Defining the critical nitrogen concentration (Nc; g N kg−1) for maximum growth of forage brassicas will aid in the fertilizer management of these crops. Typically, the Nc value decreases with increasing crop biomass. In this paper, we used a nitrogen (N) response experiment with kale (Brassica oleracea) to define a critical N dilution (Nc = 55·3 × biomass−0·47). However, at biomass <3·4 t ha−1, a constant NC of 31·2 g N kg−1 was found. This N dilution curve compared favourably with published data sets for a range of forage brassicas but was substantially different from the established N dilution curve for oilseed rape (Brassica napus). This study also found a strong relationship (R2 = 0·81) between the nitrogen nutrition index (NNI) and the NO3 content of forage brassicas from a range of data sets. The NNI is the actual N concentration of the shoot as a ratio of the Nc from the established curve. The relationship between NNI and NO3 contents was significantly different between leafy forage brassica crops and root forage brassicas. For each 0·1 increase in NNI, the proportion of total N that was in the form of NO3 increased by 2·7% for leaf/stem brassicas and 0·60% for root crop brassicas. The critical dilution curve defined in this study can be used to manage fertilizer N in forage brassica crops, so that growth can be maximized but the risk of high NO3 concentrations in the forage can be minimized.