COMPARISON OF CANOLA, INDIAN-MUSTARD AND LINOLA IN 2 CONTRASTING ENVIRONMENTS .2. BREAK-CROP AND NITROGEN EFFECTS ON SUBSEQUENT WHEAT CROPS

The main canola-growing region in Australia is southern New South Wale s where previous studies showed higher yield and grain protein of whea t growing after brassicas compared with wheat grown after wheat. This advantage, called the break-crop effect, was studied using winter oils eeds in two field experiments, one in this region and the other in cen tral western New South Wales which is generally drier during the growi ng season, warmer throughout the year and is currently considered marg inal for oilseeds. The effect of nitrogen (N) fertilizer on the respon se of wheat to previous crops was also investigated by considering bot h the soil N remaining from fertilizer applied to the break crops, and N applied to the subsequent wheat crops. The experiments were conduct ed over three years, with two phases of an oilseed-wheat sequence and a wheat-wheat control sequence at each site. Both sites had low baseli ne levels of soil mineral-N and average levels of root-disease inoculu m. At the drier site the inoculum of wheat leaf and root pathogens rem ained during both phases but there were no break-crop effects. The eff ect of previous crops on the yield and protein of a subsequent wheat c rop could be explained by the amount of residual soil mineral N. At th e wetter site, wheat responses to previous crops could be explained by the amounts of residual soil mineral N in one phase of the experiment when there was no root disease of wheat. Under these conditions, yiel d and grain protein generally increased in response to increasing leve ls of soil mineral N. The exception was the yield of wheat after Linol a which decreased when an excessive amount of residual N resulted in g reater vegetative biomass, rapid depletion of soil water and decreased yield. In the other phase, when root disease was present, break crops increased yield of a subsequent wheat crop by 30% and grain protein b y 1.3% compared to wheat growing after wheat, and among the oilseeds t he brassicas gave a greater break-crop benefit than Linola. Applicatio n of fertilizer N to wheat growing after wheat failed to compensate fo r the disadvantage, indicating that residual N was not responsible for the differences. The break-crop benefit of the oilseeds extended to t he second successive wheat crop for the phase in which root diseases w ere present, with increases of 13% in grain yield. The break-crop effe ct at the wetter site confirms previous observations of this benefit o f oilseeds in general, and brassicas in particular, in southern New So uth Wales. The absence of any break-crop effect at the drier site sugg ests that root disease of wheat was less severe, possibly because the inoculum was less infective during the dry springs. (C) Elsevier Scien ce B.V. All rights reserved.