Grass-clover leys have advantages in terms of yield stability, feed quality and milk production if fertilizer application is optimized. Economically optimal N application, taking into account yield, protein content and clover value for milk production, vary from no N at high clover content (~ 50%) to more than 240 kg N ha-1 at low clover proportion (~ 20%). Thus, there is a need to differentiate fertilizer N application according to clover proportion, which may be affected by establishment, weather and sward deterioration over the years. Automated image analysis has opened the possibility for identifying clover content which, linked to decision support systems, can be used for differentiated fertilizer application. In-field variability in clover content needs further investigation, but an example of clover content variation from 0 to 70% across an 11 ha field is shown. Furthermore, the technology may assist other management decisions such as reseeding time. Compared to annual crops and grazed systems, under cutting-only management, nitrate leaching from fertilizer inputs is much reduced. In an experiment with increasing levels of N in cattle slurry, applied for up to 4 years, the nitrate concentrations were very low and no increase in leaching was found when applying cattle slurry at up to 300 kg total-N ha-1yr-1. However, there is a need to investigate consequences at the crop rotation level of fertilizer strategies for both nitrate leaching and N2O emissions.