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Quick, test your winter weed escapes

This winter, researchers are recommending that growers and agronomists consider collecting live plant samples for herbicide resistance testing as a first step to identify and manage herbicide resistance in crop.
The first multi-species herbicide resistance survey conducted in the Northern grain growing regions of NSW and Queensland has shown that testing for herbicide susceptibility is well worthwhile as there are likely to be products that are still effective on weeds that are resistant to other modes of action.
Dr John Broster, Charles Sturt University recommends growers conduct regular herbicide resistance testing to better inform their herbicide program decisions.
Results from the ‘Quick Test’, offered by Plant Science Consulting, will usually provide a guide for herbicide strategies to use in the current season to stymie the effects of herbicide resistance. If weeds escape herbicide applications early in the season they will add large quantities of seed to the weed seed bank for next year, but if these plants can be killed this season their impact will be greatly reduced.
Testing weed seed at the end of the season is also a valuable tool to make more informed decisions for next season’s herbicide program, particularly for pre-emergent herbicides, which cannot be assessed using the Quick Test. There are three weed seed testing services in Australia, including Charles Sturt University.
The implementation of a variety of WeedSmart tactics, such as the double knock, mixing and rotating herbicides, stopping seed set and harvest weed seed control will also have a lasting impact on weed numbers.
With GRDC investment, Dr John Broster, Charles Sturt University NSW and Dr Michael Widderick, Queensland Department of Agriculture and Fisheries, Queensland led their respective teams through the process of testing samples and analysing the results for weed seed collected throughout the Northern cropping region in 2016 and 2017.
Weed seed samples were collected from 440 paddocks across northern NSW (244) and Queensland (196). The seeds were sown in controlled conditions and when the plants reached the three to five leaf stage they were tested for resistance to a range of herbicides. While collecting weed seed samples the researchers also assessed the weed density in each surveyed paddock.
The winter-growing weed species collected across the northern region and screened at CSU included annual ryegrass, wild oats, barley grass, brome grass, wild radish, Indian mustard, turnip weed and African turnip weed.
For NSW, this survey adds weight to other weed surveys conducted over recent years. Dr Broster says around 1000 paddocks in NSW and Queensland have now been surveyed between 2014 and 2018.
Wild oats is widespread across NSW and Queensland, having been found in just over half the paddocks surveyed. Annual ryegrass was also very abundant but so far only found in NSW, where it was present in almost 70 per cent of paddocks surveyed.
Barley grass, brome grass, Indian mustard weed and turnip weed were found only in NSW while wild radish and African turnip weed were mainly found in Queensland.
“The stand-out finding from our broader testing in NSW is that approximately 60 per cent of collected annual ryegrass populations were resistant to diclofop, sulfometuron and imazamox/imazapyr,” says Dr Broster. “Resistance to other commonly used herbicides for ryegrass control is relatively lower, but must be noted if these herbicides are to remain options for future herbicide programs. We measured resistance to clethodim (6%), trifluralin (7%) and glyphosate (6%) herbicides in the collected ryegrass populations sampled across the NSW regions.”
“Glyphosate resistance in ryegrass is more prevalent in the northern NSW regions than other parts of the state. Resistance to selective herbicides is lower in the northern NSW region than other parts of the state.”
2016 northern NSW and Queensland survey results summary for winter weed species

All results quoted below are the % of surveyed populations where resistance was detected.
Ryegrass only found in NSW – 94 populations tested. Resistance was found to diclofop (Hoegrass, 44%), imazamox/imazapyr (Intervix, 33%), sulfometuron (Oust, 29%), glyphosate (Roundup, 10%) and clethodim (Select, 2%). The sampled populations were susceptible to prosulfocarb + s-metolachlor (Boxer Gold) and pyroxasulfone (Sakura). All but one population was susceptible to trifluralin (Treflan, 1% developing resistance).

Glyphosate resistant annual ryegrass, 2016 Northern Region weed survey. Map courtesy of Dr John Broster, Charles Sturt University NSW.

Wild oats – 193 populations tested across Queensland (71) and northern NSW (122). Resistance was found to clodinafop (Topik, 38%) and Hussar (idosulfuron, 4%). These populations were fully susceptible to clethodim (Select), glyphosate (Roundup) and triallate (Avadex Xtra).
Barley grass – 17 populations, all in NSW. All susceptible to quizalofop-p-ethyl (Targa), clethodim (Select), mesosulfuron-methyl (Atlantis) and paraquat (Gramoxone).
Brome grass – 13 populations, all in NSW. Resistance found to mesosulfuron-methyl (Atlantis, 36%) only with all populations susceptible to quizalofop-p-ethyl (Targa), clethodim (Select), imazamox/imazapyr (Intervix) and glyphosate (Roundup).
Wild radish – 12 populations from Queensland and northern NSW. Resistance found to chlorsulfuron (Glean, 8%), diflufenican (Brodal, 80%) and 2,4D Amine, 77%. All populations were susceptible to glyphosate (Roundup), imazamox/imazapyr (Intervix) and Atrazine.
Indian hedge mustard – 7 populations found only in NSW. Resistance was found to chlorsulfuron (Glean, 17%) and diflufenican (Brodal, 43%). All populations were susceptible to glyphosate (Roundup), imazamox/imazapyr (Intervix), Atrazine and 2,4D Amine.
Turnip weed – 32 populations found in Queensland and northern NSW. Resistance was found to chlorsulfuron (Glean, 19%) and imazamox/imazapyr (Intervix, 11%). All populations susceptible to glyphosate (Roundup), Atrazine, diflufenican (Brodal) and 2,4D Amine.
African turnip weed – 17 populations found in Queensland and northern NSW. Resistance found to 2,4D Amine while all populations were susceptible to glyphosate (Roundup), chlorsulfuron (Glean), imazamox/imazapyr (Intervix) and Atrazine.

