Silage fermentation: how to get the pH of your silage down

If your silage ferments too slowly, moulds and bacteria get a chance. If the pH drops quickly and stays stable, your silage keeps well. That way you prevent heating and loss of feed value, and you save on feed costs.

How does silage ferment?

Once the pit is sealed and the last oxygen has gone, lactic acid bacteria turn sugars into acids, such as lactic acid, acetic acid and propionic acid. This makes the pH drop. That low pH holds back bacteria and yeasts, and that is how the feed is preserved. Well-fermented silage is also more palatable, so cows eat more of it.

Lactic acid bacteria need an environment without oxygen for this: an airtight pit. If conditions aren't right, unwanted bacteria such as butyric acid bacteria get a chance, and heating is just around the corner. More about butyric acid in silage. All phases after ensiling are in what happens in your silage after ensiling.

What pH should your silage have?

That depends on the dry matter. The drier the grass silage, the higher the pH can be. These are the limits for well-made silage (WUR, Handboek Melkveehouderij):

• Wet grass silage, below 25% dry matter: pH 3.8 to 4.2.
• Grass silage at 30% dry matter: up to about 4.3.
• Grass silage at 35% dry matter: up to about 4.6.
• Grass silage at 40% dry matter: up to about 4.8.
• Grass silage at 45% dry matter: up to about 5.0.
• Grass silage at 50% dry matter: up to about 5.2.

Maize has its own range: normal maize silage is usually at 4.0 to 4.2 (WUR, Handboek Snijmaïs).

In the silage analysis from Eurofins Agro, the preservation index shows how preservation has gone. The index runs from 0 to 100, and the target is above 80.

How do you get better fermentation? 4 tips

1. Compact well and seal straight after rolling. Acid production only gets going once the last oxygen has gone. With poor compaction, more oxygen stays in the feed and it takes longer before it ferments. So don't leave the pit open overnight. In a field trial with bales by Groeikracht, a Dutch silage advisory group, feed that was only sealed after 5 hours had lost almost 10% of its dry matter and had a clearly lower feed value than feed that was sealed straight away (Veeteelt, May 2021). "When it comes out from under there, it's just better silage," says Sybren Nauta from Itens.
2. Keep water out of the pit. Rainwater is much less acidic than good silage. If it gets into the feed, the pH rises. Make sure you have a watertight cover and good drainage, and use side-wall plastic if the walls of your silage pit aren't watertight.
3. Get the dry matter content right. Silage that is too dry is hard to compact. Wet silage contains a lot of water, and that slows down fermentation. What to do with dry silage is in a separate article.
4. Keep the pit closed for at least 6 weeks. That is how long it takes before the pH has dropped far enough and the feed stays stable when oxygen gets to it again. If you open it earlier, other bacteria, moulds and yeasts become active again.

See how Silage Safe seals your pit quickly and tightly on our product information page.

Why does maize ferment more easily than grass?

Maize contains more sugars. Bacteria quickly turn them into acids, so the pH drops faster. With grass, a lot depends on the sugar content at the moment of mowing. Why that is so is in sugar content of grass.