To achieve maximum profitability while reducing negative environmental consequences, corn growers attempt to apply nitrogen (N) fertilizer close to the Maximum Economic Rate of N (MERN). MERN is highly variable and difficult to predict due to variation in net soil N supply and crop N demand which are both influenced by weather. Data from a ten year N response trial in Elora, Ontario were analyzed to study rainfall effects on MERN. Further historical weather and N response data from Elora and Ridgetown, Ontario were also collected. On these medium-textured soils, net soil N supply had little variation while crop N demand was variable. Rainfall accumulated from V5 to V12 was associated with increased yield, crop N demand and MERN. A rainfall adjustment to the current Ontario Corn N Calculator improved grower profitability by $54.86 ha-1 in a validation dataset, demonstrating potential to use in-season precipitation data to improve N management.