Paddock Practices: Summer Weed Management Tips for WA

Key Points

• WA’s common summer weeds include Afghan melon, paddy melon, button grass, caltrop, feathertop Rhodes grass, fleabane, and windmill grass
• Control in the fallow conserves soil moisture and nutrients for crop use
• Weed species identification is key to success
• Herbicides are best used at weed seedling stages
• This reduces the need for high rates of herbicide knockdown before seeding.

Summer weeds in the fallow are the enemy of soil moisture conservation and removing them in the months after harvest will be one of the biggest contributors to improving water use efficiency in 2019 crops.

This will also minimise soil nutrient losses and the potential for disease and pest carry-over to crops on the ‘green bridge’.

GRDC is investing in research to control Western Australia’s most problematic summer weed species and has produced a range of resources for growers to find up-to-date information and advice.

Its latest GroundCover™ supplement Summer Weeds delves into trial results and has valuable links to tools for summer weed management.

Other valuable resources are the GRDC’s online Integrated Weed Management hub; the industry WeedSmart website; and the GRDC ‘Summer Weed Guide’, which can be used to help identify species. This can be ordered here.

Species that are costing you money

Summer weed incidence and density changes across the WA grain belt, depending on seasonal conditions and location.

Common species found in most years include:

• Afghan melon (Citrullus lanatus)
• paddy melon (Cucumis myriocarpus)
• button grass (Dactyloctenium radulans)
• caltrop (Tribulus terrestris)
• feathertop Rhodes grass (Chloris virgata)
• flaxleaf fleabane (Conyza bonariensis)
• windmill grass (Chloris truncata).

Research by the Department of Primary Industries and Regional Development (DPIRD), with GRDC investment, has found controlling these weeds in low rainfall zones is 70-99 per cent likely to be profitable in a single year. In high rainfall zones, the likelihood of making a profit is estimated to be 30-80 per cent.

This takes into account the crop yield benefits stemming from stored soil moisture conservation. Potential returns are higher when nutrient loss and pest and disease loads from weeds are also considered.

Identifying problematic species

Multi-year surveys in WA, led by DPIRD research officers Dr Catherine Borger and Dr Abul Hashem with GRDC investment, have found the prevalence of WA’s summer weed species can change between years in a single location.

They have also noted an increase in persistence of some winter weeds, such as wild radish (Raphanus raphanistrum) and wild oats (Avena fatua), in the summer months in their roadside surveys.

The researchers say weed identification is critical to successful year-round control and to reduce risks of the evolution of herbicide resistance.

Diagnostic information for a wide range of species is available on the GRDC’s Integrated Weed Management hub or go to the DPIRD website.

Summer weed control tactics

The most effective and economical time to control summer weeds is typically after harvest to stop seed set and at seedling stages, when the weed plants are least stressed.

Dr Borger also advises to monitor summer weed populations and control any survivors of a herbicide application before seeding 2019 crops.

She says glyphosate (Group M) is the most important herbicide for controlling weeds in the summer fallow, but several common species have evolved resistance to this herbicide. All growers are advised to reduce risks by mixing and rotating between alternative herbicide modes-of-action.

Options for a double knockdown herbicide strategy for summer weed control can be:

• glyphosate first and then paraquat (Group L), at full label rates, if resistance risk is low
• glyphosate plus an effective alternative herbicide, at full label rates, in the first knock and a robust rate of paraquat in the second treatment.

Below is an overview of key tactics to manage some commonly-found WA summer weed species, as found by DPIRD research carried out with GRDC investments.

Afghan melon and paddy melon

• Tends to grow after spring and summer rainfall
• Seed set can occur after only a few weeks of growth
• Individual plants can produce about 20,000 seeds
• Spray early, before plants have a chance to produce melons
• Good results have come from spray grazing using a hormone herbicide, such as 2,4-D, followed by heavy grazing
• Herbicide mixtures with 2,4-D (Group I, phenoxy) are effective
• This can include triclopyr (Group I) and 2,4-D
• There is no evidence of herbicide resistance in melons.

From October 2018 there are new regulations for use of 2,4-D - see here.
For further information, go to the DPIRD website.

