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Agronomists, researchers, growers answering important questions

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Does ambient temperature affect herbicide performance?

with Chris Preston, Associate Professor, Weed Management
The University of Adelaide
Temperature affects the absorption, translocation and metabolic degradation of herbicides applied to plants. Herbicides applied under the wrong conditions can appear to fail, however the reason may not be herbicide resistance.
Dr Chris Preston, Associate Professor, Weed Management
 at The University of Adelaide says most herbicides have a temperature range at which they are most effective in controlling target weeds.
“Applying herbicides outside the optimal temperature range is likely to contribute to a spray failure, even in susceptible populations,” he says. “Alternatively, applying herbicides within the correct temperature range can improve the control in populations known to have a level of resistance to that herbicide.”
Dr Chris Preston suggests testing whole plants rather than seed for responses to a range of post-emergent herbicides. The Quick-Test is conducted in the same growing season as herbicide will be applied so the testing will occur under similar conditions to field conditions.
Dr Preston says the effect of frost on the efficacy of clethodim is a striking example. Spraying clethodim in non-frosty conditions achieves vastly better results than spraying after three days of frost, even on populations that are resistant to this chemical mode of action.
“Combining the optimal temperature with optimal weed size will give the best results possible,” he says. “The current common practice of applying clethodim to tillered ryegrass in the coldest months is not making the best use of this herbicide.”
As a general rule of thumb, Group A (fops), paraquat (Group L) and glyphosate (Group M) are more effective at lower temperatures while Group A (dims), atrazine (Group C) and glufosinate (Group N) are more effective at higher temperatures. However, weeds that are resistant to paraquat become less resistant in warmer temperatures.
“The other implication of this research is the effect of ambient temperature on herbicide test results,” says Dr Preston. “Seed collected in winter and grown out in the glasshouse in summer will be tested for resistance in conditions that are not representative of field conditions when growers are next treating that weed species. The Quick-Test using whole plants overcomes this problem and improves the reliability of herbicide susceptibility testing.”
How can I get the best performance out of clethodim?
Short answer: Avoid applying clethodim during frosty periods.
Longer answer: Twice as much clethodim is required to kill susceptible annual ryegrass if the product is applied after three days of frost. Even higher rates are required if the plants have resistance to clethodim.
Planning to apply clethodim for grass control outside the coldest months of June and July, and avoiding night spraying in winter, will see better results in both resistant and susceptible populations, particularly in tillered plants. Clethodim is most active when temperatures are over 20 degrees C.
Weed seed that is tested during summer may return false negative results, which could translate into spray failure in the field the next season.
Twice as much clethodim is required to kill susceptible annual ryegrass if the product is applied after three days of frost. Even higher rates are required if the plants have resistance to clethodim.
When it is it too hot for glyphosate?
Short answer: Efficacy is much better at 20 degrees C than at 30 degrees C.
Longer answer: Spraying glyphosate resistant barnyard grass at lower temperatures is more effective than under hotter conditions. If barnyard grass is tested for herbicide resistance during the cooler parts of the year it may appear susceptible to the field rate of glyphosate but then when this rate is applied to the population in summer there may be many survivors.
When glyphosate is taken up rapidly it tends to limit its own translocation, which can mean that although symptoms may appear more rapidly in warmer temperatures, plant kill is less reliable.
Which herbicide resistance test should I use?
Short answer: The weed resistance Quick-Test for post-emergent herbicides.
Longer answer: The Quick-Test involves testing whole plants rather than seed for responses to a range of herbicides and rates. The Quick-Test is conducted in the same growing season as herbicide will be applied so the testing will occur under similar conditions to field conditions. The results of the Quick-Test are available within the same season, potentially giving growers an opportunity to apply an effective weed control tactic before the end of the season. The Quick-Test is not available for many pre-emergent herbicides.
The Quick-Test is available through Plant Science Consulting and results are normally available after four weeks.
Relevant links

Maximising clethodim performance and the impact of frost fact sheet
Keeping clethodim working in broafleaf crops
Plant Science Consulting herbicide resistance testing – Quick-Test
GRDC Update Paper – New developments and understanding in resistance mechanisms and management

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Using fodder crops to manage resistant weeds?

with Simon Falkiner, Research Consultant, Falkiner Ag
No till, continuous cropping has seen some huge productivity gains in Australian agriculture however herbicide resistance in weeds is a real Achilles Heel.
Simon Falkiner, research consultant with Falkiner Ag says the rapid increase in herbicide resistance in the high rainfall zones is alarming and he recommends that growers seriously consider the value of pasture and fodder crops to regain control.
Simon Falkiner says herbicide resistance has occurred faster in the high rainfall zone of south-western Victoria than many growers realise. He suggests growers consider including a fodder crop in the rotation to drive down weed numbers in just a few years.
“There is no doubt that some growers are losing money on weedy crops,” he says. “Herbicide resistant weeds can and do cause negative gross margins even in harvestable crops.”
“Fodder crops are a seriously good option to clean up weedy paddocks and still generate income. For most growers, reintroducing livestock of their own or on agistment will take best advantage of the buoyant livestock market. Where the property is located close enough to dairies, feedlots or an export market, making hay or silage is another option.”
A two-year fodder or pasture phase with grazing and harvesting biomass prior to weed seed set will remove over 90 per cent of annual ryegrass seeds from the seed bank. Gaining control of wild radish takes longer due to the longevity of the seed in the soil however the same principles apply — do everything possible to stop seed set.
“There are several ways to make fodder and pasture production a profitable part of a cropping enterprise,” says Mr Falkiner. “Grazing offsets the cost of taking a paddock out of grain production but has little impact on weed numbers. The whole benefit for managing herbicide resistant weeds lies in controlling the late germinating in-crop weeds that survive grazing and chemical applications, and preventing them from setting seed.”
What is the best method to stop weed seed set in a fodder crop?
Short answer: The method doesn’t matter; just the timing and a double knock.
Longer answer: Hay, silage, spray-topping and green and brown manuring are equally effective at controlling annual ryegrass. It is essential that the crop is ended before the annual ryegrass sets any viable seed. If some viable seed is present, then silage is a safer option than hay to avoid transporting weed seeds to other locations. The correct timing of the operation generally does not allow for a grain or oilseed harvest. Using the feed on farm is the most profitable option at the moment and, as many growers say, the hay market can be very fickle.
Grazing animals do not play a direct role in the management of weeds. The real benefits lie in using the crop to generate income through livestock production and or harvesting the biomass before the weeds have set viable seed.
Which fodder crops and pastures are the best for weed management?
Short answer: The choice of crop, and the management of the crop itself, is less important than the timing of the operation to prevent seed set in weeds.
Longer answer: The choice of fodder species may take many things into account such as improved soil fertility, the range of in-crop herbicide options, livestock preference, marketing opportunities, biomass production and so on. Arrowleaf clover and perennial ryegrass converted to silage and brown manured Balansa clover and field peas all provided an 89 to 91 per cent reduction in annual ryegrass numbers. Forage oats (silage) and serradella (brown manure) were less useful for weed management but still achieved weed reduction over 80 per cent.
Using fodder crops to manage weeds.
How does fodder cropping compare to chemical fallowing or burning stubble for weed control?
Short answer: Stubble burning does not kill enough weed seeds. Chemical fallowing continues the heavy reliance on herbicides.
Longer answer: Fodder cropping provides more opportunities to include non-herbicide tactics into the cropping rotation. In the high rainfall zone, moisture conservation is less of a priority than in lower rainfall zones, allowing both summer and winter fodder cropping options, which can help drive down the seed bank. Even in high stubble load conditions, stubble burning does not generate the high temperatures for long enough to kill enough weed seeds on the soil surface. There is no doubt that weedy crops cost growers money, and even generate negative gross margins – a two-year break from cropping in paddocks infested with herbicide resistant weeds could be exactly what’s needed to solve a very costly problem.


