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On-the-goal yield monitors are now widely available. However, it is not clear how to use this Site Specific Crop Management (SSCM) tool in a effective manner for N fertilization. The objectives of this study were to examine different yield indices as predictors of maximum economic rate of N (MERN) application; to develop a predictive relationship for MERN based on the most useful of these indices; and to evaluate the potential usefulness of these indices in SSCM. Two large, independent historical data sets containing 202 (from 1962–1986) and 56 (from 1986–1990) field trials were used to develop the indices. Data sets were divided into three geographical groupings. Yield potential (Ymax) and most economic yield (Ye) were highly correlated to each other, and both were poor predictors (r2 < 0.15) of MERN. Check yield (Yc, yield with no N applied) explained more of the variation in MERN (20–56%) than either Ymax or Ye. The maximum yield increase ∆ Ymax=Ymax − Yc, and the economic yield increase, ∆ Ye = Ye−Yc, were both strongly correlated (r=0.70 to 0.88) with MERN. However, the relationship is non-linear. An equation was derived to describe the relationship based on the correlation between coefficients describing the fertilizer response. Field experiments were carried out to demonstrate the use of ∆ Y measurements to obtain the site specific map for N fertilizer application. The procedure requires the establishment of strip check plots with no N fertilizer. The study indicates that on-the-go yield measurements from a single rate of applied N fertilizer cannot be used to estimate the map for fertilizer N application.