Nitrogen Concentration at Maturity—An Indicator of Nitrogen Status in Forage Maize

The increased awareness of potential impacts of agricultural activities on nonpoint source pollution has increased the demand for agro-environmental policy measures and for scientifically sound indicators to control their implementation. Our objective was to investigate whether N concentration of maize (Zea mays L.) at silage maturity, a routinely recorded quality parameter, can serve as an end-of-season indicator of N status. Based on 29 field trials on sandy soils in northern Germany, we derived a critical N concentration at silage maturity (CNC), i.e., the minimum N concentration necessary for maximum yield production. A quadratic-plateau function describing dry matter (DM) yield as a function of N concentration allowed for the exclusion of all data sets not responsive to N fertilization or with luxury N uptake. For the remaining pooled data points, a mixed-model analysis provided parameter estimates describing N concentration (Nc) as an exponential function of relative DM yield (Wrel), namely Nc = 4.4141·exp (0.0086·Wrel). Setting Wrel = 100 provided a CNC of 10.5 g N kg-1 DM. This value is in good agreement with results in the literature, which indicates the relative robustness of CNC with respect to a wide range of environmental conditions and genotypes. The CNC constant can be used to evaluate and monitor the end-of-season N status on a large-area scale. Applied to an extended set of silage quality data of northern Germany, it revealed that forage maize production is characterized by significant excess of N supply in this region and leaves ample opportunity for reduction in N use without risk of any yield loss.