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

Abstract

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.

Keywords:

• Brassica oleracea ssp. acephala L.
• critical leaf area index (LAI_crit)
• critical specific leaf nitrogen (SLN_crit)
• nitrogen use efficiency (NUE)
• nutritive values

Acknowledgements

This work was conducted through the Pastoral 21 Environment Programme (C10X1117; objective 7), jointly funded by MBIE, DairyNZ, Fonterra and Beef + Lamb New Zealand. We also thank Plant & Food Research employees who were involved in the experiment, particularly Mike George and Alexandre Michel.