Physiological Perspectives of Changes Over Time in Maize Yield Dependency On N Uptake and Associated Efficiencies: I. General Overview of Changes.

During the past decades, few studies have combined investigations of GY response to N fertilizer with detailed physiologically-based analyses of plant N dynamics such as N uptake quantities, timing, and (or) partitioning - and the complex interactions of those with specific genotypes (G), management practices (M), and (or) production environments (E). We reviewed 100 published and unpublished papers based on field experiments which consistently reported total plant N uptake at maturity and maize GY (frequently among other physiological variables). Our analyses were limited nearly exclusively to experiments involving hybrid (as distinct from inbred) response to M input levels where plant density data was available. Dissection of the complex interactions among years, plant densities and N rates began with division of treatment mean data (close to ~3000 individual points) into two time periods defined by year(s) of the original research: i) studies from 1940 to 1990 - “Old Era” and, ii) studies from 1991 to 2011 - “New Era”. For the Old Era, maize GY averaged 7.2 Mg ha^-1 at a mean plant density of 5.6 pl m^-2 with a total plant N uptake of 152 kg N ha^-1, a grain harvest index (HI) of 48% and N harvest index (NHI) of 63%. For the New Era, maize GY averaged 9.0 Mg ha^-1 at a mean plant density of 7.1 pl m^-2, total plant N uptake of 170 kg N ha^-1, a grain HI of 50% and a NHI of 64%. The most striking findings in terms of overall GY and plant N uptake were: 1. on a per-unit-area basis, both potential GY and NIE (GY/N uptake) increased from Old to New Era at comparable N uptake levels, 2. on a per-plant basis, total plant N uptake at maturity had not changed between Eras despite increased plant density in the New Era genotypes. In addition, New Era genotypes showed higher tolerance to N deficiency stress (higher GY when no N fertilizer was applied), and larger GY response per unit of N applied, relative to Old Era hybrids.