Context: Understanding changes in below-ground plant traits due to crop improvement is crucial for crop productivity and sustainability assessments. Objectives: To quantify historical changes in root traits due to maize breeding and plant density increases and assess factors influencing root trait expression under field conditions. Methods: We studied root mass, root length, specific root length (SRL), root: shoot ratio, and root carbon concentration in 11 maize hybrids released from 1983 to 2017 under current (8.7 plants m- 2) and a historic plant density treatments (4.7, 6.2 and 8.7 plants m- 2). Soil cores were collected from seven US Midwest environments. Results: Root mass linearly increased with the years of hybrid release under current (7.6 kg ha- 1 year- 1, p = 0.15) and historical (18.2 kg ha- 1 year- 1, p = 0.0028) plant density treatments. Breeding accounted for 44 % and plant density for 57 % of the increase in root mass. Root mass significantly correlated with root carbon (r = 0.99) and grain yield (r = 0.82). Root mass genetic gain was 9-fold lower than that of grain yield. Root length remained unchanged with the year of hybrid release under current density but increased with the year of hybrid release when historical increases in plant density were imposed (0.065 km m- 2 year- 1, p = 0.095). Specific root length decreased with the year of hybrid release by -0.62 mm mg-1 year- 1 (p = 0.0017) in both plant density treatments. The root: shoot ratio did not change with breeding or plant density. The environment substantially influenced the expression of root traits, with precipitation explaining a portion of the variability. Conclusion: Maize breeding and historical increases in plant density increased root mass, decreased SRL, and maintained the root: shoot ratio unchanged, indicating that crop improvement has altered below-ground maize traits in different directions. Implication or significance: Present findings enhance our understanding of how below-ground root traits have changed due to breeding and plant density, which can also support crop modeling studies and soil carbon budgets. The increase in root mass and carbon suggests that breeding for high maize yields boosts root carbon inputs and that crop improvement aids sustainability.

Context: Quantifying historical changes from plant breeding and increasing plant density on maize biomass production and allocation to organs is crucial for understanding historical grain yield increase and its implications for soil health, and carbon sequestration. Yet, such information is scarce. Objective: To quantify and partially distinguish the effects of maize breeding and increasing plant density on maize biomass production, biomass allocation to different plant organs, and biomass re-allocation during grainfilling period. Methods: We studied 18 commercial hybrids (111-day relative maturity) released between 1983 and 2017 across seven environments in the US Corn Belt. Hybrids were grown at current plant density (8.1 pl m-2) and historically increasing plant density (4.7, 5.9, 7.0, and 8.1 plants m-2 for hybrids released in decadal eras 1985, 1995, 2005, and 2015, respectively). Biomass and its distribution to stems, ears (including cobs and kernels), and leaves (including green and senesced) were assessed at the beginning and end of the effective grain-filling period through destructive plant sampling. Results: New hybrids planted at 8.1 pl m-2 produced 6.2 Mg ha-1 more biomass than old hybrids at 4.7 pl m-2. Maize biomass production linearly increased by 107 kg ha-1 year-1 (0.4% year-1) and by 185 kg ha-1 year-1 (0.8 % year-1) under current and historically increasing plant density, respectively. Breeding accounted for 58 % and plant density for 42 % of the total biomass increase at physiological maturity. Plant density did not influence the biomass increase that occurred during the reproductive phase (3 Mg ha-1). Breeding caused a significant shift in biomass allocation, favoring the ear over stems with little impact on leaves. New hybrids remobilized less stem dry matter (1.1 % year-1) and had 15% more green leaf biomass at physiological maturity than older hybrids. Conclusions: Breeding and plant density effects on biomass production and partitioning differed between crop stages. Maize breeding increased reproductive biomass production while plant density increased vegetative biomass production. Breeding and plant density together increased biomass production by 30% from 1983 to 2017. Maize breeding had a greater influence on biomass allocation than plant density. Modern hybrids allocate more dry matter to the ear, have more green leaves at physiological maturity, and remobilize less stem dry matter compared to the old hybrids. Significance: Our results can help explain historical grain yield increase in the US Corn Belt and accurately estimate residue carbon inputs for sustainability assessments and inform crop model calibration tasks. Our findings provide valuable new insights into understanding changes in the maize plant over the years and breeding and plant density interactions.

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 (R-2 = 0.95) and a nonlinear relationship between RFV and maximum root depth (R-2 = 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.

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.

