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Agronomists, researchers, growers answering important questions

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Does delayed sowing help manage weed populations?

The answer to this question is a very simple ‘no’. Waiting for weed seeds with longer dormancy to germinate before sowing costs yield and weeds often set more seed in late sown crops.  
Dr Gurjeet Gill, Associate Professor of Weed and Crop Ecology at The University of Adelaide says sowing a competitive crop ‘on time’ has better outcomes for both crop yield and suppressing weed seed production.
Dr Gurjeet Gill, Associate Professor of Weed and Crop Ecology at The University of Adelaide is one of the four experts presenting the new WeedSmart Crop Competition 101 online course.
“The lack of effective in-crop herbicides leaves growers with few chemical options when it comes to controlling weeds like annual ryegrass and brome grass that are emerging later in the crop,” says Gurjeet. “Our field trials in South Australia demonstrated that sowing ‘on time’ is the best way to maximise crop yield and suppress weeds that germinate in-crop, both with and without the use of pre-emergent herbicides.”
The time of sowing x seeding rate x herbicide field trials were conducted at several sites in South Australia in 2018 and 2019 with GRDC investment.
“The other aspect of these trials was investigating the effect of seeding rate on weed density and seedhead production,” says Gurjeet, “Higher seeding rate increased the yield in wheat at Minnipa at both times of sowing, and did not increase screenings.”
Early sown crops consistently produce more crop and less weeds.
Dr Gill is one of the presenters in WeedSmart’s new Diversity Era ‘Crop Competition 101’ course, which can be completed online in less than 10 hours, giving you a solid grounding in ways to effectively implement tactics that give crops a competitive advantage over weeds.
This free online course can be found at

Why are weeds in cropping systems becoming more dormant?
Short answer: Increased cropping intensity and routine use of pre-emergent herbicides selects for the longer dormancy trait in annual ryegrass and brome grass.
Longer answer: Weed populations have a mix of individuals with different levels of seed dormancy. In self-regenerating pastures, there is likely to be a penalty for high seed dormancy and germinating later than the neighbouring plants. Therefore, high dormancy late germinating plants remain a minority in the population.
However, the situation changes when growers switch to intensive cropping where knockdown herbicides routinely kill the very early germinating plants. In such systems, weeds that emerge with or soon after the crop have a greater survival because they escape the effects of the knockdown herbicides. After several years of cropping, weed populations change from being early germinating to later germinating. These later germinating weed populations are less responsive to delayed sowing and are now common in southern farming systems.
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Get the best results using pre-emergent herbicides

with Chris Preston, University of Adelaide and Mark Congreve, ICAN
Resistance to glyphosate and to a range of post-emergent herbicides is driving the increased use of pre-emergent herbicides in Australian cropping systems, but getting a good result with these herbicides is not always straightforward.   
To assist growers and agronomists, Dr Chris Preston, Professor, Weed Management at The University of Adelaide and Mark Congreve, Senior consultant with Independent Consultants Network Australia, have shared their expertise in the online Diversity Era course ‘Pre-emergent Herbicides 101’.
Presenters Mark Congreve (left) and Dr Chris Preston (centre) with WeedSmart extension agronomist Peter Newman deliver the Pre-emergent herbicides 101 course.
“In the southern farming systems, resistance to in-crop herbicides has been the main driver for the increased use of pre-emergent herbicides,” says Chris. “When pre-emergent herbicides are used to provide early weed control in a competitive crop, the amount of weed seed set in-crop can be vastly reduced.”
In northern cropping systems the main driving force for increased use of pre-emergent herbicides is found in the fallow periods where a rapid increase in glyphosate resistant and glyphosate tolerant weeds in the last five years leaves no-till farmers with few weed control options.
“Northern growers are looking to add pre-emergent herbicides to their fallow management program and cotton and sorghum growers are using lay-by applications of pre-emergent herbicides to control weeds in the inter-row,” says Mark. “The use of pre-emergent herbicides in these situations requires extra care and planning to avoid crop damage and to keep future cropping options open.”
WeedSmart’s ‘Diversity Era Pre-Emergent Herbicides 101’ course can be completed in less than 10 hours, giving you a solid grounding in the basics of how pre-emergent herbicides work and how to get the best results in winter and summer cropping systems.
Follow the link to the free, online Diversity Era Pre-emergent Herbicides 101 course.
Are there different pre-emergent herbicides suited to different situations?
Short answer: Yes. A key difference between pre-emergent herbicide products is their mobility in the soil, which is largely driven by soil type, chemical solubility and level of absorption to soil and organic material.
Longer answer: All pre-emergent herbicides need to reach the soil and create a band of treated soil around the weed seeds to reduce germination. In high stubble situations, and especially where the stubble is laying flat on the soil surface, the first challenge is to get the herbicide through the stubble layer and into the soil. Some herbicides wash off stubble better than others. Products like trifluralin are generally not an effective option in high stubble situations.
In very high residue years growers may decide to burn paddocks prior to sowing and applying pre-emergent herbicide. The herbicide will not bind to the ash, but sometimes there is a layer of chaff left after a stubble burn, and this can still be a barrier to the herbicide reaching the soil.
Once in the soil, some pre-emergent herbicides move more quickly down the profile than others. For example, S-metolachlor (one component of Boxer Gold) and Sakura both have a ‘medium’ rating for binding to soil and organic matter, but quite different solubility ratings. S-metolachlor is reasonably soluble and reaches the weed seeds in the topsoil easily, while Sakura has low solubility, which can mean that in dry years the herbicide might not reach some deeper weed seeds before they germinate.
However, in high rainfall zones or in wet years s-metolachlor is likely to move further horizontally and vertically in the soil profile, increasing the likelihood of coming into contact with the crop seed.
What else do I need to have in place to support pre-emergent herbicides in-crop?
Short answer: Crop competition, harvest weed seed control and double breaks all help make the most of pre-emergent herbicide applications.
Longer answer: Relying on pre-emergent herbicides only is not a good idea in weedy situations. If weed numbers are high it is essential to reduce the weed seed bank using other tactics first. Pre-emergent herbicides are most effective when used to provide early weed control in competitive crops where the crop itself can further suppress weed growth and seed set later in the season.
The extensive and long-term use of in-crop herbicides in southern grains regions has selected for longer dormancy in weeds like annual ryegrass. As a result, growers can make significant gains through sowing early into warmer soils to promote vigorous early crop growth and using a pre-emergent herbicide either incorporated by sowing or, for certain herbicides, applied soon after planting. If soil conditions are warm and moist at sowing it is very important to incorporate the herbicide quickly and realise that microbial degradation can be more rapid in this situation, making strong crop competition even more important.
How does chemical degradation impact on pre-emergent herbicides when used in a summer fallow?
Short answer: Pre-emergent herbicides primarily degrade, or ‘run out’, due to microbial breakdown in the soil. Photodegradation (UV light exposure), volatilisation, leaching and hydrolysis may also be significant loss pathways for certain herbicides.
Longer answer: Soil temperature and moisture are the key drivers for microbial degradation of pre-emergent herbicides. These factors need to be considered as a risk when using pre-emergent herbicides with long plant back periods and the possible impact on rotational crop options if there is insufficient rainfall to allow breakdown of the herbicide.
Sorghum seedling exhibiting symptoms of pre-emergent herbicide damage (S-metolachlor, in association with short term waterlogging). Photo: M Congreve
On the other hand, some pre-emergent herbicides applied soon after harvest may not provide full-season fallow weed control in high rainfall years.
The frequent use of a single pre-emergent herbicide may favour the build-up of the suite of microbes that degrade that herbicide. Over time, pre-emergent herbicide efficacy can ‘run out’ quicker than it did when it was first used.
When applied in a no-till fallow, the pre-emergent herbicide may sit on the soil surface for a period of time awaiting rainfall for incorporation. To minimise losses due to UV light and volatilisation it is important to follow the label instructions for the timing of incorporation by rainfall, irrigation or tillage.
Follow the link to the free, online Diversity Era Pre-emergent Herbicides 101 course.

