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How can I be certain that herbicide residues in the soil have fully degraded at planting?

In the course of a chemical fallow there are often several applications of herbicide and some residues may still be present on or near the soil surface when it is time to plant the next crop. In particularly dry years, residues may even carryover from the crop prior to the fallow. NSW Department of Primary Industries soil scientist, Dr Mick Rose, says there has been concern in recent years about the effect these residues may have on soil microbial activity and on the establishment and growth of crops following the fallow, even after the plant back period. Dr Mick Rose, DPI NSW soils researcher, is developing tests and predictive models to support growers in their decisions about crop choice after using residual herbicides. (Photo: GRDC). “Glyphosate has been the most commonly used knockdown herbicide in northern fallows for several decades and more recently growers have been looking to use more diverse programs that include chemicals with residual activity on weeds,” he says. “The increased use of imazapic and diuron have been of most concern to growers when choosing the next crop, particularly after a low rainfall fallow period.” With investment from GRDC, Mick has been working on a project led by Dr Michael Widderick from the Department of Agriculture and Fisheries, Queensland to develop a soil test for imazapic and diuron residues that will indicate damaging residue levels and help growers to decide which crops would be safe to plant in a paddock. “We are determining the threshold levels of residues of these two herbicides at which crop damage is likely for six crops, both winter and summer growing, in a range of soil types,” he says. In earlier work he also looked at the level of glyphosate residue in soils around the country at planting time and the impact these residues have on soil biological processes. “We found that residues of glyphosate were commonly detected in the soil at planting but there was no indication that the herbicide was adversely affecting soil biological activity,” says Mick. “This suggests that the label recommendations are suitable and the proper application of glyphosate in Australia is not posing a threat to soil health.” “For growers to be able to keep using glyphosate they need to implement the WeedSmart Big 6 strategy, including using diverse chemistry in fallows,” he says. “Residual herbicides are a useful tool for growers but there are some gaps in our knowledge about how these herbicides break down in different soils and under different seasonal conditions.” Why not just follow the plant back recommendations on the label? In brief: The label provides the minimum plant back period provided certain environmental conditions are met. There is a possibility of crop injury even though plant back periods are observed. The details: Many factors affect the bioavailability of a herbicide in the soil. For example, even though a clay soil and a sandy soil might have similar residue levels, more herbicide will be available for uptake in the sandy soil. More rain will increase the rate of breakdown, but it is not known exactly how much rain will ensure the specific soil is ‘safe’ to plant into. Another important factor is that many things can contribute to a germination failure. In some situations, residual herbicide may be suspected as the culprit, but can be difficult to either rule it in or out with certainty when diagnosing the reason for a problem at planting. If herbicide residues in the plant tissue can be shown to be phytotoxic, then another, less susceptible crop could be sown into the paddock. Dr Annie Ruttledge, DAF Qld weeds researcher, inspecting chickpea plants growing in soils containing different levels of imazapic and diuron herbicide residue. What effects can herbicide residues have on emerging crops? In brief: The herbicide itself can inhibit germination and growth, or it can exacerbate other factors, such as root disease. The details: At different levels of bioavailability, herbicide residues will have different effects on crop plants. If the herbicide is readily available to the plant, then susceptible crops will take it up from the soil and it can have phytotoxic effects ranging from suppressed vigour to yellowing and potentially plant death. Testing the plant tissue of a struggling crop can show if the leaves contain sufficient herbicide to have caused the observed symptoms. Some herbicide residues in soil can also ‘prune’ plant roots, particularly the fine roots that help access moisture and nutrients. Obviously, if the young plants are struggling to access resources then they will be less vigorous and possibly die. Damaged roots are also more susceptible to water stress, disease and poor nodulation in legumes, making it difficult to determine the initial cause of the