As silage season approaches, now is the time to consider the environmental risk of silage effluent to avoid a pollution incident that could cause serious harm to our waterways, writes Heather Gregg, College of Agriculture, Food and Rural Enterprise (CAFRE) Agri-Environment adviser.
Silage effluent is probably one of the biggest risk factors at silage time as its potency as a pollutant can have seriously damaging effects if it reaches a waterway. As a result, its management is critical through the silage making, ensiling and storage process.
The severity of silage effluent can be highlighted by explaining the biological oxygen demand (BOD) of many substances.
BOD is a measure of the amount of oxygen required by micro-organisms to break down organic material. The higher the BOD, the greater the pollution potential.
As oxygen in the waterways is used to break down the pollutant, animals and plants no longer get the oxygen required resulting in catastrophic damage to the ecosystem.
BOD values for common pollutants
Table 1 indicates the BOD of silage effluent compared to other products, including cow slurry and domestic sewage.
Silage effluent has the second-highest BOD value after milk, making it over 200 times more damaging than raw domestic sewage. Consequently, any amount of this pollutant entering the waterway will have seriously damaging effects.
In summer months, pollution incidents have a greater effect on our waterways due to the lower flows and a reduced oxygen-carrying capacity due to the higher summer temperatures.
Effluent is an inevitable product of silage production; however, aiming to keep the volume produced to a minimum is key, not only to avoid the pollution potential but also to improve the efficiency of the silage making process by retaining all the nutrients harvested with the crop.
Crop’s dry matter
The main factor to consider is the dry matter of the crop. Grass harvested at 18% dry matter (DM) will produce approximately 150 litres of effluent per tonne. Grass harvested at 25% DM will produce around 25 litres of effluent per tonne.
Considering the management of the effluent crop is also vital. If the crop is being stored in a silo, time should be taken to inspect the silo walls, floor and drainage channels. Completing this well in advance of harvest allows time to repair cracks and joints in the concrete.
If the silage is going to be baled, it is also important to remember the regulations set out in the Nutrient’s Action Programme (NAP).
Furthermore, bales should be stored a minimum of 10 meters from any waterway which effluent could enter to avoid effluent leaching from the bales reaching the watercourse.
If bales are stored on hard standing, all effluent should be collected, and therefore, the area should be inspected, similar to a silo.
The majority of effluent is produced in the first two weeks after ensiling; however, it is important to monitor effluent continually.
Allow effluent channels to flow freely, ensuring they are clear of obstruction, allowing effluent to travel directly to the storage tanks.
The levels in the effluent storage tanks should be continually checked to avoid overflow. Also, be aware of signs that effluent is escaping. If effluent is making its way to a field, grass will appear scorched.
Also, check waterways for unpleasant odours or the presence of foam discolouration and for any differences above and below the discharge points.
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