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 ex periments at Greenethorpe (1988) and Canowindra (1989) in the cereal b elt of New South Wales to determine (1) the response of the crop to ni trogen (N) fertilizer when grown late in a cropping sequence; (2) the seasonal course of dry-matter production and N accumulation; (3) the d istribution of dry matter and N among plant parts, including shed leav es; and (4) the apparent mobilization of dry matter and N from stems a nd leaves to seeds. At both sites, maximum dry-matter production and s eed yields occurred at 75 kg applied N ha(-1). Seed yields increased f rom 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 res ulted in 70-90% of the seed yields obtained when the equivalent amount of N was applied pre-sowing. At maximum seed yield, canola accumulate d 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 m ature plants accumulated before flowering and 50% of the dry-matter co ntent of the mature plants accumulated during flowering. Maximum dry-m atter and N contents for leaves occurred at the start of flowering, an d 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 co mbined mobilization from the stem + leaves could have contributed to a bout 17% of the dry matter and 55% of the N accumulated by seeds. Amou nts of dry matter and N lost in shed leaves ranged from 1-1.75 t and 1 0-30 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, irre spective of the rate of N fertilizer applied. N fertilizer increased p od number per plant but had little effect on seed number per pod or 10 00 seed weights. Seed oil concentrations were unaffected by the N rate at which maximum seed yield was obtained. N fertilizer rate had no ef fect on dry-matter harvest indices (mean both sites 30%) which, when e xpressed on the basis of the biosynthetic costs for straw and seed pro duction, 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 dec reased with increasing N fertilizer rate, and were similar to values r eported for wheat when differences in the biosynthetic cost of grain p roduction were taken into account. (C) 1997 Elsevier Science B.V.