problem in the field. If herbicide residues are shown to be the problem then a more tolerant crop can be sown, speeding up the breakdown of the residue and there will be more rainfall events before the next cropping season comes around. Seedling emergence and establishment is being measured for six crops (winter and summer) in the presence of different levels of herbicide. What pre-planting soil tests are being developed to give growers confidence to plant? In brief: The current project is establishing phytotoxicity thresholds for six summer and winter crops in a range of soil types, for two herbicides – imazapic and diuron. The details: By mid-2021 the aim is to have established the thresholds so that soil could be tested pre-plant to determine what crops would be safe to plant. This will give growers confidence to use these herbicides in a diverse strategy to manage weeds like feathertop Rhodes grass in the fallow, while avoiding germination or establishment failures in the following crop. Spray records play an important role in the management of these herbicides and mistakes can easily be made if the spray history for the past several years is not taken into account. In time, growers and their agronomists will gain a better understanding of how these herbicide residues behave in the soils on a particular property and will be able to make herbicide application and crop rotation decisions with more confidence. In another project with the Soil CRC, Mick is developing a predictive model for herbicide breakdown for a wider range of herbicides used in southern and western cropping systems. Until these tests and models become available, the use of an in-field or pot bioassay with a susceptible crop can be helpful in determining potential plant back issues.   Related resources Herbicide residues in soil – the scale and significance (GRDC Update paper) Herbicide residues in soil (GRDC Podcast)
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Maximising the weed control value of my crop rotation

with Kevin Morthorpe, Trait & Seed Technology Stewardship Manager, Pioneer Seeds A diverse crop rotation is the twine that holds a good farming system together and underpins an effective weed management program.   Kevin Morthorpe, Pioneer Seeds’ Trait & Seed Technology Stewardship Manager says herbicide tolerance traits in crop hybrids can be used to maximise competition against weeds and increase the herbicide options available to growers while optimising yield and profitability of the crop sequence in rotations. Kevin Morthorpe (left) – Pioneer Seeds’ Trait & Seed Technology Stewardship Manager, with Dr Ray Cowley – Canola Research Scientist, Corteva Agriscience and Pioneer Seeds’ Rob Wilson – Strategic Customer & Market Development Manager and Clint Rogers – Western Regional Sales Manager & Canola Product Lead at a canola research trial near Jindera in southern NSW. Plant breeders continue to introduce herbicide tolerance traits in a number of crops in Australia, including corn, canola, pulses, cereals, grain sorghum, summer forages and cotton. “For example, in canola there are several herbicide tolerance traits and they are primarily available in hybrids,” he says. “This means growers get both improved crop performance due to hybrid vigour and more flexibility in herbicide use patterns.” The increased vigour of canola hybrids also generates greater biomass production and early canopy closure that suppresses growth and seed set of weeds that germinate in-crop, complementing the use of pre-emergent herbicides. “Hybrids super-charge crop competition through a strong root system and vigorous growth,” Kevin says. “From an economic angle, hybrids optimise yield in both high input and tough environments. In fact, we see more growers selecting hybrids when producing canola in tough conditions.” Since the release of the first herbicide tolerant canola in 1991, the popularity of herbicide tolerance has seen a 98 per cent adoption of canola varieties with tolerance to imidazolinone (Clearfield), triazine (TT) or glyphosate (RR). In the last 15 years, the area sown to hybrid canola has risen to an impressive 47 per cent in Australia. With glyphosate tolerant canola hybrids entering South Australia in 2021 and new hybrid releases, the hybrid percentage will increase further over coming years. With glyphosate tolerant canola hybrids entering South Australia in 2021 and new hybrid releases, the hybrid percentage will increase further over coming years. Kevin says that Pioneer Seeds have seen increasing demand for Clearfield canola in recent years following a dip in popularity. Through strategic application of herbicide tolerant traits in diverse crop rotations it seems that farmers are overcoming the resistance problems that were prevalent with the Clearfield technology and can now re-introduce these varieties and take advantage of the weed control benefits and high yields they