Button grass

• Emerges in spring and early summer and grows vigorously
• Can produce 8000-32,000 seeds per square metre
• Exposure to waterlogged soils for two weeks or longer can kill seed
• Burying seed at 2cm or below can kill seed within two years
• High rates of herbicide (according to label directions) are needed
• Glyphosate is an option
• Paraquat can be effective when used in a double knockdown treatment
• There is no evidence of herbicide resistance
• But it is advised to not rely on glyphosate every year.

For further information, see GRDC’s GrowNotes™ Button Grass Tips and Tactics or DPIRD’s website.

Caltrop

• Found after frequent rainfall in spring and summer
• Can flower and set seed very quickly (within a few weeks)
• Can have up to 2000 fruits and produce more than 40,000 seeds
• Long-term seed dormancy makes seed set prevention vital for control
• Seeds can be viable for two years, even if buried at 2cm or 10cm
• If most seeds are on the soil surface, germination is reduced and eradication is possible in 2-3 years
• If mouldboard ploughing is used to bury seed, it needs to stay buried for at least 10 years
• Bipyridyl (Group L) herbicides can provide 100 per cent control
• Addition of saflufenacil (Group G) to paraquat can provide faster control than paraquat alone.

More information is available on the HerbiGuide website and on the DPIRD website.

Feathertop Rhodes grass

• An emerging weed problem in WA summers
• Mostly found on roadsides, but starting to infest cropping paddocks
• Researchers are currently not clear on the resistance status of WA feathertop Rhodes grass populations to non-selective herbicides (Group M and L), including glyphosate. However, it is suspected some populations have some tolerance. Testing is being conducted to confirm this
• Isoxaflutole (Group H) and paraquat can be used in the summer fallow
• Glyphosate and paraquat used in a double knock can be effective (but will rarely achieve 100 per cent control)
• Shogun® (containing the Group H herbicide propaquizafop) is registered in WA for control of feathertop Rhodes grass in summer fallow either alone or followed by an application of paraquat 7-14 days later (influenced by growth stage)
• Imazapyr (Group B) is registered for non-crop land uses (such as roadsides)
• Integrated weed control methods are vital

Fleabane (also known as flaxleaf fleabane)

• Common in no-tillage systems
• Widespread resistance to glyphosate in other States (not WA)
• Can produce up to 110,000 seeds per plant
• Wind can spread seed across long distances
• Best controlled at seedling stages, before seed production
• Difficult to control with herbicides, especially when plants are bigger than 30cm
• Strategic soil disturbance can reduce germination
• Several residual herbicides can be used in summer fallows
• A single application of selective herbicides, such as 2,4-D alone, can be effective
• A mix of glyphosate and 2,4-D is another option for a single spray
• A double knockdown can also achieve good control if necessary.

More information is available on the DPIRD website and the WeedSmart website.

Windmill grass

• Typically grows as an annual plant during summer and can be grazed
• Can germinate in autumn and winter, depending on temperature
• Produces up to 25,000 seeds per plant that are then spread by wind
• Mature tufts are hard to control
• Difficult to get adequate herbicide coverage on very small plants
• High water rates can help improve coverage
• It is advised to kill seedlings seven to 21 days after summer rain
• Glyphosate resistance is increasing nationally (but is at low levels in WA)
• Glyphosate is effective
• This can include glyphosate as potassium salt 500g active ingredient per litre (a.i./L).

More information is available on the GRDC’s Integrated Weed Management hub and the DPIRD website.

More information

Dr Catherine Borger, DPIRD, 08 9690 2220, catherine.borger@dpird.wa.gov.au
Dr Abul Hashem, DPIRD, 08 9690 2136, abul.hashem@dpird.wa.gov.au
Alexandra Douglas, DPIRD, 08 9821 3246, alex.douglas@dpird.wa.gov.au

Useful resources

GRDC GroundCover™ Supplement Summer Weeds
GRDC IWM hub
HerbiGuide
APVMA
WeedSmart

GRDC Research Codes

UA00156, UA00149, DAW00257, CSP00111