Definition

Propionic acid is a volatile fatty acid (VFA), similar to acetic and butyric acids. It has a sharp, acidic odour and taste. In silage, propionic acid forms during fermentation when sugars are converted into organic acids under favourable conditions.

Importance in silage

Propionic acid plays a key role in silage preservation:

• Rapid pH reduction during fermentation prevents spoilage.
• Inhibits overheating and mould growth after opening.
• Contributes to overall feed hygiene and stability.

The combined level of acetic acid + propionic acid is part of forage analysis.

• Optimal range in grass silage: 20–35 g/kg dry matter.
• Too low: Risk of heating and spoilage.
• Too high: Unpleasant taste and reduced palatability.

Mode of action

Acetic and propionic acids are weak acids. At low pH, they exert an antibacterial effect on fungi, yeasts, and harmful bacteria. Well-preserved silage with low pH minimises the risk of aerobic spoilage. Propionic acid is often included in silage additives for improved preservation.

Role in the rumen

In the rumen, propionic acid is produced by microbes during the breakdown of non-resistant starch (e.g., cereals).

• Function: Provides energy and is primarily used for lactose synthesis in milk, which determines milk yield.
• Risk: Excess unstable starch in the ration can lead to rumen acidosis due to high levels of propionic and other VFAs.

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