Slurry is a source of nutrients for grass growth thus use of slurry is important for reducing the demand for inorganic fertilisers. Spreading slurry when the conditions are favourable reduces nutrient losses through leaching into water bodies as well as greenhouse gas emissions.
Slurry is a mixture of animal excreta, water, and other wastes. It is a source of nutrients required for grass growth. However, slurry is a significant source of direct emissions of methane (CH4) and ammonia (NH3) to the atmosphere and also an indirect source of nitrous oxide(N2O), so the application needs to be managed well.
Why should farmers aim to spread slurry in Spring?
Spring has the most favourable conditions for minimising the environmental impacts of slurry due to the following reasons.
- Appropriate application of slurry allows farmers to recover maximum nitrogen (N), phosphorus (P), and potassium (K) from their farms.
- According to Teagasc’s advice, Spring is the best time to spread slurry as the conditions allow for fewer emissions and improve nitrogen retention. For example, using Low Emission Slurry Spreading (LESS) equipment increases N availability by 65% in Spring compared to summer applications.
The following video provides all the information on timing of slurry application.
How do you ensure the proper timing of slurry spreading?
- Avoid spreading slurry when rain is forecast within the next 48 hours to avoid slurry being washed away into water sources.
- Cool and damp conditions are best when spreading to minimise losses of NH3 and N2O into the air. Irish farmers are encouraged to follow a slurry spreading calendar.
- Soil temperature should be above 6°C.
- Spread when grass cover is low (up to 1200kgDM/ha) to reduce contamination of grass.
- Spread using LESS equipment.
- Ensure there is enough storage space to avoid spreading slurry when conditions are not permitting.
Economic and Environmental Benefits
Spreading all slurry in Spring ensures more nitrogen is recovered from the slurry and minimises fertiliser emissions. A summary of net savings and emission reductions for a typical 84-cow herd farm is presented in Table 6.
Table 6 Savings and emissions reduction from spreading all slurry in Spring
|Scenario 1||Scenario 2|
|Cattle slurry applied @ 33m3/ha/hour||50% Spring 50%Summer||100% Spring|
|Total slurry produced (m3)||499||499|
|Available nitrogen( kg /m3)||0.85||1|
|Fertiliser savings per farm (€)||202|
|Reductions in emissions (%)||–||0.53|
84 cow herd, soil drainage is average
Fertiliser costs 2.41€/kg nitrogen
*Net savings/ cost are the savings or cost per year for the farm
At Farm Zero C, Life Cycle Assessment is undertaken to quantify the reductions in N2O emissions when the slurry is spread in Spring rather than Summer.
The key findings of previous research on the timing of slurry application include:
- Testing different equipment for slurry spreading, researchers in Ireland found that more nitrogen can be retained for use if the slurry is spread in Spring rather than in Summer.
- Farmers could retain 8.7 units of nitrogen per 1000 gallons of slurry in Spring using a trailing shoe as compared to 5.4 in Summer (Forrestal et al., 2021).
- If farmers use a splash plate in summer they can retain as low as 3.3units nitrogen/1000 gallons of slurry (Forrestal et al., 2021)
- Farmers should aim to spread at least 75% of their slurry in Spring.
Links to more research
Manure and its Management in cattle
Slurry timing for protecting the environment
Effect of increasing the time between slurry application and first rainfall event on phosphorus runoff
The Nitrates and water directive Ireland– The nitrates directive states that the maximum stocking rate on a farm is 170kg/ha (1.9LU/ha) unless the farmer applies for and is approved for a nitrates derogation.
- Slurry cannot be spread on land from 8th October 2022 and 1st October from 2023 onwards to January 12 / 15th / 31st (depending on which part of the country a farm is located).
- Farmers are prohibited from spreading slurry when heavy rainfall is forecast within 48hrs and also when temperatures are very high.
The Nitrates Derogation – For derogation farmers, they should adhere to all slurry spreading timing and use LESS equipment for slurry spreading.
This is a management strategy which requires no information on suppliers.