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How can I avoid sub-lethal dosing when spraying weeds?

Bill Gordon, spray application consultant with Nufarm Australia says these ‘hard-to-kill’ species often have hairy surfaces, thick cuticles, reduced translocation due to plant stress or internal mechanisms that metabolise the product.

“The application rate and technique are critical when managing these weeds,” he says. “If the application is compromised there is a good chance that the weed will survive and go on to set seed.”
“In weeds that are generally susceptible to a herbicide mode of action we see low dose rates inadvertently applied through insufficient coverage, incorrect formulation or adjuvant choice and stressed plant condition, which all impact on the uptake of the herbicide.”
The choice of droplet size, carrier volume and sprayer setup all contribute to the delivery and effective uptake of the herbicide. If a population of susceptible weeds are exposed to long-term low dose application there is strong selection for plants that can survive and set seed, leading to increased herbicide resistance over time.
“Correct plant identification and knowing the resistance status of the population is key to choosing the most appropriate product, most effective rates and the recommended adjuvant,” says Mr Gordon. “Water is the cheapest thing that goes in the spray tank so use sufficient total application volumes to achieve the coverage required for the mode of action group, particularly in paddocks with plenty of stubble.”

Bill Gordon, Nufarm spray application consultant says herbicide application rate and technique are critical when managing the ‘hard-to-control’ weeds that are often the dominant weed species found on farms.
What are the common spray application mistakes that can contribute to herbicide resistance?
Short answer: Mismatching rates, application volumes and not rotating modes of action.
Longer answer: Application technique and the choice of product, rate, water volume and adjuvant are generally within the grower’s control. Weather conditions and the plant size and stress are harder to work around. In ideal conditions the aim is to apply the right rate of the right product to the right target to achieve plant death and prevent seed set. Herbicide resistance is known to evolve, often un-noticed, along fencelines and other green bridge situations.
How does stubble affect the dose delivered to the target weed?
Short answer: Coverage can be up to 60% less at the base of standing stubble compared to the inter-row.
Longer answer: Standing stubble can intercept many droplets before they reach the target weeds. Choose an appropriate spray quality for the target and the mode of action. Use visual indicator tools such as water sensitive paper to ensure adequate coverage is achieved where it is needed. Penetration into standing stubble can be difficult, but is easier to manage than trash on the ground.
For soil applied products, use a very coarse or larger spray quality to maximise contact with the soil. Keep in mind that if a tank mix partner is targeting emerged weeds adjust the spray quality to suit both mode of action and target types. Often the compromise is a coarse spray quality at higher application volumes.
Coverage can be up to 60% less at the base of standing stubble compared to the inter-row. Penetration into standing stubble can be difficult, but is easier to manage than trash on the ground.
How can I counteract the edge effect to apply the correct dose on paddock borders?
Short answer: Large headlands and slow down!
Longer answer: Maintain wide enough headlands to allow easy turning and entering the paddock with the full spray pressure. Using a minimum hold in the controller for non-residual products will prevent the rate dropping too low and ensure the nozzle’s spray pattern does not collapse.
For residuals, slow down and increase application volume to minimise overdosing that may impact on crop establishment, or increase plant back periods.
Other resources:

10 Point Plan – Carefully manage spray events
GRDC GrowNotes Spray Application Manual for Grain Growers

How to ask a WeedSmart question
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Does soil pH affect weed management?

with Dr Abul Hashem, Weed Science Principal Research Officer, DAFWA
Soil acidity limits productivity on approximately 50 per cent of Australia’s agricultural land and several important weed species have a competitive advantage over broadacre crops on soils where the surface soil pH level is less than 5.5 or the subsurface pH is less than 4.8.
Dr Abul Hashem, Department of Agriculture and Food, WA weed science principal research officer, says efforts to improve soil pH have a two-pronged effect on crop production. “Firstly, agricultural crops yield better when they are grown in soils where the pH is in the optimal zone and secondly, crops are better able to compete with weeds,” he says. “Having lower weed numbers in-crop means the herbicides applied are more likely to be effective and also there is less potential for weed seed to be added to the soil’s seed bank.”

Where soil acidity was removed as a constraint to production, DAFWA researcher Chris Gazey and Joel Andrew from Precision SoilTech measured a total biomass increase of 160 per cent compared to the control (un-limed) acidic profile in a trial at Kellerberrin, WA and the weed biomass was reduced to only three per cent of the total biomass.
“It is usually several years before a lime application has a measurable effect on soil pH, especially in the subsurface,” says Dr Hashem. “In our GRDC-funded trials at Merredin, Wongan Hills and Eradu from 2010 to 2014 with researchers from the Australian Herbicide Resistance Initiative (AHRI) at the University of Western Australia, we saw reductions in populations of wild radish, annual ryegrass and barley grass of between 40 and 70 per cent in the fourth year of the trial as a result of lime applications.”
Surface pH can also impact on the breakdown processes of residual herbicides, potentially impacting future crop safety and exposing later generations of weeds to sub-lethal doses of the herbicide.
Crop competition is one of the most effective non-herbicide weed control tactics available and managing the soil’s surface and sub-surface pH is central to enhancing plant growth and productivity, giving crops the winning edge against weeds.
DAFWA Principal Research Officer Dr Abul Hashem demonstrated the effect of lime application on the suppression of annual ryegrass 4 years after application. Annual ryegrass numbers were reduced from 139 to 56 plants/m2 (59% reduction) at the Merredin site.
How does managing soil pH affect weed biomass?
Short answer: Most broadacre crops are suited to soil pH above 5.5. In more acidic soils, weeds can take advantage of the soil moisture and nutrients to grow and set seed, adding to the soil weed seed bank.
Longer answer: A small shift in soil pH can have a large impact on the availability of aluminium in the soil. Soluble aluminium restricts the growth of plant roots and this limits the plant’s ability to access water and nutrients, particularly in seasons where there is a dry finish, resulting in reduced yield and smaller grain size. Weeds do not usually prefer acidic soils, they are just able to take advantage of the reduced competition for resources. By adding lime to acidic soils it is possible to lift the pH, allowing crops to perform better and out-compete the weeds.
Weeds do not usually prefer acidic soils, they are just able to take advantage of the reduced competition for resources. By adding lime to acidic soils it is possible to lift the pH, allowing crops to perform better and out-compete the weeds.
Does soil pH affect the breakdown of residual herbicides?
Short answer: Yes. In low pH (acidic) soils, imidazolinones breakdown slower, resulting in increased persistence. The opposite occurs with triazines and sulfonylureas, which persist much longer in high pH (alkaline) soils.
Longer answer: Residual herbicides that breakdown through reactions with water (hydrolysis) are strongly affected by soil pH. Triazines (Group C sub-group) and sulfonylureas (Group B sub-group) are broken down through chemical hydrolysis in neutral or acid soils. This process is much slower in alkaline soils, potentially restricting crop choice and exposing late germinations to sub-lethal doses of the herbicide.
The other main method of residual herbicide breakdown is through microbial degradation. Soil pH generally does not affect this process, except in the case of imidazolinones (Group B sub-group), which persist for longer in acidic (low pH) soils.
Can I expect a crop yield benefit from lime application?
Short answer: Yes, if pH is currently a limiting factor then applications of lime will improve crop yield.
Longer answer: WA growers Alex and David Leake applied lime to plots on their property at a rate of 5 t/ha, about 20 years ago. Limed plots produced 60 per cent more barley biomass than unlimed plots and, in addition, unlimed plots contained proportionally more ryegrass than plots.
Increased crop competitiveness and potentially better herbicide efficiency on soils with good pH combine to fight weeds and reduce the weed seed bank over time.
Other resources:

Dr Hashem’s research paper presented in 2014
AHRI Insight – Heal thy soil, heal thy crops, kill thy weeds
Investigating herbicide resistance and lime incorporation in WA

How to ask a WeedSmart question
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Management of herbicide resistant weeds on fencelines

with Dr Chris Preston, Chair of the Australian Glyphosate Sustainability Working Group
Weeds growing along fences and other crop border areas are quietly gaining resistance to important knockdown herbicides like glyphosate, setting plenty of seed and are ready to move into neighbouring crops.
Dr Chris Preston, Chair of the Glyphosate Sustainability Working Group, says glyphosate products have been so effective for the past 15 to 20 years that many growers have relied solely on these products for weed control in their non-crop areas.

“The problem that has arisen is that any escapes – weeds that survive the glyphosate treatment – are able to flourish in an environment where there is little competition for moisture, nutrients or light,” he says. “This means they can set large quantities of seed and it doesn’t take much for this seed to move into the crop area, already highly resistant to a key herbicide mode of action.”
The other major problem that occurs is due to all weeds growing along the fenceline being treated with the same product. This means that weed species that already have a high tolerance to glyphosate, and are not listed on the product label, are able to gain a stronger foothold.
“This can be clearly seen along roadsides around the country where hard to kill grasses like feathertop Rhodes grass have become a dominant species,” says Dr Preston. “In situations where other weed control options are limited, it is difficult to contain the spread of these weeds.”
Fencelines and crop borders represent a significant threat to crop production due to the often-unnoticed evolution of glyphosate resistance in weeds.
What other options do I have for managing fenceline weeds?
Short answer: Using diverse herbicide and non-herbicide tactics including double-knock, chipping survivors, slashing, burning, livestock, cultivation and different herbicide modes of action, including residuals.
Longer answer: Consider the situation, that could realistically arise, where glyphosate is not effective on any weeds present on the fencelines or crop border areas. What would you do? Making a change to how you manage the borders now could extend the useful life of glyphosate in your farming system. Herbicide resistance is best addressed by combining a variety of control tactics to drive down weed numbers and prevent seed set.
What other herbicides are a good option for fencelines?
Short answer: Herbicides that are not used in crop production could be valuable in maintaining clean borders.
Longer answer: One example is bromacil, a Group C herbicide that controls a range of both grass and broadleaf weeds. This herbicide is effective on small weeds post-emergence and also offers residual activity to suppress germinations. It would be best applied during the winter cropping phase when winter-growing weeds are small, rather than leaving treatment until August or September when the target weeds are large. In the northern region, spring is a good time to be targeting summer-growing weeds along fencelines. However, if you do have to use bromacil on larger weeds, it must be applied with an effective knockdown herbicide partner. Complete control of glyphosate resistant annual ryegrass has been achieved in trials using paraquat/diquat + bromacil and glufosinate + bromacil, applied post-emergent.
Bromacil can also be applied to bare soil to stop weeds establishing. Other residual herbicides currently registered, including simazine, fluometuron and imazapyr, all have recorded instances of herbicide resistance and so are not robust options for fenceline weed control. Flumioxazin is soon to be registered for application to bare ground and will offer growers another residual herbicide option without the risk of damaging trees.
A new registration for flumioxazin to be applied to bare soil in non-crop areas will increase the residual herbicide options available to growers. This photo shows the efficacy of flumioxazin compared to glyphosate (front), four months after application.
What happens if a fenceline gets incorporated into a cropped area when I realign paddocks?
Short answer: Crop competition becomes an important control tactic.
Longer answer: If fencelines are no longer required, removing them will make it easier to manage crop borders around the farm. Where crops are planted across old fencelines there is often additional weed pressure for the first few years. If possible, thoroughly clear away weeds during the paddock re-development and plan to grow very competitive crops in the first few seasons at least. The ‘fenceline effect’ is often observed for several years so it pays to keep the competitive pressure on.
Other resources:
Search this website for other articles about managing ‘fencelines‘
How to ask a WeedSmart question
Ask your questions about managing fencelines on Facebook, Twitter or leave a comment below.

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Dealing with a feathertop Rhodes grass problem?

with Andrew Storrie, Agronomist, Agronomo
Feathertop Rhodes grass, with its distinctive seed head, is steadily travelling across the country ­– even being seen along the Eyre Highway crossing the Nullarbor Plain. Agronomo agronomist, Andrew Storrie says prolific seed production and its ability to withstand herbicides makes this weed difficult, but not impossible, to control.
“The key is to concentrate on preventing seed set,” he says. “Feathertop Rhodes grass seed is relatively short-lived in the soil, so with a dedicated approach it is possible to run down the seed bank reasonably quickly.”