Maize breeding programs have indirectly altered many plant traits; however, our knowledge of some important phenological traits remains unexplored. One such trait is leaf appearance rate, which is crucial for predicting maize development. We studied 40 short-season (103-day) and 38 long-season (111-day) hybrids released from 1980 to 2020 by Bayer Crop Science. Measurements included weekly counting of collared leaves across 13 experiments in the US Corn Belt. The progression of leaf number was expressed as a function of thermal time and described with a trilinear model. Results indicated that new 111-day hybrids produce leaves faster than old hybrids throughout the vegetative phase (7.4% and 3.1% faster before and after the ninth leaf stage, respectively), whereas new 103-day hybrids produce leaves faster only after the ninth leaf stage (9.4%). Thermal time to silking and anthesis decreased by about 1 and 0.56 & DEG;C day year(-1), respectively. Our data revealed that silking and anthesis can precede the final collared leaf by 96 & DEG;C day (3.3 days under optimal conditions), which indicates an overlap between vegetative and reproductive phases. We concluded that maize breeding has indirectly altered the rate of vegetative development of maize hybrids without affecting the final leaf number. Present results expand our knowledge base on the genotypic variability in maize development traits, which can improve empirical and process-based models used for crop stage and yield prediction.

Context: 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. Objective: 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 evo-lution of maize HI.Methods: 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: 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 ac-counts for ca. 15% of the historical maize yield increase in the US Corn Belt over the past 50 years.Conclusions: The maize HI has increased over the last 50 years, and this increase was attributed to breeding, not to management. Significance: 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.

Cover crop fall biomass production and thus successful provisioning of ecosystem services depend on the previous cash crop harvest date. We used a process-based eco-physiological model to investigate the potential of short-season soybean maturity groups (MG) to lengthen the cover crop growing window while achieving yields similar to full-season MG cultivars. Cultivar coefficients for MG 0-4 cultivars for the DSSAT - CROPGRO model were calibrated with data from 13 site-years (in 2017 and 2018) across Kentucky, Nebraska, and Ohio. The model was efficient in predicting differences in soybean harvest maturity date (R8; Model efficiency [ME] = 0.61; Root Mean Square Error [RMSE] = 7.4 days) and yield (ME = 0.38; RMSE = 0.452 Mg ha(-1)) for an independent set of soybean cultivars in the same site-years. Thereafter, a multi-factor sensitivity analysis across 30-yr of historical weather data was conducted. Simulated results showed that MG 3 cultivars would not reduce yield and would advance cover crop establishment compared to MG 4 cultivars. For planting dates in May and conditions of no water stress, adaptating cultivar choices to MG lower than 3 would reduce yields by 55 to 567 kg ha(-1) per unit decrease in MG. Under water stress or when planting date was delayed, adapting cultivar choices to MG lower than 3 had a less detrimental effect on yield. Overall, switching to earlier cutlivar maturities would advance soybean harvest by 7-11 days MG(-1) (May 15 planting date) or 1-7 days MG(-1) (Jul 1 planting date), and lengthen the cover crop growing season in the fall by 95-198 degrees C day MG(-1) (May 15 planting date) or 19 -104 degrees C day MG(-1) (Jul 1 planting date). The greater potential to increase the cover crop growing season with short-season MG cultivars was also associated with a greater soybean yield penalty in the warmest locations in our study. Using crop coefficients calibrated by MG rather than by specific cultivar provided a way to increase model application within a study region to study cultivar maturity adaptations for crop rotations while reducing the need for calibration. Further studies that analyze the tradeoffs from soybean cultivar adaptation on C, N, and water balance, and other indirect ecosystem services from cover crops are necessary.

Core Ideas Wheat simulations occurred in over 2000 site–years for semiarid and subhumid climates. Limited transpiration trait increased wheat yield in 12 g m −2 in semiarid climate. Root exploration traits improved wheat yield by 60 g m −2 in semiarid climate. Faster leaf development trait increased wheat yield by 21 g m −2 across the entire study‐region Genetic variability exists for the above traits for breeding programs to explore. Genetic variability exists for plant traits that confer drought tolerance to wheat ( Triticum aestivum L.); however, there are limited quantitative assessments on the long‐term effects of these traits on wheat yield. As some of these traits might be detrimental in wet years, our objectives were to assess predicted winter wheat yield gains resulting from six altered traits in a moist subhumid to semiarid climate transition area. We used a mechanistic crop simulation model and daily weather data for 30 consecutive years at semiarid ( n = 27), dry‐ ( n = 23) and moist‐subhumid ( n = 18) locations in the US Southern Great Plains. Modified traits were limited transpiration rate under elevated vapor pressure deficit, deeper root system, faster root development, early or late stomata closure in response to soil drying, faster or slower leaf area development, and shorter vegetative cycle. Probability of water‐deficit, defined as fraction of transpirable soil water (FTSW) < 0.3 (i.e., limiting to transpiration), was 0.1 and 0.9 for the moist subhumid and semiarid environments, respectively. Increased root depth and rate of root development resulted in 35.5 to 87.3 g m −2 yield increases and probabilities of yield gain > 0.7 in dry subhumid and semiarid environments. Faster leaf area development resulted in probabilities of yield gain > 0.85 in subhumid environments. Limited transpiration rate increased grain yield by 12 g m −2 in semiarid environments. Neutral traits were early‐ and late‐stomatal closure in response to soil drying, reduced length of vegetative cycle, and slow rate of leaf area development.