Changes in Post-Silking N Accumulation and Allocation in Maize over Time: A Review.

It has been proposed that modern maize genotypes take up a greater proportion of their total N after the silking period, but the implications of this physiological shift on grain yield (GY) and N stress tolerance are not clear. We conducted a synthesis analysis on data from 87 prior field experiments conducted from 1903 to 2014 to explore the specific consequences of post-silking N accumulation (PostN) in New Era versus Old Era hybrids on maize grain yield (GY) and recovery from plant N stress at flowering (R1).  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 two eras were similar (170 and 172 kg ha^-1 for the Old and New Era, respectively), but plant densities averaged 5.0 plants m^-2 in the Old Era versus 7.4 plants m^-2 in the New Era studies. Whole-plant N stress at R1 for each hybrid, environment and management combination was ranked relative to the N Nutrition Index (NNI). The key findings from this analysis are, (i) genotypes in the New Era increased the proportion of the total plant N at maturity that was 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 when N stress is less acute at the onset of reproductive stages, (iv) the New Era accumulated both more total N (an increase of 30.5 kg N ha^-1) and a significantly greater proportion of their total N (an increase from 30% to 36% from the Old to the New Era) after silking (%PostN) than did the Old 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 will assist in gaining a better understanding for how we can exploit these traits to manage N fertilizer more efficiently while increasing grain yields.