The response of dryland canola to nitrogen fertilizer: partitioning and mobilization of dry matter and nitrogen, and nitrogen effects on yield components

Canola (Brassica napus) was grown under dryland conditions in field experiments at Greenethorpe (1988) and Canowindra (1989) in the cereal belt of New South Wales to determine (1) the response of the crop to nitrogen (N) fertilizer when grown late in a cropping sequence; (2) the seasonal course of dry-matter production and N accumulation; (3) the distribution of dry matter and N among plant parts, including shed leaves; and (4) the apparent mobilization of dry matter and N from stems and leaves to seeds. At both sites, maximum dry-matter production and seed yields occurred at 75 kg applied N ha(-1). Seed yields increased from 2.3 to 3.5 t ha(-1) at Greenethorpe, and from 0.85 to 2.5 t ha(-1) at Canowindra. Topdressing with st single application of N at the 5-6 leaf rosette stage, flower buds visible or the start of flowering resulted in 70-90% of the seed yields obtained when the equivalent amount of N was applied pre-sowing. At maximum seed yield, canola accumulated 165 kg N at Greenethorpe and 110 kg N ha(-1) at Canowindra. Averaged over both seasons and all N treatments, 52% of the N content of the mature plants accumulated before flowering and 50% of the dry-matter content of the mature plants accumulated during flowering. Maximum dry-matter and N contents for leaves occurred at the start of flowering, and for the stem at the end of flowering. Averaged over all N treatments at Greenethorpe, about 20% of the dry matter and 60-65% of the N was apparently mobilized from the stem and leaves, after flowering. The combined mobilization from the stem + leaves could have contributed to about 17% of the dry matter and 55% of the N accumulated by seeds. Amounts of dry matter and N lost in shed leaves ranged from 1-1.75 t and 10-30 kg ha(-1), and N removal in seed ranged from 63-112 and 27-96 kg ha(-1) at Greenethorpe and Canowindra, respectively. N concentrations in whole shoots and vegetative organs declined during the season, irrespective of the rate of N fertilizer applied. N fertilizer increased pod number per plant but had little effect on seed number per pod or 1000 seed weights. Seed oil concentrations were unaffected by the N rate at which maximum seed yield was obtained. N fertilizer rate had no effect on dry-matter harvest indices (mean both sites 30%) which, when expressed on the basis of the biosynthetic costs for straw and seed production, were comparable to those reported for wheat (37%). N harvest indices (mean both sites 76%) were reduced only at the highest N rates at Greenethorpe. Indices of N fertilizer use-efficiency generally decreased with increasing N fertilizer rate, and were similar to values reported for wheat when differences in the biosynthetic cost of grain production were taken into account. (C) 1997 Elsevier Science B.V.