Related links:

Testing for herbicide resistance and susceptibility
Correct preparation of Quick test samples video below

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Tackling ryegrass in Tasmania’s high rainfall zone

The wettest April in 60 years is likely to cause growers in Tasmania’s high rainfall zone some unique challenges when it comes to controlling annual ryegrass, the Australian grain industry’s #1 weed.
Blow-outs are mostly in wet years and in paddocks with other underlying issues where ryegrass can take advantage of any gaps or areas of lower crop vigour. Many growers are finding that ryegrass is particularly difficult to manage in winter cereal and canola crops where the long cool season allows multiple germinations of ryegrass and the wet conditions reduce the residual benefits of pre-emergent herbicides.
WeedSmart southern agronomist Greg Condon says creative farmers and agronomists are addressing the problem head on and finding ways to implement the WeedSmart Big 6 weed management tools to stop seed set and drive down weed numbers.
“Farmers need to operate in a low-weed environment so they have more options in their crop and livestock decisions,” says Greg. “The principles of crop diversity, crop competition and mixing and rotating herbicide modes of action are central to keeping farming systems profitable.”
Diversity in cropping and livestock operations needs to result in diverse weed control tactics.
“The creativity comes in with the problem-solving process needed to apply these principles in challenging environments. The practices that have been developed in other high rainfall zones do not always apply directly to the Tasmanian farming systems.”
Growers and agronomists can use the WeedSmart Big 6 to keep ryegrass numbers under control while maintaining maximum flexibility in their cropping program. Greg says all the tactics will work in the Tasmanian high rainfall zone, with the exception of harvest weed seed control.
“Growers in the region have a distinct advantage when it comes to diversity in their farming system and this needs to drive the use of different weed control methods to keep the weeds guessing – using both herbicide and non-herbicide tools,” he says.
“Unfortunately, harvest weed seed control is far less effective in the Tasmanian cropping zones than it is in other high rainfall zones,” he says. “This is because a large percentage of the ryegrass that germinates in the crop will shed its seed before harvest. Some growers may still find value in this tactic to collect the seed from the later cohorts of the weed but it just isn’t as cost-effective as it is in other areas.”
Even without harvest weed seed control as a mainstay option, growers can still implement a three or four year ‘war on weeds’ program to drive down the ryegrass seed bank.
“To start with, tactics such as spring cropping, fodder crops and triple break crops can be used to maximise the efficacy of available herbicides and reduce ryegrass germinations,” says Greg.
“When it comes to herbicides it is critical that a plan is put in place to mix and rotate herbicides across the whole crop sequence,” he says. “In-crop herbicides are scarce and products like clethodim are registered for use in many crops so it is easy to keep using it, but we know that quickly leads to resistance problems.”
Having diversity of crops in the system only helps with weed control if it is used as a way to rotate chemistry. Testing the weeds for their susceptibility to single products and mixes of herbicides allows growers to plan ahead with more confidence. Double knocking each herbicide application preserves the available chemistry, particularly glyphosate.
Growers can also take advantage of ryegrass’ poor competitive ability. When placed in a competitive environment, ryegrass sets less seed, reducing the pressure of this weed on farming system decisions.
“Crop agronomy has a major impact on competitiveness,” says Greg. “The choice of crop, solid plant establishment, sowing early with a robust pre-emergent herbicide and attending to any soil constraints all swing the advantage toward the crop and away from the weeds.”
The final tactic that growers can implement is stopping weed seed set using hay and silage, crop-topping in canola or spraying under the cutter bar when swathing, or using camera-guided shielded sprayer technology to target weeds growing in the inter-row.
Cutting weedy areas of the crop for hay is a highly effective tactic that can reduce the impact of a weed blow-out.
Ian Herbert, Southern Farming Systems’ Tasmanian Projects and Trials Manager says fodder crops and livestock play a critical role in managing ryegrass on many Tasmanian farms.
“Growers can plant fodder crops directly after grain harvest, graze these fodder crops through winter, while allowing multiple germinations of ryegrass to occur, and then remove these plants using broad spectrum herbicides and or cultivation prior to planting a grain crop in spring,” he says. “This tactic reduces the pressure on selective herbicides and changes the timing of when ryegrass is controlled, compared to years where the paddock is in a cereal production phase. Cultivation is often needed to remove the deep pugging from livestock, which often occurs during our wet winters.”
Livestock can play an important role in weed management and there are many options available to growers.
Michael Chilvers is one grower who is embracing an integrated approach to managing ryegrass on his 1200 ha farm south of Launceston, where he runs a diverse cropping enterprise of around 300 ha of grain production along with intensive lucerne hay, potato and hybrid seed production.
Michael says the exceptionally wet autumn across much of the high rainfall zone, and particularly in their region, is going to put heavy pressure on the pre-emergent herbicides applied at planting.
“Incorporation of pre-em herbicides is critical and often not easy to achieve,” he says. “Unfortunately, the newly released pre-em products are probably not going to be an option for us in very wet seasons so we need to focus on getting the most out of the existing products.”
Michael is also very aware of the heavy reliance on Group A herbicides such as clethodim across his farming system and is doing what he can to rotate away from this key mode of action at every opportunity.
“Not only do we use it frequently, we also know that its efficacy can be compromised in our environment through a long cool growing season, which means we are running a real risk of losing it if we don’t adopt a more diverse approach to managing ryegrass,” he says.

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Keeping a lid on weeds growing in your crops

In-crop weed control is particularly difficult in some years. Even after you have done all you possibly can to get your crop off to a competitive start the growing season can throw up some major challenges.
In this article we explore key principles that impact on the efficacy of in-crop ‘selective’ herbicides. You will most likely have to make some compromises and it is almost impossible to implement every tactic perfectly every time.
Herbicide mixes help to preserve the effectiveness of each mode of action by avoiding unnecessary usage.
Over the last few years WeedSmart has collected and promoted great advice from seasoned agronomists, wise researchers and crafty farmers on all aspects of weed control. To save you some time we have collected the resources that we think can be of assistance as you make the hard decisions about what to apply, when to apply and how to apply the herbicide and non-herbicide weed control tools at your disposal.
Post-emergent herbicides have been widely used in Australian crops because they are generally highly effective and easy to use. Unfortunately, their popularity has led to widespread resistance and most farms will have at least one weed species that is resistant to at least one post-emergent herbicide mode of action.
Despite recent increases in resistance, post-emergent herbicides remain an integral component of weed control strategies in many production systems.
Key messages:

Avoid the routine use of any weed control tactic – mix, rotate and keep changing.
Know what modes of action still work – test for susceptibility.
Have a plan for dealing with a weed blow-out.
Right Product, Right Time, Right Application.