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Does pre-emergent herbicides help combat alphabet-resistant ryegrass?

with Chris Preston, University of Adelaide
The new decade is bringing with it some truly new pre-emergent herbicides with activity on annual ryegrass. With multiple-resistance to current pre-emergent herbicides looming large, it is essential that plans are put in place to ensure that these new herbicides can do the job of keeping ryegrass numbers low well into the future.  
Dr Chris Preston, Professor, Weed Management at The University of Adelaide has watched the multiple-resistance and cross-resistance story unfold through the weed surveys and grower submissions to the testing services across southern Australia.
Dr Chris Preston, University of Adelaide, says the recent and pending releases of a suite of new pre-emergent herbicides will broaden the options for growers to mix and rotate through the crop sequence, but they will not fix the ryegrass problem on their own and should not be considered the new ‘go-to’ products.
“Annual ryegrass is well-known for its ability to evade different herbicide modes of action,” he says. “In recent years we have found a number of populations that have resistance to pre-emergent herbicides in Groups J, K and D.”
The herbicide resistance ‘picture’ in Australia is based on randomly-selected weed samples collected during official weed surveys and samples that growers or agronomists send in for testing, often following an apparent failure of a herbicide in the field.
“Through herbicide resistance testing we are finding that an increasing number of ryegrass populations have ‘alphabet resistance’, that is, resistance to multiple modes of action,” says Chris. “That doesn’t mean that none of the herbicides work, some will still have good efficacy in the field, but possibly not for long. It is a clear indication that the ‘mix and rotate’ strategy must be implemented across the crop sequence.”
Two recently-released pre-emergent herbicides (in Groups K and Z) and two more (in Groups Q, E) in the final stages of registration with release expected in 2020 or 2021, will broaden the options growers have to keep ryegrass numbers down.
Coupling the mix and rotate strategy for pre-emergent herbicides with strong crop competition is a winning combination within the WeedSmart Big 6.
What’s the difference between multiple-resistant and cross-resistant weeds?
Short answer: Multiple resistance is due to multiple genes present in the plant conferring resistance to several herbicides while cross-resistance involves a single gene conferring resistance to several herbicides.
Longer answer:
Multiple-resistance is where a weed possesses multiple resistance mechanisms that allow it to evade several different herbicide modes of action. Mixing and rotating between the available herbicides is the best option as the population will have varying levels of resistance and some herbicides will still be effective.
Cross-resistance is where a weed possesses a single resistance mechanism that enables it to evade multiple herbicide modes of action. Cross-resistance can make a weed resistant to a herbicide that it has never been exposed to. For example, resistance to Groups J and K appears to be genetically linked in some populations, with examples of Group J use selecting for resistance to Group K, and vice versa. In this situation, rotating between Groups J and K will not be sustainable. A broader herbicide strategy will be required to keep these two groups as viable options in the farming system.
Resistance to one herbicide in a MOA group does not mean the population is resistant to all herbicides in that group. For example, in Group D resistance to trifluralin is quite widespread while resistance to propyzamide, also Group D, is rare and populations can usually be controlled by using the full rate of propyzamide. Similarly, resistance to Group K Butisan is being seen in the field while Sakura, also Group K, is still effective.
The term ‘alphabet resistance’ covers all populations that have resistance to several herbicides – often both pre-emergent and post-emergent use patterns.
What are the new herbicides?
Short answer: Luximax® (cinmethylin, Group Z) is now registered and available for use in 2020, carbetamide (Group E) is approved and a Group Q active is pending approval from the APVMA. They follow the recently-released Group K herbicide Devrinol-C® (napropamide) that belongs to a unique chemical class (acetamides) within Group K.
Longer answer: The release of these new pre-emergent herbicides will broaden the options for growers to mix and rotate through the crop sequence. They will not fix the ryegrass problem on their own and should not be considered the new ‘go-to’ products. Annual ryegrass across Australia has been exposed to herbicides over a long period of time and populations can be expected to possess multiple resistance mechanisms.
In the face of increasing cross-resistance in annual ryegrass it is conceivable that some weed populations may challenge these new herbicides as a result of previous exposure to other herbicides. This highlights the importance of using the right mixing partners to ensure these new herbicides can be effective tools for ryegrass control.
Annual ryegrass has been exposed to many different herbicides and there are now populations that have ‘alphabet resistance’, that is resistance to multiple modes of action. Seed testing can reveal what products still work. Photo: CSU
How do I set up an effective mix and rotate strategy for pre-emergent herbicides?
Short answer: Get some resistance testing done on seed from ‘survivor’ ryegrass plants to see what still works; and boost crop competition.
Longer answer: Knowing what works is the first step. This involves collecting ryegrass seed and running tests with multiple herbicides. This can be done every five years or so as resistance to pre-emergent herbicides evolves relatively slowly.
Once you know what the pre-emergent options are, look for opportunities to mix and rotate the herbicide groups throughout the crop sequence. If there are limited options for one crop, be sure to ‘save’ those options for exclusive use in that crop. Where there are opportunities to mix pre-emergent herbicides, take them. Always apply the mixing partners at full label rates.
Growing a competitive crop is an important tactic in maintaining low weed numbers and delaying resistance to pre-emergent herbicides.
Find out more:

Understanding alphabet resistant ryegrass (GRDC Update)
WeedSmart Wisdom – Herbicide resistance testing services
Competition reduces weed seed set in wheat and canola

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Weed detection and site-specific control update

with Greg Condon, WeedSmart southern agronomist
The power of the WeedSmart Big 6 really ramps up when the tactics are applied in a site-specific way. The broad-brush approach of herbicide application has played a big role in the development of herbicide resistance and has effectively changed the weed spectrum on many farms in favour of weeds that are hard-to-kill using least-cost herbicide options.
Greg Condon, WeedSmart agronomist based at Junee, NSW has seen the benefits that growers have won through the use of optical sprayer technology over the last 20 years and is keen to see the development of new technologies that will be commercially available in the foreseeable future.
Greg Condon, WeedSmart agronomist based at Junee, NSW has seen the benefits that growers have won through the use of optical sprayer technology over the last 20 years and is keen to see the development of new technologies that will be commercially available in the foreseeable future.
“Being able to identify weeds and apply a treatment that is going to be the most effective is the best way forward,” he says. “If the treatment applied kills the weed and prevents seed set then eradication of the weed – herbicide resistant or not – is within reach.”
Once the location of the weeds is known it is possible to use robust herbicide rates and devise mixtures that are most effective on the weeds present, knowing that the volume of product applied will be low and therefore an economical proposition.
“The mapping technologies are developing fast and many are already commercially available,” says Greg. “In some cases satellite imagery is useful to identify weedy areas and to estimate areas and in other cases drone imagery is more useful. The NDVI sensors mounted on tractors can also provide good information about the presence and density of weeds.”
The site-specific weed control technology arena is pretty crowded at the moment with many companies and research bodies working hard to find the ultimate commercial products, but it won’t be long before there are paddock-ready options available to growers for both herbicide and non-herbicide control tools. Many of the leading ag-tech companies gave updates of their products and services at the 2019 WeedSmart Week events at Horsham and Emerald.
What’s the best way to identify and map weeds or weedy patches?
Short answer: Satellite or drone imagery.
Longer answer: Satellite imagery gives a broad view of the weeds present in a paddock, particularly in a fallow situation. Recent personal experience showed that ground truthing satellite maps using NDVI filters was not very successful.
Using a drone to collect the imagery is a practical option that offers more flexibility but the images collected need to be stitched together to create a single image for analysis. Some growers are effectively using their own systems but there is also a commercial option through providers such as Jerome Leray at In-Farm.
Getting accurate maps that can turn your current sprayer into a spot-sprayer is the closest-to-reality technology in site-specific weed control. When combined with the rapidly moving development of software to identify individual weed species, growers expect to achieve significant reductions in product volumes and take advantage of more expensive but effective selective herbicides.     
Using a drone to collect the imagery is a practical option that offers more flexibility but the images collected need to be stitched together to create a single image for analysis.
What are the herbicide delivery platforms closest to commercial operation?
Short answer: Robots and automated tractors are now giving a new angle to older technologies and artificial intelligence software is rapidly developing to support integrated herbicide sprayers.
Longer answer: Using an automated tractor or a robot, such as a SwarmBot, to expand the use of optical sprayer technologies opens up the possibility of spraying more often and continually targeting small weeds. These platforms could also adapt to new technologies as they are developed.
The reliability and practicality of green-on-green spraying is still a little way off but there is significant investment going on with ag-tech companies such as Bilberry and AgriFac teaming up to build a sprayer with wide-ranging capabilities.
While the green-on-green technology is a not yet widely or economically available, an interim option is a camera-guided shielded-sprayer. Being camera-guided, sprayers like CropStalker can operate in crops like lentils that do not have the plant architecture to allow the use of conventional trailing shields. These sprayers currently apply a band of herbicide so they are not weed-detecting, but they are still able to target weeds growing in-crop and using lower volumes or more selective herbicides. There is a possibility that these sprayers could also have cameras fitted to spray only when weeds are present.
While the green-on-green technology is a not yet widely or economically available, an interim option is a camera-guided shielded-sprayer like CropStalker.
And the ‘old’ optical spot sprayers are also updating, WeedSeeker 2 now has on-board weed mapping capability while WEEDit have upgraded their sensor accuracy and capability to analyse data. On-board mapping capability allows the sprayer to ‘remember’ where weeds were sprayed early in the fallow, enabling the operator to apply residuals to known weedy patches while also applying a knock-down to kill existing plants prior to planting.
What about delivering non-herbicide options?
Short answer: Although there are a few non-herbicide tools being developed, most are likely to take a while to come to market.
Longer answer: The problem with many non-herbicide options, such as microwave and laser, has been that they are high energy when applied in a broad band, but the energy required is cut by over 90 per cent when applied in a site-specific way.
SwarmFarm have demonstrated the possibilities of microwave units on a robotic platform that also allows for the time required to deliver the necessary treatment.
The WeedChipper is another option that uses sensors to detect weeds and activate a tyne to physically remove weeds. Once developed, this tool has great application in situations where escapes have developed into large plants where herbicide is not the best option, or to manage weeds that are difficult to kill using current herbicides.
An overseas camera-guided cultivator, like the camera guided shielded sprayer, is also available and suitable for reducing weed pressure in-crop.
More information:

AHRI Insight Thermal weed control – just hot air or site-specific reality?

