Improved understanding of the interactions between management practices and climate variability is critical for the development of sustainable grassland management and the provision of bundles of ecosystem services in a changing environment. Here we used a long-term fertilizer field trial to examine the impacts of climate variability (annual temperature, rainfall) on an upland grassland subjected to a gradient of nutrient availability. We tested for the effect of climatic indices, fertilizer regime, and their interactions, on annual biomass production, forage quality (crude protein content, digestibility) and plant diversity (species richness, equitability). During the 15-year study period we recorded significant interannual variation in both climate and grassland properties. We found that fertilizer treatments, mean annual temperature and annual precipitation all affected the grassland variables in this study, but interactions between climate and management treatment were generally limited. Contrary to expectations, interactions were driven by the PK rather than the NPK treatment. These results highlight the importance of management for projected responses to future climate change in models of grassland function.