Selenium concentration in wheat (Triticum aestivum) grain as influenced by method, rate, and timing of sodium selenate application

Field trials were conducted to identify effective methods of increasing selenium (Se) in wheat (Triticum aestivum) grown on low-Se soils in Canterbury, New Zealand. Various rates (5-20 g Se/ha) and methods (soil-applied fertiliser, seed treatment, foliar application at flowering) of applying Se (sodium selenate) were evaluated in autumn- and spring-sown crops on an irrigated site at Wakanui and on a dryland site at Methven. Several Se fertiliser treatments were included to examine the effects of timing of application (Se applied at sowing or growth stage 31) and to determine whether applying other fertilisers (superphosphate, urea) with Se would affect its uptake. Two cultivars were grown in the autumn and spring trials at each location. Grain Se content averaged 0.03 mg/kg in the controls (no Se added), increasing to 0.4-0.5 mg/kg where Se was added at 20 g/ha using the most effective application methods-foliar spray and fertiliser at growth stage 31. Fertiliser at sowing was less effective in raising grain Se than fertiliser at growth stage 31, particularly for autumn-sown crops. The results suggested that the plant availability of applied Se can decrease rapidly in the soil and, to be most effective, fertiliser Se should be applied when the crop is beginning to grow rapidly in spring. Grain Se levels for the irrigated site were close to those in the corresponding treatment at the dryland site, even though yield was much greater where irrigation was used. Application of superphosphate (200 kg/ha) or urea (100 kg/ha) did not have a large or consistent effect on grain Se. Cultivar effects on grain Se were either small or not significant. For the most effective application methods (fertiliser at growth stage 31; foliar spray), grain Se increased by c. 0.02 mg/kg for an increase in Se rate of 1 g/ha.