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How do I increase the nitrogen availability of slurry?

Slurry is an important source of N, P & K and its effective use on farms can help to balance soil fertility levels and offset expensive chemical fertiliser costs, writes David Wall, Teagasc, CELUP, Johnstown Castle, Co. Wexford.

Research conducted by Teagasc Johnstown Castle has shown that in order to maximise the nutrient value of cattle slurry and reduce N loss to the air a number of decisions must be made. 

Slurry

Firstly, where on the farm should slurry be applied in order to maximise slurry P and K? Secondly, when is the most efficient time to apply slurry to maximise N recovery?

Targeted application of slurry in the early growing season, or at silage closing time, based on soil test results, will ensure the most efficient use of the slurry N, P and K by grassland. 

The typical value of 1,000 gallons of cattle slurry applied by low emission slurry spreading (LESS) method in springtime has an available N-P-K content equivalent to a 50 kg bag of 9-5-32.

Benefits of using low emission slurry spreading (LESS) methods

1. Increasing the nitrogen (N) availability of cattle slurry

  • Nitrogen losses increase when slurry is spread during dry sunny & windy weather
  • Slurry applied in spring (cool damp weather) delivers more N for grass growth
  • LESS methods reduce these N losses directly after slurry spreading. LESS methods minimise the surface area to which slurry is applied and put the slurry directly onto the soil. The main type of LESS equipment are:
    • a) Band-spreader;
    • b) Trailing shoe or;
    • c) Shallow injection methods.
  • The N fertiliser replacement value of slutty increases by +10-20% when using LESS methods

Table 1. Nitrogen (N) availability and value (€) using low emission slurry spreading methods

Cattle slurry nitrogen value (€) using low emission slurry spreading methods

Splash plate Splash plate Trailing shoe/LESS Trailing shoe/LESS Direct injection Direct injection
Spring Summer Spring Summer Spring Summer
N recovery % 25% 14% 40% 30% 50% 45%
Available 
N/ha
20kg 13kg 33kg 23kg 40kg 36kg
N value (€/ha) €21 €14 €35 €24 €42 €37

2. Increased flexibility of slurry applications on grazed pasture with LESS

  • Low contamination of grass herbage (faster return to grazing);
  • Potential to apply slurry to heavier grass covers (especially with trailing shoe);
  • More opportunities to apply slurry to grazing paddocks.

Each additional grazing day in spring is worth €2.70 per cow/day

3. Accurate application of slurry nutrients with LESS and replace expensive chemical fertilisers

  • LESS methods achieve a more even application rate of slurry nutrients across the spread width than the traditional splash-plate method;
  • Reduce slurry odour at the time of spreading as the slurry is placed on the soil;
  • Provides an opportunity to substitute expensive chemical P and K fertilisers with slurry;
  • Return slurry to silage ground where possible to recycle nutrients efficiently and maximise the slurry fertiliser value;
  • Increase the fertiliser replacement value of slurry by using LESS
    + €2-3 in N fertiliser replacement value per 4.5 m3 of cattle slurry (figure 1).

 

Figure 1. Equivalent N-P-K fertiliser value of 4.5m3 (1,000 gallons) of cattle slurry when applied using splash-plate and low emission slurry spreading (LESS) methods.

4. Improved environmental and economic performance of slurry management

  • Slurry contributes to Ammonia emissions to the atmosphere;
  • LESS is a key management strategy to reduce Ammonia emissions;
  • Up to 60% reduction N loss (as Ammonia) to atmosphere using LESS;
  • Opportunity to lower chemical N fertiliser requirements following LESS (more N delivered from the slurry);
  • Significant savings can be achieved on the farm fertiliser bill. An additional +€10 to €28 per 33m3/ha value of N can be saved by using LESS (table 1).
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