Nitrogen (N) fertilization and plant density determine sorghum (Sorghum bicolor L. Moench) productivity and govern the fertilizer energy and irrigation water used per unit of grain. A single-season split–split plot trial (three replications) evaluated three genotypes (ACSAD56, ACSAD58 and EZRAA7), four N rates (0, 40, 80 and 120 kg N ha−1) and three densities (25,000, 33,333 and 50,000 plants ha−1) under limited irrigation (≈480 mm) in a semi-arid Mediterranean environment. ANOVA, correlation, regression, principal component analysis (PCA) and N-, energy- and water-use indicators were applied. Nitrogen increased leaf area index (LAI), chlorophyll, biological and grain yield with diminishing returns: agronomic N efficiency declined from 15.5 to 8.5 kg grain kg−1 N between 40 and 120 kg N ha−1, while fertilizer-N energy per tonne of grain increased 2.6-fold. High density maximized LAI, biological and grain yield and nearly doubled irrigation water productivity without additional fertilizer energy. ACSAD56 and ACSAD58 had similar mean grain yields (2582 and 2541 kg ha−1), 26–28% above EZRAA7. Genotype × N × density interactions determined the best combination: ACSAD56 × 120 kg N ha−1 × 50,000 plants ha−1 produced the highest yield (4342 kg ha−1; 0.90 kg m−3 irrigation water), whereas ACSAD58 × 80 kg N ha−1 × 50,000 plants ha−1 reached 91% of this yield with one-third less N. PCA identified a productivity axis (47.8% of variance) linking grain yield with biomass, LAI, leaf greenness and crop duration. Combining a responsive genotype with high density and intermediate N can sustain grain output while reducing fertilizer energy per unit grain; multi-season validation is needed.
