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Making pre-em herbicides work in high stubble

Whether stubble is standing or laying flat on the ground it represents a challenge for farmers using pre-emergent herbicides to control weeds early in the cropping season. Most growers and advisors are aware that products such as trifluralin are quite tightly bound if they contact stubble during application, however the behaviour of newer products has been largely unknown. For pre-emergent herbicides to be effective the product must be placed in contact with the soil and must provide an even layer of chemical to intercept germinating weeds. Decisions at harvest will have a direct bearing on pre-emergent choices and efficacy for the next season. Yaseen Khalil is a PhD candidate at the UWA School of Agriculture and Environment, studying the availability of pre-emergent herbicides applied to crop residue and then exposed to different rainfall simulations. Decisions at harvest will have a direct bearing on pre-emergent choices and efficacy for the next season. Yaseen Khalil, PhD candidate at the UWA School of Agriculture and Environment, is studying the availability of pre-emergent herbicides applied to crop residue and then exposed to different rainfall simulations. The three pre-emergent herbicides investigated were trifluralin, Sakura (pyroxasulfone) and Arcade (prosulfocarb). As expected, trifluralin was very resistant to leaching from crop residue with minimal amounts of the product washing off stubble in rainfall events simulated just one day after herbicide application. On the other hand, a sufficient quantity of Arcade was leached by rainfall applied after 7 days to provide some control of annual ryegrass. Sakura was the clear ‘stand-out’ when it comes to efficacy of leachate. Product applied to 4 t/ha crop residue plots provided 100 per cent control of annual ryegrass when just 5 mm of simulated rainfall was applied 14 days after the herbicide application. “The implications for growers are that they can rely on significant amounts of Sakura and Arcade being washed off crop residues and into the soil if there is a rainfall event of at least 5 mm within about one week of applying these pre-emergent herbicides,” says Mr Khalil. “If the crop residue is already wet when the pre-emergent herbicides are applied, the products are more tightly held and less product leaches off the residue in subsequent rainfall events, compared with spraying onto dry stubble.” Mr Khalil’s supervisor University of Western Australia senior lecturer, Dr Ken Flower says an important factor to consider is the unevenness of residue spread at harvest. “While the residue level across the paddock may average out to an acceptable 3 t/ha it is common for the residue to be as high as 10 to 15 t/ha directly behind the header,” he says. “This in itself has implications when it comes to growers’ decisions about their weed management tactics.” Australian Herbicide Resistance Initiative communication lead Peter Newman says getting the pre-emergent product through the crop residue and onto the soil at application time is still the priority. “It is good to know that some products remain effective and can be leached off the crop residue with rainfall following the application,” he says. “Yaseen’s research has added to the growing knowledge bank about the most effective use of pre-emergent herbicides in no-till, stubble retention systems.” Whilst quite extensive in itself, this study only compared three pre-emergent herbicides. The chemical properties of the over 50 active ingredients that possess pre-emergent, or residual, activity vary enormously, and they are found in 10 mode of action groups. The efficacy of these products relies on different environmental, soil and crop residue conditions. “Understanding how these products can best fit into an integrated weed management program on a farm requires considerable thought, taking many, many factors into consideration,” says Mr Newman. “There are great opportunities to use these products to add diversity to weed management but it is essential that they are not over-relied on as 100 per cent weed control is uncommon, even when the conditions at application are as good as possible.” “We used to talk a lot about rotating herbicides from one year to the next, now we are more focused on herbicide mixes as the best way to go — mixing two pre-emergent herbicides together where possible, and then perhaps rotate to another mix next year,” he says. “They say that rotating herbicides buys you time, but mixing herbicides buys you shots. Mixing and rotating buys you time and shots, which is the best that we can hope for.” Mr Newman recommends growers take the time to discuss a variety of options with agronomists and take on board research like Yaseen’s, along with their own observations, to build a robust and ever-changing weed management program. “The aim is to keep overall weed numbers low as this is the best way to minimise the risk of herbicide resistance,” he says. Other relevant resources Podcast: Herbicides and stubble – some wash off, some don’t AHRI Insight – Herbicide and stubble GRDC Pre-emergent herbicides manual What’s the safest way to manage pre-emergent herbicides at seeding
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Satellite imagery to detect and monitor weeds

