In spring tillage systems, soils are often left fallow during the autumn and winter, with no actively growing crop to take up residual nitrogen. As a result, there is a high risk that nitrate remaining from the previous spring cereal crop, together with nitrogen released through natural mineralisation during the autumn, will be lost to groundwater.
This risk is particularly high on free-draining soils when soils become saturated or are exposed to prolonged or heavy rainfall. Nitrate can leach down through the soil profile. Once nitrate moves below the rooting zone, it is no longer available for crop uptake and has the potential to enter groundwater, where it can negatively impact water quality.
Cover crops create a demand for nitrate in the soil, taking up available nitrogen and incorporating it into plant biomass rather than allowing it to leach into groundwater. In addition to reducing nitrate leaching, cover crops provide several other benefits, including increasing soil carbon, improving soil structure, reducing soil compaction, increasing water infiltration, and lowering the risk of soil erosion and phosphorus loss during periods of heavy rainfall.
Maximising the benefit of cover crops
The magnitude of the benefit of cover crops to reduce nitrogen leaching is dependent on the amount of biomass produced. Under Irish conditions the principal factor affecting the amount of biomass produced by the crop is sowing date.
Figure 1 shows the results for three sowing dates: early - 30 July; target – 18 August and delayed - 8 September. The two cover crops used were mustard which is a fast-growing non-legume and hairy vetch which is a winter hardy legume.
The results show that there was a linear reduction in the amount of biomass produced as sowing date was delayed. The mustard lost 2t DM/ha for each three-week delay in sowing.
Impact of cover crop on nitrogen leaching
Cover crops have been shown to reduce nitrogen leaching substantially. Work at Teagasc Oak Park on a free draining soil has shown that a cover crop established in mid to late-August reduced nitrogen leaching by 52 kg N/ha compared to bare soil in a continuous spring barley rotation, meaning this is not lost to water and could be used to grow the next crop.
Choice of cover crops
There is a wide variation in cover crops available. However, it is important to select the correct cover crop species. Growers of beans and peas should avoid legumes in their cover crop mixtures.
Oilseed rape growers should avoid brassicas due to clubroot. Other factors to consider when choosing a cover crop mixture is frost tolerance, biomass production, plus pests and diseases (speak to your adviser and/or merchant supplier).
For most tillage farms in Ireland, a forage oats, vetch, phacelia and oilseed radish mix offers one of the best balances of cost, soil improvement, and nutrient retention after spring barley. If grazing is the priority, adding forage rape or stubble turnips can make the mix more productive for livestock.
Farming for Water EIP
The Farming for Water EIP payments for implementing cover crops is available in certain catchments. There are two payment systems: a spring cropping system when they must be sown by 1 September and remain in place until 1 January. The second is for winter cropping systems and the crop must be sown by 1 August and remain in place until 30 September.
The minimum area to be delivered is 0.5ha and maximum area for payment is 20 ha. Establishment must use non-inversion techniques (ploughing not allowed) and the seed mixture must contain at least two crop species.
Last year, it is estimated that approximately 30,000ha were sown as part of the Farming for Water EIP, one third of which was sown in the Slaney and Barrow catchments (speak to your ASSAP or local adviser for more information).
*Neilus Noonan is Slaney Catchment co-ordinator at the Better Farming for Water Programme.
Darren and Brendan Ferguson
Darren Ferguson farms with his father Brendan near Rosslare Harbour in Co Wexford, where they operate a mixed dairy and tillage enterprise. In 2025, Darren established 30ac of cover crops under the Farming for Water EIP project and a further 50ac through the ACRES scheme.
He has been incorporating cover crops into the farming system for many years, recognising their value in improving soil health, while also contributing to better water quality. Darren is particularly aware of the importance of reducing nitrate losses to nearby Lady’s Island Lake.
The cover crop consisted of a seven-species mix comprising buckwheat, crimson clover, tillage radish, vetch, phacelia, berseem clover, and sunflowers. The cover crops were sown after early potatoes (as part of localised Lady’s Island Lake recommended mitigation actions) and also after winter barley.
Establishment involved sub-soiling with a disc aerator, followed by disc cultivation, sowing, and rolling. Sowing commenced on 16 July 2025 and was completed very soon after. Ideal establishment conditions in 2025 resulted in some of the best cover crops Darren has ever grown on the farm.
Darren was very impressed by the flexibility offered by the farming for water EIP pilot scheme launched in 2025 and hopes to sow the same or increased area under cover crops again this year.
Always looking for opportunities to innovate and try new things on the farm, Darren also successfully trialled direct drilling a crop of winter wheat after the cover crop and was delighted with the result. Darren and Brendan hope that all this work to improve water quality being undertaken by them and their neighbouring farmers will result in the required reductions in nitrate concentrations in Lady’s Island Lake and also in the close by Slaney Catchment (where they also farm some tillage land).