Covering grass silage: when, how and what to watch out for

After a long day of mowing and ensiling, the grass silage is in the pit. How quickly and how tightly you seal it shows up in your feed for the rest of the year. Grass is only well preserved once the pit is airtight and watertight, and fast. In practice it often goes wrong at the edges, in the rain and when you add several cuts on top of each other.

What do you do before covering grass silage?

A tight cover starts in the field and in the pit.

1. Mow for high sugar. Grass makes sugar in the sun. Grass mown later in the day therefore usually contains more sugar than grass mown in the morning. Keep the wilting time short, so more sugar stays in the crop. What it depends on is explained in the sugar content of grass.
2. Don't mow too short. Leave a stubble of 6 to 7 cm (WUR advises 5 to 6 cm). Then less soil and manure gets into the silage, and so fewer butyric acid bacteria.
3. Aim for 35-45% dry matter. According to Groeikracht, a Dutch silage advisory group, grass with this dry matter content compacts best. Grass that is too wet ferments slowly. Grass that is too dry is hard to compact and holds more air.
4. Make sure the silage pit is clean and dry, with open drains and the side-wall plastic ready.
5. Keep an eye on the weather. If rain is forecast, plan the ensiling so that the pit is sealed before the shower.

All tips for spring are in first cut silage tips.

How quickly should you cover grass silage?

Straight after rolling the last layer. Don't leave the pit open overnight. After rolling, there is still oxygen in the grass. Only once it is used up does fermentation get going.

A field trial by Groeikracht showed how much difference it makes (Veeteelt, May 2021). Bales of chopped grass were wrapped at different times, to imitate a silage pit. Every hour the grass silage stays open costs about 1.5% dry matter. After waiting 2.5 hours, the dry matter loss was already over 6%. After 5 hours it was almost 10%, and the feed value was clearly lower than that of grass wrapped straight away.

So ensile at a cool time of day if you can, and have your covering materials ready.

How do you cover grass silage? Step by step

1. Roll the silage evenly. Finish the top smooth and slightly rounded, so no water stays on it (WUR). With Silage Safe you give the silage a rounded profile, so the water runs off through the drainage. Start doing that straight away, not just with the last loads.
2. Lay underlay film over the full width of the pit. Underlay film clings tightly to the grass. If the edge of the film lies on the silage, condensation drips into the silage there and you get a weak spot.
3. Pull the top sheet over the silage from one side first, then from the other side. Then you have underlay film plus two layers of top sheet everywhere: at least 3 layers of plastic. Which film to choose is explained in silage plastic and oxygen barrier film.
4. Put the cover on top and secure everything, especially the edges. Then the wind gets no grip and the pressure stays even. If it is very windy, follow the steps in covering silage in strong wind.
5. Check after a few weeks and pull everything tight again. During fermentation the silage settles a little more. The plastic can then become looser and air pockets can form.

Work with enough people, but let one person take charge. The full step-by-step plan is in the guide covering a silage pit.

Why are the edges of grass silage the weak spot?

Along the wall the grass is hardest to compact. So that is where most air is. Most air also gets in at the sides. Tyres and soil don't stay in place well on the edges. So use a cover that gives pressure right at the wall, and don't drive extra runs right next to the edge: that leaves ruts with air pockets.

Jarne Schuiling from Vries (NL) notices the difference in his grass silage: "With Silage Safe we have little to no losses now. We can now retain much more of the protein in our grass silage." Read his experience with Silage Safe.

How do you keep rainwater out of grass silage?

Rainwater is much less acidic than good silage. If it runs into the silage, the pH rises and butyric acid bacteria, moulds and yeasts get another chance. Rainwater also carries soil, old silage residue and manure with it.

• Use side-wall plastic if the walls of your pit aren't watertight. Sealed joints wear quickly.
• Let the plastic run down to the floor and leave a good length lying on the floor. With Silage Safe you leave at least 20 cm of plastic on the floor.
• Make sure drains and gutters are open and clean and that water doesn't stand anywhere.
• Roll evenly and finish the top slightly rounded, so rainwater runs off.

All measures against rainwater in silage are in a separate article.

How do you cover the pit when you add one cut on top of another?

When you add cuts on top of each other, the pit holds several layers of grass. When feeding out you cut through all the layers, so your cows get a more even ration. But the pit is opened more often and so gets air more often. Farmers who cover with tyres or soil often cover less carefully between two cuts to save work. That is exactly when heating starts.

1. Cover every cut just as carefully as the last one. Even if the next cut comes in a few weeks.
2. Leave the side-wall plastic against the wall. Then you don't have to pull the plastic all the way over the pit again each time.
3. Only open the pit when the new cut is ready to go in, and cover it again the same day after rolling.
4. Remove dirt and loose residue from the old layer before the new grass goes on top.

With Silage Safe the plastic and the covers stay hanging at the wall. You release the covers, bring in the new cut and then cover it again quickly. Bart Cornelis from Aalter (BE): "When adding another cut of silage, you can quickly open the pit and cover it again." See how it works on the product information page.

How long should grass silage stay closed?

Keep the pit closed for at least 6 weeks. By then the pH has dropped far enough and the silage is stable. If you open it earlier, yeasts and moulds get another chance with the new oxygen. When you add another cut, you do open the top earlier, but you don't feed from the pit yet.

Match the height of the silage to your feed-out rate when you fill the pit: at least 1.5 metres per week in winter and at least 2 metres per week in summer. If your silage face gets the sun, feed out even faster.