As the requirement to reduce nitrogen (N) fertiliser use comes thundering down the tracks, farmers need to look to examples of where this has already happened.

The Department of Agriculture has targeted a reduction in fertiliser N use by 2030.

For many, it seems like a mammoth task. There is great risk involved. There is a scepticism from farmers on clover and not enough research on multispecies swards.

At a recent Teagasc Signpost webinar, researcher at Solohead Research Farm Dr James Humphreys outlined examples of systems making greater margins at zero N than those at 250kg of N/ha.

It sounds simple. It’s not simple but from the research James outlined it can be done. Informed decisions are essential.

While nitrogen use decreased, drainage work was needed on the farm, the herd with zero N is higher in EBI than those with fertiliser N in the mix and this is key to the strategy.

Bloat management also needs to be addressed. James outlined some of the key management practices to manage clover in swards which can be seen below.

Nitrogen use at Solohead

Fertiliser nitrogen use at Solohead is at 100kg N/ha, while the aim is to be at less than 50kg N/ha in 2023.

To put this into context, fertiliser nitrogen recommendations have moved from 250kg to 380kg N/ha in the early 1990s to 250kg to 280kg N/ha in the mid 2000s and to 150kg to 250kg N/ha at present (all based on a stocking rate of 2.5 cows/ha).

Comparing systems

Looking at the different systems outlined in Table 1, it is not surprising that N rates differ with stocking rates. Cow numbers are lower with decreasing nitrogen.

The EBI of the herd on zero N (grass and clover) is 195 and is lower at 165 on the grass and clover swards with 110kg N/ha and the grass-only sward with 280kg N/ha.

These figures made a significant difference to the greenhouse gases (GHG) emitted. Some 12.3t/ha of GHG emissions came from the grass-only sward with 280kg N/ha, while 9.5t/ha were emitted from the grass and clover sward with zero N. This equated to a difference of 0.19kg CO2eq/L between the two treatments.

Most importantly, the zero N system appears to be cost-effective, with a net margin €248/ha higher than the grass-only sward receiving 280kg N/ha.

Reduction in carbon footprint

As nitrogen use has decreased on Solohead Research Farm, so too has the carbon footprint and ammonia emissions.

Managing clover swards

  • Swards need a low fertiliser N input.
  • Tight grazing (<4cm).
  • Soil pH of 6.5.
  • Soil phosphorus and potassium managed to be at index 3.
  • Increase to six-week rotations in the autumn.
  • Low covers over winter.
  • Reseeding at 10-year intervals.
  • Drainage of wet soils.
  • Use silage harvests to increase clover contents.
  • Why decrease nitrogen?

    Decreasing artificial nitrogen fertiliser use results in a decrease in nitrous oxide and ammonia emissions and can also contribute to reduced damage to water quality.

    Nitrous oxide is a potent greenhouse gas and while ammonia is not a greenhouse gas, it is a significant polluter.

    Protected urea

    Protected urea and low-emissions slurry spreading equipment are also being used on the farm to reduce emissions.

    Where do emissions come from on dairy farms?

    James described a GHG emissions profile for a dairy farm with a stocking rate of 2.42LU/ha, fertiliser N use of 272kg/ha and a carbon footprint of 1.05kg CO2eq/L.

    Enteric fermentation (methane) and excreta accounted for 63% (52% enteric fermentation and 11% excreta) of emissions. Fertiliser N accounted for 19% of these emissions, while concentrates accounted for 8%, fuel accounted for 3% and 7% of the emissions were classified as other.

    Genetics, grass utilisation and possibly into the future feed additives can help to reduce methane emissions, while clover can help to reduce the need for artificial N through biological N fixation.

    Soil carbon

    Soil carbon has been measured at Solohead for 20 years. James noted it to be about 200t/ha to a depth of 90cm, with little change over the years.

    He explained that the land is wet and plenty of drainage work was carried out over the years. Keeping clover in swards usually means reseeding and this needs to be done at least every 10 years which also results in a loss of carbon. The seedbed is prepared with a disc.


    The main motivation for reducing fertiliser N has traditionally been the impact on the environment. This change will now be forced upon farms through legislation, but real motivation comes from the bottom line.

    The data from this research shows a greater net margin with zero N.

    Fertiliser input costs are dramatically reduced and efficiency has to be improved, as soil pH, Ps and Ks cannot afford to be wrong or clover will not thrive.

    While more establishment costs are involved, this research is important for the future and an aim of less than 50kg N/ha is an ambitious one which will be watched with interest.