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Implementing the ‘mix and rotate’ strategy to combat herbicide resistance

with Tony Lockrey, consulting agronomist, AMPS Moree
Mixing and rotating herbicide modes of action is a key strategy in the WeedSmart Big 6 – but it’s a herbicide response to a herbicide problem. So, while it’s critical, it must be implemented within a diverse weed management program.
Tony Lockrey, senior agronomist with AMPS Agribusiness at Moree has seen herbicide resistance get out of control on some farms in northern NSW while other growers have responded early and managed to maintain a broader spectrum of effective herbicides in their program.

AMPS Moree consulting agronomist Tony Lockrey has seen good results when herbicides are rotated and mixed in each phase – the fallow, pre-sowing, in-crop and for desiccation.
“It has to start with herbicide resistance testing – specifically for susceptibility,” he says. “Knowing what does work is very important as you’ve probably already got a fair idea about what doesn’t.”
Once all the effective actives are ‘on the table’ it’s time to look at what crops can be grown to allow the use of the widest range of herbicide groups in the rotation, and where you might be able to find synergistic mixes that can further delay resistance and potentially allow the use of actives that are no longer effective on their own.
“When we sit down to plan out an integrated weed control program we want to make sure there is rotation and mixing going on in each phase – in the fallow, pre-sowing, in-crop and for desiccation, where required,” says Tony. “When this is done in conjunction with a determination to stop seed set and remove survivors then it is possible to keep weed numbers low.”
With an increasing number of proprietary herbicide mixes coming onto the market and the broad spectrum of synergistic and antagonistic interactions between potential mixing partners it pays to be well-informed and to seek advice.
If I already rotate modes of action why do I have to mix too?
Short answer: Rotation buys you time; mixing buys you shots. Mixing and rotating buys you time and shots.
Longer answer: Rotation of effective modes of action can significantly delay the onset of herbicide resistance and needs to be built into your crop rotation plan. Herbicides in Group A and Group B are particularly susceptible to multiple exposure resistance with as few as six exposures being enough to select for the resistant mutation.
By mixing MOA groups, either in the same tank mix or applied separately to the same population (like a double knock), those plants that survive one MOA are often killed by the second.
How does testing for susceptibility help when there’s a weed blow-out?
Short answer: Knowing what will work against a resistant population helps drive down the seed bank and helps you regain control.
Longer answer: One real-world example is a paddock near Moree where Group A resistant wild oats were discovered in 1998 following a history of repeated use of Topik® (Group A – fop), Verdict® (Group A – fop) and, later, Axial® (Group A – den). Testing of this population showed the wild oats was very susceptible to Group B sulfonylurea, so Atlantis was used to drive down the weed numbers. A new plan was then put in place with Groups B, A, C and M used across the winter cropping program, but there was still too much reliance on Group B. The current plan for the farm now includes pre-emergent herbicides from Groups K, J and D used individually and in mixes.
How do I integrate more mixes into my herbicide program?
Short answer: Look for opportunities for synergistic mixes throughout the fallow and cropping seasons. In many instances the most important mixing partner is more water.
Longer answer:
Many growers are looking for tank mixes to improve control of glyphosate-resistant seedlings. Knowing which mixtures are beneficial and which are antagonistic is important.
In the fallow, there are often opportunities to use the mix and rotate strategy to great effect in a double-knock application, such as:
Group M (glyphosate) + Group I (2,4-D or fluroxypyr or picloram) followed by Group L (paraquat)
Group M (glyphosate) followed by Group L (paraquat) + Group G (Sharpen® or flumioxazin)
Group M (glyphosate) followed by Group L (paraquat) + Group K (Dual® Gold)
Group A (Shogun®) followed by Group L (paraquat) + Group K (Dual® Gold)

The fleabane on the right was unresponsive to glyphosate on its own but mixing picloram with triclopyr or 2,4 D to the glyphosate application was effective (left).
Pre-plant examples include paraquat plus a triazine herbicide (Group C) or paraquat plus an imidazalinone (Group B), which are commonly used to provide broad spectrum knockdown and residual control. Dual® Gold (Group K) is another common fallow residual option which is very compatible with glyphosate, triazines and paraquat.
An example of an in-crop mix is the addition of clethodim to haloxyfop (both Group A) to improve control of fop-resistant grasses in broadleaf crops where both are registered.
At the end of the season there is also some opportunity to mix desiccants for some crops.
None of these mixes are provided as recommendations – seek advice for your own situation and always read and follow the label.
What about application set up for mixtures?
Short answer: Some herbicides require better coverage. In many instances the most important mixing partner is more water.
Longer answer: Suitable product and water rates, droplet size and the right adjuvant, are critical for optimising herbicide efficacy.
For example, while a fallow mix such as glyphosate plus a Group A, or a Group G (depending on the target weed), is physically compatible, the components have different requirements for optimal performance. Seek advice about the best water rate to use, the potential impact of an oil-based adjuvant (required for most Group A and Group G herbicides) on glyphosate efficacy for some summer grass weeds, and other possible risks.
Factsheet – Mixing knockdown partners with Group G
How do I avoid generating multiple and cross-resistance?
Short answer: Implement as many different weed control strategies as possible. The WeedSmart Big 6 is a practical foundation for an integrated program of herbicide and non-herbicide tactics.
Longer answer: Rotating and mixing herbicide groups can give you room to move in holding off resistance or getting more out of some marginally effective products.
The only way to stave off herbicide resistance completely is to have low weed numbers and to be vigilant about preventing survivors from setting seed. Have a diverse cropping program, use herbicides to provide early weed control, set your crops up to compete strongly and monitor and remove survivor weeds.

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What’s the best way to out-compete resistant annual ryegrass in cereals?

with Chris Davey, Agriservices Agronomist, YP AG
Often regarded as the ‘poor cousin’ to herbicides, crop competition is making a come-back as a simple way for growers to increase crop yield and reduce weed seed set, without breaking the bank.
Chris Davey, partner and director of YP AG at Kadina has worked with growers on the Yorke Peninsula of SA for over 20 years in an on-going battle with weeds such as annual ryegrass, brome grass and bifora, and mounting resistance to herbicide.

YP AG agriservices agronomist Chris Davey says several Yorke Peninsula growers have adopted east-west sowing after seeing the benefits of competitive crops combined with pre-emergent mixes in a recent trial.
“Working with our grower group we have demonstrated that stacking crop competition tactics at sowing really does make a difference,” he says. “When you add an effective pre-emergent herbicide to the top of the stack to provide early weed suppression, the resulting increase in yield and reduction in weed pressure definitely puts money in the bank.”
In the 2018 trial, two cultivars of wheat (Scepter and Emu Rock) and barley (Compass and Spartacus) were sown into lentil stubble. The trial compared the performance of these four cultivars when sown east-west v north-south, and with a range of pre-emergent herbicide packages.
“The result was clear – when you plant a competitive crop like barley in fertile soil – such as following lentils, row orientation doesn’t make much difference, but if you plant a poorly-competitive crop like wheat, it really pays to stack as many things in its favour, including east-west sowing and an effective pre-emergent herbicide,” says Chris. “In wheat we measured a consistent 0.5 t/ha yield gain through east-west sowing across the two cultivars and pre-emergent herbicide packages. In barley, stacking a premium pre-emergent herbicide mix onto an already-competitive crop boosted yield by 1.1 t/ha and reduced ryegrass plant numbers ahead of the following seeding by over 80 per cent, compared to the least competitive, nil pre-emergent barley treatment.”
With harvest weed seed control already adopted by many growers on the Yorke Peninsula, the results of Chris’ trial has prompted the adoption of even more WeedSmart Big 6 tactics to tackle herbicide resistance head-on.

In the trial, the package of competitive crops and cultivars plus east-west sowing plus a premium pre-emergent herbicides mix combined to make an impressive difference in annual ryegrass numbers while also producing more grain.
What is the effect of crop competition on weed seed production?
Short answer: Crop competition makes a massive difference to weed seed production in-crop.
Longer answer: Ahead of this trial in 2018 the weed seed potential of the site was calculated as 87,000 annual ryegrass seeds/m2. The annual ryegrass population was known to be 100 per cent resistant to trifluralin (e.g. Treflan), 50 per cent resistant to triallate (e.g. Avadex) and 30 per cent resistant to prosulfocarb + s-metolachlor (Boxer Gold).
Applying crop competition plus pre-emergent herbicide drove this number down to around 4000 seeds/m2leading into the 2019 cropping season. The owner of the trial site chose to cut the surrounding crop for hay to prevent further blow-out of the annual ryegrass population. Although the crop competition plus pre-emergent herbicide package made a vast difference to weed seed production it is not a one-year fix for a ryegrass blow-out. It is important to keep the pressure on.
After seeing the results of the trial, several members of Chris’ grower group immediately adopted east-west sowing on paddocks where the change was practical. It is understood that it is often necessary to sow according to land type, such as on dune swale paddocks, or other constraints, such as established CTF systems that run north-south. Where changing row orientation to east-west is not possible there are still many other ways to boost crop competition.

