Historical increases in plant density increased vegetative maize biomass while breeding increased reproductive biomass and allocation to ear over stem

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 grain-filling 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.