Of all the issues facing farmers at present, the cost and availability of chemical fertiliser is one the most pressing. Reducing reliance on chemical fertiliser, particularly nitrogen, is enshrined in both the Climate Action Plan and the Nitrates Action Plan.

However, the transition to lower-input pastures such as high clover and multispecies swards, which require less nitrogen, takes time and management changes.

A big increase in price, or worse still, the restricted availability of chemical fertiliser next spring will have massive repercussions for Irish livestock farms, particularly higher-stocked dairy farms, which rely more on chemical nitrogen to grow grass.

Spring grazing

While the response to chemical nitrogen is lowest in early spring, the grass that the nitrogen grows is extremely valuable – high in energy and protein and still cheaper than concentrates.

CAN fertiliser being unloaded at Belview port in Waterford last summer. There is the potential for delays to fertiliser imports next spring. \ Donal O'Leary

Even at high nitrogen prices, there is an economic return for most dairy farmers who complete their first round of grazing to spread nitrogen in early spring.

However, it’s important that the highest possible response is achieved and according to Teagasc researcher Michael O’Donovan, the highest response is achieved when nitrogen is:

  • Spread from early to mid-February when soil temperatures exceed 5.5°C.
  • No rainfall is due within the next five days after spreading.
  • Only applied on fields with a high percentage of perennial ryegrass.
  • Only applied on fields with good soil fertility.
  • Spreading 2,500 gallons/acre of slurry in early spring is the equivalent of spreading 23 units/acre of nitrogen. The nutrient value of slurry has been one of the biggest revelations in recent years, particularly since the introduction of low-emission slurry spreading (LESS) equipment. Not only does slurry provide nitrogen, but it also provides phosphorus, potash and other nutrients, along with organic matter.

    Depending on grass covers, up to 40% of the farm can be spread with slurry in early spring. Where LESS is used, slurry can be spread on covers up to 1,000kg/ha without tainting the grass. Where a splash plate is used, slurry should not be spread on covers of over 500kg or 600kg/ha, which obviously limits where slurry can go and how much area of the farm can be spread.

    “The key thing is to avoid spreading too much slurry over too small an area. The best use of slurry is when it is spread at a rate of 2,500 gallons/acre over as big an area as possible. If not all the slurry is spread, then that’s fine. More can be spread in March or hold on to it for the silage ground,” O’Donavan says.

    The Teagasc recommendation for spring 2022 is outlined in Table 1. Essentially, 40% of the farm should get slurry in January or February and an additionally 15% of the farm should get slurry after grazing in February. The remainder should get 23 units/acre of urea, except the 15% that is to be grazed first in February. The area that was grazed and got slurry and urea in February should get an additional 23 units/acre in March. Everywhere else should get 40 units/acre in March.

    If following this plan, the total amount of chemical nitrogen being applied will average 48 units/acre, along with 16 units/acre of organic nitrogen in slurry, giving a total of 64 units/acre of nitrogen applied by early April. This is 10% less total nitrogen compared to advice in previous years.

    Silage costs

    For most farmers, the single largest application of fertiliser is on silage ground. That application typically takes place in late March or early April.

    What impact will rising fertiliser costs have on the cost of making silage next year and what will be the impact on silage yield if less fertiliser is spread? The typical costs of silage in 2021 are presented in Table 1.

    These costs include a land charge of €120/ac and presume no slurry is applied. If slurry was applied, less nitrogen would be needed and no 0:7:30 would be required. The nitrogen rate is set at 80 units/acre and the response is estimated at 35kg grass for every 1kg of nitrogen. This suggests that if no nitrogen was spread, there would be a cover of 1,500kg/ha on the field by 1 June. The later cutting date has a higher yield but is likely to lead to much lower-quality silage and will produce less grass for the remainder of the season. Based on this table, silage costs in 2021 are between 17c/kg DM and 12c/kg DM depending on yield.

    Table 2 looks at the impact of higher fertiliser costs on silage costs.

    Land charges, harvesting costs and yield are the same as 2021. In this example, protected urea (38% nitrogen) costs €800/t and 0:7:30 costs €700/t. With the higher fertiliser costs, silage costs increase by 28% to 22c/kg DM for a 5t/ha crop and 16c/kg DM for a 7tDM/ha crop.

    Table 3 looks at the impact on silage costs and silage yield if less fertiliser was applied to silage ground.

    Some farmers might decide to spread less fertiliser in response to the rising costs but there is also the possibility that if fertiliser isn’t available next April, farmers will have no choice but to spread less than normal.

    In this example, it is presumed that 30% less nitrogen and 30% less 0:7:30 is applied to silage ground. Taking the same response rate of 35kg of grass dry matter for every 1kg of nitrogen applied, the yield of grass harvested on 1 June and 15 June is 21% less than if the full rate of fertiliser was applied. Furthermore, the cost per kilo of grass silage increases by almost 10% where less fertiliser is applied. In this case, the 1 June cut costs 0.24c/kg DM while the 15 June cut costs 0.17c/kg DM.

    There are no winners with rising fertiliser costs, but this analysis clearly shows that supply of fertiliser is more critical than cost.

    If Ireland has insufficient quantities of fertiliser to meet crop needs next spring, we can expect a major fodder crisis next winter.

    A 28% increase in silage costs compared to 2021 is hard to fathom and farmers are rightly worried that increased output prices will erode gains in output prices in 2021 and possibly into 2022.

    However, it’s a much bigger worry for farmers without a reserve of silage.

    In this example, spreading 70% of the normal fertiliser amount reduces yield by 21% and increases cost per kilo of silage by almost 10%. If fertiliser is scarce, the advice is to apply the recommended amount on a smaller area in order to reduce unit costs. The shortfall could be made up by a larger second or even third cut, when hopefully fertiliser supply will have increased.


    Making more use of slurry is, of course, an option, but cows will produce the same amount of slurry this winter as they did last winter.

    While there are many efficiencies to be gained with slurry, it won’t solve the fertiliser supply problem by itself.

    Longer term, the use of clover both in silage and grazing swards is a logical mitigation against rising fertiliser costs and supply chain disruptions, while also reducing greenhouse gas emissions and input costs.

    In brief

  • Use slurry to replace chemical nitrogen in early spring.
  • No more than 2,500 gallons/acre of slurry should be spread in early spring to replace chemical N.
  • At current fertiliser prices, the cost of silage next year is set to increase by 28%.
  • If fertiliser rates are reduced by 30%, there is likely to be a 21% reduction in silage yield.