“Often the first obvious sign of feathertop Rhodes grass will be a few large plants with the seed head ‘skeletons’ pointing toward the sky,” he says. “FTR grass is moving rapidly along roadsides and most invasions occur from a boundary fence, with seed blown into a paddock. Once on a property it can spread easily on machinery and along irrigation channels.”
Feathertop Rhodes grass is an exotic species that first gained a foothold in the northern grain growing region in response to a shift to low tillage production systems. It is now an emerging weed across the southern and western grains regions of Australia.
“FTR grass is an annual plant that flowers within three to four weeks of germinating and will keep tillering and producing seed while soil moisture is available,” he says. “Not all the seed germinates with the first rain event so there are usually multiple germinations over summer, however most seed in the seed bank is from a single season of seed production rather than accumulating over years.”
Although the light seed is wind dispersed, most seed will fall within five metres of the parent plant, making patch management a viable control option.
Andrew Storrie, Agronomo agronomist says growers need to look for the Archilles Heel of hard to kill weeds like sowthistle, flaxleaf fleabane and feathertop Rhodes grass.
What are the herbicide control options for feathertop Rhodes grass?
Short answer: FTR grass is naturally very tolerant of glyphosate and there are several cases of highly resistant populations. Many growers are using Group A knockdown herbicides but this could be a short-lived option.
Longer answer: Although Group A herbicides are currently an effective control for feathertop Rhodes grass, resistance is inevitable and field observations suggest it could have already occurred in some paddocks.
Group A herbicides are registered for use in various summer and winter broadleaf crops and in fallow ahead of a mungbean crop (Permit 12941). This permit states that a fallow application must be applied to small weeds and followed with paraquat within 10 days.
Group A herbicides are very sensitive to plant stress and, like paraquat, good coverage is essential for reliable results. Given these constraints, it may be prudent to ‘save’ this herbicide for in-crop grass weed control and look for other ways to control FTR grass in summer fallows.
What patch management methods do you recommend?
Short answer: Physical removal of isolated plants, patch cultivation, burning and optical spray technology.
Longer answer: FTR grass invasion often begins with a few ‘mother’ plants. The plants have shallow root systems and are easy to remove. If this is done before the seed drops, significant costs can be avoided.
Seed buried below a few centimetres of soil will not germinate and most will be non-viable in 12 months, even if another cultivation returned then to the soil surface. Light cultivation of a patch is a very viable option to stimulate germination and allow effective herbicide control of small plants. Under the right conditions fire is also effective to kill the plants and seed on the surface.
Optical spray technology is the only option for treating plants larger than fist size. Permit 11163 provides a range of herbicide options for use with optical sprayers. There are also products from Nufarm and CropCare registered for application with an optical sprayer.
Feathertop Rhodes grass seed head.
What can I do to control FTR grass that establishes under the crop?
Short answer: Harvest early following a pre-harvest desiccation and come back immediately with a control tactic to target small FTR grass plants.
Longer answer: Pre-harvest desiccation will allow earlier harvest, particularly in crops like canola that allow more light in toward the end of the season. This is particularly useful if there has been a blow-out in FTR grass numbers.
An application of paraquat + amitrole soon after an early harvest can provide good control and be followed with a residual herbicide, such as Balance® applied ahead of a chickpea crop, to extend the control into the summer months.
FTR grass is sensitive to crop competition. All efforts to increase crop competition through crop and variety choice, narrower rows and stubble management will suppress FTR grass that might otherwise germinate as the temperature rise above 20 degrees C in spring.
Other resources:

ARHI Insight – New boy-band ‘The Feathertop Rhodes’
Integrated Weed Management – Feathertop Rhodes grass (DAF Qld)
Search for ‘feathertop’ on the Weedsmart site for a wide range of other resources.
Watch the videos in the playlist below.

How to ask a Weedsmart question
Ask your questions about control options for feathertop Rhodes grass on Facebook, Twitter or leave a comment below.

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What are the year-round options for sowthistle control?

with Annie van der Meulen, Research Scientist, Queensland Department of Agriculture and Fisheries
A GRDC-funded survey conducted across the GRDC Northern Grains Region in 2014–2016 discovered that glyphosate resistant sowthistle was more widespread than previously thought. Originally identified on the Liverpool Plains of NSW in 2013–14, resistant populations have now been identified throughout the region, from central Queensland to central NSW.
Department of Agriculture and Fisheries (DAF) research scientist, Dr Annie van der Meulen says the survey showed that while there is a definite ‘hot spot’ on the Liverpool Plains, glyphosate resistance in sowthistle is a widespread problem for growers.

“Currently, it seems that many resistant populations are exhibiting relatively low level resistance,” she says. “This means glyphosate is still effective in many situations however attention to detail will make a difference to application efficacy.”
“Plant size is very important when applying glyphosate to sowthistle,” says Dr van der Meulen. “The label on glyphosate products typically specifies two different rates according to plant size classes and following these instructions is very important.”
If a lower rate is applied to larger weeds they can take a long time to die (if at all) and may well set seed before the plant dies, making the application a waste of money.
“One of the difficulties with managing sowthistle is the fact that it can germinate at any time of the year if soil moisture is available. In favourable conditions, this weed can set seed within months and has the capacity to produce about 6000 seeds per plant,” she says.
To stay in control of sowthistle growers need to be ready to respond to new flushes at any time that surface soil moisture is available. One weakness that growers can exploit is that sowthistle is a poor competitor, so choosing crops that rapidly achieve canopy closure can be a very effective control measure.
To gain a deeper understanding of the extent of herbicide resistance, GRDC has funded further surveillance work in the northern grains region. As part of this work DAF is asking grain growers in Queensland to give permission for weed samples to be collected from their property. Contact Adam Jalaludin at DAF on 13 25 23. A similar survey, managed by John Broster at Charles Sturt University, will be conducted in NSW.
DAF research scientist Annie van der Meulen says glyphosate resistance in common sowthistle is more widespread than previously thought. She is encouraging growers to be involved in the DAF’s herbicide resistance testing during the summer cropping season.
What are the control options for sowthistle in the fallow?
Short answer: Target small, actively growing plants with robust rates of glyphosate.
Longer answer: Use optical sprayer technology and the maximum label rate. Double-knocking treats survivors and provides control for cohorts that germinated after the first spray. In the double knock operation, partner glyphosate with a product that has a different mode of action and ensure the second treatment occurs within 21 days of the first. Pre-emergent herbicides also have a role to play, but follow the label instructions carefully to avoid any unwanted impacts on the subsequent crop. Stubble cover may be a double edged sword as it helps retain moisture that the sowthistle can access. Be aware of the antagonism between glyphosate and 2-4D that reduces the efficacy of glyphosate on sowthistle. Burying seed to a depth of 2 cm or more can help to reduce emergence however repeated use of cultivation can bring seed back to the soil surface where germination can occur.
Common sowthistle thrives in situations where there is no competition.
Are there any control options in-crop?
Short answer: Pre-emergent herbicides plus a competitive crop.
Longer answer: Crops that form a dense canopy, and close over quickly, will help suppress most weeds. Winter cereals, such as barley and narrow-sown wheat, can provide very effective control of sowthistle. Be aware that sowthistle will exploit any gaps in the crop. It is usually best to avoid planting chickpea in a paddock that has an existing sowthistle problem. Herbicide resistant crops can also assist with in-crop herbicide applications.
What can I do at harvest to control sowthistle?
Short answer: Plants cut by the harvester can re-shoot, so control established plants as soon as possible following harvest, and be ready to spray new flushes when the plants are small.
Longer answer: Researchers are investigating whether sowthistle is a good candidate for harvest weed seed control. Although not generally considered a summer weed, sowthistle plants can germinate at any time of year if soil moisture is available, in multiple flushes throughout the growing season. Moreover, plants that are cut off during harvest can reshoot and take advantage of the ample resources available once the crop is removed. Plants left uncontrolled at any time of the year can set seed, presenting a problem in subsequent crops.
How to ask a Weedsmart question
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What are the herbicide options for the summer fallow?

with Mark Congreve, Senior Consultant, ICAN
The weakest link in weed control programs in most northern region farming systems is the summer fallow. With no competition from crops, uncontrolled weeds can use the abundant water and nutrients to produce lots of seed, putting additional pressure on future weed control program.
For the last three decades glyphosate, and its tank mix partners, has provided effective summer weed control and allowed the implementation of conservation farming techniques, but with glyphosate resistance affecting more and more paddocks, summer fallow management needs to change.