offer, and manage herbicide residues in the soil. “A diverse rotation of crops and pastures is one of the WeedSmart Big 6 tactics, which Pioneer Seeds endorses wholeheartedly to protect the longevity and effectiveness of herbicide tolerance traits,” he says. “Through an effective crop rotation you can tick off all the herbicide and non-herbicide tactics needed to drive down weed numbers.” How do I make the most of a hybrid crop? In brief: Employ best practice agronomy. The details: Grain hybrids are vigorous plants that produce increased biomass and grain yield. To do this, they must be supported with adequate crop nutrition. When properly fed, hybrids will provide increased crop competition and achieve greater water use efficiency compared to their conventional counterparts. Growing a hybrid crop with herbicide tolerance traits does not equate to a full weed control program. These crops must be used within the WeedSmart Big 6 framework, within a diverse crop rotation and using herbicide tactics such as double knocking alongside cultural practices such as harvest weed seed control and crop competition to reduce seed set. They also combine well with pre-emergent herbicides to achieve excellent early weed control and suppress seed set in any late germinating weeds. Can I use hybrid crops with herbicide tolerance to fix a weed blow-out? In brief: No. This technology is not suitable for salvage operations. The details: When Roundup Ready canola varieties were first released there was an expectation that these traits could be used to reverse a weed infestation. This proved not to be the case. Hybrid crops are best used in low weed density situations where they can effectively drive down the weed seedbank. They should be grown in rotations that include an effective double-break, brown manure crop or a pasture phase. Having hybrid crop options for both summer and winter growing seasons increases the opportunities to tackle weeds throughout the year or to use different fallow herbicides while maintaining the ability to safely grow crops in the following season. New glyphosate tolerance traits (Truflex® and Optimum GLY®) and the stacking of herbicide tolerance traits of triazine tolerant and Clearfield® (TT+CL) have expanded the safe window for herbicide application in canola. Are residues in grain a concern when using stacked trait herbicide tolerant hybrids? In brief: Not if the stewardship program is followed. The details: New glyphosate tolerance traits (Truflex® and Optimum GLY®) and the stacking of herbicide tolerance traits of triazine tolerant and Clearfield® (TT+CL) have expanded the safe window for herbicide application in canola. This gives more options, more flexibility and more crop safety through the rotation. The stewardship program for the herbicide tolerant trait hybrids describe herbicide use patterns that growers must follow to confidently avoid the accumulation of herbicide residue in the grain and ensure that Australian maximum residue limits (MRLs) will not be exceeded. [Note that MRLs in other countries may be different to the Australian MRL. Find out more at Grain Trade Australia]  To avoid problems with crop safety within the rotation it is important to maintain accurate paddock records to avoid applying herbicide to the wrong crop variety and ensure susceptible crops are not sown into paddocks with herbicide residues in the soil. On the flip-side, herbicide tolerance in crops increases the options for crop selection within the rotation. Also, keep in mind the importance of controlling any volunteers from a herbicide tolerant crop in the summer fallow or following crop.
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Harvest weed seed control in a nutshell

*Note: In Australia we call the whole machine a harvester, not just the cutting front. At harvest time many weeds that have grown in the crop still have seed held in the seed head. These seeds enter the harvester along with the grain and most exit the harvester and are spread across the paddock in the chaff and straw. Collecting these weed seeds at harvest and either destroying them or depositing them in a known location where they can be monitored and controlled later, is an excellent way to stop weeds in their tracks. Brome grass is the most costly weed for Mallee farmers to manage, even though herbicide resistance in brome grass is currently low in the region. If you are considering adding harvest weed seed control (HWSC) to your weed control program there are excellent resources on the WeedSmart website to help guide you through the initial decisions and the implementation of this important weed control tool. Key messages: Decide on which system fits your farm best. Get maximum weed seed into the header. Know how to manage the collected weed seed. Which system is best? HWSC is being rapidly adopted in Australia and other countries around the world. There are six systems currently being used on Australian farms and they have all been developed by farmers. Research has demonstrated that all are very effective weed control tactics, achieving over 80 per cent control and for some nearly 100 per cent. There are six systems currently used to collect and manage weed seed at harvest: chaff carts chaff lining chaff decks (chaff tramlining) impact mills Bale Direct narrow windrow burning While they are all effective, they vary considerably in capital and ownership cost, nutrient removal costs, operational costs and labour costs. 