Variability for Internal N Efficiency of Maize and the Impact on N Fertilizer Recommendations.

Internal N efficiency (IE) is defined as grain dry matter (DM) produced per unit of N in the above-ground plant at physiological maturity (R6). Internal N efficiency defines the target for plant N content at R6 in yield-goal based N rate recommendations (currently used in 30 U.S. states) and several commercial N recommendation models. Commonly an invariant value of approximately 48 kg DM kg^-1 N is assumed in yield-goal based recommendations. We examined the variation in IE across a large number of environments and identified weather, soil, and crop factors contributing to the variation. Experiments were conducted in 2014 and 2015 at 30 sites in the Corn Belt. Nitrogen rate treatments ranged from 0 to 314 kg N ha^-1 in increments of 45 kg N ha^-1, applied either at planting or split with 45 kg N ha^-1 at planting and the remainder at the V10 growth stage. Nitrogen timing had little effect on IE. Internal N efficiency at the economic optimum N rate (EONR) ranged among sites from 46 to 73 kg DM kg^-1 N, averaging 60 kg grain DM kg^-1 N content. This variation in IE would result in plant N content at maturity varying from 137 to 217 kg N ha^-1 for maize grain DM of 10,000 kg ha^-1. Plant N content at VT and R6 of the zero N applied treatment (an indication of soil N supplying capacity) explained 41 and 15%, respectively, of the variation in IE across site-years and timing (P≤0.05). Lesser soil N resulted in higher IE. Abundant and well-distributed rainfall (AWDR) throughout the season divided by soil organic matter and AWDR during grain fill explained less than 10% of the variation in IE (P≤0.05). Yield at EONR, EONR, and AWDR during other growth stages did not explain variation in IE (P>0.05).