Planning your in-crop herbicide use
Widespread and increasing herbicide resistance demands a planned approach to herbicide use throughout the crop sequence. If you have been using a particular herbicide or group of herbicides routinely, it is probably because they work well. To ensure these effective products remain an option into the future, it is necessary to use them less often!
Testing weeds for their susceptibility to a range of herbicides is cheap compared to applying a herbicide that has limited or no effect. Resistance to one or more herbicide does not mean you have no options.
We now know that mixing and rotating herbicides is an effective strategy to prolong the effective life of each mode of action. But even these mixes and rotations need to be change.
There are currently very few in-crop herbicides available for grass control. There are more options for broadleaf control. Plan a herbicide use program that spans your crop sequence so you can ‘save’ particular herbicides for use in crops where there might be limited alternatives, while using a range of other modes of action in other crops.
All herbicides applied in crop will have some impact on crop safety. Herbicides must be applied according to the correct crop growth stage for each herbicide.
Shielded spraying opens up the possibility of using other chemistry in-crop that would otherwise not be an option. Some growers are also looking for ways to include non-herbicide in-crop tactics such as inter-row cultivation or scuffling in wider-row cropping situations.<

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For planning herbicide use through the season and the crop sequence the NSW DPI Weed Control in Winter Crops booklet is full of useful tables of selective herbicides for each crop type. Click to download a copy.

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If you don’t have a copy of Mark Congreve and John Cameron’s ‘Understanding Post-emergent Herbicide Weed Control in Australian Farming Systems’ GRDC technical manual, you really need to download this resource.

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The GRDC Integrated Weed Management in Australian Cropping Systems manual provides a comprehensive guide to IWM, including the use of in-crop herbicides.

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This table shows the in-crop herbicides and their application timing for cereals. It is an extract from the NSW DPI Weed Control in Winter Crops booklet. You can download a copy here.

 
WeedSmart resources:

What are the ‘mix and rotate’ options for in-crop herbicides?
Testing for susceptibility and resistance
How can I implement the mix and rotate strategy to combat herbicide resistance?
How can I avoid getting stuck in an imi herbicide cycle?
Can we grow broadleaf crops without clethodim?
What can be done to regain control of herbicide resistant sowthistle?
Grower case study: Tim Rethus, Horsham Vic
Grower case study: Leigh Bryan, Swan Hill Vic
Grower case study: Phil and Brad Jackson, Westmar Qld

– Achieving maximum impact
Herbicide application decisions are complex and it is almost impossible to have all the important factors in place every time. Start with the product label and follow the instructions on rate, nozzle use, boom height, speed, adjuvants, optimal environmental conditions and so on.
When mixing, the order is important to avoid precipitation in the tank but it is also important to ensure that the tank mix partners each retain their efficacy. Some mixes are antagonistic and should be avoided because this will likely result in a significant reduction (up to 50% in some products) in efficacy. Adding an adjuvant can reduce herbicide selectivity and thereby increase crop damage.
The GRDC GrowNote Spray Application manual provides detailed information on:

Planning your spray operations – things to think about
Preparing for spraying – checks, accuracy and efficiency
Spraying system – major components and set-up considerations
Selecting a spraying system options available and operational considerations
Review and planning for future needs

CLICK IMAGE TO DOWNLOAD INTRODUCTION MODULE
The manual comprises 23 modules and a ‘water flush for residuals’ calculator. Each module includes a series of videos (see playlist below).

The video playlist is a great place to start:

 
WeedSmart resources:

Getting mixing right to avoid glugs and efficacy issues
Don’t start mixing until the water quality is right
Spray well: correct nozzles, adjuvants and water rates
Are you going spraying or killing weeds?
Never cut the rate

 
Right time – Manage multiple germinations
Timing is one of the hardest things to get right. Large spray programs and wet weather can make it very difficult to apply herbicides at the optimal time in every paddock for every weed.
The guiding principle is to always target small weeds, even if this means multiple applications. In wet years, multiple germinations will occur and waiting for the next germination invariably means the first flush will be much more difficult to kill and more likely to generate lots of seed for next year.
All herbicides applied in crop will have some impact on crop safety. Herbicides must be applied according to the correct crop growth stage for each herbicide. Also consider the effect of environmental conditions, particularly frost, on crop safety.
Environmental conditions at the time of spraying can make a huge difference to herbicide update in weeds. Look for the directions on the label for optimal conditions as spraying outside these parameters will result in less effective weed control.
avoiding chemical residues in grain.
WeedSmart resources:

Spray small multi-resistant radish twice
Does ambient temperature really affect herbicide performance?

Be ready for weed blow-outs
Wet years are the classic weed blow-out situation. Pre-emergent herbicides breakdown faster, spray applications can’t be applied on time and weeds will take advantage of any gaps that might develop in the crop.
Monitoring the weed pressure and having a plan to limit the potential damage might help.
The aim is to stop a massive seed set event using tactics such as – crop topping, hay or patching out. And be ready to implement harvest weed seed control – all the methods work well.
WeedSmart resources:

Grower case study: Trevor Syme, Bolgart WA
Grower case study: Mark Branson, Stockport SA
Grower case study: Day Family, Lockhart, NSW

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Extent of herbicide resistance in summer weeds revealed

