Butyric acid in silage: causes and 5 tips to prevent it

You smell silage with butyric acid as soon as you reach the silage face. Your livestock eat less of it and get less feed value out of it, and the bacteria are bad for the quality of your milk. It usually starts with soil, slurry or water in wet grass silage.

What is butyric acid and what are butyric acid bacteria?

Butyric acid is an unpalatable acid that butyric acid bacteria make from lactic acid. It makes your silage stink: Eurofins Agro compares the smell to sweaty feet. The feed loses palatability, so your livestock eat less. On top of that, dry matter and feed value are lost. That affects both animal health and production.

Butyric acid bacteria (clostridia) are found mainly in soil, slurry and water, but they are also naturally present on grass and maize. They turn lactic acid into butyric acid and CO2 and don't need oxygen to do so. They grow well in a warm, wet environment with a high pH.

In dry, cool silage with a low pH they grow poorly, but they don't die. Butyric acid bacteria form spores. These are dormant bacteria that are very resistant to heat, drought and low pH. When conditions improve, the spores germinate again and continue to grow. That is why butyric acid is hard to get rid of once it is there.

How does butyric acid develop in silage?

Butyric acid problems have two causes. Too many bacteria have been ensiled with the crop (contamination), or the conditions in the pit allow them to keep growing. Often it is both.

Contamination happens when the feed comes into contact with soil, slurry, water or remains of old silage. That can happen at any stage:

• When ensiling: through badly adjusted machines, a dirty silage pit or access route, or dirty silage plastic.
• During storage: through a cover that lets water in, soil on the pit, or pests that eat holes. Droppings, sand and water then get in.
• When feeding out: for example through loose feed that is left lying in the pit.

Continued growth happens mainly in wet grass silage. Grass with less than 35% dry matter often contains fewer sugars. Lactic acid bacteria then have less to acidify the silage with. Water has a pH of around 6 to 7, far higher than good silage, and slows down the pH drop. Proteins also work against fermentation. Wet grass silage (less than 25% dry matter) should drop to a pH of 3.8 to 4.2 (WUR, Handboek Melkveehouderij). Which pH fits drier silage is explained in silage pH and fermentation.

Even silage that has fermented quickly can still develop butyric acid later. That happens in wet spots:

• where the material itself is wetter, because the feed isn't even;
• where condensation forms, such as under puddles or at the edge of the film;
• where water gets in from outside, through a leaking silage pit, poor drainage or a cover that isn't watertight.

When feeding out, oxygen and rain get in and the silage warms up. The bacteria can then grow quickly again.

Why is butyric acid bad for your milk?

The spores survive the pasteurisation of milk. As a result, they can cause problems in cheese making, such as a rancid taste. According to Eurofins Agro, the spores get into the milk through bedding, slurry and above all poor silage. That is why dairy processors monitor the level: if it is too high, you get a warning and a deduction from your milk price.

How do you prevent butyric acid in silage? 5 tips

1. Aim for 35-45% dry matter

According to Groeikracht, a Dutch silage advisory group, grass with 35 to 45% dry matter compacts best. The pH then drops quickly and butyric acid bacteria get less chance. If your grass is too wet or too dry, read what to do with dry silage and the tips for your first cut.

2. Work cleanly

Don't mow too short: keep a stubble of 6 to 7 cm, so less soil and slurry gets into the grass. Use clean, well-adjusted machines. Make sure the silage pit and access route are clean and dry, and the tyres are clean. Only use clean silage plastic.

3. Prevent wet spots in the pit

Preferably have the grass chopped with a forage harvester. That gives you a more even material, the sugars are released more easily and it compacts better. Roll the pit evenly and finish the top smooth and slightly rounded (WUR). Then no puddles are left standing, and puddles cause condensation.

4. Keep water and soil out of the pit

Use side-wall plastic, so no water gets into the feed through the walls of the silage pit. Cover airtight and watertight and pay extra attention to the edges: they are less well compacted and less well protected against air and water. Make sure there is good drainage in and around the silage pit. You can read more in preventing rainwater in silage.

Soil on the pit is a known source of butyric acid bacteria. Andre Schulte from Denekamp in the Netherlands stopped using it: "Because we no longer use soil, the risk of butyric acid and listeria bacteria is much lower." At the HyCare dairy farm De Bladelse Hoef in Bladel, the pits also used to be covered with soil. See how to cover without soil or tyres on our product information page.

5. Keep pests away

Mice, rats and birds make holes in your cover and contaminate the feed with soil and droppings. Preferably don't use tyres, because pests hide and drink in them. Mow around the silage pit and keep everything clean. You will find more tips in pest control at your silage pit.

How do you recognise butyric acid in silage?

By the smell: a sharp, rancid odour that reminds you of sweaty feet. You only know for sure with a silage analysis, which measures the butyric acid content. According to Eurofins Agro, your silage should contain less than 3 grams of butyric acid per kilo of dry matter.

What should you watch after ensiling?

• Don't open the pit within 6 weeks. Until then it hasn't fermented enough and isn't yet stable.
• Remove bad patches when feeding out, so they don't spread further.
• Feed out fast enough, so the silage doesn't warm up too much. We advise at least 1.5 metres per week in winter and 2 metres in summer.
• Keep working cleanly. Keep machines, the silage pit and the access route clean. Make sure no stray plastic, tyres or soil from covering end up in the feed.
• Keep the silage face smooth and don't leave loose feed in front of it.