A REVIEW OF TRENDS IN WHEAT YIELD RESPONSES TO CONSERVATION CROPPING IN AUSTRALIA

Trends in wheat yield responses to conservation cropping in Australia were analysed using data from 33 medium-term (3-5 year) and long-term (>5 year) agronomic experiments. The overall effect of tillage (direct drilled v. cultivated) was small in all regions (-0.18 to +0.06 t/ha) , while stubble retention (stubble retained v. stubble burnt) reduced yield in all regions (-0.31 to -0.02 t/ha). There was large year-to-ye ar variation in the yield response to both tillage: and stubble retent ion at all sites, but no consistent relationship between yield variati on and amount or pattern of seasonal rainfall based on monthly totals. There was also little evidence that the yield of direct-drilled and s tubble-retained treatments increased relative to cultivated or stubble -burnt treatments with the duration of the experiments, despite the im provement in soil conditions reported at many sites. The factors thoug ht responsible for the year-to-year variation in yield response varied within and between regions, although some consistent effects emerged. Reduced early seedling growth of direct-drilled crops was considered a major factor underlying the yield response at most sites, and this w as rarely associated with the availability of water or nitrogen. High soil strength and increased severity of rhizoctonia root rot were resp onsible for these effects at some sites, but at others the cause remai ns unclear. Stubble retention influenced yield in many ways but the si gnificance of the allelopathic effects of stubble, suspected by many a uthors to influence crop growth, is poorly understood. Adoption of dir ect drilling and stubble retention is likely to remain low in areas wh ere no long-term yield benefit can be demonstrated and where greater m anagement flexibility is offered by systems of reduced cultivation and late stubble burning-incorporation. Further research should identify and develop strategies. to overcome the constraints to yield in conser vation farming systems, particularly stubble retention, and assess the sustainability of recent innovations incorporating minimum disturbanc e and partial stubble retention. The more rapid adoption of such inter mediate strategies will hasten the move of the grains industry toward sustainable production.