The first multi-species herbicide resistance survey conducted in the Northern grain growing regions of NSW and Queensland has shown that while resistance to key herbicides is prevalent, growers are keeping weed numbers low, for now.
Rising resistance to glyphosate has been clearly demonstrated but this key herbicide can continue to be a highly effective tool for weed control, provided growers and agronomists implement a variety of tactics, such as the double knock, mixing and rotating herbicides and seed bank management, as a matter of urgency.
Dr Adam Jalaludin (DAF) collecting weed seed to be screened for herbicide resistance in the first multi-species weed survey in the northern grains region.
With GRDC investment, Dr Adam Jalaludin, Department of Agriculture and Fisheries, Queensland and Dr John Broster, Charles Sturt University NSW led their respective teams through the process of testing samples and analysing the results for weed seed collected throughout the Northern cropping region in 2016 and 2017. This survey was part of a wider GRDC investment in ‘Innovation in Crop Weed Control’ project across the northern region.
Weed seed samples were collected from 440 paddocks across northern NSW (244) and Queensland (196). The seeds were sown in controlled conditions and when the plants reached the three to five leaf stage they were tested for resistance to a range of herbicides. While collecting weed seed samples the researchers also assessed the weed density in each surveyed paddock.
The summer weed species collected across the northern region and screened in Queensland included sowthistle, fleabane, awnless barnyard grass, feathertop Rhodes grass, windmill grass and liverseed grass.
Of these predominantly summer-active species, only sowthistle, awnless barnyard grass and liverseed grass are listed on the label as being controlled by glyphosate alone. There is no label claim that the other three species – fleabane, feathertop Rhodes grass and windmill grass, will be controlled at the registered label rates of glyphosate (729 g active ingredient per ha). 
“Not surprisingly, all fleabane populations collected failed the glyphosate test,” says Dr Jalaludin. “Interestingly, 32 per cent of the feathertop Rhodes grass populations and 42 per cent of the windmill grass populations were actually susceptible to glyphosate.”
“Of concern was that 36 per cent of awnless barnyard grass populations and 14 per cent of the sowthistle populations were resistant to glyphosate.”
Dr Adam Jalaludin (DAF) recommends herbicide susceptibility testing as the first step in addressing patches of resistant weeds.
“In this survey we detected evolved resistance to haloxyfop in feathertop Rhodes grass and to imazapic in windmill grass, albeit at a low frequency.”
Glyphosate resistance is certainly entrenched in the northern region, and for sowthistle is much worse than in other areas of Australia. The good news is that weed density is relatively low and there is susceptibility to other herbicides.
“It is essential that an integrated approach is taken to manage these key weeds in summer crops and fallow,” said Dr Jalaludin. “An increasing number of weeds from a range of species are surviving glyphosate treatment, giving a clear indication that over-reliance on this herbicide is unsustainable.”
“Herbicide testing is a good place to start to identify which herbicides provide the best control. Herbicide applications should be supported with as many other WeedSmart tactics as possible and any survivors removed.”
Summary of results
The populations of viable sowthistle seed were screened with glyphosate, 2,4-D amine, Velocity and chlorsulfuron. Glyphosate and cholsulfuron provided poor control while all populations were susceptible to 2,4-D amine and Velocity.
Fleabane populations 1 week after spraying with glyphosate. Susceptible control population (right) and a resistant population (left) collected during the northern region herbicide resistance survey.
Fleabane populations across the Northern region were screened with glyphosate and 2, 4-D amine. All fleabane populations tested survived treatment with glyphosate while no sample survived the application of 2,4-D amine.
Screening of the feathertop Rhodes grass populations revealed 68 per cent were not controlled by glyphosate. One population survived treatment with haloxyfop, while all populations were controlled with clethodim.
Thirty-six per cent of the awnless barnyard grass populations collected across the Northern region were resistant to glyphosate. Fortunately, all populations were susceptible to propaquizafop, clethodim and imazapic.
Most of the windmill grass populations sampled in the survey were found in NSW. Screening showed that more than half (58%) of the populations were not controlled with glyphosate. Similarly, 40 per cent of the populations survived imazapic treatment. All populations were totally controlled by propaquizafop and clethodim.
The screening of the few viable liverseed grass populations collected across the Northern region did not reveal any evolved herbicide resistance.
Weeds collected in this survey were screened against several herbicides for which there are no label claims for some species. In field conditions, these weeds are often subject to exposure to a range of herbicides, which may be observed to have some activity. It is illegal to apply herbicides in any way contrary to the label and the results from this research are not a recommendation of use.

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Calculating the cost of HWSC for your farm

When it comes to harvest weed seed control methods there is good evidence that all the available options achieve a similar end result in terms of weed seed bank decline. But there is a wide variation in initial outlay, in associated activities and in nutrient removal or concentration.
Then there is the on-going problem of using average costs compared to using your own costs when doing the calculations.
WeedSmart western extension agronomist, Peter Newman has developed a spreadsheet growers can use to do calculations for their own farm to compare the cost of different HWSC options.
To assist, WeedSmart western extension agronomist Peter Newman has developed an interactive spreadsheet tool that growers and advisors can use to compare the costs of the available harvest weed seed control (HWSC) tools, using data and costings from their own farm situation.
“We have good data to suggest that harvest weed seed control costs are $7 to $19 per hectare and there are only minor differences in the cost between the available tools when compared using the same farm data,” says Peter. “But because every farm is different and small differences in costs per hectare can make a big difference at the whole farm scale, it is important for growers to have confidence that they can afford and justify the expenses incurred.”
Some HWSC tactics involve the purchase of substantial machinery – such as an impact mill, chaff cart or chaff deck – but the operational and labour costs might be lower than methods such as narrow windrow burning, which involves low set-up costs but higher nutrient losses and labour costs associated with burning. Invariably narrow windrow burning is the most expensive option in the long-run due to the high nutrient removal cost.
There is good data to suggest that harvest weed seed control costs are $7 to $19 per hectare and there are only minor differences in the cost between the available tools when compared using the same farm data. A chaff cart (pictured) is a particularly good fit on mixed farms.
“As a general rule, the cost of HWSC is lower per hectare on larger farms with lower yields,” says Peter. “This is because the capital cost is spread over a larger area, less nutrients are removed because the yields are lower and the low biomass crops allow low harvest heights without slowing down the harvester or using extra fuel. The converse is generally true for smaller farms with higher yields.”
“By imputing your own figures into the HWSC costing tool you can quickly evaluate the financial impact of the available options,” says Peter.
Another important factor to consider is that some of the technology is relatively new to market and there are unknowns such as the expected life of components when operating in commercial field conditions. As time goes by the manufacturers will be able to provide more concrete advice on service and repair costs, and any component failures will be overcome.
The easy-to-use spreadsheet takes into account capital cost, nutrient spread/loss, cost of ownership, harvest cost, reduction in harvest capacity, fuel use and replacement of wearing parts. Users also input data about their farm size and crop yields.