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. To calculate the cost of each method for your farm you can use a calculator developed by AHRI’s Peter Newman. https://www.weedsmart.org.au/calculating-the-cost-of-hwsc-for-your-farm/ The HWSC tools all involve some modification to the harvester. The simplest modification is for chaff lining and narrow windrow burning, where a simple chute is attached to the rear of the harvester to direct the residue into a band on the ground, running the same direction as the harvester has travelled. These chutes are often constructed and fitted on-farm. All the other systems are commercial modifications that are fitted to the harvester – chaff decks and impact mills – or trail behind the harvester – chaff cart and Bale Direct. WeedSmart resources: Videos from the HWSC course outline the science and practice of HWSC https://www.weedsmart.org.au/resources/hwsc/ Calculating the cost of HWSC https://www.weedsmart.org.au/calculating-the-cost-of-hwsc-for-your-farm/ Stepping into chaff lining https://www.weedsmart.org.au/stepping-into-chaff-lining/ Using your harvester to destroy weed seeds https://www.weedsmart.org.au/using-your-harvester-to-destroy-weed-seeds/ Get the weed seeds into the header Harvest weed seed control only works on weed seed that enters the header. Getting the weed seed into the header relies on the seed being held in the seed head at the time of harvest. The seed head must also be at harvestable height. Consider the weed spectrum and the likelihood of seed capture. Even if some seed has shed, chances are there will be other seed heads that have not yet shed and even this will assist with reducing the amount of seed entering the seed bank. There are four chaff-only systems and two all-residue systems. The chaff-only systems – chaff carts, chaff lining, chaff decks and impact mills – require the harvester to be set up to separate chaff and straw, and to keep the weed seed in the chaff stream. This may require modifications to the harvester rotor and sieves and the installation of a baffle to keep the weed seed in the chaff stream. If you choose the Bale Direct system or narrow windrow burning, all the straw and chaff ends up in the same place, so no other modification to the harvester is needed. WeedSmart resources: Harvester setup for HWSC https://www.weedsmart.org.au/webinars/harvester-set-up-for-harvest-weed-seed-control-hwsc-for-all-header-colours/ Getting weed seed into the chaff stream https://www.weedsmart.org.au/setting-up-harvesters-to-capture-weed-seed-in-the-chaff/ Using HWSC in different weed spectrums https://www.weedsmart.org.au/is-harvest-weed-seed-control-a-real-option-for-managing-northern-region-weeds/ Manage the weed seed after harvest If you choose an impact mill as your HWSC tool then the tactic is completed in one pass at harvest, with nothing extra to do. All the residue is spread in the field and the weed seeds are rendered unviable. All the other HWSC tools involve some action after harvest to remove or destroy the weed seed collected at harvest. Chaff decks deposit the weed seed-laden chaff in one or both harvester tramlines or wheeltracks. Some growers find that the chaff rots and the weed seeds die, but in other environments growers find that it is necessary to control weeds that germinate in the tramlines using herbicide or non-herbicide tactics applied just to the tramlines. Chaff carts can be emptied as they fill in the paddock or emptied at a central point. Many growers use chaff piles as a high nutrient value stockfeed, others burn the piles and others leave them unburned in the paddock and sow through them the following season. Chaff lines are usually left unmanaged with the expectation that the following crop will provide adequate competition to the weeds to minimise weed growth and seed production. The Bale Direct system results in large bales of crop residue that can be sold into suitable markets. Distance to market is usually an important factor in the success of this system for HWSC. Narrow windrow burning uses fire to destroy the weed seed in the Autumn following harvest. There are significant labour costs and safety risks to consider along with the loss nutrients and ground cover. Key resources to learn more: Diversity Era online course – Harvest weed seed control 101 https://www.diversityera.com/courses/harvest-weed-seed-control-101 Kondinin Group Residue Management at Harvest – Weed Seed Options research report https://www.weedsmart.org.au/app/uploads/2018/06/RR_1802_weedsmart.pdf Kondinin Group Harvest Weed Seed Warriors research report https://www.weedsmart.org.au/app/uploads/2020/05/RR_February_2020_Weedsmart.WS_.2020.pdf Grower experiences: Chaff decks and chaff lining in a high rainfall zone https://www.weedsmart.org.au/case-studies/esperance-growers-using-chaff-decks-and-chaff-lining/ Keeping pressure on brome grass with HWSC https://www.weedsmart.org.au/case-studies/bruce-family-alford-sa/    