Left: No crop competition, just solid annual ryegrass. Middle: Least competitive treatment (N/S sown non-competitive wheat variety – Emu Rock, with standard pre-emergent – trifluralin + triallate). Right: Most competitive treatment (E/W sown competitive barley variety – Compass, with premium/stacked pre-emergent – Boxer Gold + triallate).
Where did the reduction in weed seed production come from?
Short answer: Stacking competition tactics and pre-emergents in barley reduced ryegrass weed seed set by over 80 per cent.
Longer answer: Changing from a less competitive (Spartacus) to more competitive (Compass) cultivar reduced ryegrass plant numbers at seeding in 2019 by 13 per cent on north-south orientation and 24 per cent on east-west orientation, with no pre-emergent applied. Keeping the cultivar the same and changing row direction reduced weed numbers by 26 per cent in Compass and 16 per cent in Spartacus. This suggests that changing to east-west sowing will not achieve much in barley unless a more competitive cultivar is chosen. Changing both the competitiveness of the cultivar and the row orientation achieved a very useful reduction in weed numbers of 34 per cent (without using any pre-emergent). The result may be even greater in a more common rotation where barley is planted after wheat and the initial soil nutrient status is less than after lentils.
When you add a standard pre-emergent mix (trifluralin plus triallate) to the east-west sown Compass, weed numbers are driven down by 50 per cent. Using a premium mix of (Boxer Gold plus triallate) achieved an 82 per cent reduction in annual ryegrass plants going into the 2019 season. This is particularly impressive given the field’s known resistance to the applied pre-emergent chemistry and highlights the value of stacking pre-emergents together and growing a competitive crop.
What impact did the treatments have on yield?
Short answer: East-west sowing increased wheat yield in this trial, probably due to extra competition at a very weedy site.
Longer answer: In wheat there was a consistent 0.5 t/ha yield gain through east-west sowing across the two cultivars and pre-emergent herbicide packages. In barley, stacking a premium pre-emergent herbicide mix onto an already-competitive crop boosted yield by 1.1 t/ha compared to the nil pre-emergent, north-south treatment.

E/W sown barley (left) v N/S sown wheat (right).
Why worry about crop competition if there are new pre-emergent herbicides coming to market?
Short answer: The new herbicides will provide another useful tool for growers but are not the answer on their own.
Longer answer: The choice of pre-emergent herbicide should be the final decision after you have stacked as many crop competition tactics as possible.
Look for the most competitive combination of crop species/cultivar, row spacing, seeding rate, row orientation, sowing time for early vigour and healthy soil, then add a pre-emergent that is known to be effective. If the crop competition is strong then the pre-emergent just needs to provide the early weed suppression that gives the crop a head start.
Strong competition plus a current premium pre-emergent package (Boxer Gold plus triallate) performed as well as the ‘experimental’ pre-emergent products in this trial.

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How can I avoid getting stuck in an imi herbicide cycle?

with Sarah Wilson, Senior Technical Services Specialist, BASF
Group B herbicides are very handy tools in the weed control toolkit, but weeds can evolve resistance relatively quickly to their mode of action. To keep these herbicides as an option, and to maximise the benefits of imi-tolerant crops, it is essential that they are used correctly within farming systems.
Senior Technical Services Specialist with BASF, Sarah Wilson says it is easy for growers to get caught up in the imi-cycle of using imi-tolerant crops to avoid plant-back issues with imi residuals in the soil.
Senior Technical Services Specialist with BASF, Sarah Wilson says it is easy for growers to get caught up in the imi-cycle of using imi-tolerant crops to avoid plant-back issues with imi residuals in the soil.
“The problem arises when an imi-tolerant crop is sown to avoid imi-residues from the previous crop or fallow, but then the grower also wants to use the imi chemistry in the crop,” she says. “This leads very quickly to over-use of the imi herbicides, and research has shown that as few as four applications of group B herbicides (to which imi herbicides belong) to the same population of weeds can result in the selection of resistant individuals, so resistance can evolve within a very short period of time.”
In Australia there are four imidazolinone or ‘imi-type’ active ingredients registered to control a variety of grass and broadleaf weeds in crops and fallow. These actives are imazamox (e.g. Intervix®*, Raptor®), imazapic (e.g. Bobcat I-Maxx®*, Flame®, Midas®*, OnDuty®*), imazapyr (Arsenal Xpress®*, Intervix®*, Lightning®*, Midas®*, OnDuty®*) and imazethapyr (Lightning®*, Spinnaker®). The other types of herbicides in Group B are the pyrimidinylthiobenzoates, sulfonylureas (SUs) and triazolopyrimidines herbicides. They all inhibit the plant’s production of specific essential proteins.
Use the WeedSmart Big 6 to develop an integrated weed control program that keeps Group B herbicides as a viable option well into the future.
*products that contain more than one active.
How do Group B herbicides work?
Short answer: The Group B herbicides, including the imis, interfere with the activity of the ALS enzyme that is used in the production of certain essential plant proteins.
Longer answer: The Group B mode of action is to inhibit the production of the acetolactate synthase (ALS) enzyme in the plant cells. This enzyme is needed to produce essential plant proteins. By inhibiting ALS production, a foliar herbicide application causes the plant to deplete its supplies of the essential proteins and the plant will slowly die, often taking about three weeks. Group B herbicides with residual activity inhibit the production of amino acids so the plant uses up the reserves in the seed as it germinates and is exhausted before it breaks through the surface of the soil.
Whether using Group B herbicides as a pre-emergent, or post-emergent application; consider the use of registered tank mixes with herbicides from other modes of action.
What conditions do Group B herbicides need to work best?
Short answer: Small weed size is critical for effective foliar application. Imis will not kill older weeds, so applying these herbicides to large weeds is a waste of money.
Longer answer: ALS concentration is highest in young plant tissue and so foliar Group B herbicides are most effective when plants are small and actively growing. When plants are moisture-stressed there will be reduced uptake and translocation of foliar-applied imis.
Foliar uptake of imis is very sensitive to high temperatures. In summer, temperatures in the 30s will require much more active ingredient for the same level of control achieved at lower temperatures. Follow the label instructions.
There is a wide range of soil characteristics and environmental conditions that affect the efficacy of soil-applied Group B herbicides.

What are recommended Group B use patterns?
Short answer: Apply no more than two (2) Group B herbicides in any four (4) year period on the same paddock, and choose the right product for the situation.
Longer answer: A Group B herbicide application in either a summer crop or summer fallow is equivalent to a winter crop pre-emergent application, so no further Group B applications should be made in that paddock, that year.
Use Group B herbicides strategically, if you use imazapic (Flame) in the summer fallow and Ally, Logran, Atlantis or Intervix (for imi-tolerant cereals) over the top of your cereal crop in the winter, you then need to wait three years before using any other Group B chemistry in that paddock.
If you are planting imi-tolerant varieties to get around an imi residue problem, do not use imi chemistry over the top – it’s not good practice for resistance management and you will get stuck in the imi-cycle!
What are my options if there’s sufficient planting rain but the plant-back requirements for the Group B herbicide haven’t been met?
Short answer: Consider planting a Clearfield or imi-tolerant crop, but try to avoid using imis or other Group Bs in-crop. Imazapic (e.g. Flame) applied in a summer fallow is cheap and effective, but it will have implications for crop rotation flexibility.
Longer answer: Imis have a broad range of soil binding characteristics and the period of residual decay varies markedly. Microbial activity is the primary mechanism for breakdown of soil-applied imis. Consequently, soil moisture and temperature play an extremely important role in how long the herbicide remains effective in the soil and when it is safe to plant a sensitive crop.
Source: Soil behaviour of pre-emergent herbicides in Australian farming systems (GRDC)
Even if the residual has not broken down sufficiently to safely plant sensitive crops, there may be poor weed control due to sub-lethal amounts of herbicide remaining in the soil. This scenario represents a serious risk of partially-resistant weeds setting seed. Other weed control options must be set in place to control weed escapes.
While Clearfield and imi-tolerant crops are the most tolerant crops available, there are several non-Clearfield crops, such as chickpea, field pea, mungbean, peanut and soybean that have a degree of natural tolerance to imi herbicides. Look for a safe option that also enables the use of non-Group B herbicides and or cultural methods to manage weeds in-crop. If you need to use a pre-emergent, be sure to choose from an alternative herbicide MOA group.
Other resources:

Imi-tolerant crops – use sparingly and to best effect
Growers and agronomists interested in learning more about the benefits and risks of pre-emergent herbicides can access a free online course at, presented by Mr Congreve and Dr Chris Preston.
Clearfield herbicide plantback guide
Clearfield stewardship
Soil behaviour of pre-emergent herbicides in Australian farming systems

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How to manage annual ryegrass in chickpea crops

with Bhagirath Chauhan, weeds researcher, QAAFI
Annual ryegrass is becoming increasingly prevalent in the northern cropping region, and many populations already have a high level of resistance to the major Group B and Group A herbicides registered for use in chickpea crops.
To keep this important crop as a viable option, growers are looking for ways to add non-chemical in-crop options to an integrated weed control program to prevent a yield-limiting blow-out in ryegrass populations.
QAAFI weeds researcher Baghirath Chauhan has completed several studies to investigate ways to make pulse crops, including chickpeas and mungbeans, more competitive against weeds.
The principles of crop competition are fairly well known but making the necessary changes to planting gear can be daunting, so it is important to know that any changes will achieve the desired effect.
To assist growers to better implement crop competition in chickpeas, A/Professor Bhagirath Chauhan, principal research fellow and weed team leader, QAAFI, UQ looked at the effect of narrow rows, variety and early weed control to assess which was the most powerful suppressant of annual ryegrass.
“In a weed-free environment, it has been shown that narrow-sown chickpeas will produce higher yield, so we wanted to see if narrow sowing also suppresses weed growth and seed set,” says Bhagirath. “We also wanted to understand whether a more-prostrate variety like PBA Seamer would suppress more weeds than the more-erect PBA HatTrick. The third aspect we considered was the effect of weed infestation at different growth stages of the crop.”
The results were pretty conclusive: PBA Seamer sown at 25 cm and kept weed-free for at least the first three weeks after planting is a winning combination for ryegrass control.
Crop competition is one of the WeedSmart ‘Big 6’ tactics that will be the centre of discussion at the two WeedSmart Week events this year, one in Emerald from 13 to 15 August and the other in Horsham from 27 to 29 August. To register go to
What is the best way to reduce ryegrass growth and seed set?
Short answer: Narrowing the row spacing and ensuring good early weed control are the most effective tactics in chickpeas.
Longer answer: Plant architecture made some difference, but only in very weedy conditions. Narrowing row spacing from 75 cm to 25 cm reduced weed biomass by 16 per cent and reduced seed set by 26 per cent.
Keeping the crop weed-free for at least three weeks had the biggest effect, driving down weed biomass at the end of the season by 52 per cent and weed seed set by 48 per cent. This shows that, once established, chickpea can hold its own against weeds that emerge later in the season.