Peter Newman joined us for a webinar to discuss the HWSC cost estimate tool, you can watch the full recording below.

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Interest in cover crops continues to grow

There is a growing interest in planting cover crops in cotton and broadacre systems, providing the potential to preserve soil moisture, improve soil health and manage weeds.
As part of the CRDC project ‘Staying ahead of weed evolution in changing cotton systems’, the Queensland DAF Weed Science team investigated the impact of cover crops on weed suppression.
Jamie Grant (far right) is pictured with Jeff Werth (DAF weeds researcher). Jamie grows French white millet as a cover crop in rotation with cotton in his dryland cropping system at Jimbour, on the Darling Downs.
Research has shown that cover crops can provide a benefit in terms of weed control. However, in order for them to be effective, it is important to start with a clean crop and ensure that the cover provided is adequate and evenly spread.
Similar to findings from grower Jamie Grant in the following case study, research showed that when the cover was not adequate, lower amounts of cover provided a haven for weeds to germinate. A clean crop also provides the cover crop with a head start and improves its ability to out-compete the weeds.
The project also examined the effectiveness of the 2+2 and 0 strategy (two non-glyphosate tactics in crop, plus two non-glyphosate tactics in fallow and zero survivors or incursions). This strategy was found to be effective, and the use of tools such as WEED-IT can provide an effective way to incorporate other herbicides, and particularly follow-up for effective survivor control.
Darling Downs grower Jamie Grant has more than a decade of experience growing cover crops and was a pioneer in including millet in his rotation as a dedicated cover crop. Jamie has modified his machinery and farming style, after much on-farm trial and experimentation.
Jamie Grant: experience and experimentation lights the way
Jamie is a dryland cotton grower near Jimbour, Darling Downs in South East Queensland. His current crop rotation is cotton every second year and a millet cover crop every other year. He has included French white millet as a cover crop in his rotation for nearly a decade and as a result, he has been able to change from cotton every third year to every second.
Jamie said his main reason for including the cover crop is to preserve soil moisture.
“The cover crop increases infiltration from rainfall, prevents the majority of run-off in larger events, and also prevents evaporation of moisture from the soil,” he said. “Weed management was not a major focus for the inclusion of the cover crop, however the cover from the millet does give an additional benefit in terms of weed control.”
Jamie also highlights the importance of a dedicated cover crop, as compared to a cash crop that is harvested for grain.
“The main purpose of the cover crop is to preserve moisture and cover,” he said. “When a crop is allowed to reach harvest maturity, it has taken extra moisture from the soil profile contrary to the objectives of a cover crop.”
Crop choice
Jamie has settled on French white millet as his cover crop, planted in 15-inch (38cm) rows. As the focus is to preserve soil moisture, millet is a short duration crop and can be grown to near maturity in six weeks from planting in October to December. In this time, the millet provides maximum cellulose to give the maximum length of cover from the stubble.
“While growing, the millet only uses approximately one foot or 30 cm of stored soil moisture,” Jamie said. “The gains in soil moisture has improved fallow efficiency from 30 per cent in fallow to 70 per cent with the cover crop.”
Before the inclusion of the cover crop, the soil profile required approximately 600mm of rainfall to refill. Now the profile is refilled after 300mm. The millet also creates enough cellulose that the cover remains adequate until cotton is planted the following season.
Jamie’s own research has shown that legumes tend to break down too quickly to provide the length of cover required, and French white millet has the right characteristics.
Jamie Grant grows cotton every second year rather than every third, using the moisture stored under the cover crop.
“I find that if I plant in October, I generally have 40 per cent cover the following November, when I’m ready to plant cotton,” Jamie said. “I don’t use sorghum as a cover crop, as the wider row spacing does not provide the cover needed, and the gaps in the stubble create a suitable microenvironment for weed germination and growth.”
“I also noticed that in lighter rainfall events in sorghum and wheat stubble, the rainfall runs down the stalks of the standing stubble and creates a wet patch at the base. This is where the weeds grow and creates weedy patches across the field. A good millet cover crop is more even and allows the rain to penetrate the stubble evenly, and the stubble cover reduces weed emergence and the need to spray.”
Cover crops must reach maturity to create the maximum amount of cellulose for longevity. Other crops such as sorghum, wheat and barley take too long to reach maturity and as a result use too much moisture.
The main weeds on Jamie’s farm include sowthistle, feathertop Rhodes grass and fleabane. Jamie places a high importance on weed control, however says “if you can grow good weeds, you can grow good crops”.
Jamie’s focus on weed management in the cover crop is to ensure adequate cover across the whole field, as gaps in cover create a haven for weeds.
“I do this by ensuring good germination, with quality seed, and I put as much effort into growing a good cover crop as I do growing cotton,” he said. “Double knocks are still an important part of the herbicide program and controlling weeds prior to crop emergence (both for millet and cotton) ensure the crop can get a head start to out compete the weeds. An in-crop spray of MCPA and Starane is always done in the millet to control volunteer cotton, however if a heavy cover crop is grown a spray to control volunteer cotton is not always needed.”
Jamie also uses a controlled traffic system (CTS), as he considers minimising soil compaction to be very important, has been using WeedSeeker technology on a large boom for a number of years, and is now using a SwarmFarm robot mounted with a WEED-IT sprayer across his fields.
The big boom is generally used for broadacre spraying, with the relevant herbicide mixture for the weeds present. The WeedSeeker, and now the SwarmFarm robot with the WEED-IT, will be mainly used to control weeds in fallows between rain events, and broadacre sprays on mass germinations. The spray rig is also rotated across the tramlines in the CTS, so that it does not constantly run up and down the same wheel tracks. This allows subsequent sprays to control weeds that were run over by the rig in the previous spray.
Jamie’s key learnings and advice to growers considering growing cover crops is to ‘work it backwards’.
“Grow the cover crop that can accumulate the most moisture, and then grow the cash crop that will take the best advantage of the moisture,” he said. “It is important to work out your moisture availability and your crop frequency. The moisture holding capacity of the soil will be better with a cover crop independent of soil type. The lower the capacity of the soil to hold moisture, the greater the effect evaporation has. This increases the importance of having a cover crop.”
Growing good cover
Jamie has spent a couple of years determining how to germinate and grow a good cover crop. He also stressed the importance of purchasing quality seed.
“Patience is the key,” he said. “It is important to do a good job with proper seedbed preparation at planting. An example of this when planting millet, is that it does not like to break through a crust while emerging.”
If Jamie gets enough rainfall for planting millet, he checks the forecast to ensure a further heavy rainfall event is not lik
Jamie finds that putting the effort into the millet crop means he reaps the benefit in the following cotton crop.
“A new tactic I’m considering is intercropping – planting millet between the cotton on a 60-inch row spacing (152 cm), and then spraying the millet out after three to four weeks,” he said. “This will increase ground cover in the cotton crop, with the benefits of increased weed competition, better rainfall infiltration and reduced moisture evaporation in-crop, for the sacrifice of some surface moisture that will evaporate in summer anyway.”
Jamie said it is also of key importance to let neighbours know what cover crops you have, to minimise the risk of spray drift, which will reduce their effectiveness by either killing areas or impeding growth and creating areas of less than adequate cover.
“Mapping fields with SataCrop is an important tool to do this,” he said.
Effect on soil moisture quantified
Cover crops serve multiple purposes in a cotton rotation, with research underway to quantify the effect on water infiltration and moisture holding capacity of soils.
Research is also underway in the Riverina as part of the ‘Staying ahead of weed evolution in changing cotton systems’ project. Researchers at NSW DPI in collaboration with CRDC, GRDC and Queensland DAF have run a series of experiments at the IREC trial site near Whitton in Southern NSW to better understand the effectiveness of incorporating cover crops into cotton systems. The aim of this research is to evaluate the benefits that cover crops could provide when incorporated into cotton systems, especially improved water infiltration and water holding capacity of soil.
An experiment looking at cover crop species and rotation types has been completed and is being analysed by a biometrician to gain insight into the soil water dynamics as influenced by the cover crops. Initial results suggest the type of cover used is less important than the amount of cover or biomass that is grown when it comes to influencing on yield.
This season a spray out timing experiment is being conducted to determine how much biomass is required by cover cropping to have an influence on infiltration and water holding capacity. During the winter fallow a cover crop mix was sown and subsequently sprayed out at different growth stages.
NSW DPI cotton research agronomist at Yanco, Hayden Petty says the intent was to achieve varying amounts of biomass into which cotton was planted. This will be compared to a fallow that is the control for the experiment.
“Cover crops offer many benefits to a cotton farming system, as research is showing with weed suppression and soil health,” Hayden said. “After harvest this year we will have fully analysed the data and will be in a position to offer a quantifiable effect on soil moisture.”
For more information, contact Hayden Petty
This article appears courtesy of the Cotton Research and Development Corporation (CRDC). It was published in the Autumn 2020 edition of CRDC’s Spotlight magazine: www.crdc.com.au/spotlight. Images courtesy Tom Quigley and Hayden Petty.

