Relationship Between Response Indices Measured In-Season and at Harvest in Winter Wheat

Current methods for making nitrogen (N) recommendations in winter wheat (Triticum aestivum L.) do not adjust for in-season temporal variability of plant available non-fertilizer N sources. The purpose of this study was to compare the use of different N response indices determined in-season (RINDVI and RIPLANTHEIGHT) to the N response index measured at harvest (RIHARVEST). In addition, this study evaluated the use of the in-season response indices for determining topdress N rates for winter wheat. Nine experiments were conducted over two years at eight different locations. A randomized complete block design with nine different treatments and four replications was used at each location. Preplant N source was ammonia nitrate (34-0-0). At Feekes 4–6, RINDVI was measured to determine the topdress N rates. Both RINDVI and RIPLANTHEIGHT were able to predict RIHARVEST (r2 = 0.75 and r2 = 0.74, respectively). Because the sensor-based approach for making N recommendations relies on information obtained from in-season sensor readings, RINDVI should be used to estimate a site's potential for response to additional N. Use of the response index will allow producers to move away from reliance on preplant application of N and start managing N based on the likelihood of achieving an economical response to N fertilizer.