Can more competitive crops also produce higher grain yield?
Short answer: Yes. If you can’t do narrower rows then put an emphasis on early weed control.
Longer answer: Averaged across both cultivars and all weed infestation periods, sowing chickpeas on 25 cm row spacing (same seeding rate) produced 20 per cent more grain than sowing on 75 cm row spacing. This is most likely due to the crop plants being more evenly spaced and able to better exploit the available soil and light resources.
This research was conducted across two growing seasons and demonstrated that controlling annual ryegrass for the first three weeks after planting increased crop yield by a whopping 200 per cent compared to the season-long weedy scenario. Annual ryegrass that emerges 6 weeks or more after planting does not impact on chickpea yield, but if allowed to set seed, can contribute to the weed seed bank present at seeding the following year.
Is annual ryegrass a serious weed in chickpea?
Short answer: Yes, annual ryegrass is a yield-limiting weed and is adapting to farming systems further north than its traditional range.
Longer answer: Averaged over row spacing and cultivar, the penalty attributable to annual ryegrass was 1.2 t/ha less grain yield between the weed-free plots (1.8 t/ha) and the season-long weedy plots (0.6 t/ha). Without any competition, season-long weedy plots produced more than 129 annual ryegrass seed spikes per m2.
By planting PBA Seamer at 25 cm row spacing and keeping the crop weed-free for three weeks, the number of annual ryegrass spikes is reduced to just 8 per m2.  

How can I achieve this early weed control?
Short answer: Start the year ahead in the paddocks you plan to grow chickpeas and do everything possible to reduce the ryegrass seed bank using effective herbicides, weed seed burial, competitive cereals and harvest weed seed control tactics or hay-making. Back this up with registered pre-emergents for chickpea and as many non-herbicide tactics in-crop as possible.
Longer answer: Annual ryegrass is a master at evolving herbicide resistance. In southern regions it has evolved resistance to the registered in-crop herbicides for chickpeas. This will also occur in the northern region if steps are not taken to preserve the efficacy of Group A post-emergent chemistry across the crop sequence.
An over-reliance on pre-emergent herbicide use will also select for herbicide resistance, just as it has for post-emergent herbicides. To minimise this risk, it is important to use a diverse range of weed management tactics in-crop, such as crop competition, inter-row cultivation or chipping, to remove survivor weeds before they set seed. Where possible, rotate registered pre-emergent herbicide modes of action groups J, D and K between years and consider mixing pre-emergent modes of action groups where permitted, always at full label rates for all active components of the mix. ALWAYS READ AND FOLLOW LABEL INSTRUCTIONS.
Other resources:

AHRI Insight – Chickpea v Ryegrass
Mix and rotate herbicide modes of action

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What’s the latest in optical sprayer technology?

with Jeremy Jones, PA Specialist, Dalby Rural Supplies
Can you believe that it’s almost 20 years since optical sprayer technology came to Australia? In that time, being able to spray ‘green weeds on brown paddocks’ has been a game-changer for fallow weed management on many grain farms.
What started as a great double-knock tool has since evolved into a multi-purpose weed management tool for driving down the weed bank and re-gaining control of weeds that are notoriously hard to kill with glyphosate.
Jeremy Jones, Dalby Rural Supplies PA specialist. Photo: Neil Lyon
Precision agriculture specialist with Dalby Rural Supplies, Jeremy Jones has a long association with optical spray technology. Jeremy sees the adoption of optical sprayer technology across Australia’s grain growing regions is testimony to the role the technology can play in an integrated weed control strategy.
“Early on, growers used optical sprayers such as Weedseeker and WEEDit as an efficient way to apply paraquat in the traditional double-knock tactic to control any weeds that were surviving the initial glyphosate application,” says Jeremy. “This helped save time and chemical but growers were often left with a changing weed spectrum that was dominated by hard-to-kill species such as feathertop Rhodes grass, sowthistle and fleabane.”
“This led growers to look toward more expensive herbicides that have better efficacy on these species and optical sprayers enabled the economic application of these products because product was only applied to such a small portion of the paddock area, typically around three per cent.”
The latest use pattern emerging for these sprayers is the option to apply pre-emergent herbicides to known patches of weeds such as feathertop Rhodes grass to reduce germination in the following season.
The whole aim of fallow weed control is to save soil moisture and to reduce the weed seed bank from harvest to planting ensuring crops are sown into clean paddocks. Optical sprayers have proven their worth as a valuable and cost effective way to achieve both these outcomes.
WeedSmart Week from 13–15 August in Emerald will focus on leading technologies and tactics that make a real difference to effective weed control. Jeremy will be speaking about the opportunities that optical sprayers present and the latest WEEDit sensors will be put through their paces on-board robotic platforms for spraying and on the ‘Weed Chipper’ at the SwarmFarm Robotics base at Gindie. To register go to

How much chemical can I save?
Short answer: The amount of chemical applied is always significantly less. The cost saving may not be as great as more expensive products may be applied.
Longer answer: The greatest savings will not always be in chemical costs. Using optical spray technology usually reduces chemical use by 90 per cent and, consequently 90 per cent less water. The real savings though are seen in resistance management through the use of more modes of action and maintaining a low seed bank. Frequently a single pass achieves the required level of control, saving hours of time spent in the sprayer. Growers generally find that the payback period for these sprayers is just a few years.
How are growers using optical sprayers to apply pre-emergent herbicides?
Short answer: By mapping the weedy areas and using the sprayer to select these areas for treatment prior to planting.
Longer answer: Soon, the WEEDit will have on-board mapping capability to ‘remember’ where weeds were sprayed early in the fallow, enabling the operator to apply residuals to known weedy patches while also applying a knock-down to kill existing plants prior to planting. This capability already exists with SwarmFarm’s robotic platforms that currently carry the WEEDit sensors.
WEEDit sensors are mounted on the latest SwarmFarm robotic platform, which will feature at the Emerald WeedSmart Week field demonstrations in August.
Can optical sprayers be used to apply all herbicides?
Short answer: Many products now carry registrations for optical sprayer application.
Longer answer: When the rates used in the optical sprayer are within the application rate range on the label, there is no problem using an optical sprayer or any other. Some labels have an application range specified for optical sprayers.
Some minor use permits are available for use patterns that lie outside the conditions on the product label. For example, APVMA permit number 85049 provides for the control of volunteer and ratoon cotton in fallow using optical spot spray technology using specified tank mixes and application rates. Always read the label to check that the use pattern you plan to follow is legal.
Can optical sprayers help reduce spray drift?
Short answer: Yes, less product is applied to begin with, putting less particles into the air. The new nozzles increase the proportion of coarse droplets, in line with the new 2,4-D guidelines.
Longer answer: Optical sprayers are acknowledged as a useful tool to reduce spray drift. When the optical sprayer is engaged and the coverage area is below the threshold, the required buffer zone is reduced.    
Jeremy Jones, Precision Ag specialist with Dalby Rural Supplies says the real savings that growers are seeing through the use of optical sprayers are in resistance management through the use of more modes of action and maintaining a low seed bank.
Website resources

Jeremy Jones on WeedSmart podcast 
SwarmFarm case study 
Is mechanical site-specific weed control a practical fallow management option?

