Evaluation of Chlorophyll Meter to Assess Nitrogen Status of Cocksfoot Sward

The sward nitrogen index (NNI), computed from nitrogen (N) content and standing herbage mass, is a sound approach to assess the N sward status for tactical adjustment of N fertilizer. However, chlorophyll meter (SPAD) readings might simplify the procedure, avoiding having to weigh and analyze the herbage. Four experiments, combining two rates of N additions and two cutting regimes, were carried out on a cocksfoot (Dactylis glomerata L. cv. Lude) sward in order to investigate the relationship between SPAD readings and NNI. For two of them, SPAD readings were taken at the mid-point of the youngest mature lamina. For the two others, they were taken at the mid-point of laminae, which composed the upper sward layer (from four to 14 cm at the top of the canopy). There was a satisfactory relationship (P < 0.001) between plant N content and SPAD measurements, whatever the sampling procedure. Furthermore, we found a close relationship between SPAD readings and NNI calculations for both methods of SPAD measurements, but the parameters were significantly different for the two methods, in agreement with the relationships established for N content in the uppermost sward layer and the youngest fully expanded lamina. Suggestions are made to improve the sampling procedure by taking into account the vertical gradient in N content at the sward and lamina levels.