Economic and Environmental Determinants of Nitrogen Use Efficiency on Irish Dairy Farms: A Comparison of Derogation and Non-Derogation Systems

Irish livestock systems are characterised by a predominantly grass-based production model, where cows spend the majority of the year grazing outdoors. Effective grassland management is therefore fundamental to the success of Irish livestock farming. Central to this is the strategic application of fertiliser, both chemical and organic, to ensure an adequate supply of grass throughout the grazing season and for the production of grass silage for the housing period. Nonetheless, excessive use of nitrogen (N) fertilisers poses environmental risks, with high N surpluses causing potential runoff into watercourses, thereby compromising water quality. A key metric in assessing the sustainability of livestock systems, where N inputs often exceed outputs, is Nitrogen use efficiency (NUE). In this paper, the NUE trends on Irish dairy farms from 2013 to 2023 was investigated, analysing Farm Accountancy Network data (FADN) to identify key determinants of NUE on grass based systems. The analysis shows that reducing total N inputs is the most important driver, but that farm size reducing the N content in concentrate feed and innovations such as Low Emission Slurry Spreading (LESS) and the use of protected urea are also important drivers of NUE at farm level. Notably, the interaction between soil productivity and nitrogen inputs indicates that more productive soils support significantly higher NUE at comparable input levels, suggesting a greater capacity to convert nitrogen inputs into output on better land.