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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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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
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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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Big 6 winter – 2. Mix and rotate herbicide MOA

Before herbicide selection has taken place it is very rare for an individual weed to be resistant to two herbicides. Mixing herbicides at full label rates in a single application takes advantage of this fact. Within an integrated weed control program, try to make sure there is rotation and mixing going on in each phase – in the fallow, pre-seeding, in-crop and for desiccation. When done in conjunction with a determination to stop seed set and remove survivors, it is possible to keep weed numbers low.   Principle #1 Rotating buys you time, mixing buys you shots Herbicide products are classified and grouped according to their mode of action (MOA). That is, products that target the same lethal pathway are grouped together. Rotating between herbicide modes of action has the beneficial effect of ‘buying time’ because if a MOA is used once every two years the lifespan of the herbicide effectively doubles. 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 – this ‘buys you shots’. Principle #2 Rotate between herbicide groups Start with a herbicide susceptibility test to find out what herbicides and herbicide mixes are still effective. Rotating between products within the same MOA group is the same as using one product all the time and is a very high risk weed control tactic. Crop rotation can drive rotation of herbicide MOA. For example, if you use trifluralin in canola, consider another registered MOA option for wheat. Based on research from the Australian Herbicide Resistance Initiative, the best advice to growers and agronomists is to rotate between these three groups of pre-emergent herbicides – 1. trifluralin, 2. Sakura, Boxer Gold and triallate and 3. propyzamide. Full label rates must be applied. Principle #3 Use different groups within the same herbicide mix Mixtures are more effective than just rotating MOA in delaying resistance as mixes generally achieve a greater kill rate. While some mixes are physically compatible, the components might have different requirements for optimal application – such as droplet size and and water rates. 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. Rotating herbicide groups is an effective way of slowing resistance build up and managing resistance while ever there are actives to go to. Mixing herbicides from the same or separate active groups can also give you room to move in holding off resistance or getting more out of some marginally effective products. In fallows, where there is no additional crop competition, a second (double knock) application to the same weed germination can be very effective. Some common double knock approaches that use the mix and rotate strategy are: 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 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. Principle #4 Always use full label rates Reducing the application rate of herbicides increases a weed’s ability to evolve resistance. Any saving in chemical costs is significantly outweighed by the risk of the low dose causing faster herbicide resistance evolution. Having dual action does not negate the need to change herbicide products and rotate modes of action. Repeated use of any single strategy will reduce the effectiveness of that strategy over time. Some herbicides require better coverage. In many instances the most important mixing partner is more water. https://www.weedsmart.org.au/using-tank-mixes-to-extend-herbicide-life/ https://www.weedsmart.org.au/how-does-mixing-moas-buy-more-shots/ https://www.weedsmart.org.au/dont-start-mixing-water-quality-right/ https://www.weedsmart.org.au/factsheets/mixing-requirements-for-spraying-operations/   The Winter 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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Big 6 winter – 1. Rotate crops and pastures

