Critical nitrogen curve and nitrogen nutrition index for corn in eastern Canada

Plant-based diagnostic methods of N nutrition require the critical N concentration (N-c) to be defined, that is the minimum N concentration necessary to achieve maximum growth. A critical N curve (N-c = 34.0W(-0.37) with W being shoot biomass in Mg DM ha(-1)), based on whole plant N concentration, was determined for corn (Zea mays L.) in France. Our objectives were to validate this critical N curve in eastern Canada and to assess its plausibility to estimate the level of N nutrition in corn. Shoot biomass and N concentration were determined weekly during the growing season at three sites for 2 yr (2004 and 2005); four to seven N treatments were used at each site. Data points were divided into two groups representing either nonlimiting or limiting N conditions according to significant differences in shoot biomass at each sampling date. All data points included in the limiting N group were under the critical N curve and most data points of the nonlimiting N group were on or above the critical N curve, hence confirming the validity of the critical N curve determined in France. The nitrogen nutrition index (NNI), calculated as the measured N concentration divided by the predicted N-c, ranged from 0.30 to 1.35. A significant relationship between relative grain yield (RY) and NNI (RY = -0.11 + 1.17 NNI if NNI < 0.93 and RY = 0.98 if NNI > 0.93; R-2 = 0.89) was determined. The critical N curve from France is valid in eastern Canada and the NNI calculated from that curve is a reliable indicator of the level of N stress during the growing season of corn.