Nitrogen (N) is typically the most limiting nutrient for corn production. Application of nitrogen fertilizer at the economic optimum nitrogen rate (EONR) boosts corn yield and profitability while minimizing environmental N losses (Box); however, the EONR varies and is unknown at the time of N fertilizer application. Soil, hybrid and agronomic management interact with weather to create this variability. Weather accounts for 50% of the variability in EONR while agronomic management and hybrids are responsible for the remaining 50%. Measuring and understanding variability in EONR can identify opportunities to improve N fertilizer management for greater N use efficiency and profitability.