EFFECT OF BRASSICA BREAK CROPS ON THE GROWTH AND YIELD OF WHEAT

The effect of the Brassica crops, canola and Indian mustard, on the gr owth and yield of subsequent wheat crops was investigated in field exp eriments at four sites in southern New South Wales. In all experiments , shoot growth, root growth, disease incidence and water and nitrogen use of wheat following the Brassica crops were compared with wheat fol lowing wheat. Linseed and field peas were included as break crops at s ome sites for comparison. At one site, methyl bromide fumigation was u sed to investigate break crop effects in the absence of soil-borne pat hogens. Growth improvements following break crops were evident at an e arly stage (4 leaf stage), but were not related to levels of soil mine ral N or the incidence of plants affected by Gaeumannomyces graminis v ar. tritici (take-all) or Rhizoctonia solani. At two of the four sites , early vegetative growth was greater following Indian mustard than fo llowing canola. Treatment with methyl bromide led to increased vegetat ive growth of wheat following all crops, but the ranking of the break crop effect was maintained, with wheat growth after Indian mustard>can ola>wheat. The average increase in shoot biomass at anthesis following the break crops was 29%, varying from 12 to 46% according to site and break crop species. The effect of break crops on grain yield was infl uenced by water availability after anthesis. At one site, where 89 mm of rain fell after anthesis, the early improvements in growth persiste d until maturity, and yield was significantly improved following the b reak crops. At the other three sites, less than 20 mm of rain fell aft er anthesis, and the greater biomass of wheat following break crops re sulted in rapid depletion of soil water. The increased water deficit d uring grain filling reduced grain size, and no yield benefit was obser ved. As a result of increased pre-anthesis growth, wheat following bre ak crops accumulated more N at anthesis in above-ground biomass at all sites. This N was redistributed into the grain after anthesis resulti ng in an average increase of 1.5% in grain protein in wheat following break crops. At one site, the increased N uptake in the crop was assoc iated with less mineral N remaining in the profile at harvest. The res ults indicate the potential for break crops to improve the yield and p rotein levels of subsequent wheat crops, although the magnitude of the se effects is dependent on seasonal conditions. The nature of the earl y growth improvements remains uncertain. However, the results from two of the sites support a previous report of the superior break-crop eff ect of Indian mustard.