Short rotations cause herbicide resistance! This is because of the inherent lack of diversity. Weed populations quickly respond to routine management practices – those that survive will set seed and their progeny have an advantage when the same control tactic is used again soon after. Principle #1 Diverse herbicide choices, diverse cultural practices Following a longer crop rotation, and including a pasture phase if possible, means having more tools in the toolbox—better herbicide rotation, a range of seed set control options, varied planting times, competitive crop species or varieties and the ability to implement a variety of harvest weed seed control options. If you can’t see your way clear to lengthen your crop rotation, look for ways to increase diversity within the crops you grow. Changing varieties may allow a different sowing time and in herbicide tolerant crops such as canola, you can rotate between the RR and TT hybrids. In a tight rotation harvest weed seed control and maximum crop competition are even more important. Principle #2 Use double breaks, fallow and pasture phases to drive the weed seedbank down over consecutive years Rotating to a double (or triple) break crop is a great way of smashing the weed seed bank, setting the paddock up for a long crop phase. Examples of double break combinations are hay/canola, pulse/canola and fallow/canola and single breaks are still often used in the rotation. Some growers also include two crops of the same type (e.g. two cereals) in succession and rotate chemistry or change planting date. If you can include a pasture, it is important that you always go into the crop phase with low weed numbers and also go into the pasture phase with low numbers. Sowing a pasture should not be seen as a re-set option after a weed blow out. Take a planned approach right across the pasture phase and use a number of tactics known to be highly effective at preventing seed set. Several of the tactics available for use in a pasture phase can provide over 90 per cent control of the target weeds. The plan needs to outline how the pasture phase will fit into the crop rotation and what tactics will be used seasonally and rotationally to maximise the effect on weed numbers. Including a pasture phase does not always mean gearing up to run  livestock. The pasture could be grown for hay or silage or even as a fallow cover crop. https://www.weedsmart.org.au/does-diversity-help-with-weed-control-and-herbicide-resistance/ https://www.weedsmart.org.au/why-is-diversity-so-important-in-the-battle-with-herbicide-resistance/# https://www.weedsmart.org.au/is-crop-rotation-an-economic-option-for-managing-weeds/ https://www.weedsmart.org.au/will-longer-rotations-really-help-manage-weeds/ https://www.weedsmart.org.au/can-use-pasture-phases-beat-herbicide-resistant-weeds/ 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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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.    Other resources: AHRI Insight – Easy to adopt crop competition tools
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Never cut the herbicide application rate

Scientific studies have demonstrated that resistance can rapidly evolve in weeds subjected to low doses of herbicide. Some weeds can develop resistance within a few generations. Full rates when mixing herbicides too! When mixing herbicides it is important that each product is still applied at the full label rate to ensure high mortality. Applying different chemicals in one mix can provide an additive advantage. It is important to understand the mode of action of each herbicide on the plant when preparing a herbicide mix. This is just as important for pre-emergent grass weed mixes as it is for post-emergent mixes aimed at broadleaf weed control. ALWAYS READ THE LABEL. Surrounding weed seeds with a combination of pre-emergent herbicides with different modes of action can give a high level of control and help extend the useful life of all the chemicals used. The high level of control must be supported with additional control measures for all survivors. All products with different modes of action must be applied at full label rates for this to be an effective strategy.   Mixing two chemicals with the same mode of action can achieve some additional efficacy, however, the mix should deliver the combined full rate to ensure a lethal dose. The amount of stubble present and crop safety are all important considerations when mixing chemicals. For example, when using a tank mix of Avadex® and trifluralin to control ryegrass in wheat, the rates used will vary depending on the sowing system and level of stubble retention. Be sure to get good advice. Many herbicides on the market are a combination of two or more modes of action within the one product. These products must be applied at the full label rate to be effective. Having dual action does not negate the need to change herbicide products and rotate modes of action. Repeated use of any single strategy will reduce the effectiveness of that strategy over time.  