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Is mechanical site-specific weed control a fallow management option?

with Andrew Guzzomi, Senior Lecturer, School of Engineering, The University of Western Australia
There’s nothing more old school than a chipping hoe when it comes to weed control. Chipping has so many practical benefits – simple, low tech, no survivors, cheap, environmentally friendly, non-chemical, works on all weed species and sizes and keeps the kids out of the house for hours.
The downside of course is that it is slow, hot, boring work and requires teams of people to make any difference to weed numbers. So, how good would it be to have multiple chipping hoes mounted on a bar travelling at 10 km/h and poised at the ready to chip out any weeds?
UWA Agricultural engineer, Dr Andrew Guzzomi led the engineering component of a project to develop a tactical tillage machine, that can help manage herbicide resistance risk in conservation cropping fallow management. (Photo: Ryan Early, Anvil Media)
To see if this was feasible, GRDC funded a project for an expert team* of agricultural engineers and weeds researchers, along with grower and industry advisors, to build and test prototypes in various cropping areas around Australia. Their efforts have culminated in the development of the ‘Weed Chipper’, a cultivator fitted with hydraulic response tynes and commercially-available optical sensors that is ready for commercial trials and validation.
Agricultural engineer and senior lecturer at the School of Engineering at The University of Western Australia, Dr Andrew Guzzomi led the engineering development of the rapid response tyne, which was based on the Shearer Trashworker hydraulic breakout system.
“Like the optical sprayers, the ‘Weed Chipper’ is fitted with commercially-available optical sensors that can detect weeds and trigger individual tynes to rapidly chip out the weeds,” says Dr Guzzomi. “Its best fit is in fallow situations where the weed density is low – around 1 weed per 10 square metres.”
“Being well-suited to the control of larger weeds, this machine provides a fantastic non-chemical option to clean up survivors in a double-knock operation within a conservation cropping system.”
This machine has the potential to revolutionise herbicide resistant weed management and help farmers keep weed numbers low. Field trials showed the implement is a highly effective way to manage key summer weed species, such as windmill grass, feathertop Rhodes grass and awnless barnyard grass, even when these weeds are up to 70 cm in diameter.
Any tactic that can consistently achieve 90 to 100 per cent weed control needs to be taken seriously. See Table 1 below.
How much damage does the chipping do to the soil in a no-till system?
Short answer: Very little at low weed densities.
Longer answer: Targeted tillage is suitable for use at low weed densities, i.e. 1 plant per 10 m2 or less. At low weed densities the ‘Weed Chipper’ disturbs only a very small portion of the paddock’s surface during weed control. The response tyne is designed so that the amount of soil disturbance can be altered as required to control the target weeds. The cultivation depth and duration can be increased to target large tap-rooted and fibrous weed species, or reduced for smaller, shallow-rooted species.
Photos: Michael Walsh
How fast does it operate?
Short answer: The ‘Weed Chipper’ has been developed to operate at a ground speed of around 10 km/h.
Longer answer: The response tyne system was designed to operate at a nominal 10 km/h where weed densities are 1 plant per 10 m2 or less. Although higher operational speeds are possible, this would increase system loads and the potential for misses of targeted weeds. With less environmental limitations impacting on safe operation, the ‘Weed Chipper’ can be operated 24/7 if necessary.
What is the best-fit for an optical chipper in an integrated weed control program?
Short answer: As a non-herbicide fallow weed treatment option in low weed density situations.
Longer answer: The best-fit for the ‘Weed Chipper’ is to target low density (1 plant per 10 m2) weed populations in fallow paddocks – the same situation where optical sprayers are currently being used. This will remove survivors and reduce weed seed-set, to potentially prevent or delay resistance evolution. The Weed Chipper also has the benefit of being able to be used across a range of environmental conditions that prevent the application of herbicide treatments. This facilitates more timely and effective weed control.
* The Weed Chipper team led by Dr Michael Walsh (University of Sydney) is comprised of agricultural engineers Dr Andrew Guzzomi (UWA) and Dr Carlo Peressini (formerly UWA) and weed researchers Dr Michael Widderick (QDAF), Dr Adam McKiernan (QDAF) and Dr Bhagirath Chauhan (UQ).

A new era of tactical tillage
Technology heralds new approach to weed control. Targeted tillage machine can chip out weeds at densities of one plant per 10 square metres, while travelling at 10km/hour.
Posted by Grains Research and Development Corporation on Tuesday, 14 May 2019

More resources
Table 1. Response tyne efficacy on three summer weed species at three growth stages, Gatton and Hermitage, Qld 2017

Trial site location, summer 2016/2017

Weed size

Barnyard grass

Feathertop Rhodes grass


Control (%)









The dash (–) indicates that established weeds were either missed by the tyne (alignment issue) or the tyne did not activate.
This table is reproduced from the Weed chipper site-specific tillage for fallow weed control GRDC Update Paper.

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Does chaff in a chaff line suppress weeds?

with Annie Ruttledge, Weeds researcher, DAF, Queensland
In the wake of rapid adoption of chaff lining, the newest harvest weed seed control tool developed by Australian farmers, a substantial research effort has been made to validate the efficacy of this practice.
Chaff lining involves depositing weed seed-laden chaff in a narrow line behind the header. Some growers using this practice have suggested that as the chaff in the chaff line rots away, much of the weed seed also decays in the process. Researchers working across the northern grains region have now gained a deeper understanding of what happens to weed seed in a chaff line.
Dr Annie Ruttledge, DAF Qld weeds researcher has been investigating weed emergence from chaff lines.
Department of Agriculture and Fisheries, Queensland weeds researcher Dr Annie Ruttledge and several collaborating scientists have been looking into different aspects of weed seed decay and weed suppression in the chaff line.
“Non-herbicide tools like chaff lining are very important to help manage the onset and spread of herbicide resistance in weeds,” says Dr Ruttledge. “The idea with harvest weed seed control tactics is to collect any weed seed present at harvest height, usually above 15 cm. With chaff lining, these weed seeds are deposited in a narrow line of chaff behind the harvester.”
“Burial in the chaff line can suppress emergence in some weed species, but it does not guarantee that no weeds will emerge from chaff lines,” she says. “Harvest weed seed control tools like chaff lining and chaff tramlining concentrate the weed seed into confined zones where emergence can be monitored and action taken as required, without treating the whole paddock.”
Chaff lining (and chaff tramlining) deposit weed-laden chaff in a narrow line behind the harvester or directed onto the tramlines.
Does the chaff line suppress weed emergence?
Short answer: Yes, if the weed seed is buried deeply enough in the chaff. Many weeds in no-till and reduced-till farming systems prefer to germinate on the surface where there is plenty of light.
Longer answer: Our trials investigated the effect of chaff on germination rates of annual ryegrass and common sowthistle. The sowthistle seed was more readily prevented from emerging than annual ryegrass seed, probably due to the different requirements of the species for light. Maximum suppression of annual ryegrass emergence was achieved with a chaff load of 42 t/ha, which can be feasibly produced in a 3.5 t/ha cereal crop. In contrast a chaff load of just 12 t/ha of wheat chaff was sufficient to suppress emergence of common sowthistle seed.
Work done by our collaborator Dr John Broster, CSU found that chaff from cereal crops generally provided better suppression of annual ryegrass compared to canola and pulse chaff. For all chaff types the higher the rate per hectare the better the suppression.
One of the experiments involved determining the level of chaff required to effectively suppress emergence of annual ryegrass and common sowthistle.
What’s the difference in suppression in chaff lines compared to chaff tramlines?
Short answer: Chaff tramlining effectively halves the amount of chaff in each line, potentially reducing the suppressive potential of the chaff.
Longer answer: Placing a single line of chaff behind the harvester (or directing all the chaff from a chaff deck into one tramline) maximises the amount of chaff and therefore the level of weed suppression. Different crop types, sowing rates and crop yield all influence the quantity of chaff produced.
In addition to looking at suppression of emergence, we looked at weed seed decay under field conditions. In these trials there was no evidence that weed seeds rotted more rapidly in a chaff line than on the soil surface. However, we expect that environmental conditions play a large part in weed seed decay so the results could vary according to season, with more rotting likely in a wet year than in a dry year. The depth and persistence of chaff cover and the type of weed species are other factors that would influence seed persistence in chaff lines or chaff tramlines.
Harvest weed seed control tactics aim to collect weed seed at harvest and concentrate it in a small zone where weeds can be targeted at a fraction of the cost of whole-paddock treatments.
What are the options for treating the weeds in the chaff line or tramlines?
Short answer: Farmers are leading the way with practical solutions to manage weeds along chaff lines and chaff tramlines.
Longer answer: Some growers use an optical sprayer or a boom with nozzles only operating on the chaff line or tramlines to apply a mix of herbicides that may be too expensive to apply to the entire paddock. Weed seed that is collect at harvest and placed in the chaff line, may have survived in-crop herbicide applications and may be herbicide resistant. Susceptibility testing can help identify herbicides that can provide effective control.
Non-herbicide options are to wait for germination and chip or trample the weeds. In a mixed farming operation, sheep can graze the chaff lines rendering most of the weed seed unviable, and the sheep will benefit from an additional feed source.
Growers who have been using chaff lining and chaff tramlining for several years report that the high concentration of weed seed leads to a high level of competition between the weeds and this is compounded with competition from the following crop. Over time, seed set reduces and any weed seed produced will be collected and returned to the chaff line the following year. When chaff is deposited on the wheeltracks, weeds that emerge face dry, compacted conditions and are often subject to frequent passes with heavy machinery.
This chaff tramline was sprayed out using a shielded sprayer.
Does it matter if I use a draper or a stripper front?
Short answer: No, not in terms of amount of weed seed harvested, provided you set up and operate with weed seed collection in mind.
Longer answer: Our collaborators Dr John Broster, Dr Michael Walsh and Annie Rayner conducted trials with stripper and draper fronts at two trial sites. The results at one site showed that it is possible to achieve the same level of weed seed collection with the two harvester front options. At the other trial site, the stripper front was not as effective as a result of wider row spacing, higher operating height and faster operating speed compared to the draper front at the same site. The key to success with chaff-only harvest weed seed tools is getting the weed seed in the front and effectively separating the weed seed and chaff from the straw component. The WeedSmart website provides practical information about setting up different harvesters and operating them for effective harvest weed seed control.
Bear in mind that a stripper front will generate a lot less chaff than a cutter type front, and so this is likely to influence weed suppression and the rate of weed seed decay.

