Developing a critical nitrogen dilution curve for forage brassicas

AbstractDefining 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 Ncvalue 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 NCof 31·2 g N kg−1was 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 NO3content of forage brassicas from a range of data sets. The NNI is the actual N concentration of the shoot as a ratio of the Ncfrom the established curve. The relationship between NNI and NO3contents 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 NO3increased 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 NO3concentrations in the forage can be minimized.