Mark Congreve, senior consultant with ICAN, says that growers are implementing new strategies, not simply to replace glyphosate but to extend its use in their farming system. “Glyphosate has been, and continues to be, the most useful knockdown herbicide available to growers in the summer fallow,” he says. “While still effective in many situations, resistance to this herbicide is becoming increasingly evident and other options need to be built into the summer fallow program.”
“There are two main signs that growers need to look for to detect if their weed populations are becoming more resistant to herbicides,” he says. “The first is to take note of ‘survivor’ weeds – individual weeds that appear unaffected after a spray event that has killed the surrounding weed plants. The second sign is ‘rate creep’. This is where a herbicide is still effective but only when higher and higher rates are applied.”
Growers should not ignore either of these signs, which could indicate changes in the herbicide resistance status of the weed population. Mr Congreve suggests growers should seek agronomic advice and test weeds for their susceptibility to a range of herbicides.
“While waiting for the test results, immediately look for alternative fallow management strategies to those that may have worked in the past but are increasingly less effective,” he says.
Mark Congreve, ICAN senior consultant urges growers to take note of the early signs of glyphosate resistance and to make changes to their fallow management program.
What should I do as a first step, even if there are no definite signs of glyphosate resistance?
Short answer: Do something different. Double-knock, whenever possible. Implement the 2 + 2 + 0 strategy.
Longer answer: Herbicide resistance is considered an inevitable part of long-term herbicide use but if weed numbers are kept low and all survivors are removed then resistant weeds can generally be managed. The 2 + 2 + 0 strategy adopted in the cotton industry to support Round-up Ready* technology involves using two non-glyphosate tactics in the crop, two non-glyphosate tactics in the fallow and ensuring there are zero survivors.
Wherever possible use the double-knock combination of two herbicides or a herbicide followed with a non-herbicide tactic (e.g. cultivation), particularly while glyphosate resistance levels are still relatively low.
Are there any other knock-down options for grass weed control in the fallow?
Short answer: Consider using Shogun, a recently registered knock-down herbicide for use in fallow situations.
Longer answer: Shogun (propaquizafop) is a Group A ‘fop’ herbicide. Fops are particularly effective on small grass weeds, however, resistance is likely to develop faster than has occurred with glyphosate. When using Group A herbicides in the fallow it is essential to double-knock to remove any survivors and enable longer term use this herbicide option. This strategy is useful for controlling glyphosate-resistant weeds such as barnyard grass, liverseed grass and feathertop Rhodes grass. It provides an alternative mode of action to glyphosate and can increase the diversity of chemical control options used across the farming system.
Feathertop Rhodes grass is challenging the glyphosate-reliant summer fallow management program on many grains farms.
What knockdown options exist for glyphosate resistant broadleaf weeds such as fleabane or sowthistle?
Short answer: There are a number of Group G and I herbicides registered for these problem weeds. Check product labels for details and timing.
Longer answer: Group I herbicides containing either 2,4-D, fluroxypyr (Starane*), clopyralid (Lontrel*), or picloram (Tordon*) can be effective on one or both of these weeds. If weeds are larger, a double knock will be required for high level control. Clopyralid and picloram based products also give residual control of fleabane.
Group G herbicides containing flumioxazin (Valor*) or saflufenacil (Sharpen*) are also effective on one or both species. As they are contact herbicides they perform better when applied to very small weeds, ideally when used in a tank mix with either glyphosate or paraquat.
*Registered Trademark
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‘Can herbicide affect weed seed dormancy and viability?’

with Hanwen Wu, NSW DPI Principal Research Scientist
Farmers have been reporting that several weed species seem to be emerging later than usual, evading pre-emergent herbicides, to establish in-crop.
This apparent shift in emergence has also caused weeds researchers some trouble when they conduct herbicide resistance testing and find that seed from some populations has greater germination than others.

Hanwen Wu, NSW DPI principal research scientist says he first came across this with fleabane when seeds collected from some populations failed to germinate. He began to investigate whether herbicides applied to the weed populations over summer could have been affecting seed dormancy and viability.
“We have now confirmed that different herbicides sprayed on mature fleabane plants at early and late budding stages definitely affect the seed dormancy and viability,” says Hanwen. “Other researchers have previously found that brome grass and barley grass seed collected in cropping fields often have longer dormancy than seed collected from adjacent non-crop habitats.”
“Herbicide-induced dormancy could be a problem for growers as it can cause delayed emergence and prolonged seed persistence in the field. Weeds like fleabane that are difficult to kill, especially once they have developed a strong root system, can quickly take advantage of summer rainfall to send out multiple branches bearing large amounts of seed.”
“Our field and glasshouse trials have shown that there are herbicides that can be used to reduce the amount of viable seed produced by large fleabane plants, but the timing must be right,” he says.
Dr Hanwen Wu has demonstrated that herbicide applications can induce dormancy in fleabane seeds, enabling seeds to evade pre-emergent herbicides and establish later in-crop when control options are minimal.
How important is the timing of herbicide application to sterilise fleabane seed?
Short answer: Spray at early budding for best effect.
Longer answer: There is only a 3-week window between early budding and late budding when the fleabane buds begin to open. Spraying glyphosate at late budding or at flowering is generally wasted and is more likely to help the plants produce seed with longer dormancy. Spraying at early budding can effectively kill or stunt the seed heads and more seed will be sterile.
What herbicides are effective for this use?
Short answer: A range of herbicides commonly-used in summer can effectively reduce the germinable seeds and the total viable seeds on flaxleaf fleabane plants, especially when applied at the early budding stage. Delayed herbicide application has a lesser sterilising effect on the seed.
Longer answer: Similar studies around the world demonstrate that herbicides could potentially reduce seed production and viability of many agricultural weeds. While there is potential to use this tactic to reduce seedbank replenishment there is also a risk that herbicide applications could also induce seed dormancy, which could prolong the seed persistence in the field, making weed management more complex in-crop.
Glyphosate applied at early budding can effectively reduce seed set on the main stem of flaxleaf fleabane – a management tool that could be incorporated into an integrated weed management program over summer.
How can this strategy be used in an integrated weed management plan?
Short answer: Herbicide applied at early budding stage can reduce the amount of viable seed set, however this tactic must not be used in isolation.
Longer answer: Large flaxleaf fleabane plants are hard to kill with herbicides, particularly if soil moisture is limited. They readily regrow after being cut off at harvest, drawing on large root reserves. Over summer, use the double knock technique after rain to treat regrowth while the branches are relatively ‘small’. Crop competition is very effective in reducing fleabane populations so choose competitive crops, narrow row spacing and set them up for success to minimise the opportunities for late germinating plants to gain a foot-hold. Strategic cultivation and regular sheep grazing are also effective tactics. Integration of chemical and non-chemical options could minimise the buildup of dormant seed in the seed bank.
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Why look for summer weeds in spring?

with Chris Preston, Chair of the Australian Glyphosate Sustainability Working Group
Sowthistle, windmill grass and feathertop Rhodes grass are problematic weeds for grain growers across Australia. All three are known to evolve resistance to glyphosate, the mainstay herbicide in no-till farming.
These three weeds have several key biological traits in common that make them difficult to control, particularly once the winter crops have been harvested and these weeds have uninhibited access to whatever soil moisture and nutrients are available over summer.
Dr Chris Preston, Chair of the Glyphosate Sustainability Working Group says these three weeds have some natural tolerance of glyphosate, particularly once they have some size, and some populations have been identified that are resistant to robust rates, even when applied to small, actively growing plants.

