Definition

The Overheating index measures the susceptibility of silage to aerobic spoilage (heating) once exposed to air. A high index indicates greater sensitivity, enabling early preventive actions to minimise feed-out losses and maintain feed quality. It also serves as a guide for improving future silage management practices.

Causes of heating

Heating occurs due to microbial activity, primarily yeasts, when oxygen enters the silage during feed-out. A temperature difference of more than 10°C between the silage and ambient air is a clear sign of spoilage.

Consequences of heating

The impact of heating can be severe:

• Dry Matter Losses: Up to 3.5% per day, and over several days, losses can exceed 10%.
• Energy Losses: Around 15 VEM (Dutch net energy unit) per kg of dry matter.
• Feed Quality Risks: Reduced intake and, in extreme cases, complete spoilage due to mould growth.

Interpreting the Overheating index

The index provides a risk assessment:

• Below 20: Low risk of spoilage.
• Above 50: High risk of spoilage.

Management recommendations

To reduce heating and spoilage:

• Feed more sensitive silage during winter.
• Adjust feed-out rate or clamp height.
• Apply propionic acid on cutting faces.

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