Effects of nitrogen fertiliser application rate on nitrogen partitioning, nitrogen use efficiency and nutritive value of forage kale

Winter grazing of forage kale ( Brassica oleracea var. acephala L.) has been linked to environmental pollution and negative animal health issues. An experiment with seven nitrogen (N) fertiliser application rates (0–500 kg N ha −1 ) was carried out at Lincoln, Canterbury, New Zealand to investigate N uptake, partitioning, apparent N use efficiency (aNUE) and nutritive value of forage kale. Final harvest dry matter (DM) yield increased four‐fold to 25.5 t ha −1 when 500 kg N ha −1 was applied compared with the 0 kg N ha −1 crops, and N uptake by five‐fold to 425 kg ha −1 . The inability of the crops receiving ≤100 kg N ha −1 to attain critical leaf area index and critical level of specific leaf nitrogen content resulted in lower DM yield compared with the higher N fertiliser application rates. The aNUE decreased from 111.6 kg DM kg −1 N for the 50 kg N ha −1 crops to 37.5 kg DM kg −1 N for the 500 kg N ha −1 crops. Nitrogen fertiliser application rate had no systematic effect on the crude fibre content of the kale crops. The crude protein content was unaffected by N supply, and ranged between 7.7% and 10.5%. Under conditions of optimal water and nutrient supply on deep soils with high water‐holding capacity and long growing season (c. 7 months), it is recommended that N fertiliser is applied at 300 kg N ha −1 to achieve a yield of about 22 t ha −1 and aNUE of ≥51 kg DM kg −1 N applied.