“All three species respond to warming temperatures in spring and can germinate late in-crop where they are difficult to control with herbicides,” says Chris. “Windmill grass is a short-lived perennial that can regrow from the crown, giving it a distinct advantage when soil moisture is limited.”
“Another trait that these species share is the short period of seed dormancy. This means that seed will germinate almost immediately after rainfall events in spring once the soil has started to warm and, once established, these plants are also very tolerant of heat and moisture stress.”
“Sowthistle can also germinate in autumn but these populations are easier to control as part of the seeding operation,” he says.
Dr Chris Preston says sowthistle, feathertop Rhodes grass and windmill grass are of concern in all Australian grain growing regions, not just the Northern region.
Why is sowthistle more of a problem in some years than others?
Short answer: Sowthistle seed does not persist in the soil for very long. More rainfall over spring and summer leads to larger populations.
Longer answer: Burying seed can extend the life of the seed so cultivation is generally not recommended. Sowthistle incursions often begin in non-crop areas where there is little competition. Sowthistle resistance to Group B herbicides is widespread, resistance to Group M (glyphosate) has been confirmed in the northern region and resistance to Group I herbicides has been confirmed in the southern region. This means that one or more of the common summer fallow herbicides do not work on many populations of sowthistle.
If these summer weeds are so hard to control with herbicides, what options do growers have?
Short answer: Herbicides can still play a part but applications need to be well-timed and survivors removed.
Longer answer: The key to control lies in preventing emergence to run down the seed bank. These weeds are not great candidates for harvest weed seed collection even though they are often present at harvest, because the seed is light and easily spread on the wind. They generally don’t perform well in competitive situations so they often become prolific in non-cropped areas like roadsides and fencelines. Avoid spraying these areas with glyphosate alone whenever possible and use non-herbicide options like slashing, pasture and hay making to prevent seed set.
Large sowthistle plants produce huge quantities of air-borne seed but the seed does not persist for long, giving growers the opportunity to intensively manage incursions and non-crop areas to keep weed numbers low.
Are feathertop Rhodes and windmill grasses really a problem outside the northern region?
Short answer: Feathertop Rhodes and windmill grass are closely related C4 species, which means they grow rapidly in warm, sunny conditions, so they are prevalent in the northern region. However, they are found on roadsides all over Australia – representing a considerable risk in all regions.
Longer answer: These two grasses produce seed that does not persist in the soil for very long, in fact all the viable seed shed germinates the following spring and summer. Like sowthistle, these grasses prefer to have no competition. Feathertop Rhodes incursions often begin in a patch around a mature plant and experience in the northern region suggests that intensive patch management is effective and worthwhile. The most effective control is a double knock strategy of glyphosate plus a Group A herbicide, followed by paraquat. As Group A herbicides are prone to resistance, use of Group A herbicide must be followed with a robust second knock.
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Growers response to increasing herbicide resistance?

with Rick Llewellyn, Farming Systems Researcher, CSIRO
A comprehensive study involving 602 Australian farmers has shone the light on the way grain producers are implementing practices to tackle the rising problem of herbicide resistance. Herbicide resistance is estimated to cost Australian farmers over $187 million per year in additional herbicide costs alone.
Dr Rick Llewellyn, CSIRO is a farming systems researcher interested in reducing the economic impact of weeds on Australian farms. He and a team of researchers and analysts conducted a two-part study that has shown how Australian farmers are responding to the increasing risk of herbicide resistance and implementing a larger number of weed management tactics to minimise weed seed set.

“Not surprisingly 83 per cent of growers identified annual ryegrass as their number 1 herbicide resistant weed,” says Dr Llewellyn. “Resistance and the extent of ryegrass nationally means that it is still the most costly weed for farmers to control, followed by wild radish, brome grass, wild oats and fleabane. However, the species and rankings vary across the 13 major agro-ecological zones in the GRDC Western, Southern and Northern grain growing regions.”
CSIRO farming systems researcher Dr Rick Llewellyn, says Australian farmers have responded to the presence and ongoing risk of herbicide resistance by using an increasingly wide range of practices.
This GRDC-supported research showed that weed control costs, rather than yield losses, account for about three-quarters of the total economic impact of weeds on Australian grain growing businesses. In-season herbicide costs, including application costs, are estimated to be $1590 million and chemical fallow costs are about $507 million nationally. A growing number of farmers are implementing non-herbicide weed control measures to reduce costs and keep weed numbers down.
What practices have replaced cultivation in no-till systems?
Short answer: Crop-topping, double knockdown and narrow windrow burning.
Longer answer: 42 per cent of growers (mainly in the GRDC southern and western regions) use crop-topping for weed control at a total cost of $29 million, which includes herbicide application costs and crop yield damage. Glyphosate resistance risk is driving the uptake of the double knockdown tactic which 61 per cent of growers implement at a cost of $97 million. While harvest weed seed control uptake remains lower there is an upward trend, particularly with narrow windrow burning becoming common in several regions. Currently 30 per cent of growers nationally use narrow windrow burning to destroy harvest weed seed but the survey indicated this is likely to increase to 46 per cent adoption in the next five years, despite low usage in the Northern GRDC cropping region. The surveyed growers expressed considerable interest in the in-board Harrington Seed Destructor (iHSD), that has since become a commercial reality, and also chaff-lining as potential harvest weed seed control tools. 26 per cent of growers continue to burn whole-paddock stubble primarily for weed management reasons.
Narrow windrow burning is currently the harvest weed management practice of choice on about 30% of farms nationally and adoption is expected to rise to about 46% in the next five years.
How are herbicides used in current no-till farming systems?
Short answer: Herbicides are still the major form of weed control.
Longer answer: 91 per cent of the cropped area is treated with a knock-down herbicide prior to seeding; a pre-emergent herbicide is applied to 74 per cent of the area at seeding and post-emergent selective herbicide is applied to 80 per cent of the cropped area. 64 per cent of growers indicated that they manage a herbicide resistance weed population and 17 per cent reported the presence of glyphosate resistant weeds on their farms. Surveyed growers were generally optimistic about the likelihood of new herbicide products being released in the next decade to assist with weeds that are resistant to current herbicides.
What are farmers doing to manage herbicide resistance?
Short answer: 94 per cent of growers indicated that they implement at least one IWM practice to manage herbicide resistance.
Longer answer: To manage herbicide resistance, there is increasing emphasis on practices that kill weed seeds. In addition, 36 per cent of growers use chemical rotation, 26 per cent use crop rotation and 19 per cent use livestock. Other practices include: double knockdown, pre-emergent herbicides, modifying current herbicide practices (higher rates and better applications methods), crop-topping or hay freezing, cultivation, burning, green and brown manure, hay and altering sowing times or density.
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Can ‘survivor’ weeds still be susceptible to glyphosate?

