The role of mandatory and voluntary management changes to improve nitrogen use efficiency on derogation and non-derogation dairy farms in Ireland.

Irish dairy systems are predominantly grass-based, with cows grazing outdoors for most of the year, making effective grassland and nutrient management central to both productivity and sustainability. Excessive nitrogen (N) fertilizer use poses environmental risks, with N surpluses potentially contributing to water pollution, making nitrogen use efficiency (NUE) a key metric of farm sustainability. Using farm accountancy data network data from 2013 to 2023, this study examines the determinants of farm gate NUE on derogation and non-derogation dairy farms, quantifying the effects of management practices, regulatory requirements, and biophysical conditions using a fixed effects panel regression model. Results indicate that derogation farms, despite higher organic N stocking rates, achieve NUE comparable to non-derogation farms, reflecting effective N management at higher intensities. Reducing total N inputs, lowering N in concentrate feed, adopting low emission slurry spreading, and using protected urea all improve NUE, whereas more productive soils further enhance efficiency. The findings highlight the importance of targeted regulation, support for voluntary mitigation practices, and nutrient management strategies tailored to soil and farm-specific conditions.