However, when comparing the lower and the upper critical concentration among the different evaluated parameters, it can be clearly stated out that the variation of the lower is smaller than of the upper critical concentration. The lower critical concentrations are rather similar for all investigated parameters (RG, RFI, PER, RCP, RE), except for FGR, within
the range of 1.23–1.28% in diet DM (2.22–2.31% in CP), whereas the range of the upper critical concentrations is larger (2.10–3.82% Met + Cys in diet DM; 3.80–6.91% Met + Cys in CP). Therefore, the s.e. for the upper critical concentrations are clearly higher than for the lower critical concentrations (Table 3.2). Two reasons might be responsible: (i) turbot responded less sensitive to an oversupply of Met+Cys than to a deficiency,which is illustrated for RG in Fig. 1, and(ii)oversupply was realized by much larger Met increments (2.0%-unit, diet nos. 7 and 8) than in diets no. 1–7 (0.2%-units). Therefore, it seems to be necessary to more precisely determine the upper critical concentration in further studies with smaller increments of Met.