with Peter Boutsalis, Plant Science Consulting director and Adelaide University weed science team researcher
Annual ryegrass was the first cab-off-the-rank with resistance to glyphosate first recorded in Australia in 1996. In the last 20 years several other weed species have been added to the glyphosate resistant list but annual ryegrass remains the leader by far with over 600 resistant populations confirmed.
Dr Peter Boutsalis, Plant Science Consulting director and weeds researcher at Adelaide University has been in the thick of this unfolding dilemma, taking a leading role in testing for herbicide resistance and conducting random surveys across southern Australia to monitor the spread and evolution of resistance.

“We now know that 15 years of consistent use of glyphosate, without alternative weed control measures, will invariably lead to glyphosate resistance in ryegrass,” he says. “There are confirmed cases of resistant populations under many land management systems but we see the largest number of cases in winter cropping paddocks, along fencelines and beside roads.”
One area of particular interest to Dr Boutsalis is the fact that even if resistance is confirmed at a particular rate, sometimes the resistant plants will be effectively controlled with a higher rate.
Dr Peter Boutsalis conducts random weed surveys across the southern states testing weeds for herbicide resistance under a range of management and land use conditions.
“This doesn’t mean that we can keep increasing the rate indefinitely,” he says. “It just means that if the population is susceptible to a higher rate, this information can be used to form part of an integrated management response to escalating resistance.”
How can a resistant plant still be susceptible?
Short answer: The resistance mechanism must work hard to ‘protect’ the plant but it can be overloaded.
Longer answer: The target site mechanism in annual ryegrass works by pumping the herbicide to the leaf tips and restricting translocation. A plant may be able to sustain this pump at one rate but at a higher rate there may be sufficient ‘leakage’ to kill the plant and stop it setting seed.
What is a good first response to confirmed glyphosate resistance?
Short answer: Glyphosate resistant weeds are often not good competitors when glyphosate is not used. Research has shown many instances of a ‘fitness penalty’ that makes these ‘weak survivors’ less competitive and so they set less seed.
Longer answer: If it is necessary to continue using glyphosate, apply a higher rate of glyphosate initially to reduce plant numbers and then use a different herbicide MOA to further reduce the number of potentially resistant plants setting seed. For best results with this double-knock, apply the second mode of action herbicide, e.g. paraquat, within three days of applying glyphosate.
What’s the best way to manage hot spots like fencelines to avoid glyphosate resistance taking hold?
Short answer: Don’t leave spraying fencelines until spring – try to get in early and treat weeds when they are small.
Longer answer: Resistant ryegrass is more sensitive to glyphosate when the conditions are optimal – aim for small, unstressed weeds, optimal spray coverage, including sufficient adjuvant, and spray in the cooler part of the day (below 30 degrees C). Resistant pollen can quickly spread at least 50 m into the paddock. Ryegrass cross-pollinates and there is a low, but increasing, incidence of populations possessing both target site and translocation resistance mechanisms.
The 2015 random weed survey 2015 revealed an incidence of 9 per cent glyphosate resistance in annual ryegrass in the Wimmera region of Victoria. It is not possible to diagnose herbicide resistance in weeds in the field. The only way to know what herbicides particular weeds are susceptible to is to undertake scientific testing. The ‘Quick Test’ can be done on living plants and provides timely feedback on suitable herbicide options.
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How can I respond to emerging weed threats?

with Bhagirath Chauhan, Principal Research Fellow, Queensland Alliance for Agriculture and Food Innovation
Emerging weeds are generally weeds that have been present in an area but for some reason have not been of any great concern—until something changes with a run of wet or dry season, or a change in farming practice.
Dr Bhagirath Chauhan, Principal Research Fellow with the Queensland Alliance for Agriculture and Food Innovation is leading a project to study the characteristics of 10 weed species that are becoming increasingly important in the northern grains region of Australia.

“Growers in the region put forward the list of three winter weed and seven summer weed species, which they thought were of greatest concern,” said Dr Chauhan. “Over the next four years we will be studying the influence of rainfall pattern and cropping history on the seed dormancy and persistence of these species.”
The study will focus on better understanding the seed bank biology of sowthistle, turnip weed and Mexican poppy in the winter and feathertop Rhodes grass, windmill grass, liverseed grass, button grass, caltrop, bladder ketmia and sweet summer grass in summer.
“We want to know if these species are more or less of a potential problem in higher rainfall areas such as Dalby or drier areas such as St George,” he said. “We will also see if crop competition and additional resources, such as fertiliser, affect dormancy and germination patterns of these weeds.”
“Seed dormancy and persistence in the soil are very important aspects of weed ecology and can be used to manage weed population with both herbicide and non-herbicide measures.”
Dr Bhagirath Chauhan, Principal Research Fellow with the Queensland Alliance for Agriculture and Food Innovation is studying the ecology of ten emerging weeds in the northern grains region to help identify the most effective control tactics.
Why is information about seed dormancy and persistence important?
Short answer: These characteristics vary considerably between plant species but once understood they help identify management practices that will work best.
Longer answer: Seed dormancy is a measure of the conditions required for the seed to germinate. For instance, one species may not germinate until a certain soil temperature is reached while another species may respond to soil moisture or light. Persistence is a measure of how long a seed can remain viable in the soil. Some species remain viable for decades while others only a few months while awaiting the right conditions for germination.
How can I use this information to manage weeds?
Short answer: Look for ways to run down the seed bank.
Longer answer: For example, sow thistle has no dormancy—as soon as it rains the seed will germinate, making it easier to time an effective herbicide application. Mexican poppy shows strong dormancy over summer then germinated in a flush when it rained in autumn. Turnip weed has been considered a winter weed however we are seeing it germinate as early as February. Knowing the triggers for germination helps plan a control program.
Understanding the persistence characteristic of these weeds will take longer but will alert growers to the potential benefit of cultivation to control weeds that do not persist for long once the seed is buried or crop competition for weed seeds that do not remain viable for long on the soil surface.
Weed seeds are grown in semi-controlled field conditions to determine the effect of rainfall patterns and other environmental influences on seed dormancy and persistence.
Is turnip weed likely to be as big a problem for northern region growers as its Western Australian relative, wild radish?
Short answer: Possibly, but there are differences between the species.
Longer answer: Until the last few years turnip weed has been mainly found on roadsides rather than in cropping paddocks but it is now a common weed in crops. Like wild radish it produces vast quantities of seed however the seed is less dormant and less persistent in the soil. The seed coat still needs to rupture for germination to occur but fluctuations in moisture and temperature, along with insect damage means dormancy is readily broken down and turnip weed is germinating earlier in the year than expected. There have been cases of resistance to Group B herbicides in turnip weed so it is important to manage any survivors after applying these herbicides. A planned survey of 600 turnip weed populations will illustrate the current density of turnip weed and herbicide resistance testing against current herbicides will better inform researchers and growers.
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Controlling herbicide resistant ryegrass in canola crops?

with Gurjeet Gill, Associate Professor – Weed & Crop Ecology, University of Adelaide
The last in-crop herbicide option for the control of annual ryegrass in break crops is slipping away. Herbicide resistance testing in 2015 of weeds collected across the Eyre Peninsula in late 2014 has found that 7 per cent of the populations tested in the southern Eyre Peninsula are resistant to the Group A herbicide, clethodim.