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Spray well – correct nozzles, adjuvants and water rates

Spray application is a technical field and growers need to make sure their equipment and application techniques are spot-on. The GRDC Spray Application GrowNote provides detailed information and about 80 videos to demonstrate key skills. Prevent spray-drift The focus of spraying herbicide needs to be on doing the job right so the weeds receive the correct dose and die, and this includes reducing the air borne fraction to a bare minimum. Bill Gordon’s 10 Tips for Reducing Spray Drift Choose all products in the tank mix carefully. Understand the product mode of action and coverage requirements. Select (and check) the coarsest spray quality that will provide effective control. Expect that surface temperature inversions will form as sunset approaches and will likely persist overnight and even beyond sunrise on many occasions. DO NOT SPRAY. Use weather forecasts to inform your spray decisions. Only start spraying when the sun is about 20 degrees above the horizon and when the wind speed has been above 4–5 km/hr for more than 20–30 minutes, and clearly blowing away from any adjacent sensitive crops or areas. Set the boom height to achieve a double overlap of the spray patterns. Avoid higher spraying speeds. Leave buffers unsprayed if necessary and come back. Continue to monitor conditions, particularly wind speed, at the site during the spray operation High water rates don’t have to slow you down Some growers are concerned that increasing the water rate when applying herbicide will slow down their spray operation and cost them money. However, the biggest financial loss during spraying usually comes from a failed spray job. To keep your spray operation as time efficient as possible when using more effective and reliable application volumes, you can: Use nurse tanks around the farm to reduce the time spent travelling back to a central re-fill point. Use a larger pump, e.g. 2.5 inch, to make re-filling quicker. Pre-mix the batch while the sprayer is operating. Many mixes can be held in the mixing tank for up to 6 hours. However, wettable granules and suspension concentrates will need agitation to keep them in solution. For pre-emergent herbicides in high stubble situations, carrier volume has a large effect on the level of control achieved. Across four trial sites Dr Borger’s research demonstrated that ryegrass control with trifluralin or Sakura® increased from 53% control when the carrier volume was 30 L/ha to 78% control when the carrier volume was increased to 150 L water/ha in high Water quality and mixing order Water quality is often overlooked as a possible contributor to herbicide failure and can lead to confusion over the herbicide resistance status of weeds on a property. Water should be considered as one of the chemicals in any mix, given that water quality varies markedly depending on its source. Getting the mixing order right is essential for effective spray results. Don’t start mixing until the water quality is right Podcast – Mixing herbicides Adjuvants Sometimes adding an adjuvant is beneficial and sometimes it is detrimental; and there is an art to knowing how to best deploy these additives. When weeds are susceptible to the applied herbicides, the effectiveness of adjuvants generally goes un-noticed. Correctly applied adjuvants can reduce the impact of low level herbicide resistance by helping to maximise the amount of herbicide taken up by the plant.
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Clean borders – avoid evolving resistance on the fence line

About one-quarter of glyphosate-resistant populations within broadacre cropping situations across Australia come from fencelines and other non-cropping areas of the farm. Along paddock borders, where there is no crop competition, weeds can flourish and, if not controlled, set lots of seed. The traditional approach has been to treat these weeds with glyphosate to keep borders clean but after 20-odd years this option is now failing and paddock borders are becoming a significant source of glyphosate-resistant weed seed. Weed researcher Eric Koetz said the limited options for managing weeds along irrigation infrastructure and other non-crop areas is a problem and is putting additional pressure on knock-down herbicides in irrigated systems. In some situations, cultivation can be used to kill the weeds and provide a firebreak, but on light soils this may pose an erosion risk and mowing or slashing may be safer options. Another possible tactic is to continue using herbicides but to ensure that a clean-up operation is carried out before any survivors can set seed. Some growers are choosing to increase the heat on weeds along the borders by planting the crop right to the fence and then baling the outside lap and spraying with a knockdown herbicide to kill any weeds and provide a firebreak. Another good option in some situations is to maintain a healthy border of vegetation using non-invasive grasses. In Queensland, buffel grass is a good example of a grass that can outcompete other weeds while not invading crop lands. If only herbicides are used on fencelines, resistance is inevitable. Surviving weeds on fencelines have no competition and access to plenty of soil moisture, so they set a lot of seed and resistance can easily flow into neighbouring paddocks. Other resources It’s time for a glyphosate intervention Farm hygiene cottons on – Cleave Rogan, St George What’s new in management of herbicide resistant weeds on fencelines? Keeping the farm clean – Graham Clapham, Norwin Don’t jeopardise glyphosate for clean fencelines Keeping fencelines clean Resistance risk to knock-down herbicides on irrigated cotton farms

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