Identifying an In‐Season Response Index and the Potential to Increase Wheat Yield with Nitrogen

Current nitrogen use efficiency (NUE) of cereal crop production is estimated to be near 33%, indicating that much of the applied fertilizer N is not utilized by the plant and is susceptible to loss from the soil–plant system. Supplying fertilizer N only when a crop response is expected may improve use efficiency and profitability. A response index using harvest data was recently proposed that indicates the actual crop response to additional N within a given year. This response index, RI Harvest , is calculated by dividing the average grain yield of the highest yielding treatment receiving N by the average yield of a check treatment (0 N). Although theoretically useful, RI Harvest does not allow for in‐season adjustment of N application. The objective of this work was to determine the relationship between RI Harvest and the response index measured in‐season (RI NDVI ) using the normalized difference vegetative index (NDVI). Research was conducted in 23 existing field experiments in Oklahoma. Each field experiment evaluated crop response to varying levels of preplant N. At Feekes growth stages 5, 9, and 10.5, RI Harvest was accurately predicted using RI NDVI ( r 2 > 0.56). These results indicated that the in‐season response index based on sensor readings is a viable method for identifying environments (i.e., fields) where the potential to respond to additional N exists.