An irrigated corn (Zea mays L., cv. Pioneer 3732) N fertilizer field experiment was conducted for 3 yr to compare plant analysis interpretation of N status by the diagnosis and recommendation integrated system (DRIS) and sufficiency level (SL) methods. Preplant soil NO3‐N was supplemented with ammonium nitrate to provide seven available N levels. Leaf samples were taken at the silking stage and analyzed for N, P, K, Ca, Mg, S, Mn, Zn, and Cu. Nutrient indices were calculated using published standard and locally‐developed (South Dakota) DRIS norms derived from 600 observations. At bighest grain yield, 11.86 Mg ha−1, the average soil NO3‐N level was 247 kg ha−1 over the 3‐yr test period, while check yields averaged 6.54 Mg ha−1 and required 68 kg N03‐N ha−1. The SL approach (using a critical ear leaf N concentration of 27.6 g kg−1) diagnosed N as inadequate even when excess N was applied, indicating that the N sufficiency level for irrigated corn is inflated and needs to be readjusted to 25.2 g N kg−1. Nitrogen indices calculated from the standard and local DRIS norms at 90% of maximum yield were −2 and 2, respectively. Regression of Nutrient Balance Indices against relative yield indicated greater variability in the standard norms than in the local norms. Our data suggest that the SL approach overestimated ear leaf N requirement, and that DRIS indices calculated using published standard norms were less useful than those from locally‐developed norms.