Nitrogen uptake and nitrate-nitrogen accumulation in forage kale grown under varying amounts of water and nitrogen fertiliser

Grazing of forage kale can potentially cause ground-water pollution through nitrate leaching from excreted urine, particularly in wet conditions. It can also cause nitrate–nitrogen (NO3–N) poisoning of grazing ruminants when crops take up excess soil N. Thus, N supply should be matched with crop requirements to achieve an optimum combination of yield and N content. An experiment was conducted on a stony Balmoral silt loam soil in Canterbury, investigating relationships between dry matter (DM) yield, N uptake and NO3–N accumulation under four N application rates (0, 75, 150, 300 kg/ha), with and without irrigation. The final DM yield for the fully irrigated treatments showed a positive response to N application, ranging from 10 t DM/ha to 26 t DM/ha for the treatments receiving 0 and 300 kg N/ha, respectively. Similarly, for the non-irrigated (rain-fed) treatments, DM yield increased from 5 t DM/ha to 11 t DM/ha for the same N treatments. Total N uptake also increased with N application rate, from 88 to 350 kg N/ha for the treatments receiving 0 and 300 kg N/ha, respectively. Weighted whole plant tissue NO3–N concentration increased from 0.04 mg/g for the 0 kg N/ha crops to 0.65 mg/g and 4.63 mg/g when 300 kg N/ha was applied for irrigated and rain-fed treatments, respectively. Furthermore, NO3–N was higher in the stems than in the leaf lamina and petiole and increased from the upper to lower portions of the stems. In this study, the results support N application rates of 150 kg N/ha for rain-fed and 300 kg N/ha for irrigated crops, to produce optimum DM yield to feed non-pregnant animals. Additional keywords: Brassica oleracea var. acephala L., Balmoral stony silt loam, feed utilisation, potential evapotranspiration, shallow soil

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