Using Maximum Yield Response to Nitrogen to Verify Corn Nitrogen Recommendations.

Nitrogen (N) rates recommended for corn production are rarely field verified for their accuracy partially due to the difficulty associated with in-field determination of actual corn N requirements.  Measuring delta-yield, the maximum yield response associated with applying a non-limiting N fertilizer rate, offers the potential for a simplified verification of corn N fertilizer recommendations.  A database of 600 Ontario corn multi-rate N response trials were fitted with a quadratic-plateau model that provided estimates of maximum yield response to nitrogen (YRTN) and the associated N rate at which it occurs (N_max).  Estimates of economically optimum N rates are based on a rectangular hyperbolic relationship between YRTN and agronomic efficiency of N (kg Grain/kg N applied) at N_max (r²= 0.55).  The maximum potential agronomic efficiency of N (AE) is estimated at 63 kg Grain/kg N applied.  Increasing YRTN is associated with increasing AE.  AE is also increased by about 8% when N was sidedress applied at growth stage V5-V8 compared to spring pre-plant applications.   The potential exists for integrating two rate, YRTN estimations of optimum N rates on a site specific basis in order to verify N rate recommendations.