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FTR grass demands attention to stop seed set

Feathertop Rhodes (FTR) grass is quickly becoming one of the biggest weed threats in Australian farming systems, demanding swift and decisive action.The vast number of seeds produced per plant and the species’ ability to germinate and establish on very small rainfall events, gives this weed a real competitive advantage, particularly in a fallow situation.
Feathertop Rhodes grass is a serious weed challenging no-till farming in Australia.
In the northern cropping region researchers have observed FTR grass (Chloris virgata) germinating almost all year round, at temperatures ranging from 15/5° to 35/25°C (day/night temperatures). While many seedlings that establish in winter are killed by frost, some will survive and it only takes a few plants to produce a large number of viable seeds for the next generation.
Recent research by Queensland Alliance for Agriculture and Food Innovation (QAAFI) weed researcher, Dr Bhagirath Chauhan has demonstrated that some populations of FTR grass are producing seed that is capable of germinating just two weeks after they mature.
“Night temperature does affect seed production of feathertop Rhodes grass so it is important to concentrate efforts on preventing germination or controlling these weed populations in spring and early summer,” says Dr Chauhan. “These early germinated populations are also more able to compete with summer crops and then set seed in-crop.”
Being able to tolerate both knockdown and residual (pre-emergent) herbicides, FTR grass can quickly gain a foothold in no-till farming systems. No-till and stubble retention also provide a favourable environment for germination, establishment and survival of FTR grass because of the moist soil conditions around the weed seed.
An integrated approach, like the WeedSmart Big 6, is needed to tackle this serious weed before it forces a return to full cultivation for weed control.
Diverse crop rotations – FTR grass is a year-round weed. Having diverse and competitive crops in rotation reduces the risk of a blow-out situation.
Mix and rotate herbicide MOA – FTR grass is not reliably controlled with a single post-emergent herbicide application. To be effective, the weeds must be sprayed when they are very young and not stressed.
At this stage, high rates of glyphosate with the best surfactants available, along with some group A products, can reduce the weed population. Residual herbicides like metolachlor, applied in late winter fallows, are very useful in moist soil conditions.
Herbicide is largely ineffective on FTR grass unless it is applied to small, actively growing seedlings.
Double knock glyphosate – Plan to follow any glyphosate application with a double knock to reduce the number of FTR grass survivors.
Grow competitive crops – 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 germination. Early weed control in sorghum can effectively suppress weed seed production of FTR grass plants that germinate later in the crop. A competitive cover crop could also be a valuable option.
FTR grass is susceptible to crop competition. Front – FTR from fallow, Middle – FTR from 1m row sorghum and Back – FTR from 0.5m row sorghum.
Stop weed seed set – This is the single-most effective tool to prevent an FTR grass incursion. FTR grass is a prolific seed producer and can quickly get out of hand. Initial invasions often occur as a weedy patch forms around a few ‘mother’ plants. Removing large FTR grass plants before they seed, using patch cultivation, chipping, hand pulling or fire, is the best option. Seed is easily spread in overland flow and on vehicles, machinery (particularly headers), people and animals. Extreme care is required when managing weedy patches to avoid spreading the problem.
Burning individual plants is one option to stop seed set on FTR grass.
Harvest weed seed control – FTR grass could be a good candidate for weed seed collection and destruction at harvest. One study has shown that as much as 93 per cent of the weed seed was retained (held) on the plant at the time of mungbean harvest (Chauhan et al., unpublished data). Increased crop competition tends to encourage taller FTR grass plants, making it easier to capture the seedheads at harvest.
“We also found that FTR grass seed on the soil surface is not viable after 12 months. Burying the seed lengthens the period that the seed remains viable, so unless the seedbank is completely buried to a depth of 5 cm or more and left undisturbed for more than 18 months, cultivation on its own might not be a good control tactic,” says Dr Chauhan. “If FTR grass seed is left on the surface, and no more seed is allowed to set, the seed bank will deplete in 12 months. In dry years the seed is likely to persist longer and some seed can be buried at planting or simply falling down cracks in the soil.”
Feathertop Rhodes grass is already widespread across Australia and it is easily transported to new areas during floods, on machinery and in hay. Roadsides, water channels, head ditches, and on-farm tracks are all sources of weed seed, which can then easily enter cropping areas. If hay is brought in, it is wise to feed out in defined areas so any FTR grass plants can be more readily seen and removed before they set seed. It is also important for agronomists, researchers and contractors to strictly follow biosecurity practices and ‘Come Clean, Go Clean’.
Find out more:

