Comparison of canola, Indian mustard and Linola in two contrasting environments. II. Break-crop and nitrogen effects on subsequent wheat crops

Abstract

The main canola-growing region in Australia is southern New South Wales where previous studies showed higher yield and grain protein of wheat growing after brassicas compared with wheat grown after wheat. This advantage, called the break-crop effect, was studied using winter oilseeds in two field experiments, one in this region and the other in central western New South Wales which is generally drier during the growing season, warmer throughout the year and is currently considered marginal for oilseeds. The effect of nitrogen (N) fertilizer on the response of wheat to previous crops was also investigated by considering both the soil N remaining from fertilizer applied to the break crops, and N applied to the subsequent wheat crops. The experiments were conducted over three years, with two phases of an oilseed-wheat sequence and a wheat-wheat control sequence at each site. Both sites had low baseline levels of soil mineral-N and average levels of root-disease inoculum. At the drier site the inoculum of wheat leaf and root pathogens remained during both phases but there were no break-crop effects. The effect of previous crops on the yield and protein of a subsequent wheat crop could be explained by the amount of residual soil mineral N. At the 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, yield and grain protein generally increased in response to increasing levels of soil mineral N. The exception was the yield of wheat after Linola which decreased when an excessive amount of residual N resulted in greater 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 by 1.3% compared to wheat growing after wheat, and among the oilseeds the brassicas gave a greater break-crop benefit than Linola. Application of fertilizer N to wheat growing after wheat failed to compensate for the disadvantage, indicating that residual N was not responsible for the differences. The break-crop benefit of the oilseeds extended to the second successive wheat crop for the phase in which root diseases were present, with increases of 13% in grain yield. The break-crop effect at the wetter site confirms previous observations of this benefit of oilseeds in general, and brassicas in particular, in southern New South Wales. The absence of any break-crop effect at the drier site suggests that root disease of wheat was less severe, possibly because the inoculum was less infective during the dry springs.