Rethinking quality deterioration in maize genetic improvement: From "content reduction" to "productivity surge"

Context Genetic improvement has increased maize grain yield and starch content but decreased protein content. Whether this decline indicates reduced protein productivity, and how N supply regulates yield-quality relationships across cultivar eras, requires further clarification. Objective This study evaluated changes in maize grain quality during cultivar replacement from content and productivity perspectives, and examined N effects on yield-quality coordination. Methods From 2023–2025, six representative maize cultivars released between the 1950s and the 2000s were evaluated in a long-term N fertilization experiment with 0, 150, 225, and 300 kg N ha −1 . Grain yield, starch and protein contents, their respective yields, and production efficiencies were analyzed across cultivar eras and N rates. Results With advancing release year, grain yield increased by 101 kg ha −1 yr −1 and starch content by 0.044% yr −1 , whereas protein content decreased by 0.046% yr −1 . Despite this decline, modern cultivars exhibited significantly higher starch and protein yields and production efficiencies than early cultivars, with protein yield increasing by approximately 65%. N supply affected protein more strongly than starch. Higher N supply narrowed the protein content gap among cultivars from different eras and alleviated the discrepancy in genetic gains between starch and protein production efficiency. Higher N supply showed the potential to further enhance protein output in modern cultivars; compared with N150, N300 increased the protein yield of XY335 by 28%. However, agronomic efficiency of applied N for protein yield (AE N-PY ) declined significantly with increasing N rate. Overall, N225 showed a better balance between protein yield and return per unit N input. Conclusions Long-term genetic improvement enhanced maize protein productivity despite declining grain protein content. The simultaneous improvement of maize yield and quality should build on the genetic backgrounds of modern high-yielding cultivars, integrate genetic improvement with sound N management, and comprehensively evaluate performance across multiple dimensions, including grain component contents, output per unit area, and N-use efficiency.