Optimizing planting density and nitrogen management for enhancing grain quality and free-amino acid compositions in quality protein maize (Zea mays L.)

Maize is a major contributor to global food security; however, conventional cultivars are deficient in essential amino acids, limiting protein quality. Quality protein maize (QPM) addresses this limitation through enhanced lysine and tryptophan content, although its grain biochemical composition is strongly influenced by agronomic management. This study evaluated the effects of planting density and nitrogen regimes on growth, yield, and free-amino acid composition of QPM grain grown during the winter (Rabi) seasons of 2020–21 and 2021–22 in the terai region of West Bengal, India. A split-plot field experiment was conducted using three planting densities and five nitrogen application regimes. Grain yield components, protein-related traits, and individual free amino acids were quantified, and multivariate analyses, including principal component analysis (PCA), were applied to elucidate treatment-driven biochemical variation. Results showed that lower planting density improved individual plant growth and kernel-level yield attributes; however, higher planting density increased grain yield per unit area by up to 55% compared with the lowest density due to greater cob number per hectare. Nitrogen regime exerted a stronger influence on grain biochemical composition than planting density. Late-stage nitrogen application, with 70% of nitrogen supplied at tasseling and silking, significantly increased grain yield (10.2 t ha −1 ), and enhanced protein yield (1186 kg ha −1 ) relative to early nitrogen application. This regime also promoted the accumulation of nutritionally important free-amino acids, including arginine and tryptophan. PCA explained >90% of total variance, clearly separating nitrogen regimes based on amino-acid compositions, while planting density primarily influenced stress-associated amino acids such as proline and cysteine. The results demonstrate that nitrogen timing is a key regulator of amino-acid metabolism and protein quality in QPM grain. It is therefore recommended that moderately high planting density (40 × 20 cm) combined with late-split nitrogen application be adopted to simultaneously maximize yield and nutritional quality of QPM, providing a practical strategy for sustainable cereal-based nutrition security.