Ground Cover Supplement : GC Supplement - Summer weeds
13 Issue 137 | Nov – Dec 2018 | GRDC GROUNDCOVER SUPPLEMENT: SUMMER WEEDS GROUNDCOVER WINDMILL GRASS FAST FACT The windmill grass seedbank can be reduced substantially with one to two years of effective management. WINDMILL GRASS FAVOURED BY NO-TILL SYSTEMS Windmill grass in mungbean. PHOTO:BHAGIRATHCHAUHAN Windmill grass is favoured by no-till practices and has very flexible germination requirements, but seedbank persistance is relatively short providing an opportunity for growers to substantially reduce populations in one to two years By Dr Bhagirath Chauhan, Dr Sudheesh Manalil and Dr Gurjeet Gill n Windmill grass (Chloris truncata) is an important summer fallow weed across mainland Australia with several populations having evolved resistance to glyphosate. Infestation levels in New South Wales and southern and central Queensland are high. It can cause substantial yield loss in summer crops as well as depleting soil moisture during summer fallow. Research with GRDC investment at the University of Queensland in 2016-17 found that windmill grass densities of 11, 25 and 39 plants per square metre caused grain yield reductions in mungbeans of 20, 34 and 56 per cent respectively. However, this crop was planted on 0.5-metre row spacings, meaning that losses could be even higher in typically grown 1m rows. Understanding the biological characteristics of different populations of windmill grass through GRDC research at the University of Queensland and the University of Adelaide will help researchers to identify control opportunities. GERMINATION Two populations of windmill grass collected from Queensland were found to ger minate at day/night temperatures ranging from 15°C /5°C to 30°C /20°C, demonstrating the weed’s potential to infest both summer and winter crops in the northern region. South Australian populations did not ger minate below 9.2°C. In both northern and southern studies windmill grass was more likely to germinate on or near the surface, demonstrating why it is favoured in no-till systems. Up to 70 per cent of seeds from Queensland populations ger minated in the dark and from depths of up to 2 centimetres. In contrast, almost no SA seeds ger minated in the dark and never from depths of 0.5cm or deeper. Pot trials at Gatton, Queensland, demonstrated that retaining crop residue had potential to suppress seedling emergence, with approximately 1.8 tonnes per hectare of sorghum residue reducing emergence by half. OPPORTUNITIES Queensland research showed that seeds were depleted faster at the soil surface than when buried at 2 and 10cm depths, as occasional rains favoured ger mination from the surface (Figure 1). No viable seeds were found on the surface after one year, or after two years when buried. However, while SA seeds were also non-viable after one year, lower-than- average rainfall over the summer was found to increase the persistence of buried seeds. This suggests that seedbank persistence could increase in northern systems with recent dry conditions. Proactive management to prevent weed seed-set for one or two years could deplete windmill grass seedbanks rapidly, provided there is sufficient rain. Mature weeds also need to be managed as plants can grow from the crown of the parent plant. Harvest weed seed control in summer crops has some potential to reduce seed- set. Trials at Gatton in 2016-17 showed that up to 85 per cent of windmill grass seed was retained when mungbean was harvested at maturity, 87 days after sowing. The lowest weed seed panicle was at 15cm height. In contrast, in 2017-18 less than three per cent of windmill grass seed was retained at sorghum harvest. This may be because mungbean is harvested earlier than sorghum or because mungbean has a lower water requirement, allowing the weed to grow for longer. More knowledge is required about the seed production and ger mination characteristics of windmill grass plants that germinate in late winter, where they are often able to set seed. These may differ from those that ger minate during fallow, affecting control options. o GRDC Research Codes UA00156, UA000149 More information: Dr Bhagirath Chauhan, University of Queensland, 07 5460 1541, email@example.com 0 12 6 18 24 100 80 60 40 20 0 0cm 2cm 10cm Viable seed (%) Months after seed placement SOURCE: BHAGIRATH CHAUHAN FIGURE 1 Windmill grass seed persists longer when buried at 2 and 10cm depth than on the surface, Gatton, Queensland. Error bars represent the standard error of the mean.
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