Current methods for making nitrogen (N) recommendations in winter wheat (Triticum aestivum L.) do not adjust for in-season temporal variability of plant available nonfertilizer N sources. The purpose of this study was to compare the use of different N response indices determined in-season (RI NDVI and RI PLANTHEIGHT ) to the N response index measured at harvest (RI HARVEST ). In addition, this study evaluated the use of the in-season response indices for determining topdress N rates for winter

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... heat. 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, RI NDVI was measured to determine the topdress N rates. Both RI NDVI and RI PLANTHEIGHT were able to predict RI HARVEST (r 2 = 0.75 and r 2 = 0.74, respectively). Because the sensorbased approach for making N recommendations relies on information obtained from inseason sensor readings, RI NDVI 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.