Abstract: In this present study, the spatial patterns of N yield in a grass silage field at first, second and third cuts over a three-year period were evaluated, and temporal variability of sward at the first and the second cut were quantified. A 7.9 hectare grassland field close to the Agricultural Research Institute at Hillsborough, Northern Ireland, was selected for the study. Sward N yields were found to vary greatly across the field in different seasons. It was concluded that site-specific management approach to fertilization is necessary. The spatial patterns of N yield in the field are related with the variation altitude. They can be divided into management subunits by the topography.