Silage plastic, underlay film and side-wall plastic: what should you choose?

A small hole in the plastic, or a wall where water runs in: you only see it months later, when you find warm and mouldy patches while feeding out. Which plastic you choose and how you lay it largely decides how airtight and watertight your silage will be. If the plastic isn't right, air and water reach the silage, and that is where spoilage starts.

How do you recognise good silage plastic?

Silage plastic is never completely airtight. There are big differences between types. Look at these four things:

1. Air permeability (cm³/m²/24 hours). This figure tells you how many cubic centimetres of oxygen pass through one square metre of plastic in 24 hours. For standard film this is often around 250 cm³. The lower, the better.
2. Number of layers and thickness (micron or µm). These help decide how much air gets through, how well the plastic resists punctures and tears and how far it stretches. That way you avoid plastic that breaks quickly.
3. UV stability. Most films are UV-stable for 12 months. After that, the sun slowly breaks the plastic down. If your pit stays closed for longer than 12 months, the cover slowly gets worse.
4. Colour. Black plastic holds more heat than light plastic, so silage under black plastic warms up faster. If there are covers on top of the plastic, it makes little difference.

How thick should silage plastic be?

That depends on the type of plastic. There is no single thickness that is best for every pit. As well as thickness, look at the number of layers and the air permeability. Together they decide how much air gets through and how well the plastic resists punctures and tears. In general, thin plastic lets more air through than thick plastic of the same material. So compare the air permeability on the product sheet as well as the microns.

What is the difference between underlay film and silage plastic?

Underlay film is a thin film that you lay directly on the silage, under the silage plastic. Because it is so thin, it follows the surface of the silage more closely. It more or less clings to the silage. The silage plastic on top is thicker and protects against punctures, tears and sunlight.

Do check which underlay film you choose. Ordinary underlay film mainly makes sure less air stays between the plastic and the silage. An underlay film with an oxygen barrier (oxygen barrier film) also lets much less air through than ordinary silage plastic.

What this gives you:

• less air stays under the plastic;
• with a barrier film, less air also gets through the plastic;
• preservation gets going faster and better;
• you have less risk of heating.

Always pull the underlay film over the full width of the silage. The edges of the underlay film are often a weak point. Air collects there and condensation with a higher pH drips into the silage.

Do you need side-wall plastic in a silage pit?

When covering, you mainly pay attention to the top and the ends of the pit. But water can also get in along the sides. In practice, silage pit walls are often not completely watertight: the sealant joints are often worn after one or two seasons. Then water leaks in along the walls.

That is why you lay side-wall plastic, or lay the silage plastic itself over the wall. Pull it down to the floor and leave a good length lying on the floor, so rainwater can't run into the silage. This also works while the silage is still below the top of the wall.

Without side-wall plastic, rainwater soaks into the silage unnoticed, and that raises the pH. As long as the pit is airtight, the damage is limited. But when you feed out, heating and spoilage spread faster. So check your walls before ensiling too, as part of preparing your silage pit.

How wide should silage plastic be and how do you put it in place?

Add the silage height to the width of the pit. For a pit 8 metres wide with silage 2 metres high, take plastic 10 metres wide. Then you can lay the plastic over both walls down to the floor, you don't need separate side-wall plastic and there is a double layer everywhere.

We recommend working with an underlay film and two large sheets of silage plastic. This is how you put it in place:

1. Choose plastic that is long and wide enough. If possible, take plastic that covers the full length of the pit in one piece.
2. Use new plastic, not second-hand.
3. Lay the plastic over both walls. Pull it down to the ground and leave a good length lying on the floor, so rainwater can't run into the silage. With Silage Safe, leave at least 20 centimetres on the floor.
4. Fill the pit and roll it well. Roll the silage evenly and make the top smooth and slightly rounded, so no water stays on it (WUR). With Silage Safe you give the silage a rounded top, so the water runs off through the drainage.
5. Lay the underlay film over the full width of the silage.
6. Pull both sheets over the silage. First the sheet on the side the wind is coming from, into the wind. Then the sheet from the other side, with the wind.

Now there is an underlay film and a double layer of silage plastic over the whole pit. That is at least 3 layers of plastic everywhere. Then carry on covering as in covering a silage pit: step by step. If it is very windy, see covering silage in strong wind.

If you put a covering system on top, the plastic also stays tight. Geert Mulder from Balkbrug: "Wind is much less of a hassle now. As soon as we secure a row of covers, the plastic stays put." See how the Silage Safe system works on our product information page.

Can you reuse old silage plastic?

No. Old plastic always holds traces of soil, silage and water, and with them you bring a lot of bacteria and moulds into your silage. It often has small holes already, and part of its UV stability is used up. It looks like a saving, but you lose more feed because of it. You can work out what that costs with calculating silage losses. Where else mould comes from is covered in the 4 causes of mould in silage.

Which mistakes do you see most often with silage plastic?

1. Using old plastic. It seems cheap, but contamination and poorer quality mean you lose more.
2. No side-wall plastic down to the ground. Then water gets in along the sides and the pH goes up.
3. Too little overlap. Then a small hole quickly leads to heating, air pockets form and more air gets through.
4. Underlay film not over the full width. The edges of the underlay film are exactly the weak points.