Maize kernel weight genetic gain is achieved through different mechanisms depending on the hybrid maturity

Context or problem: A range of maize (Zea mays L.) hybrid maturities are planted across the US Corn Belt, yet, studies that concurrently evaluate genetic gain for grain yield and associated kernel growth traits are rare and this limits our ability to understand physiological mechanisms of yield formation by hybrid maturity. Objective or research question: Our objective was to explore the genetic gain in yield, kernel weight, and kernel growth patterns, and to address differences in genetic gain associated with hybrid maturity. Methods: We studied genetic gain for grain yield and six kernel growth and development traits in 38 commercial hybrids released from 1985 to 2015. The hybrids represent two relative maturities (RM similar to 103 and RM similar to 111, hereafter short and long maturity, respectively). Kernel growth was assessed weekly from a total of seven experiments in the USA during 2021 and 2022. Results: Results indicated a positive genetic gain for grain yield (0.76% and 0.66% year(-1) for the long and short maturities, respectively). Kernel weight increased with the year of hybrid release for long maturity hybrids (0.37% year(-1); p < 0.05), while this increment was marginal for short maturity ones (0.23% year(-1); p = 0.12). The increase in kernel weight was achieved through different mechanisms depending on hybrid maturity. In the long maturity, it was associated with an extended grain-filling duration (0.18% year -1; p < 0.05) caused by reducing the kernel desiccation rate before physiological maturity. In the short maturity hybrids, it was related to an increased kernel growth rate (0.18% year(-1); p < 0.10) caused by increasing the potential kernel size during early grain filling. Conclusions: We concluded that maize breeding increased final kernel weight and yield via different mechanisms depending on hybrid maturity. Consequences of these differences are large, since grain-filling duration and rate are governed by different physiological processes. Implications or significance: Our results evidence the need to segregate breeding effects by hybrid maturity. The existence of different mechanisms provides maize breeding alternative pathways to further increase kernel weight without affecting grain moisture at harvest.