Nitrogen fertilizer is applied to supplement soil nitrogen supply to maximize forage brassica crop dry-matter production. However, nitrogen fertilizer applications in excess of that required to maximize growth result in potentially toxic nitrate–nitrogen (NO3–N) concentrations in grazeable plant tissues. Three experiments, two for forage kale at Lincoln (Canterbury) and one for forage rape at Hastings (Hawke's Bay) in New Zealand were grown under different rates of nitrogen (0–500 kg N ha−1) to determine the effect of different rates of nitrogen on NO3–N content of different plant parts of the crops. One of the kale experiments was grown with either full irrigation or no rain and no irrigation over summer, hereafter referred to as summer drought. The NO3–N concentration on a whole plant (weighted average) basis increased from 0·1 mg g−1 dry matter for the control plots to 2·30 mg g−1 for the 500 kg N ha−1 plots for forage kale. It increased from 0·99 for the control plots to 3·37 mg g−1 for the 200 kg N ha−1 plots for forage rape crops. However, NO3–N concentration increased with N supply under the summer-drought plots from an average of 0·33 mg g−1 when ≤120 kg N ha−1 was applied to 2·30 mg g−1 for the 240 kg N ha−1 treatments but was unaffected by N supply under irrigation. The NO3–N concentrations were higher in the stems and the petiole (which included the midrib of the leaf) than leaves in all three experiments. The NO3–N concentration was highest at the bottom of the kale stem and decreased towards the top. We recommend N application rates based on soil tests results, and for conditions similar to the current studies up to 300 kg N ha−1 under irrigation and adjusted lower N rates for regions prone to dry summers.