Rainwater in silage: how to keep water out of your silage pit

Rainwater that runs down the wall into your silage raises the pH at that spot. That gives yeasts and moulds a chance, and you lose feed value in what your cows eat. On its way in, the water also carries dirt with it. Watertight covering and good drainage keep it out.

Why is rainwater in your silage a problem?

Water slows down fermentation

After ensiling, lactic acid bacteria lower the pH of your silage. The silage becomes acidic, so other bacteria, moulds and yeasts struggle to develop. For silage with less than 25% dry matter, a pH of 3.8 to 4.2 is good. Drier silage can be higher: up to about 4.6 at 35% dry matter and up to about 5.0 at 45% (WUR dairy farming handbook).

Rainwater is much less acidic than good silage. If it runs into your silage, the pH goes up at that spot. Butyric acid bacteria, moulds and yeasts get a new chance and the feed value drops. That silage is also less palatable, so your cows eat less of it. How fermentation works is explained in what happens in your silage after ensiling.

Water carries dirt

The water that falls from the sky may be clean, but on its way to your silage it picks up all sorts of things: soil, old silage and animal droppings. That brings butyric acid bacteria, other harmful bacteria and pathogens into your silage. The result is lower feed value, less palatable feed and a higher risk of disease in your animals. Read more about the effects in butyric acid in silage.

Wet spots cause heating

Where water gets in, yeasts and moulds get their chance. You often see this as heating, especially along the walls. Heating can easily cost an additional 10% of feed value (Silage Safe, brochure 2026, based on Eurofins Agro). How to tackle heating is explained in preventing heating in silage.

Where does water get into your silage?

Water always finds the lowest route. In a silage pit, these are the usual places:

1. Along the walls. Rainwater runs off the cover, down the wall and into the silage. In a pit that isn't watertight, water can also get in between the wall panels.
2. Puddles on the silage. If the silage isn't rolled evenly, water is left standing on top. With tyres on the pit, water also collects in and around the tyres.
3. Holes in the plastic. For example from pests, or from old plastic that you reuse.
4. Condensation on the underlay film. Condensation that runs into the silage at the edge of the film raises the pH there and increases the risk of bacteria and heating.
5. From the pit itself. If the water can't drain away, it runs back into the silage from the floor or the sides.

How do you keep water out of your silage?

1. Store your silage in an airtight and watertight silage pit. Then no water can run in between the wall panels. Use side-wall plastic if needed, so no water gets into the silage along the walls. What to check before ensiling is explained in preparing your silage pit.
2. Roll the silage evenly and finish the top smooth and slightly rounded, so no water is left standing (WUR). With Silage Safe, you give the silage a rounded profile so the water runs off through the drainage.
3. Lay the underlay film across the full width of the pit. This way, as much condensation as possible ends up beside the silage and not in it.
4. Cover airtight and watertight. Good covering keeps rainwater and other contamination out. Pay extra attention to the edges: they are less well compacted and the least protected. How to do this is explained in covering a silage pit.
5. Make sure water can drain away in and around the pit. The rainwater has to be able to leave, so it doesn't run into the silage from the sides or from the pit itself.
6. Check regularly after ensiling. Look for puddles, check that the plastic is intact and that the drainage is clear.

How do you drain your silage pit?

Good drainage is the basis for keeping water out of your silage. It involves two things:

• The ground around the pit. Make sure water runs away from the pit, not towards it. Well-drained ground keeps the pit and the access route dry and clean.
• A drainage system at the pit. Set it up so that the water coming off the cover is carried away outside the silage.

The trickiest spot is the wall. Rainwater runs down the wall and looks for a way in there. With a shared dividing wall between two pits, the water can only go one way: into the silage. That is why the water has to be caught between the wall and the plastic and carried out of the pit. See how the Silage Safe drainage pipes do this on our product information page.

A drainage system only works if it is well designed and properly installed. Maintain it as well: clear any blockages so the water can keep flowing.

Ideally, keep rainwater and silage effluent separate. Clean rainwater can then be drained away separately from the effluent from your silage. Ask your local authority or water authority which rules apply to draining effluent and water from your silage base.

What do you gain from dry silage?

• Fermentation keeps going. The silage ferments well and keeps longer. That matters most if your silage has to be stored for a long time.
• Less contamination. Bacteria and moulds that cause spoilage and disease stay out of your feed.
• Better quality at the feed barrier. Poorly preserved silage costs your animals performance, and that costs you money.

You see the difference mostly at the bottom of the pit and along the walls, where the wettest silage would otherwise be. You can work out what spoilage costs you each year with calculating silage losses.