Weed biology insights to improve management of feathertop Rhodes grass and barnyard grass
Spring into action with fallow residuals
How do I deal with an emerging feathertop Rhodes grass problem?

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Weed management programs for pigeonpea

WeedSmart Audio · WS – Pigeonpea – 5-min – Read – 3 01
Pigeonpea might not be well known in Australia, but there are more than a billion people in Asia and Africa who eat this dried grain legume in a variety of dishes, and global demand is high.
This demand is driving renewed interest in the crop, which was introduced to Australia in the 1970s but never developed into commercial production. In fact, it has only been grown as a trap-crop to monitor the incidence of Helicoverpa armigera (pod borer or bollworm) in Bt cotton crops.
This trial used paired row sowing (right) to give pigeonpea a competitive advantage over weeds compared to sowing in a wider configuration (50 cm, left).
As an emerging crop of importance to the grains and livestock industries, researchers have begun the task of investigating the agronomic requirements of pigeonpea in the Australian environment. Leading the way on the weed management front are researchers Gulshan Mahajan, Rao C. N. Rachaputi and Bhagirath Chauhan from the Queensland Alliance for Agriculture and Food Innovation (QAAFI), The University of Queensland at Gatton.
“Pigeonpea is of interest because it is a drought and heat-tolerant summer legume, and provides both grain for human consumption and high quality fodder for livestock,” says Dr Bhagirath Chauhan. “Its slow growth habit is a significant limitation, making the crop very susceptible to yield loss as a consequence of competition from weeds.”
In the summers of 2017 and 2018 the researchers tested the effect of row spacing and herbicide applications on crop yield. Their findings suggested that narrow row spacing (25 cm) and sequential herbicide applications provided effective weed control that preserved yield in pigeonpea.
Across the two years there was a marked difference in seasonal conditions and weed flora. In 2017, the only weed present at the site was giant pigweed but following a deep tillage operation there was a more complex community of weeds, particularly grasses, growing at the trial site in 2018.
“At the narrow (25 cm) and wide (50 cm) row configuration, a single application of the pre-emergent (pendimethalin) or a sequential application of pre-emergent and post-emergent (imazapic) herbicide reduced weed biomass and increased yield, compared to the no-control treatment,” he says. “When the sequential herbicide program was applied, it was equally effective in the narrow, wide and paired row configurations, giving growers more flexibility.”
Narrow row spacing (25 cm) and sequential herbicide applications provides effective weed control that preserves yield in pigeonpea.
“In seasons where conditions are too dry for the pre-emergent herbicide to be effective and there is a heavier reliance on the post-emergent herbicide alone, or where there are multiple weeds present, having the crop sown on narrower rows provides better weed control and preserves crop yield.”
In the right conditions, the sequential use of herbicides addresses the common problem of multiple flushes of weeds over summer.

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2019 Top 5 WeedSmart experts, bulletins and podcasts

This year we have collected the topics that you, our audience, found the most interesting – based on how many people browsed, downloaded or listened to material on the website. Here are the results!
Starting with our expert columns where we ask industry leaders for their take on big decisions facing weed managers.
1. WeedSmart Expert
https://www.weedsmart.org.au/whats-the-latest-in-optical-sprayer-technology/
2. WeedSmart Expert
https://www.weedsmart.org.au/is-mechanical-site-specific-weed-control-a-practical-fallow-management-option/
3. WeedSmart Expert
https://www.weedsmart.org.au/how-can-i-avoid-getting-stuck-in-an-imi-herbicide-cycle/
4. WeedSmart Expert
https://www.weedsmart.org.au/is-rapid-on-farm-herbicide-resistance-testing-possible/
5. WeedSmart Expert
https://www.weedsmart.org.au/does-diversity-help-with-weed-control-and-herbicide-resistance/
Next we will count down the top 5 articles on the bulletin board.
1. WeedSmart Bulletin
 
https://www.weedsmart.org.au/stacking-the-big-6-in-a-strip-and-disc-system/
2. WeedSmart Bulletin
https://www.weedsmart.org.au/robotics-opens-up-more-non-herbicide-options/
3. WeedSmart Bulletin
https://www.weedsmart.org.au/and-then-there-were-three-impact-mills-on-the-market/
4. WeedSmart Bulletin
https://www.weedsmart.org.au/vertical-ihsd-maintains-the-brands-98-weed-kill-rate/
5. WeedSmart Bulletin
https://www.weedsmart.org.au/is-sunlight-breaking-down-pre-em-herbicides-where-farmers-are-dry-sowing/
Last but far from least, here are the top three podcasts!
1. WeedSmart Podcast
https://www.weedsmart.org.au/podcasts/chaff-dump-management-and-strip-and-disc-systems/
2. WeedSmart Podcast
https://www.weedsmart.org.au/podcasts/vertical-ihsd-test-results-and-the-weedit/
3. WeedSmart Podcast
https://www.weedsmart.org.au/podcasts/weather-forecast-accuracy-and-crop-competition-trial-results/
The WeedSmart team looks forward to bringing you more great ideas from growers, agronomists and researchers across Australia to help manage weeds.

