceptible to spoilage, but there can be problems associated with the walls as well. This can be a particular problem where walls are made of railway sleepers or loose blocks and panels. Using a plastic film on the side walls will limit spoilage in this area and again a material with a low OTR will be preferable.

Feeding Out – A Tidy Face and a Fast Feed Out

Once a silo is open, it is exposed to air and the composting process can re-start. The more the compacted silage is disturbed when material is removed, the greater the spoilage and this can reach a long way into the clamp. Silage should be removed as carefully as possible – flail type ‘defacers’ or ‘block cutters’ cause much less disturbance than simple tined buckets and so forth. The rate of spoilage will be affected by how long the silage is exposed to the air and the air temperature. Producers should be aiming to cut back the entire exposed face by one metre a week in the winter, increasing this to two metres a week in the summer. 

Round Bale Silage

Baled silage has its own advantages and problems when it comes to keeping the air out. Baled forage is generally less compacted; there is more air trapped within the crop to support spoilage. However individual bales are fed out very quickly so spoilage after opening is minimal. Air ingress after baling and during storage can be a major problem for two reasons. Firstly, a bale has a large surface area, such that much of the ensiled forage is close to surface (50% is within 12.5 cm of the surface in a 1.2 x 1.2 round bale). Secondly, the films used for wrapping bales are very thin (about 18 – 20 um when stretched) and OTR figures are correspondingly high. The effective OTR can be increased by using extra layers of wrap film, but this increases costs and recycling problems. The alternative is to use a film with a much lower OTR (see table) to exclude the oxygen and then wrap the bale in enough layers of film (minimum four layers) to allow it to be moved and handled without splitting.

Dr Tom Chamberlain will be talking at the forthcoming National Fodder Conference hosted by AFAI in Perth. His talk will look at the importance of keeping air out of silage at all stages of making, storing, and feeding out.