A Comprehensive Study of Plant Density Consequences On Nitrogen Uptake Dynamics of Maize Plants From Vegetative to Reproductive Stages.

Nitrogen use efficiency (NUE), defined as grain or biomass production per unit of available soil N, is difficult to predict for specific genotypes and environments. Two field experiments involving paired hybrids (i.e. with/without transgenic insect resistance) were conducted to investigate possible individual and interacting effects of plant density (PD) (low-54,000, medium-76,000 and high-104,000 plants ha^-1) and fertilizer N rate (low-0, medium-165 and high-330 kg N ha^-1) on maize NUE. Total above-ground biomass was fractionated and dry matter and N uptake were measured at four developmental stages (V14, R1, R3 and R6 stages). Both N supply and PD affected growth and grain yield in maize, but hybrid effects were not significant. In both locations, aboveground biomass and N content were highly correlated during the vegetative stages. However, biomass gain was not the only factor driving vegetative-stage N uptake since treatments receiving N fertilizer achieved higher shoot N concentrations (and therefore total N uptake) even when the zero N treatment reached an equivalent aboveground biomass. Total aboveground N content at R1 was significantly correlated with green LAI. Grain yield responded positively to N supply over a wide range of total N uptake in treatments with N fertilizer applied. Treatment differences in grain yield were associated more with variation in total plant biomass than with harvest index, which did not exceed 0.54. Increases in total biomass production per unit area were positively associated with an increase in both crop growth rate and N uptake rate during the critical period bracketing silking. The N rate and PD effects were more evident in plant biomass and N uptake than in grain yield and cumulative grain N uptake. Highest NUE was not always associated with the highest maize grain yields (at least in 1 of 4 hybrids tested), a pheonomena that makes progress in breeding selection for improved NUE inherently difficult.