Nitrogen nutrition index and yield relationship in wheat genotypes with contrasting grain protein

Aims To improve winter wheat nitrogen (N) management, it is necessary to understand whether genotypes that differ in protein concentration (GPC) also differ in the N status required for yield formation. Methods Four genotypes, Iba (low), Gallagher (Gal, mid), Doublestop CL+ (Dob, high), and Green Hammer (Grn, high), were evaluated under two fertilizer N rates (0 and 120 kg N ha⁻¹) across eight Oklahoma site years (2020 to 2023). Aboveground biomass and N concentration were measured at different stages. The nitrogen nutrition index (NNI) was calculated using published dilution curve, and relationships among NNI, relative grain yield, yield components, and GPC were modeled. Results Adequate N status (NNI ≥ 1.0) occurred in 77% of observations under 120 kg N ha⁻¹, compared to 24% under zero N. High protein genotypes (Grn: 43% above the critical curve; Dob 38%) maintained adequate N status more frequently than Iba (31%). Iba reached maximum yield at the lowest critical threshold (0.70), while Dob required 0.77, indicating Iba’s greater effective use of N versus Dob's higher yield potential (0.83 vs. 0.76). Yield components differed in N thresholds, with spike weight saturating at earliest (0.63–0.77), spike number (0.79–0.89), and shoot biomass (0.74–0.94). High protein genotypes exhibited gradual responses across the N gradient, progressively expanding into high-GPC yield space at NNI > 1.0. Conclusions Genotypes with contrasting GPC exhibited distinct NNI thresholds for optimizing yield components and grain yield. Existing critical N dilution curves for dryland winter wheat in the central Great Plains require further investigation.