Fig. 2. Relationship between whole-crop N content [g N kg^–1 dry matter (DM)] at silage maturity and DM yield (t DM ha^–1) at each site-year. Parameters of the quadratic-plateau function are given if model fit was successful, namely the threshold N concentration (Nc_th) where the quadratic part passes into the plateau and the corresponding plateau yield wmax. Points are treatment means over three or four replicates. Control treatments (0 kg N ha^–1) are displayed by open symbols. The critical N concentration at silage maturity of 10.5 g N kg^–1 DM is represented by dashed vertical lines. M.F. = model failed to fit, N.A. = data excluded due to drought, and N.P. = plateau estimation failed.