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Managing the risk of using residual herbicides and disc seeders in winter crops

with Peter McKenzie, private agronomist, Quirindi NSW

with Peter McKenzie, Agricultural Consulting & Extension Services
The Liverpool Plains is a hot spot for herbicide resistance. Growers have been doing battle with glyphosate and Group A and Group B resistant ryegrass since the 2000s, along with high levels of glyphosate resistance in milk thistle and rising resistance in feathertop Rhodes grass, barnyard grass and liverseed grass.

Well-known private agronomist, Peter McKenzie, is working with growers on the NSW Liverpool Plains around Quirindi to integrate the use of pre-emergent herbicides into their winter cropping program.


When all other tactics are in place, Peter reckons that’s the time to consider the strategic use of pre-emergent herbicides.

“Pre-emergents are a useful tool to have in the weed control toolbox,” says Mr McKenzie. “But we need to use them effectively when we decide to pull them out of the cupboard.”

“These herbicides have a place in both the winter and summer cropping programs provided they are well-supported with several other tactics. We need to be chipping weeds, using the optical sprayers for first and second knocks to make more economical use of herbicide, mowing verges and getting into harvest weed seed control.”

When all these other tactics are in place, Peter reckons that’s the time to consider the strategic use of pre-emergent herbicides.

Up to 80 per cent of Liverpool Plains growers are using disc seeders to handle high levels of crop residue when seeding winter crops, and to take advantage of limited planting opportunities. Peter says this presents some challenges, particularly when using pre-emergent chemistry for ryegrass control, but it can still be done effectively in certain circumstances.

Although most product labels recommend knife points and press wheel planters for ryegrass pre-emergents, the use of disc seeders is often not illegal. Growers must be aware that herbicide companies are unlikely to accept any responsibility for crop damage that may occur if the seeder does not achieve sufficient soil throw to separate herbicide from the seed line. As always, read the label carefully and ask for advice.

Be careful if you have a disc seeder and want to include pre-emergent herbicides in your winter weeds program. Check the label and consider the product properties, level of stubble and level of soil disturbance during the seeding operation.

If you are considering adding pre-emergent herbicides to the weed control program on your farm, the free, online Diversity Era course is a great way to learn more and avoid the pitfalls. Check it out

Should I use pre-ems in a paddock with weedy patches?

Short answer: Possibly.

Longer answer: Satellite imagery is a great way to identify and map weedy patches, using services such as Satamap. Once the patches are mapped it is easier to manage these patches differently.

One effective way is to harvest these areas lower, and using a homemade or commercial chaff chute to concentrate weed seeds onto the inhospitable environment of CTF wheeltracks. Using this approach, glyphosate resistant ryegrass patches can be dramatically reduced in one season – down by 70–75%. Not only can this drive down the weed numbers for the following year, it can also make the summer post-emergent spraying more effective because the chaff reduces the amount of dust thrown up by the sprayer tyres.

An alternate strategy for patches where harvest weed seed control is not practical is to apply a pre-emergent prior to the first spring storms to the patches that have been previously mapped. This is likely to substantially reduce the weed pressure over the following summer. Access to an optical sprayer (or chippers) can be very cost-effective in quickly cleaning up any escapes.

Why do I need to be particularly careful with ryegrass pre-ems in winter cereals?

Short answer: There can be serious unintended consequences (crop injury) even when all efforts have been made to correctly apply the herbicide, especially if heavy rain falls soon after planting.

Longer answer: Trying to remove a grass weed from a cereal crop can be difficult. It’s a balance between having enough herbicide available in the soil moisture to control the weed, but not enough to cause damage to the crop. The crop can tolerate higher levels of herbicide than the weed, but where extreme conditions occur, poor weed control or crop damage can result.

Under dry conditions, herbicides with low solubility are likely to be ‘safe’ to the crop but may fail to give effective weed control. Herbicides with higher availability in the soil moisture can give better weed control under these drier conditions, but may cause more crop injury under high available soil moisture conditions.

Where crop injury occurs, it is generally a combination of short term waterlogging and inadequate seed separation from the herbicide in the soil. Low disturbance disc seeders, incorrect planter set up and or shallow planting depth are often implicated. Crop injury is normally worse on lighter soil types, all other things being equal.

So what are the options?

Short answer: Do some herbicide resistance testing and find out what herbicides will work. Then assess the risks.

If weeds like ryegrass are surviving glyphosate, get some testing done to find out what other herbicides could be used to drive down the weed seed bank.

Longer answer: Resistance testing is the best way to find out what does work – you probably already have a good idea about what doesn’t! On a weed-by-weed basis, work out the best-bet control options throughout the year and for several years into the future.

Learn about the pre-emergent herbicide properties, particularly the plant-back periods, mobility in the soil, interaction with stubble and breakdown behaviour of the products you want to use. Talk to your agronomist and other growers about scenarios that have worked well. For instance, deep-planted chickpeas can be a good option for residual herbicide use.

There is also the option of swapping discs for tynes on some seeders to target a specific weed problem in a paddock with products that require effective soil throw out of the furrow. Regardless of the seeder type, it is also important to understand the interaction of the chosen herbicide with stubble.

If you use a disc seeder and decide to not use pre-emergents in your winter crops, then ensure you take advantage of the benefits of the disc seeder to achieve better crop establishment and more competitive crops. More crop, less weeds!

More resources:

Peter McKenzie WeedSmart Week 2019 presentation
Safest way to manage pre-em herbicides at seeding.
Making pre-ems work in high stubble
How can I improve pre-em herbicide efficacy in high stubble situations
Big 6 and the strip and disc system


Ask an Expert

Is rapid, on-farm herbicide resistance testing possible?

with Roberto Busi, Research Fellow, Australian Herbicide Resistance Initiative

Herbicide resistance testing is a valuable tool that farmers and their agronomists have been using for many years to confirm resistance and identify susceptibility. Knowing what herbicides will and won’t work on weeds in an area helps growers plan a ‘mix and rotate’ strategy for herbicide use.

AHRI Research Fellow, Dr Roberto Busi has been working on developing a cheap, easy and rapid screening test that farmers could one day use on-farm to help manage herbicide use. While this test is in its preliminary stages it is showing great promise and will potentially allow growers to conduct many more resistance tests than they otherwise might.

AHRI Research Fellow, Dr Roberto Busi has been working on developing a cheap, easy and rapid screening test that farmers could one day use on-farm to help manage herbicide use.

“In Australia, growers have access to two excellent herbicide resistance testing services through Charles Sturt University in Wagga Wagga and Plant Science Consulting in Adelaide,” he says. “These testing services offer traditional testing where seed samples are grown in pots and then treated with different herbicides and also the ‘Quick Test’ where growers supply fresh plant samples that are ‘revived’ in the greenhouse and then treated with herbicides.”

The test that Roberto is developing uses weed seeds placed on herbicide-laced agar. It gives results quickly, often within 5 to 7 days, and could be easily deployed on-farm using a very basic kit and low rates of herbicide.

“The concept arose following discussions with international researchers in Japan where Syngenta’s Global Head of Resistance outlined a testing method that could be used to identify herbicide resistant weed seedlings in a week,” he says. “This immediately got me thinking about developing a simple test that farmers could use, so when I returned home I started work on this idea in my own time.”

Starting with the ‘world champion of herbicide resistance’, annual ryegrass, Roberto has tested the concept using seven key herbicides – two knock-down, four pre-emergence and one post-emergence herbicides.

How will this new test help farmers?

Short answer:It will provide a quick indication whether the weeds tested are resistant or susceptible to a herbicide.

Longer answer:The intention is that the test be available as a DIY kit. This means the farmer can initially test many more samples than they might using the commercial herbicide testing services. Different patches in the one paddock could be tested to see if the same type of resistance was present. Growers could also identify products that the weeds are still susceptible to and develop a program to utilise and protect the remaining modes of action.

The test clearly shows the different response to the herbicide in question – susceptible population (no seed germinated) compared to two other populations with different levels of resistance (seeds germinated and grew in the presence of the herbicide).

How does the test work?

Short answer:Weed seeds are collected and then germinated in agar with a known dose of herbicide.

Longer answer:Seeds can be collected at harvest from standing plants or after harvest from narrow windrows or chaff lines. The control (no herbicide) will show if the seeds in the sample are viable to start with. The seed placed on agar in the presence of different herbicides will either grow (resistant) or not (susceptible). After one week the grower will know more about the herbicide resistance status of the tested paddocks or patches. If resistance is suspected, a full resistance test would be the smartest thing to do.

Does the test work for the main herbicides used in ryegrass control?

Short answer:Yes, the test has been demonstrated to work for the seven key herbicides available for ryegrass control.

Longer answer:The test is under development for several herbicides including glyphosate (e.g. Round-up), paraquat (e.g. Gramoxone), trifluralin (e.g. Treflan), pyroxasulfone (e.g. Sakura), prosulfocarb (e.g. Arcade), triallate (e.g. Avadex) and clethodim (e.g. Select). Using the test procedure, it is possible to rapidly identify which seeds are susceptible or which are resistant to each of the seven herbicides.

The test has been demonstrated to work for the seven key herbicides available for ryegrass control.

