Annual identification key to effective management

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With winter weeds now joining the seasonally variable spectrum of summer weeds found across the Western Australian wheatbelt, identification is fundamental for effective management

A survey of summer weeds was completed by the team at the Western Australian Department of Primary Industries and Regional Development (DPIRD), with GRDC investment through a University of Adelaide project led by Dr Chris Preston.

The survey recorded weed incidence on roadsides in late summer/early autumn from 2015 to 2017 at 244 sites throughout the WA wheatbelt. Sites selected included mature weeds with no evidence of chemical control in 2015, and were revisited annually after summer rainfall, ensuring that weeds had a chance to emerge. Key species for resistance testing were collected, including common sowthistle, fleabane, button grass, tar vine and feathertop Rhodes grass (FTR).

Weed spectrum

A total of 144 different species were identified. However, when we looked at how often we found each weed species there were only 19 that were found at more than 10 per cent of sites (Table 1).

TABLE 1  The most frequently found summer weed species in a WA wheatbelt survey from 2015 to 2017.
Percentage of sites
Common name Scientific name201520162017
African lovegrass Eragrostis curvula 48 70 59
Windmill grass Chloris truncata 30 55 46
Fleabane Conyza sp. 40 36 37
Wild radish Raphanus raphanistrum 27 43 45
Stinkgrass Eragrostis cilianensis 20 30 50
Common sowthistle Sonchus oleraceus 22 34 40
Wild oats Avena sp. 15 22 37
Roly poly Salsola australis 16 29 19
Capeweed Arctotheca calendula 15 22 22
Couch Cynodon dactylon 16 20 21
Afghan thistle Solanum hoplopetalum 13 25 13
Stinkwort Dittrichia graveolens 15 20 13
Panic grass Panicum sp. 11 22 16
Afghan melon Citrullus lanatus 12 11 21
Mulla mulla Ptilotus polystachyus 5 18 24
Caltrop Tribulus terrestris 11 17 12
Goosefoot Chenopodium sp. 4 12 22
Button grass Dactyloctenium sp. 8 13 14
Prickly paddy melon Cucumis myriocarpus 11 7 13
SOURCE: Dr Catherine Borger
Image of Dr Abul Hashem and Dr Catherine Borger
Dr Abul Hashem and Dr Catherine Borger discussing feathertop Rhodes grass resistant to Group M and L herbicides. PHOTO: Ashlea Owczarski, WA DPIRD

The array of species found between years was surprising. An individual site generally had summer weeds growing in each of the three survey years, but the species on each site would often vary between years. For example, site one in the northern wheatbelt had 17 species over three years, but only Afghan melon, button grass, roly poly and tar vine were present every year. Species such as wild radish, fleabane, African lovegrass and flat spurge were present one year and absent in the following year or years. Caltrop was evident in 2015, missing in 2016 and returned in 2017. This variation shows that it is very important to identify what species are growing before selecting the most appropriate management tactics.

The survey also highlighted how many common winter weeds are growing over summer, such as wild radish and wild oats. The prevalence of these species in summer surveys emphasises the need for cost-effective integrated management programs to control these weeds throughout the year as they are the second and third most problematic weeds in Australia after annual ryegrass.

KEY POINTS

  • Summer weed spectrum is broad and changeable every season meaning annual identification is key to management
  • Winter weeds and herbicide resistance are becoming issues to manage in weed populations over summer

No new populations of fleabane and common sowthistle were found with Group M or Group I resistance. No new populations of button grass and tar vine with Group M and Group L resistance were found. Resistance in fleabane and common sowthistle is already evident in WA, but the survey highlighted that resistance is still rare in these species. However, resistance or tolerance to non-selective herbicides (Group M and L) was found in several FTR populations. This species now joins others targeted for development of effective management programs with investment from GRDC.

Summer weed incidence on roadsides differs to that in paddocks. However, it is difficult to survey fields because summer weeds are often sprayed/grazed soon after emergence. While the results of a roadside survey are not directly applicable to control of weeds in paddocks, similar species are seen in paddocks and roadsides and resistant populations on the roadside can quickly spread into paddocks.

More information

Dr Catherine Borger, WA DPIRD
0467 816 082
catherine.borger@dpird.wa.gov.au

Summer weeds within the Western Australian wheatbelt