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Previous studies confirmed continued yield improvements in maize ( Zea mays L.) since the introduction of hybrids. However, corresponding yield genetic gains (GGs) and associated physiological changes occurring under low- nitrogen (N) conditions are less understood. This study assessed the growth, yield and key trait GGs of 38 non-GMO legacy hybrids under low- (34–99 kg N ha −1 ) versus high-N (202–280 kg N ha −1 ) conditions to understand the role of N in trait changes over time. Hybrids, commercialized in the USA by Bayer Crop Science from 1983 until 2020, were evaluated in eight site-years N trails at locations across Indiana, Illinois, and Iowa between 2020 and 2022. Seventeen traits were studied pre- and post-anthesis and at physiological maturity. The GGs for each trait are reported as Best Linear Unbiased Predictions (BLUPs), linked to the year of hybrid release. Grain yield increased by 88 kg ha −1 year −1 under low-N and 102 kg ha −1 year −1 under high-N. Over the past decade, modern hybrids grown under low-N outyielded older hybrids grown under high-N. Despite typical trade-offs between yield and grain N concentration, the grain C:N ratio increased in most recent hybrids subjected to low-N, indicating greater carbon (C) accumulation per unit of plant N uptake. Harvest index (HI) increased similarly under both N conditions, with no sign of a plateauing. Newly released hybrids displayed improved N use efficiency at maturity under low-N, and GGs in biomass accumulation during the post-silking period followed observations of enhanced ear dry weight in newer hybrids at the R2 growth stage. Breeding for yield indirectly improved maize tolerance to soils with limited available N and enhanced N use efficiency via more grain C capture per unit plant N uptake. This study highlights the potential for maize breeders to target specific post-silking traits while simultaneously improving yield and inherent N efficiencies. Furthermore, this research contributes to a deeper understanding of whole-plant N dynamics, and the role of breeding in advancing sustainability. However, future research should also focus on assessing the N rate dependency of GGs in maize hybrids in the context of multiple plant densities or other potential management system interactions. • Genetic gains in maize traits during past four decades evaluated at low- and high-N rates. • Higher absolute yields in newer hybrids, but with lower genetic yield gain under low-N. • New hybrids in last decade under low-N outyielded oldest hybrids under high-N. • Steady gains in grain C:N and harvest index over time confirmed partitioning flexibility. • Much higher N efficiencies in modern hybrids under low- compared to high-N.
Ordóñez et al. (Mon,) studied this question.