Dr Gurjeet Gill, Associate Professor at University of Adelaide, says that the use of pre-emergent herbicides alone in break crops like canola is unreliable at best and the loss of the only post-emergent option would have dire consequences for break cropping in the southern and western regions.
Dr Gurjeet Gill, Associate Professor at University of Adelaide says that variety selection, crop competition, controlling survivors and spray application timing and technique all make a difference when it comes to extending the useful life of herbicides.
“Our Weed Science team has been investigating ways to protect this chemistry and extend its life,” he says. “Variety selection, crop competition, controlling survivors and spray application timing and technique all make a difference and need to be implemented as quickly as possible.”
“In a field where the ryegrass was known to be resistant to clethodim we tested three canola cultivars—an open pollinated cultivar, ATR-Stingray and two triazine tolerant hybrids—Hyola 559TT and Hyola 750TT against three herbicide treatments,” says Dr Gill.
“Where no herbicides were applied, the high biomass hybrid Hyola 750TT suppressed the number of ryegrass seed spikes by about 30 per cent through crop competition alone.”
When a regime of pre-emergent herbicide applications was applied, both hybrid cultivars suppressed spike production by about 50 per cent compared to the open-pollinated cultivar.
How does cultivar competitiveness help protect herbicide chemistry?
Short answer: It’s all about reducing weed seed numbers.
Longer answer: Crop competition minimises the survival of late germinating ryegrass and reduces seed production from surviving weeds. Choosing the most competitive cultivar available throughout the cropping sequence applies non-herbicide pressure to drive down the weed seed numbers. Early sowing of wheat with effective pre-emergence herbicide package increases wheat yields and reduces grass weed seed set, thereby reducing the pressure on grass control in the broadleaf crop phase.
How can I keep clethodim working and avoid resistance evolving?
Short answer: Use as many tactics as possible in each crop phase.
Longer answer: Clethodim is an effective herbicide in susceptible weed populations and provides another tactic to reduce seed set in-crop. Plan to use pre-em herbicides, choose competitive cultivars and implement one or even two harvest weed seed control tactics such as narrow windrow burning, crop topping or seed destruction to remove any weeds that escaped the clethodim treatment.
How can clethodim still be an effective control tactic in clethodim-resistant ryegrass populations?
Short answer: Application timing can have a significant effect on the efficacy of some herbicides, including clethodim.
Longer answer: Frost and weed size both affect clethodim efficacy. If resistance to clethodim is evident, applying the product to small (up to 3-leaf stage) annual ryegrass plants under warmer conditions improves the level of control. Wait for a couple of days after a frost for the ryegrass to be actively growing. Consider spraying when there is a cold front predicted as the cloud cover reduces the chance of frost. Combining clethodim and butroxydim and applying to small clethodim-resistant ryegrass plants can be an effective tactic. Note: frost does not affect the efficacy of clethodim in susceptible populations.
The timing of herbicide application influences the efficacy of herbicides—in this case, applying clethodim to ryegrass in frost-free conditions increased the efficacy of the herbicide on clethodim-resistant plants.
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The safest way to manage pre-em herbicides at seeding?

with Sam Kleemann, Research Associate, University of Adelaide
Achieving effective pre-emergent weed control while protecting crop safety at seeding is no mean feat, with plenty of variables to manage.

Dr Sam Kleemann, research associate at the University of Adelaide, has investigated seeding systems and pre-emergent herbicide combinations that give the best results under different circumstances.
“Effective early weed control is more important than ever, and pre-emergent herbicides have a big role to play, often delivering multi-generation weed control while the crop gets established,” he says. “The difficulty is that these herbicide products vary significantly in their solubility and their tendency to bind to organic matter, and this can lead to considerable variability in weed control efficacy and crop safety.”
Dr Kleemann says that understanding the way different products behave will influence the decisions growers and advisors make when selecting products and setting up their equipment for seeding.
Dr Sam Kleemann says seeder set up and product choice are pivotal decisions when using pre-emergent herbicides to control weeds early in the crop cycle. Photo: Alistair Lawson
“There are a few rules of thumb that can help at seeding,” he says. “The first is to minimise soil disturbance so that weed seeds remain on the soil surface as much as possible.”
“The second is to remember that pre-emergent herbicides can cause crop damage. It is essential to separate the product from crop seed.”
“The third rule is to choose the right herbicide for the job and follow label recommendations closely. There are many differences in the properties of pre-emergent products such as their volatility and their rate of degradation in sunlight.”
Tined seeding systems fitted with knife-points generally provide for better crop safety when using pre-emergent herbicide than low disturbance discs. Triple discs are safer than single disc seeders provided adequate soil throw is achieved.
Why is minimising soil disturbance at seeding so important?
Short answer: Weed seeds on or very near the soil surface have the most exposure to the concentrated band of herbicide.
Longer answer: If weed seeds are buried and mixed through the soil they have a greater chance of avoiding the herbicide than when they are concentrated on or near the surface. If a weed seed germinates and establishes a root system before intercepting the pre-emergent herbicide, the weed is often able to grow through the herbicide layer. While it may be suppressed, it probably will not die.
What is the best way to keep herbicide away from the crop seed?
Short answer: Follow the label instructions carefully. Pre-emergent herbicides are mostly non-selective and will cause crop damage if they are thrown or washed back into the furrow.
Longer answer: Seeding systems that throw soil away from the furrow provide the greatest protection for the crop. Knife-point seeders generally provide sufficient herbicide incorporation while maintaining separation from the crop seed. Avoid excessive soil throw from one furrow to the next. Pre-emergent herbicides are not recommended for use with low soil disturbance disc seeders due to the unreliability of herbicide separation from crop seed and increased risk of crop damage.
Why do the label instructions for pre-emergent herbicide products vary so much?
Short answer: Each product has different properties that impact on efficacy and crop safety.
Longer answer: Properties such as solubility, need for incorporation, binding on organic matter, rate of breakdown in the environment and so on make a big difference to how a herbicide is used. Use this information to make product choice decisions that take into account the stubble load, short-range rainfall forecast, sowing date and seeding equipment and set-up.
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