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Understanding historical changes in root depth attributes is needed for crop productivity and sustainability assessments, but such information is rare. We explored whether newer maize (Zea mays L.) hybrids grow roots faster and deeper than older hybrids and quantified the role of management and environment on root trait expression. We measured root front velocity (RFV) and maximum root depth in 11 Bayer Crop Science legacy hybrids released from 1983 to 2017 across five environments in the US Corn Belt during 2021 and 2022. Root depth was measured weekly during vegetative stages with manual probes and the maximum root depth at crop harvest with a Giddings probe. Results indicated that the RFV and maximum root depth slightly increased with the year of hybrid release (0.13% per year, p = 0.1) at 8.7 plants m−2. Historical increases in plant density from 4.7 to 8.7 plants m−2 lowered RFV and maximum root depth, but the new hybrids compensated for this loss, resulting in 4% higher RFV and 3% higher maximum depth when comparing systems from 1983 to 2017. The environment strongly influenced root trait expression (>41%). Rain anomaly and soil bulk density explained a portion of this variation. We found a linear relationship between root depth and leaf number (R2 = 0.95) and a nonlinear relationship between RFV and maximum root depth (R2 = 0.77), which can stimulate crop model improvements. Faster and deeper roots were not correlated with maize yields in our environments. This study enhances our understanding of maize breeding impacts on root traits. Core Ideas: We studied root front velocity (RFV) and maximum root depth in maize hybrids released from 1983 to 2017.RFV and maximum root depth increased by 0.13% per year, a 10‐fold lower increase than grain yield genetic gain.Root depth and collared leaf number were strongly correlated (R2 = 0.95).The environment had the strongest influence on the root depth traits.Faster and deeper roots did not necessarily translate into higher yields. [ABSTRACT FROM AUTHOR]

Quantifying historical changes in maize harvest index (HI), the fraction of above-ground biomass allocated to grain yield, can enhance our ability to explain grain yield trends and estimate stover carbon inputs for sustainability assessments. However, the HI genetic gain has not been the primary focus of previous era studies. The aim of this study is to enhance our knowledge of maize HI genetic gain. Our first objective is to quantify HI genetic gain in Bayer Crop Science Legacy hybrids and investigate the contribution of breeding and agronomic management. Our second objective is to develop a general-use model to describe the temporal evolution of maize HI. We studied 54 commercial hybrids (103-day and 111-day relative maturities) released from 1983 to 2020 across 13 environments, including plant density (current and historical increasing rate) and N-fertilizer (low and sufficient N rates) treatments. The HI was estimated at physiological maturity by destructively sampling plants. Then we synthesize new experimental data with literature findings (n = 16) to provide a robust HI genetic gain estimate. Results showed that HI has increased over the years from 0.516 to 0.571 in 103-day hybrids and from 0.537 to 0.584 in 111-day hybrids. The genetic gains were similar across environments and management treatments within the studied range, indicating that this increase is attributed to maize breeding. The N-fertilizer treatments affected the magnitude of the HI, but plant density did not. Our results, combined with 16 literature datasets, revealed a 0.26% year−1 relative increase in HI since 1964. We estimated that the increase in HI accounts for ca. 15% of the historical maize yield increase in the US Corn Belt over the past 50 years. The maize HI has increased over the last 50 years, and this increase was attributed to breeding, not to management. Our findings enhance our knowledge of maize HI, will support robust estimations of carbon inputs in sustainability studies, and inform crop models to better capture historical yield increases. • We quantified harvest index (HI) genetic gain in maize hybrids. • The HI relative genetic gain was 0.26% year−1 since 1964. • The HI increase is attributed to breeding, not to management. • Plant density and N-fertilizer treatments did not affect the HI genetic gain. • We estimated that the increase in HI accounts for 15% of the historical maize yield increase in the US Corn Belt. [ABSTRACT FROM AUTHOR]