Internal N efficiency (IE) is defined as the amount of grain dry matter (GDM) produced per unit of N in the above ground plant at physiological maturity (PMN). Currently, a static value of IE (48 kg GDM kg^-1N) is used to define the optimal PMN in yield goal-based N recommendations used in 30 U.S. states and several N recommendation models. To evaluate the accuracy and variability of this value of IE at the economic optimum N rate (IE_E), experiments were conducted at 47 sites located in eight states over a three year period (2014-2016). To establish IE_E, N treatments ranged from 0 to 315 kg N ha^-1 in 45 kg N ha^-1 increments, applied either at-planting or split with 45 kg N ha^-1 at-planting and the remainder at the V9±1 V-stage. Average IE_E across all site-years was 53 kg GDM kg^-1 N with 79% of the observations between 46 and 60 kg GDM kg^-1 N, higher than the currently accepted value of IE. Half of the time the timing of N application affected IE_E, with greater IE_E with split N in 70% of these instances due to lower PMN arising from reduced stover dry matter. In most cases the timing of N did not affect IE_E. Across all site-years, GDM at the EONR or EONR were unrelated to IE_E. Plant N content at VT of the non-fertilized and 45 kg N ha^-1 at planting treatments were single variables most highly correlated with IE_E (<i>p</i> ≤ 0.10, r = -0.42 and -0.50, respectively). These variables reflected the amount of residual or available N retained in the plant and/or SDM at the optimal N rate. Other factors such as plant available water content at various depths and crop reflectance at the V9 leaf stage (sufficiency and simple ratio indices for both NDVI and NDRE at 0 and 45 kg N ha^-1) were negatively related to IE_E across all site-years, but only weakly. Predictive models for IE_E at planting and prior to sidedressing accounted for < 50% of the variation in IE_E. Internal N efficiency varied considerably, but was difficult to predict, thus contributing to the inaccuracy of the yield-goal based N recommendations.

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