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Article

Implementing ‘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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Article

Big 6 winter – 4. Crop competition

A competitive crop will suffer less yield loss at the hands of the weeds, and will also reduce seed set of the weeds compared to an un-competitive crop. In other words more crop, fewer weeds.

Principle #1Stay ahead of the pack
Crop competition with weeds is a ‘winner-takes-all’ battle. When the crop wins their is higher grain yield plus lower weed biomass and lower seed set.
For the crop to win this battle it is very important to give it a head start with effective early weed control. Keeping the crop weed free for the first three to six weeks seems to be a practical target.
To maximise the effectiveness of using the crop as a weed control tactic, start planning the year ahead and do everything possible to reduce the weed seed bank using effective herbicides, weed seed burial, competitive cultivars and harvest weed seed control tactics or hay-making. Back this up with registered pre-emergents and as many non-herbicide tactics in-crop as possible.
Principle #2 Adopt at least one competitive strategy (but two is better)
There are six main avenues to increase crop competition:
1. Increased seed rate
2. Narrower row spacing – without changing seeding rate)
3. Row orientation – sowing east-west where practical
4. Crop choices – more competitive species and / or variety
5. Soil health – less compaction, fix pH and nutrient limitations
6. Time of sowing – early sowing is usually best
It’s hard to include all six in every crop or every paddock – the more you can do the better the odds for your crops to suppress weed growth and seed set.
https://www.weedsmart.org.au/whats-the-best-way-to-out-compete-resistant-annual-ryegrass-in-cereals/
https://www.weedsmart.org.au/whats-the-best-way-to-manage-annual-ryegrass-in-chickpea-crops/
https://www.weedsmart.org.au/sow-east-west/
https://www.weedsmart.org.au/paired-rows-give-entry-level-crop-competition/
https://www.weedsmart.org.au/taking-the-competition-to-the-weeds/
https://www.weedsmart.org.au/crop-competition-give-your-crops-the-edge/
https://www.weedsmart.org.au/up-the-competition-with-professor-of-agricultural-innovation-deirdre-lemerle/
 
The Big 6
1. Rotate crops and pastures
2. Double knock – to preserve glyphosate
3. Mix and rotate herbicides
4. Stop weed seed set
5. Crop competition
6. Harvest weed seed control – the holy grail
WeedSmart Wisdom

Article
Article

Big 6 winter – 3. Double-knock to protect glyphosate

The idea of a double-knock for weed control is to use one tactic, usually a herbicide, to kill the majority of weeds and follow-up with another tactic, usually a herbicide from a different mode of action group, to kill any survivors.
This can also be a herbicide followed by a non-herbicide tool (eg. the ‘Canola Combo’ – crop top followed by HWSC). All that really matters is any resistant survivors to the first herbicide are hit with another control measure so that the weeds don’t set seed.
 

Principle #1 Follow glyphosate with a high rate of paraquat to control survivors in a fallow or pre-sowing situation
Glyphosate is the world’s most important herbicide and nothing else comes close as a low cost, reliable knockdown, so we really need to look after it.
In the fallow and / or pre-sowing apply glyphosate as the first knock, followed by a second knock with paraquat or paraquat + diquat to take out any resistant plants that have survived the glyphosate. If the main weed problem is annual ryegrass then using paraquat on its own as the second knock is an appropriate choice. If there are also broadleaf weeds present then the paraquat + diquat combination (e.g. Spray.Seed®) will be more effective overall.
A herbicide double knock is all about timing and relies on using the second knock while weeds are still small – usually one to seven days after the first knock. With a non-herbicide second knock the timing is less critical, provided surviving weeds are not permitted to set seed.
Building the double knock treatment into a whole-of-season weed management plan provides opportunities to get more ‘bang for your buck’. For example, follow a pre-sowing double knock with pre-emergent herbicides, and increase the level of crop competition with narrow row spacing, optimal sowing time and varieties with vigorous early growth.
The first knock is to kill all plants still susceptible to glyphosate—applying a lower rate risks higher survival rates, increasing the pressure on the second knock products. The second knock of Spray.Seed® or paraquat is to kill plants that survived the glyphosate. Reducing the rate of the second knock risks survival of potentially glyphosate resistant individuals and damages the integrity of the double knock tactic. Remember that paraquat and Spray.Seed® are contact herbicides and require robust water rates to ensure adequate coverage and allow for losses on stubble.
If there is a mix of weeds present it can be useful to include a compatible herbicide ‘spike’ such as 2-4D low volatile ester, carfentrazone or oxyflouren to enhance control of broadleaf weeds. Be very mindful of plant-back requirements of some herbicide ‘spikes’ before planting sensitive crops such as pulses and canola.
Optical sprayers such as Weedseeker and WEEDit are an efficient way to apply the second knock to kill any survivor weeds.
Even the highly effective double-knock tactic is at risk if growers don’t remain vigilant and ensure removal of any surviving plants.
https://www.weedsmart.org.au/protecting-knock-down-herbicide-options/
https://www.weedsmart.org.au/confirmed-resistance-to-the-double-knock-tactic-in-tall-fleabane/
https://www.weedsmart.org.au/whats-the-latest-in-optical-sprayer-technology/
 
The Big 6
1. Rotate crops and pastures
2. Double knock – to preserve glyphosate
3. Mix and rotate herbicides
4. Stop weed seed set
5. Crop competition
6. Harvest weed seed control – the holy grail
WeedSmart Wisdom

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