Making and preserving silage: the complete guide

All year round you invest in good forage: growing, fertilising, harvesting and ensiling. Whether that investment pays off at the feed barrier and in the bulk tank depends on how well your silage is preserved. At every step, from a clean pit to feeding out, you can gain or lose feed value.

What is silage and what happens during preservation?

Silage is forage stored with as little oxygen as possible, so that fermentation preserves the feed. After covering, the crop and micro-organisms first use up the oxygen still in the pit. That can take a few hours to a few days, depending on compaction and covering. As long as there is oxygen, unwanted micro-organisms can grow.

Only when the oxygen is used up does fermentation start. Lactic acid bacteria turn sugars into lactic acid, the pH drops and the silage becomes stable. The sooner the pit is free of oxygen, the more feed value you keep. All the phases step by step are explained in what happens in your silage after ensiling.

How do you make good silage? 7 steps

Step 1: prepare the silage pit

Anything left behind in the pit gets ensiled with the crop, including moulds and bacteria.

• Clear away old plastic, feed remains and dirt and sweep the pit.
• Clean the access route too, otherwise the machines drive slurry and mud into the silage.
• Mow around the pit, so stubble doesn't pierce the film and pests have nowhere to hide.
• Check walls and floor for cracks, poor sealant joints and sharp edges.
• Make sure drains and gullies are open and that no water is standing anywhere.

Step 2: mow or harvest at the right moment

Good fermentation needs sugar. Grass makes sugar in the sun: grass mown later in the day usually contains more sugar. A short wilting period helps to keep that sugar. Keep a stubble of 6 to 7 cm (WUR mentions 5 to 6 cm), so less soil and manure ends up in the silage. All the tips for spring are in first cut silage tips.

For maize, watch the chop length and the harvest moment. That harvest moment depends on the dry matter.

Step 3: ensile at the right dry matter

Ensile grass at 35-45% dry matter and forage maize at 32-36%. What goes wrong if your silage is too wet or too dry is explained below under dry matter content.

Step 4: compact the pit well

The less air stays in the pit, the sooner fermentation starts and the less deeply air gets in behind the silage face later on.

1. Put enough weight on the pit: half of the incoming material per hour. If 40 tonnes per hour comes in, roll with 20 tonnes.
2. Compact everywhere equally well, also along the edges. Weak spots later become heating spots.
3. Don't drive in ruts. Often there is extra driving just next to the edge. That creates air pockets.
4. Put the driest material and the poorest fields at the bottom. That is where it is pressed down best.
5. Ensile at a cool time of day.

Step 5: cover the pit straight away

Cover as soon as the pit is finished. In a field trial with bales by Groeikracht, a Dutch silage advisory group, every hour of waiting cost about 1.5% extra dry matter. After 2.5 hours more than 6% of dry matter was lost, after 5 hours almost 10% (Veeteelt, May 2021). A tightly covered pit holds on to that feed value. Jarne Schuiling from Vries in the Netherlands says about his silage under Silage Safe: "We can now retain much more of the protein in our grass silage."

• Use side-wall plastic: the walls of a silage pit are often not watertight.
• Lay underlay film over the full width of the pit.
• Lay the top sheet with an overlap, so there are at least 3 layers of plastic everywhere.
• Seal the edges tightly and make sure there is even pressure over the whole pit.
• Check after a few weeks and pull everything tight again, because the silage still settles.

The complete step-by-step plan, also per crop and for the plastic, is in the guide covering a silage pit.

Step 6: keep rainwater out

Rainwater raises the pH in your silage and carries dirt in with it, such as soil and slurry. Use side-wall plastic, cover watertight and make sure there is good drainage in and around the silage pit.

Step 7: 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 fresh oxygen. How fermentation works exactly: silage pH and fermentation.

What is the right dry matter content for ensiling?

• Grass: 35-45% dry matter. That material compacts best (Groeikracht).
• Forage maize: 32-36% dry matter. Above 36%, the risk of heating and mould during feeding out increases (WUR, Handboek Snijmaïs).

Material that is too wet ferments slowly. Water is much less acidic than good silage and slows down the pH drop, which gives butyric acid a chance. Material that is too dry is stemmy and hard to compact, so more air stays in and the silage is more prone to heating. If you want to ensile drier, chop finer.

How does heating start and what does it cost?

Heating happens when yeasts use oxygen to break down sugars and lactic acid. This releases heat and the pH rises. It starts where oxygen gets in: in the top layer and along the edges, at damage in the cover and, after opening, at the silage face. The worse the pit is compacted, the further the oxygen penetrates.

The heat and the higher pH make moulds and bacteria grow faster. Feed value is broken down and mycotoxins can form. The feed becomes less palatable, cows eat less, production drops and the health risks for your livestock increase.

In heating silage, the feed value can drop by 2-3% per day. Cows can also take in 10-20% less dry matter (WUR, Handboek Snijmaïs). You then have the same hectares and contractor costs, but less milk from your own forage and higher concentrate costs.

An example calculation from our brochure. A silage pit of 50 × 10 × 3 metres, with a small heap above the walls, holds about 1,750 m³ of grass silage. At 230 kg of dry matter per m³, that is 402,500 kg of dry matter. With a replacement value of €0.22 per kg of dry matter, that pit holds €88,550 in feed value. Heating can easily cost an extra 10% of feed value (Silage Safe brochure, Eurofins Agro). That is €8,855 per pit. These figures are an indication only, not a guarantee. The full calculation is in the brochure on our product information page. All the tips against heating are in preventing heating in silage.

How do you prevent heating when feeding out?

1. Keep the feed-out rate high enough: at least 1.5 metres per week, in summer at least 2 metres. When the cows go out to grass, your feed-out rate drops. Take that into account when ensiling.
2. Keep the silage face tight and smooth, with pressure across the full width.
3. Loosen as little as possible. With a silage facer or a block cutter, the silage stays firmer.
4. Don't leave loose feed in front of the silage face, it becomes a source of heating.
5. Don't feed warm or mouldy silage. Remove it and dispose of it.
6. Work cleanly, with clean machines and no soil in the feed.

What do you do if your silage is already heating?

Once heating has started, there is little you can do about it. You can stop it spreading:

• Remove the affected feed.
• Increase the feed-out rate. If that isn't possible, consider spreading the silage out again over the whole pit and covering it again. A lower pit gives a higher feed-out rate.
• Put more pressure on the silage face.

Don't pull a loose piece of plastic over the silage face. Too much oxygen stays underneath, so it doesn't stop heating. In the sun, black plastic also works like a heater, so the feed warms up even faster.

Key terms

• Silage: forage stored with as little oxygen as possible, so that fermentation preserves the feed.
• Dry matter (DM): the part of the feed that isn't water. It is best to calculate and compare in kg of dry matter.
• Fermentation: conversions by micro-organisms, including sugars into acids, so the pH drops and the silage becomes stable.
• pH: a measure of acidity. A pH that is too high means a greater risk of unstable silage and heating.
• Heating: warming up because yeasts use oxygen to break down sugars and lactic acid. The pH rises and there is a greater risk of mould and mycotoxins.
• Mycotoxins: toxins that moulds produce in silage. They can reduce feed intake, health and milk production.