The competing demands of increasing grain yields to feed a growing population and decreasing nitrogen (N) fertilizer use and loss to the environment poses a grand challenge to farmers and society, and necessitates achieving improved N use efficiency (NUE) in cereal crops. Although selection for increased yield in maize has improved NUE over time, the present understanding of the physiological determinants of NUE and its key components hampers the design of more effective breeding strategies conducive to accelerating genetic gain for this trait. We show that maize NUE gains have been supported by more efficient allocation of N among plant organs during the grain filling period. Comparing seven maize hybrids commercialized between 1946 and 2015 from a single seed company in multiple N fertilizer treatments, we demonstrate that modern hybrids produced more grain per unit of accumulated N by more efficiently remobilizing N stored in stems than in leaves to support kernel growth. Increases in N fertilizer recovery and N harvest index at maturity were mirrored by a steady decrease in stem N allocation in this era study. These insights can inform future breeding strategies for continued NUE gains through improved conversion efficiency of accumulated plant N into grain yield.

We conducted a synthesis analysis on data from 86 published field experiments conducted from 1903 to 2014 to explore the specific consequences of post-silking N accumulation (PostN) in New Era vs. Old Era hybrids on grain yield (GY) and recovery from plant N stress at flowering (R1 stage). The Old Era encompassed studies using genotypes released before, and including, 1990 and the New Era included all studies using genotypes released from 1991 to 2014. Mean N fertilizer rates for experiments in the Old and New Era were similar (170 and 172 kg ha−1, respectively), but plant densities averaged 5.0 plants m−2 in the Old Era vs. 7.3 plants m−2 in the New Era studies. Whole-plant N stress at R1 for each hybrid, environment and management combination was ranked into one of three categories relative to the N Nutrition Index (NNI). The key findings from this analysis are: (i) New Era genotypes increased the proportion of the total plant N at maturity accumulated post-silking (%PostN) as N stress levels at R1 increased—demonstrating improved adaptability to low N environments, (ii) New Era hybrids maintained similar GY on a per plant basis under both low and high N stress at R1 despite being subject to much higher population stress, (iii) PostN is more strongly correlated to GY (both eras combined) when under severe R1 N stress than under less acute N stress at R1, (iv) the New Era accumulated more total N (an increase of 30 kg N ha−1) and higher %PostN (an increase from 30% in Old to 36% in New Era), and (v) the change in stover dry weight from silking to physiological maturity (ΔStover) has a positive, linear relationship with PostN in the Old Era but less so in the New Era. This increased understanding of how modern genotypes accumulate more N in the reproductive stage and have more PostN and GY resilience to mid-season N stress, even when grown at much higher plant densities, will assist trait selection and N management research directed to improving maize yields and N efficiencies simultaneously.

N itrogen fertilizer is a high-cost input for maize production that is diffi cult to manage because efficient crop accumulation and utilization is dependent on agronomic, genetic, biological, and environmental factors. Within the wide spectrum of N fertilizer management decisions, a key interest has been improving the effi ciency with which N fertilizers are used by optimizing N rate and the timing of N application. One way to pursue increased N fertilizer effi ciency may be with the use of late-season, split N applications instead of the more common practice of N application near or shortly aft er planting (Scharf et al., 2002). Reasons that can motivate implementing late-season N applications include reducing work load during planting season, avoiding the frequently wet spring soil conditions, and allowing in-season assessment of N fertilizer needs (Scharf et al., 2002). Once N is applied to the soil it is vulnerable to losses from denitrifi cation and leaching, as well as volatilization and surface run-off if not injected (Raun and Johnson, 1999). When considering the potential benefi ts of late-season N applications, of prime interest are how the timing of N application impacts post-silking nitrogen accumulation (PostN), and the relationships between PostN and grain yield.Previous literature refl ects mixed results on the eff ects of the timing of sidedress N application. While Scharf et al. (2002) found no evidence of decreased grain yield when 100% of the N application was delayed until V11, Binder et al. (2000) reported a 12% reduction in maximum grain yield due to delaying sidedress application until V6 in a silty-clay soil. Similarly, Walsh et al. (2012) found a yield loss in six out of nine sites when sidedress was delayed until V10-VT. Th e latter authors attributed this yield decrease to irreversible yield loss due to early season N stress. Th ese studies indicate that a more feasible split-N application strategy would involve applying the majority of the N fertilizer early in the growing season and delaying a supplemental portion until the late-vegetative aBStractTh eoretically, N losses are reduced by synchronizing fertilizer additions with plant uptake requirements. We investigated the impacts of supplemental, late-season N applications on nitrogen fertilizer recovery effi ciency (NRE), and N accumulation and partitioning in maize (Zea mays L.) at silking (R1) and physiological maturity (R6). Also tested was whether modern hybrids responded diff erently to split-N applications compared to hybrids released 20 yr ago. We compared 3 to 4 N rates ranging from 0 to 245 kg N ha -1 applied either in a single application at V3, or split with the last 45 kg N ha -1 delayed until V12, over 3 yr. Two newer hybrids (2012 and 2014) and two 1990 era hybrids (1991 and 1995) were compared at all N treatment combinations. Additional plant N accumulation following latesplit N applications was already apparent at R1, particularly in stems. Late-split N application increased both whole-plant R6 N accumulation and N...

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