poor growth of canola in retained wheat straw: causes and consequences

A field trial was conducted to investigate factors associated with poor growth of canola sown into wheat stubble. Wheat (cv Snipe and Diamond Bird) was grown in 2000 and wheat stubble decomposition treatments were established on the site in early 2001 using rain out shelters to exclude rainfall for different periods (0,3,6 months). Six treatments were superimposed on the cultivar and decomposition treatments in May 2001. The six treatments were: a) a fallow (root and shoot residue removed at harvest), b) a bare treatment c) a bare treatment plus additional nitrogen, d) 4 t/ha of residue retained e) 4 t/ha of residue retained plus additional nitrogen, f) simulated plastic residue. Canola (cv. Oscar) was direct-drilled across the site. Stubble type and decomposition period had no impact on canola growth but seedling emergence, biomass and yield were reduced in the presence of stubble and plastic residue. Nitrogen immobilisation or leaching of allelochemicals could not explain the early impact on crop growth. The major impact appears to be slower rate of leaf emergence and investment of assimilate in elongated hypocotyls for penetration of the straw layer, and increased likelihood of disease due to increased sites for infection on the hypocotyl. Colder temperatures at the surface of the wheat stubble exposing the canola meristem to colder temperatures may also play a role.