Effect of Brassica break crops on the growth and yield of wheat

Abstract

The effect of the Brassica crops, canola and Indian mustard, on the growth and yield of subsequent wheat crops was investigated in field experiments 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 following wheat. Linseed and field peas were included as break crops at some sites for comparison. At one site, methyl bromide fumigation was used to investigate break crop effects in the absence of soil-borne pathogens. Growth improvements following break crops were evident at an early stage (4 leaf stage), but were not related to levels of soil mineral N or the incidence of plants affected by Gaeumannomyces graminis var. tritici (take-all) or Rhizoctonia solani. At two of the four sites, early vegetative growth was greater following Indian mustard than following canola. Treatment with methyl bromide led to increased vegetative growth of wheat following all crops, but the ranking of the break crop effect was maintained, with wheat growth after Indian mustard>canola>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 influenced by water availability after anthesis. At one site, where 89 mm of rain fell after anthesis, the early improvements in growth persisted until maturity, and yield was significantly improved following the break crops. At the other three sites, less than 20 mm of rain fell after anthesis, and the greater biomass of wheat following break crops resulted in rapid depletion of soil water. The increased water deficit during grain filling reduced grain size, and no yield benefit was observed. As a result of increased pre-anthesis growth, wheat following break crops accumulated more N at anthesis in above-ground biomass at all sites. This N was redistributed into the grain after anthesis resulting 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 associated with less mineral N remaining in the profile at harvest. The results indicate the potential for break crops to improve the yield and protein levels of subsequent wheat crops, although the magnitude of these effects is dependent on seasonal conditions. The nature of the early growth improvements remains uncertain. However, the results from two of the sites support a previous report of the superior break-crop effect of Indian mustard.