Getting more from every kilogramme of nitrogen (N) in 2022 and the years ahead will be the new challenge as fertiliser N prices reach record highs and legislative N limits tighten in a quest to improve water quality and reduce the impacts of agricultural emissions on climate change.

On farm, we mainly associate N supply with bag fertilisers such as CAN (27% N) or urea (46% N). Adjusting bag N fertilisers for soil N supply and slurry N during the growing season will be key to using applied N more efficiently.

To do this, we need to remember the four Rs.

The four Rs

  • The right product.
  • At the right rate.
  • At the right time.
  • In the right place.

    • Soil fertility and N efficiency

    The starting point to improving N use efficiency on an intensive livestock farm starts with maintaining good soil fertility, eg on mineral grassland soils having a soil pH of 6.3 to 6.5 and optimum soil P and K levels at index 3.

    Soil test results will help to provide a solid basis to correcting nutrient deficiencies such as soil pH and making decisions on the most suitable fertiliser types and formulation for 2022.

    Recent research from Johnstown Castle shows that optimising soil fertility will improve N efficiency from 35% to 65%. This means that for every 100kg N/ha applied, we can utilise 65kg N to grow grass rather than 35kg N.

    Liming unlocks soil N and P

    Maintaining the correct soil pH will firstly improve the availability of N, P and K and, secondly, increase the utilisation of applied nutrients such as organic manures or chemical fertilisers.

    For example, liming acidic soils will result in the release of up to 70kg N/ha/ year from the soil organic N reserves.

    This will help to reduce overall farm N fertiliser requirements while reducing costs in the region of €155/ha.

    Research from Johnstown Castle shows that correcting soil pH will increase the availability of soil P and improves the efficiency of applied P.

    In effect, this will reduce P requirements, as soils will supply more available P during the growing season where soil pH is maintained in the optimum range.

    Cattle slurry is a valuable on farm source of available plant nutrients (N, P and K) and can effectively replace bag fertiliser during the growing season.

    On an intensively stocked livestock farm (2-2.9 LU/ha) cattle slurry can supply 10 to 15kg N/ha/year.

    The first step is to know the fertiliser value of each 1,000 gallons of cattle slurry similar to when we purchase a bag of fertiliser. Typical cattle slurry is 6% DM and nutrient values are shown here. Plan to get slurry tested to determine fertiliser value.

    To increase the available N from cattle slurry, it is important to apply with low emission slurry spreading (LESS) technology in springtime when conditions are most favourable to reduce N loss as ammonia. It is important to remember that only 50% of the N in cattle slurry is available at the time of application. The remaining 50% (organic N) is broken down during the growing season.

    Apply cattle slurry to first-cut silage fields to supply majority of crop P and K requirements and maximize N recovery.

    Timing of early N applications

    In the early part of the growing season, there is a lower N demand as grass growth rates are low. Only apply early N when soil and weather conditions are favourable for grass growth, eg soil temperatures greater than 5°C, good soil trafficability and a good weather forecast ahead.

    Apply early N to fields that are dry, have good levels of rye grasses, grass covers >400kg DM/ha and fields with optimum soil fertility.

    The safest form of N is an ammonia-based N such as cattle slurry or protected urea. Apply 25kgN/ha (20 units/ac).

    For example, 25m3/ha (2,200gals/ac) of cattle slurry can supply 25kg N/ha.

    Protected urea cheaper than CAN

    With increasing fertiliser prices, there is an increasing differential between the cost of CAN and urea per unit of N. At present, a unit of N, in the form of urea, is approximately 25% cheaper than CAN. Choose protected urea as it is a more efficient form of N and will give the same performance as a CAN-based fertiliser at lower cost. Research over the last seven to eight years in Ireland shows that protected urea is a very reliable source of N for grassland soils and can reduce greenhouse gas emissions by 71% and reduce ammonia emissions by 79%.