Cutting grass heavily contaminated with soil for silage

This article supplied by SAC Consulting Ruminant Nutritionist, Lorna Shaw, discusses the effects of soil contamination and provides tips on preventing soil contamination in the 2024 silage season.

Twenty-eight percent of all grass silage samples analysed by wet NIR through the Edinburgh forage lab between July 2023 to January 2024 showed ash levels above 80 g/kg dry matter (DM).

Ash is a measure of inorganic matter contained in the forage which contains the mineral and trace element fraction with a target of below 80 g/kg DM.

Levels above 80 g/kg DM indicate potential soil contamination in the sample. A sample with 90 g/kg DM or more is in the high-risk zone for potential listeriosis contamination.

With silage ground already shut off, it’s time to consider how we manage the crop to optimise preservation and reduce the risk of adverse health of livestock from feeding contaminated silage. 

The ensiling process of silage relies on the presence of anaerobic microorganisms that use sugar to produce lactic acid when oxygen is not present.

This reduces the pH to stabilise the forage and reduce the growth of undesirable microorganisms. It is desirable to have a fairly rapid fermentation. 

However, soil contains increased levels of aerobic microorganisms such as clostridia which can delay a rapid drop in pH.

As a result, the presence of high levels of soil can lead to increased levels of butyric acid and reduced levels of desirable lactic acid in the silage. This increases the pH and reduces the stability of the forage.

High levels of butyric acid in forage will also harm nutrient availability through the breakdown of protein.

Soil contamination of forage may also increase the risk of heating, increasing the chance of energy breakdown and further reducing energy value.

It is easy to identify silage with a highly butyric fermentation due to its dark colour, foul smell similar to that of vomit or off-butter and slimy texture. 

Soil can carry a variety of harmful microorganisms that can proliferate especially in the favourable conditions that silage preservation offers. Many of these microorganisms affect animal health and production if ingested at high levels.

Common microorganisms contained in soil-contaminated silage include: 

• Listeria 
• Coliforms
• Clostridia 
• Salmonella
• Fungi (which produce mycotoxins)

• Avoid cutting fields that are particularly high risk such as lodged fields, flooded fields, or fields with a high number of molehills
• Consider grazing badly affected fields before shutting off
• Roll silage ground to even the surface and reduce the risk of picking up high pockets of soil
• In higher-risk areas consider making hay rather than silage. The higher DM will reduce the risk of listeriosis. Hay will likely be of lower nutritional value than silage due to the longer wilting period. So, diets for pregnant, lactating and growing stock must be carefully balanced
• Minimise the number of times the swath is moved
• Ideally, bale the crop and stack the bales separately from bales from other fields. This will allow field-specific analysis and any silage from contaminated fields to be fed to the safest class of stock
• Avoid layering grass silage cuts if possible. This will only increase the risk of pockets of listeria developing
• Ensure a pit is well consolidated to minimise pockets of air and ensure bales are baled to an appropriate density.
• Try to keep clamp machinery clear of soil and contaminants before rolling the pit
• Get a representative sample analysed and carefully check its pH and ash content
• Avoid feeding a forage with levels of ash above 90 g/kg dry matter and a pH of above 5.0 to any sheep or pregnant cows

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