REDUCED GROWTH AND YIELD OF WHEAT WITH CONSERVATION CROPPING .1. FIELD STUDIES IN THE 1ST YEAR OF THE CROPPING PHASE

An experiment was conducted on a red earth at Harden, N.S.W., to investigate the effects of tillage and stubble management on the growth and yield of wheat in the first year of conservation cropping. Treatments involved stubble-management systems of incorporation, burning or retention combined with tillage systems of either direct drilling or minimum tillage. The experiment was conducted on an oat stubble of 3.9 t ha-1. Direct drilling and stubble retention both reduced seedling growth by 15%, compared to cultivated and stubble burnt treatments, but had no effect on plant density or tillering. The effects on shoot growth were additive and persisted until maturity, leading to grain yields which varied from 2.16 t ha-1 for the stubble-mulched, direct-drilled treatment to 3.20 t ha-1 for the burned-stubble, minimum-till treatment. Direct drilling reduced the total root length in the profile (0-160 cm) at anthesis by 40%, but there was no effect of stubble retention. Reduced shoot growth and rooting depth on direct-drilled and stubble-retained treatments reduced the recovery of water and mineral N by the crop and increased the leaching of mineral N below the root zone. Early shoot growth reductions on direct-drilled plots were not related to levels of soil water, mineral nitrogen (N) or soil temperature. Reduced shoot growth was associated with increased severity of Rhizoctonia in some direct drilled plots, but growth reductions often occurred in the absence of obvious symptoms. High soil strength (>2 MPa) in the top 10 cm of soil may have contributed to reduced growth, although the exact mechanism remains unclear. Reduced growth associated with the presence of stubble was not caused by immobilization of N or increased leaf disease, although reduced soil temperatures may have been partly responsible.