Research Project
Full description Wild oats [Avena fatua L. and Avena sterilis (L.) ludoviciana (Durieu) Nyman] are the most economically important weeds within the Northern Grains Region (NGR; comprised parts of Queensland and all of New South Wales) of Australia. Wild oats possess a range of survival mechanisms to succeed in the cropping environment, such as variable seed dormancy, high seed production, and persistence in the seedbank. However, little research has been conducted on A. sterilis ssp. ludoviciana (hereafter A. sterilis), which is the more abundant wild oats species in the NGR. The prevalence of A. sterilis in the NGR has increased after the adoption of no-tillage based conservation agriculture (NTCA). In recent times with the increasing frequency of warm and dry conditions during the late winter/early spring period in most parts of the NGR may also play a role in the persistence of this weed. To gain insight into the potential persistence mechanism of A. sterilis in the NGR, a series of experiments were undertaken at The University of Queensland (UQ), Gatton Campus. A germination study was undertaken to better understand the response of the primary and secondary seeds coming from four NGR A. sterilis biotypes to light and temperature. The optimum temperature around 8°C and under a natural photoperiod was found to be best for germination, indicating the non-burial condition of NTCA may help this weed to germinate maximum during the time of winter crop planting. Dehulling the seeds was found to improve caryopsis germination over a greater range of temperatures (from 4 to 15°C), while illumination became less important, indicating that seeds in the seedbank for longer times and have lost their hull will germinate in a wider range of temperatures and under most illumination conditions. Overall, this study determined that a natural photoperiod and a temperature range of 4 to 15°C was best for testing the germinability of the studied biotypes. Three pot trials were undertaken to evaluate the impact of late season warm and/or dry conditions on the phenology and reproductive biology of the four NGR A. sterilis biotypes. Plants were grown under optimal environmental conditions in the ambient greenhouse (23/14°C day/night) until panicle initiation, then exposed to either an elevated temperature or a range of reduced soil water levels, or a combination of these two conditions. To expose plants to elevated temperature, a portion of the plants were transferred from the ambient greenhouse to a controlled temperature glasshouse (29/23°C). The process of moving plants was repeated on three further occasions, each 10 days apart. To impose different levels of soil water stress, pot water was maintained at either 20, 40, 60 and 80% plant available water content (PAWC). Under the combined stress treatments, plants were moved to the high temperature glasshouse and soil water was maintained at 60% PAWC. Depending on the duration and severity of a stress condition plants phenology and reproductive biology was found to be modified. Plants exposed to an elevated temperature for a longer duration, a severe drought stress or a combined stressed conditions were forced to mature 18 to 24 days earlier than unstressed control plants but with a penalty to lower filled seed production (30 to 34% less), smaller sized seeds (37 to 40% lower mass) with less dormancy (32 to 56%), and had less longevity (longevity could be as much as 4 years less in the soil seedbank). To determine the fate of the seeds and seedling emergence behaviour over a 2-year period under seasonal environmental conditions, primary and secondary seeds were placed at 0, 2, 8 or 15 cm soil depth in big tubs with either 0, 2 or 3 t ha-1 wheat residue laid on the soil surface. The maximum number of seedlings (60 to 80% of total seed placed), from both kinds of seed, were found to emerge from the top 2 cm of soil and when they were covered with 3 t ha-1 wheat residue load. Crop residue had less effects on seedling emergence when the seeds were buried to 8 cm or below (only 35 and 18% seedlings emerged from 8 and 15 cm depths, respectively). Maximum seedling emergence was observed in June (winter), at the time when the winter crop is planted in the NGR. After 2 years, only 10% of the primary seeds, but 24% of the secondary seeds, were found to be viable on the soil surface and when they were covered with residue load. No primary seeds were found viable at 0, 8 or 15 cm soil depths whereas only 2 and 8% secondary seeds were found viable at 8 and 15 cm soil depths, respectively. A field trial was carried out over a 2-year period to understand the phenological and reproductive development of this weed in a wheat field grown under different tillage systems (viz. NTCA; strategic tillage, ST; conventional tillage, CT). In the NTCA system no pre-sowing tillage was conducted, and the crop residue was retained onto the experimental plots. In the ST system two tine implement passages (14-days apart) were applied prior to crop planting, and to a depth of 10 cm, either in the first or second year of the experiment. In the CT system, the tillage operations consisted of four passes (two passes at one time with a set of discs followed by one pass with a rotary hoe and one pass with harrows; each 14-days apart), prior to crop planting to a depth of 15 cm in the first and second year of the experiment. Soil samples were collected from 0 to 5, 5 to 10 and 10 to 15 cm after crop planting to assess the seedbank and two 1 × 1 m2 permanent quadrats were established per plot to determine seedling emergence in the field. In the NTCA system, most of the seeds were retained on or close to the soil surface from where 48 to 77% more seedlings were emerge as compared to the ST system. Seedling emergence continued for 50 days after crop planting in the NTCA system whereas tillage operation restricted the seedling emergence within the first 20 days of wheat planting. Overall, plants coming from NTCA system matured ca. 10 days earlier than ST system, produced 40% less seeds than ST system and these seeds were 20% smaller in mass but had 10 to 20% less dormancy than seeds produced from ST system. In summary, the increasing frequency of warm and drier conditions in the NGR during the late winter to spring season is rapidly maturing A. sterilis plants and shortening their life cycle, producing lower numbers of smaller filled seeds, but those seeds are falling earlier to the soil surface and are less dormant, and have less ability to survive long in the soil seedbank. The non-burial conditions and the residue retention system of the NTCA allows these early matured and shattered seeds to remain on or close to the soil surface from where they can germinate easily during the time of next winter crop planting and reinfest the crops. An application of ST would be helpful to bury these seeds to greater soil depths from where they cannot emerge, especially as they have reduced longevity. The sustainability of the CA system in the NGR under the variable changing climate will largely depend on the effective management of the difficult-to-control weeds like A. sterilis.