A one-year scoping study determined and prioritised the important weed issues of cropping systems with dryland cotton (DAQ117C). The study was commissioned and funded jointly by CRDC, Cotton CRC, GRDC and Weeds CRC in 2001-02. After consulting widely with growers, the project team produced a report that bench-marked the weed flora and management practices used in dryland cotton cropping systems. This report is published on the Cotton and Weeds CRC websites, and has been summarised in various conference papers and a paper was published in the Australian Journal of Experimental Agriculture. The main findings were that the weed flora was diverse, cropping systems complex, and weeds had a major financial and economical impact. Many of the common weeds were not controlled adequately or consistently in all parts of the rotation. Weed control was highly reliant on glyphosate in fallows and on atrazine in sorghum, but a diverse range of herbicides and mixes were used in cotton. Few non-chemical control options were used, and residual weeds often resulted in significant replenishment of the seed-bank. The report highlighted that fleabane was an emerging weed issue with the potential to become a major problem, which has eventuated since the scoping study.

The current project (DAQ123C), also funded jointly by CRDC, Cotton CRC, GRDC and Weeds CRC, focused on improved management of 5 key summer weeds, bladder ketmia, sowthistle, fleabane, barnyard grass and liverseed grass. The approach was to strategically manage these weeds, with a particular emphasis on developing better control practices for the weak-links of the rotations, and reducing replenishment of the soil seed-bank, which will reduce the economic impact of weeds in the long-term. The project team has made substantial progress on studying the biology of these weeds, devising specific control tactics for fleabane in wheat, sorghum and fallows, better herbicide efficacy for bladder ketmia, sowthistle, barnyard grass and liverseed grass in sorghum and fallows, and manipulating sorghum agronomy for improved competition against weeds.

Advances have been made in understanding the dormancy, germination requirements, emergence patterns and persistence of the 5 key weeds through the 3 long-term seed-bank experiments. These weed species differed markedly in their emergence patterns. Barnyard grass, fleabane, bladder ketmia, and sowthistle emerged predominately from soil surface, while liverseed grass emerged mainly from 5 cm burial depth. Liverseed grass emerged in one major flush, whereas bladder ketmia and barnyard grass showed staggered emergence throughout spring and summer, and sowthistle emerged all year round. Percentage of weeds seed surviving in surface soil after 2 years was negligible for sowthistle, 2-5% for barnyard grass, liverseed grass and fleabane, but over 50% for bladder ketmia. However, a higher percentage of viable seeds was found in deeper soil. These trials will continue for additional 1-2 years.

The project invested a large effort into researching better management of fleabane, a difficult-to-control weed in dryland cropping systems with cotton. A number of very effective in-crop treatments that achieved 95-100% control were identified, based on use of preplant fallow application of atrazine prior to sorghum, and preplant chlorsulfuron or post-emergent metsulfuron mixes in wheat. These residual herbicides provided good residual control of the following flushes for 4-8 months. In fallows, timeliness of herbicide application and using mixes were crucial. Glyphosate control efficacy reduced from 88% for weeds 5cm in diameter to 13% for weeds 10cm diameter or larger. Better weed control was achieved with several herbicide mixtures, such as glyphosate mixed with Ally, 2,4-D, Tordon 75D, atrazine or Grazon DS. As well, double knockdown application of glyphosate followed by Sprayseed or paraquat was highly effective and consistent. Management strategies will be published in a brochure and websites, and distributed widely to agricultural consultants and growers. The strategies are based strategic population management using combinations of knockdown and residual herbicides, herbicide mixes, crop competition, and targeting smaller and more susceptible weeds.

Four field experiments identified a number of glyphosate-based treatments and alternatives for highly effective control (95-100%) of sowthistle and bladder ketmia in fallows.

Techniques to improve control of summer grasses and bladder ketmia in sorghum were investigated. The most effective treatments were atrazine applied pre-plant in late winter fallow incorporated with rain and atrazine + metolachlor mechanically incorporated at sowing. Atrazine alone at sowing was not consistent for grass control, particularly for high weed pressure situations. In contrast, most atrazine-based treatments were successful for effective bladder ketmia control.

The residual effects of these atrazine based treatments are being evaluated on cotton. Re-cropping cotton into the 2003 central Queensland experiment showed that the herbicides residues had no adverse affects on cotton emergence, growth and yield after one year of herbicide application. More in-depth studies on cotton sensitivity to atrazine and other residual herbicides are in progress following the 2004 experiments in central and southern Queensland.

Seven field experiments investigated options to improve weed control in sorghum using crop competition to suppress weed growth and seed production. This has important implications for long-term weed control resulting from less replenishment of seed-bank. Results showed that increased crop seeding rate reduced weed seed production by 25-30%. Also, there were large differences in sorghum cultivars’ ability to suppress weed seed production. Bonus and Goldrush reduced weed seed production by 30-40% compared with other cultivars under identical growing conditions. Row spacing also had major impact on weed seed production, with 80% more weed seeds produced in double skip row configuration compared with 1m row spacing. These results from both central and southern Queensland showed consistently that weed management can be improved based on the choice of more competitive sorghum cultivar, narrow row spacing, and high planting rates.

The project team, in collaboration with the University of New England, obtained funding from the Weeds CRC for a PhD scholarship on “Morphological and molecular characterisation of barnyard grass species in Australia and implications for weed management”. The PhD student, Michelle Keenan, commenced her study in late 2004. This PhD study will compliment this project, contributing to development of better management packages for summer weeds in dryland cropping systems with cotton.

The team has been very active in delivering research results to growers, advisers and researchers via 35 publications and presentations in refereed journals, international and national conference proceedings, grower magazines, field days and industry forums.