The range of the critical nitrogen dilution curve for maize (Zea mays L.) can be extended until silage maturity

The concept of critical N concentration (Ncrit) assumes at any time a minimum shoot N concentration necessary for maximum biomass production. For maize (Zea mays L.), Ncrit has of now been confirmed only from emergence to silking plus 25 d. These results were based on mineral N fertilization. In the present study, we verified that the validity of the concept can be extended to silage maturity and that it is not only applicable to mineral but also to organic N fertilizer. Our investigation included a 3-yr N fertilization experiment comprising four mineral N fertilization rates (0, 50, 100, and 150 kg N ha(-1)) and three slurry treatments (0, 20, and 40 m(3) ha(-1)), respectively. A quadratic-plateau model was used to determine Ncrit values. An analysis of covariance indicated that the Ncrit-to-biomass relationship was extendible to the postsilking stages. The Ncrit [g N kg(-1) dry matter (DM)] was then successfully described by a mononomial function of biomass W (t DM ha(-1)): Ncrit = 34.12(.)W(-0.391). We suggest restricting the model to growth stages with biomass exceeding a threshold of 1 t ha(-1). Model validation against an independent data set comprising different hybrids and soil conditions indicated a satisfactory separation of N-limiting and non-N-limiting growth. The Ncrit concept thus seems to provide an efficient tool for assessing the N status of forage maize for mineral and organic N fertilizers and for a broad range of hybrids, climates, and soil conditions.