Effects of sowing time and nitrogen fertiliser on canola and wheat, and nitrogen fertiliser on Indian mustard. II. Nitrogen concentrations, N accumulation, and N fertiliser use efficiency

Canola, Indian mustard, and wheat were grown at Ariah Park and Cowra (canola only) in the cropping belt of New South Wales, Australia, to determine the effects of sowing time (canola and wheat) and nitrogen (N) fertiliser on N concentrations and N accumulation in shoots, N fertiliser use-efficiency, and N removal in grain of the crops. Concentrations of N in shoots of all crops decreased during the season, irrespective of sowing time or N fertiliser rate. Late sowing decreased N accumulation by 55% and 40% for canola and wheat, respectively, at Ariah Park, and by 50% for canola at Cowra, but increased canola and wheat grain N (protein) concentrations more than the applied N. All crops accumulated most of their N before anthesis, and there was little N accumulation after the end of flowering; however, sowing canola late increased the proportion of N accumulated during flowering. Indices of N fertiliser use efficiency were reduced by sowing late, but N use efficiencies of the oilseeds at each sowing time were similar to values for wheat after accounting for differences in the biosynthetic costs of grain and straw production. Removal of N in canola grain from an April sowing was 35% greater than N removal by wheat grain sown at the same time, but was similar for both crops from late May and July sowings. Consequently, more N fertiliser should be applied to canola than wheat to obtain high grain yields when both crops are sown early in the season. It was concluded that sowing early was essential to achieve high N use efficiency, reduce potential losses of N, and maximise economic returns from N fertiliser.