ADLib Glossary (S)

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Silage Effluent

This is the liquid waste produced when a crop is harvested whilst still green for animal fodder and not dried but preserved by excluding air from the storage silo. Silage effluent can cause major problems if it enters a watercourse due to its high BOD many farmers wilt their grass simply to avoid or minimise the effluent problem. Silage effluent is very acidic and all effluent conducting channels and ducts must be coated with acid-resisting materials.

Silage effluent does contain some plant nutrients average values may be in the region of 2.0 Kg per 1000 litres N, 1.0 Kg per 1000 litres P2O 5 and 1.5 Kg per 1000 litres of K2O. It can be applied to land by a tanker or a slurry irrigation system.


Because silage effluent is so highly nutritious it is also dangerous as a pollutant if it enters our streams, rivers or lakes. Micro-organisms and algae present in waterways will feed on the effluent causing a reduction in the oxygen content of the water. Fish life literally suffocates from a lack of oxygen in waterways contaminated with silage effluent.

Public and domestic water supplies can also be seriously damaged if contaminated with silage effluent. For example, the effluent from just 100 tonnes of silage has the same polluting potential as the daily sewage output of a town of 25,000 people.

Directly ensiled wet grass can produce a peak effluent flow rate of up to 30 litres per tonne per day (or 30 cubic meters per 100 tonnes) for the first few days after ensiling. This demands that farmers are very vigilant to ensure effluent tanks do not overflow.

Farmers should also regularly inspect drains and watercourses downstream of the farmyard once silage making begins, so that immediate action can be taken in contamination is suspected.

In order to ensure silage effluent control follow these guidelines:

  • Before cutting silage, ensure effluent tanks are empty
  • Inspect effluent tanks and all drainage channels into tanks to ensure that they work effectively
  • Inspect silage slab to ensure no cracking or concrete deterioration which would allow effluent seepage.
  • Take remedial action to seal cracks and any other areas of leakage.
  • Plan remedial works in good time on effluent tanks and channels before cutting.
  • Where possible cut silage in dry weather.
  • During ensiling, frequently inspect collection channels to ensure they are free running and not blocked.
  • Inspect tanks hourly until effluent release from ensiled grass has ceased.
  • Where tanks are two-thirds full, remove effluent and either spread on cut ground or feed to animals.
  • When covering the pit, never use dung or farmyard manure. Consider old tyres, sand bags, netting, etc.
  • Ensure clean water run-off from silage covers is diverted away from effluent tanks.

In case of an emergency:

  • Block all effluent discharge
  • Prevent any further discharge to mail waterway, if necessary, by blocking drains to create temporary holding ponds.
  • Land spread all collected effluent.
  • If practical, clean out contaminated soil from drains where effluent escape has occurred.
  • Seek help from neighbours in wet weather to be allowed spread effluent on their land
  • If there is a danger of a fish kill or contamination of the water supply, you are required to notify the Environment Agency or SEPA immediately


Silage effluent is one of the most concentrated and harmful pollutants on the farm. Safe and secure storage is of prime importance to minimise the risk of causing water pollution (See the Water, Soil and Air Code for more information). Conventional storage tanks for silage effluent are usually made of concrete or masonry. This provides a convenient and economical method of storage but there is a risk that the effluent will escape because, due to its acidic nature, it can react with the surface of the concrete or masonry and eventually damage it.

A research project carried out by ADAS has shown that second-hand stainless steel tanks from the brewing industry can provide a safe alternative for storing silage effluent. Tests have shown that stainless steel tanks were resistant to corrosion. However, ordinary steel tanks coated with glass or enamel would require attention within 20 years.

The durability of flexible tanks, which are already used on some farms, is unknown.

A DEFRA funded study of sealants for concrete bunkers has found that polyurethane and polysulphide sealants are more likely to last for 20 years without leaking than bituminous sealants. Silicone sealants are variable in their performance but unbder certain conditions could last longer than 20 years.

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