Will this test replace other herbicide resistance screening methods?

Short answer:No. This ‘Rapid Test’ provides a very basic level of information about herbicide resistance.

Longer answer:The test is under development and much work is needed. Once resistance problems are identified using the Rapid Test, further testing would be justified to gain a better understanding of the level of resistance present and the options for control. The Rapid Test could show that different patches of weeds had evolved different resistance mechanisms and so should be tested and treated differently to keep weed numbers low.

Ask an Expert

Does diversity help with weed control and herbicide resistance?

with Ken Flower, Senior Lecturer, The University of Western Australia School of Agriculture and Environment

Back in 2006, the WA No-Tillage Farmers Association (WANTFA) wanted to investigate the long-term effect of different farming ‘philosophies’ and residue management on factors affecting crop profitability, including yield, moisture storage, soil nutrients, weeds and disease.

With co-investment from the GRDC, WANTFA engaged Dr Ken Flower, now a senior lecturer at the UWA School of Agriculture and Environment, to establish and monitor a large-scale, long-term farming systems trial.

“This trial has been running for 12 years now and we have been able to measure the effect of four farming systems with different levels of crop diversity in 3-year rotations,” says Dr Flower. “The least diverse system is monoculture wheat, next is continuous cereal rotation, third is a cereal / legume / brassica rotation. The fourth system is farmer-designed and follows a cereal / cereal / break crop (or fallow) rotation.”

Since 2007, Dr Ken Flower, UWA has been managing a long-term farming systems trial in Cunderdin WA that is demonstrating how diversity in weed control tactics is essential across any crop rotation cycle.

“An important feature of this trial is that there is built-in flexibility in the design that allows us to respond to any build up in weeds. Every three years we decide on the best combination of crops, varieties and available technologies that are compatible with each farming philosophy treatment for the next 3-year rotation.” 

Having now completed 12 seasons of the trial on an alkaline red sandy clay loam at Cunderdin WA, the weed pressure is considerably higher in the monoculture wheat treatment while all other treatments have allowed more diverse and effective weed control programs to be implemented using herbicide mixing and rotation, crop tolerance technologies and harvest weed seed control.

Has increased crop diversity helped control weeds?

Short answer: Yes, with more weed control strategies available in a diverse system it is easier to keep weed numbers low.

Longer answer: In the monoculture wheat, it didn’t take long for grass weeds to build up in numbers. The main weed species at the site have been wild oats, brome, rye and barley grasses. There are limited herbicide options in the monoculture, except rotating between registered MOA for wheat, although the recent inclusion of Clearfield wheat in the rotation has been very effective. The wheat variety sown has changed over time to reflect improvements in plant breeding.

In the continuous cereal treatment, including barley as a more competitive crop at least once every three years has helped keep weed numbers lower.

Were broadleaf crops an effective weed control measure in the more diverse treatments?

Short answer: Yes and no.

Longer answer: Roundup Ready canola has been a very effective tool in the diverse farming systems. The legumes have been a weak link in the rotation in terms of weed control. Narrow leaf lupins were initially used in the legume phase and then the switch was made to field peas. Field peas have the distinct advantage of being sown at a later date, allowing better knockdown weed control after the breaking rain but before seeding. Chickpea and albus lupin have also been used, but it has not been possible to overcome the tendency for weeds to build up in the legume phase.

The fallow in the farmer-designed rotation has been particularly effective at reducing weed numbers, but may have an impact on soil organic carbon, which we are looking at.

Has herbicide resistance evolved in any of the treatments since the trial started in 2007?

Short answer: Herbicide resistance testing has not been conducted during the trial.

Longer answer: There was known resistance in annual ryegrass to Group A herbicides so this has been taken into account when planning herbicide mixes. Throughout the trial researchers and WANTFA farmers have responded to weed pressures with the aim of keeping numbers low.

Did introducing narrow windrow burning (NWB) impact on weed numbers?

Short answer: Yes. The introduction of NWB has provided a distinct reduction in weed numbers in all treatments.

The recent addition of harvest weed seed control (narrow windrow burning in this case) has been very effective in driving down weed numbers, particularly in high risk rotations. 

Longer answer: Narrow windrow burning was introduced after the 2010 harvest mainly as a means of providing a low-stubble and high-stubble comparison for assessing soil moisture effects. One of the original research questions at the trial site was ‘how much stubble is optimal for profitable crop production?’ but there was not sufficient difference between the treatments so the decision was made to split the plots using narrow windrow burning to provide a low-residue treatment within each of the four farming systems.

Adding a non-herbicide weed control tactic to the trial design brought an immediate reduction in the NWB plots within each treatment. As expected with NWB over time, nutrients are concentrated in the windrow, creating uneven crop growth. This is exacerbated in a controlled traffic farming system, as the stubble in the narrow windrow is placed in the same position each year.

Figure 1. Total number of grass weeds in 2017 measured about 6 weeks after crop emergence in the cereal rotation, diverse rotation, wheat monoculture and farmer rotation. Windrow burning started after 2010 harvest. Note the monoculture has no windrow burning subplot and the farmer rotation has only windrow burn since 2010.
The rotations since 2010 have been: Cereal rotation – wheat / wheat / barley; Diverse rotation wheat /chickpea then albus lupin / RR canola; Monoculture wheat; Farmer rotation – wheat / barley / fallow.

Ask an Expert

How do you manage summer weeds without spraying at night?

Concerns are being raised about the practical implications of this for summer weed control programs.
Mary O’Brien, a private consultant with extensive experience in managing spray drift, is keen to see growers fully adopt spray application practices that maximise herbicide efficacy and minimise off-target drift.
Mary O’Brien says the ‘community drift’ that can occur when a number of applicators are each putting a small amount of product in the air at the same time can have very damaging effects on off-target sites.
“The bottom line is that allowing spray to drift is like burning money,” she says. “Any product that doesn’t hit the target is wasted and the efficacy of the spray job is reduced, mildly resistant biotypes may survive as a result of low dose application and there is potential damage to sensitive crops and the environment.”
“The difficulty is that many growers want to spray at night to cover more ground when conditions are cooler and potentially weeds are less stressed. Having a restriction on night spraying does restrict the time available to cover the areas required.”
Having heard these concerns from growers across the country Mary keeps coming back to the fact that if there was a limitation to capacity at planting or at harvest, growers would scale up to get the job done in a timely manner.
“Buying another spray rig or employing a contractor is an additional cost, especially after a couple of tough seasons, but I really think this is insignificant against the cost of losing key products and the resultant escalation in herbicide resistance to the remaining herbicides,” says Mary. “This problem is not confined to 2,4-D or even to herbicides. I recently spoke to a stone fruit grower who was forced to dump his whole crop after a positive MRL return for a fungicide he had never even heard of, let alone used.”
What about just slowing down and lowering the boom during night spraying?
Short answer: This, coupled with a good nozzle, will reduce drift but it will never eliminate it.
Longer answer: The correct ground speed and boom height will have a large effect on the amount of product that remains in the air. The problem is that it only takes 1 per cent of the product remaining in the air to cause off-target damage.
Once there are a few operators putting just 1 per cent of their product in the air at the same time, the amount of product quickly accumulates and can potentially be very damaging. Mary calls this ‘community drift’.
Isn’t it better to spray weeds at night when it’s cooler?
Short answer: Not really.
Longer answer: Research by Bill Gordon showed that even if you keep everything else the same, night spraying can put at least three times more product in the air than daytime application, even if weather conditions are similar and there is no temperature inversion in place. The main difference between day and night is how the wind is moving across the landscape, rather than the wind speed.
Under inversion conditions, the air moves parallel to the ground surface and this means that the product can move significant distances away from the target before coming to the ground.
To achieve the best results through daytime spraying, applicators should focus on treating small, actively growing weeds. When there is good soil moisture, weeds are unlikely to be stressed even when the temperature is quite high.
Temperature inversion conditions are more common at night and in the early morning. These conditions generate a laminar flow of air across the landscape allowing small droplets to travel many kilometres away from the target site before coming to ground.
Can I use other products at night and just avoid using 2,4-D?
Short answer: The current changes to 2,4-D labels has drawn a lot of attention but the problem is the same for all crop protection sprays – herbicides, fungicides and insecticides.
Longer answer: Different products have different properties and some may work better at night but the problem is the sensitivity of some crops to certain products, such as 2,4-D. All products are tested for their efficacy and the label provides detailed information about the required spray quality and spray application conditions. Many products have explicit label instructions regarding wind speed, temperature inversions (or laminar flow) and night spraying.
Given the high risk of drift at night, applicators need to be very confident that there is no inversion present, and weather conditions should be measured at least every 15 minutes to ensure wind speed remains above 11 kilometres per hour. An on-board weather station is the best way to monitor conditions.
A visual demonstration using smoke to simulate the the lateral movement of small spray droplets when a temperature inversion is in place.
What can I do to improve spray efficacy and avoid spray drift?
Short answer: If you do just one thing – change your nozzle.
Longer answer: All the factors that increase drift also reduce efficacy. To improve efficacy and reduce drift, use a better nozzle (larger spray quality) and appropriate water rates (matched to spray quality and stubble load), slow down and keep the boom low. Wind is required to push product downward and onto the target, and remember that the 3–15 km/h wind speed is for day time conditions only, this does not apply at night.