What Happens in a Silage Clamp Week by Week

The clamp is sealed and nothing visible happens for eight weeks. Inside, four distinct phases occur in sequence, and how well each goes is decided almost entirely in the first forty-eight hours.

Air is the variable. Everything else follows from how much of it got in and how quickly it was used up.

The four phases

Aerobic phase — hours. Oxygen trapped in the clamp is consumed by plant respiration and aerobic microbes. This phase should be as short as possible, because everything happening in it consumes the sugars the fermentation needs. Good consolidation and fast sealing shorten it; a loosely filled clamp extends it for days.

Fermentation — days 2 to 14. With oxygen gone, lactic acid bacteria convert sugars to lactic acid and the pH falls. This is the phase doing the preservation work.

Stable phase — weeks to months. Once the pH is low enough, microbial activity effectively stops and the crop is preserved. Nothing much happens as long as the seal holds.

Feed-out — aerobic spoilage restarts. Opening the clamp reintroduces oxygen, yeasts and moulds resume, and the face heats.

Target pH depends on dry matter

There is no single correct silage pH, which surprises people who have seen one figure quoted.

Stability depends on water activity as much as acidity. A wet crop has plenty of free water for spoilage organisms and needs a lower pH to be stable. A drier crop is stable at a higher pH because there is less water available.

Roughly, a 25% dry matter grass silage needs around pH 4.0, while at 40% dry matter something nearer 4.5 is stable. Quoting 4.0 as the universal target leads people to think a perfectly good drier silage has failed.

Maize silage behaves differently again, with higher sugar and starch content changing both the fermentation and the spoilage risk at feed-out.

Failed ferments trace back to air

Almost every fermentation problem is an air problem:

Slow filling extends the aerobic phase, burning sugars that the lactic bacteria needed.

Poor consolidation leaves air pockets throughout the clamp, so fermentation is uneven and parts never reach a stable pH.

A damaged or poorly weighted sheet lets air in continuously, and the affected area spoils.

Too dry a crop cannot be consolidated properly, which is why over-wilting causes problems as well as under-wilting.

The symptoms — a pH that never falls far enough, visible moulds, a butyric smell, heating in storage — all point back to the same cause.

Feed-out is a separate discipline

A perfectly fermented clamp can be lost at feed-out.

Once the face is exposed, aerobic spoilage restarts. The rate depends on how much surface is exposed, how disturbed it is, and how quickly you work across it.

The practical measures are a tight clean face rather than a torn one, using a shear grab or block cutter rather than a loader bucket, and a feed rate fast enough to move across the face before spoilage takes hold — commonly expressed as a minimum advance per day, which varies with conditions.

Heating at the face is the visible sign, and it represents energy the animals are not getting.

Common mistakes to avoid

• Slow filling, which extends the aerobic phase.
• Judging a drier silage against a wet-crop pH target.
• Opening the clamp during active fermentation.
• Using a loader bucket at the face, which tears it and admits air.
• A feed rate too slow for the exposed face area.

How to do it with Silage Fermentation Timeline

The Silage Fermentation Timeline shows the phases and dry-matter-adjusted targets.

1. Enter the crop and dry matter at ensiling.
2. Follow the phases from the sealing date.
3. Compare pH against the target for that dry matter, not a single figure.
4. Plan the feed rate against the exposed face area before opening.

Your national agricultural advisory service publishes silage guidance. Other reference guides are in the tools directory.

Frequently asked questions

Why does target pH depend on dry matter?

Because water activity determines how easily spoilage organisms grow. A wet crop needs a lower pH to be stable; a drier one is stable at a higher pH, so one figure does not describe both.

What causes a failed ferment?

Air, almost always — slow filling, poor consolidation, or a damaged sheet. Air lets aerobic organisms consume the sugars the lactic bacteria need, so the pH never falls far enough.

Why does silage heat at feed-out?

Aerobic spoilage restarting once the face is exposed. Yeasts and moulds consume sugars and generate heat, which is energy the animals do not get.

Final thought

Fill fast, consolidate hard, seal immediately. The first forty-eight hours decide most of what the clamp will be worth eight months later.

Try the free Silage Fermentation Timeline