Having ‘eyes in the sky’ checking crops every ten days is a useful tool for growers and agronomists monitoring crops and managing weeds. Satellite images provide a very objective means of identifying weedy areas and measuring the response to management tactics. From his family farm in northern NSW, remote sensing tech enthusiast Ben Boughton runs a satellite imagery business, Satamap, utilised by growers and agronomists from across Australia. Satamap founder and grain grower Ben Boughton uses satellite imagery to assist with the management of herbicide resistant weeds. “Using the satellite images agronomists can look for anomalies in biomass production on their clients’ farms and can then go directly to the area on the farm to identify the problem,” says Ben. “In the case of weeds, it is easy to calculate the size of weedy patches and to monitor the impact of herbicide or cultural practices.” “An example on our own farm was to clearly see the impact when we sprayed a paddock where awnless barnyard grass was a problem and were not able to get back with the second knock due to rain,” he says. “The recovery and spread of what we now know for sure was glyphosate resistant barnyard grass showed very clearly in the satellite images taken before and after the event.” Glyphosate-resistant awnless barnyard grass is one of the Boughton’s main challenges, with a patch visible on satellite imagery following a fallow double knock where the second knock application was prevented. Agronomists use the Satamap service to observe trends across their district and to make their farm visits more time efficient. In fallow paddocks the satellite images show the extent of vegetation (weed) growth that can be hard to see amongst tall stubble. The images show an average reflectance over 10 m by 10 m area so a similar sized patch on the map may represent a number of large individual plants spread out across the area, or a carpet of grass. That is the reason why ‘boots on the ground’ are still required to investigate exactly what species are present and to plan appropriate management. “The satellite images are not a diagnostic tool on their own but they do provide valuable information for the grower and their advisor,” he says. “Some agronomists are even using imagery from the previous year to plan variable rate application of pre-emergent herbicide to treat areas where the weed seed bank could be expected to be high in paddocks with known problems.” The use of satellite imagery links well with UAV (unmanned aerial vehicles, aka drones) to assist with the ground-truthing process across a farm. The Boughtons property, north-east of Moree, NSW is predominantly winter cropping with up to 25 per cent summer cropping if the season permits. Ben says summer cropping is a great way to help clean up black oats coming out of winter cereals. The farm is 100 per cent dryland cropping with no internal fencing and all paddocks set up in a 3 m controlled traffic configuration. Ben has found the tramlines can present a weed control challenge with black oats and phalaris taking advantage of the lack of crop competition. The crop rotation is generally wheat, barley, chickpea to give a two year break for the chickpeas and avoid planting wheat on wheat. “We aim for good, even crop establishment to generate strong crop competition early in the season,” he says. “This is even more important in paddocks where we dry sow and can’t do a pre-sowing knockdown on black oats.” With Group A-resistant black oats on the increase Ben uses crop competition in barley to support a one-off use of Group A (Axial) and switches to Group B (Atlantis) in wheat. Ben is conscious of the risk of losing Group A efficacy altogether and is looking for alternatives to use in the barley. “Sorghum is a good crop to combat Group A-resistant black oats using a glyphosate spray at the end of the winter fallow and planting in September,” he says. “Chickpea is also providing a useful crop for black oats control using Verdict and Select, both Group A, with high rates of oil and ammonium sulphate.” Ben is using pre-emergent products such as Flame in the summer fallow but is very conscious of the impact these applications can have on crop rotation choices, particularly in years where the summer rainfall is limited. “It is important to have a rotation plan worked out so you can take advantage of summer cropping opportunities and Flame can even cause crop damage in barley the following winter if there has not been enough summer rain,” he says. Fleabane is another challenging weed to manage due to its natural tolerance of glyphosate and the need to be very careful with applications of Group I products, such as picloram in fallows, adjacent to neighboring cotton crops. Ben says Tordon 242 applied early in wheat crops to treat late germinating fleabane is providing very effective control with some residual activity.  
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BYGUM: Barnyard grass management for cotton growers

Glyphosate resistant awnless barnyard grass is now a common problem in Australian cotton farming systems. Growers and researchers have identified management tools and practices that can reduce the impact of this weed, however the economic fall-out has been more difficult to ascertain. Dr David Thornby, former weeds researcher with the Department of Agriculture and Fisheries and now consulting researcher with Innokas, has developed a computer modelling program that allows growers and agronomists to test barnyard grass management scenarios and assess the economic impact prior to implementation, across a 5-year rotation. “The ecology and seed bank behaviour of awnless barnyard grass is quite well understood,” he says. “Getting a handle on the economic value of different management strategies is a useful feature of the Barnyard Grass Understanding and Management [BYGUM] decision making tool. The analysis also shows the yield penalty incurred in each crop as a direct result of herbicide resistant barnyard grass incursions and the effect different management strategies would have on the weed seedbank.” The BYGUM program was adapted from the Australian Herbicide Resistance Initiative’s Ryegrass Integrated Management [RIM] tool, which fulfils a similar purpose for assessing management scenarios for resistant annual ryegrass in southern cropping zones, but does not include parameters for cotton production or fallow management. “We had 15 years of weed ecology and seed bank research to draw on to adapt the RIM model to suit a barnyard grass in a sub-tropical environment growing both summer and winter crops,” says Dr Thornby. “We also had large data sets for herbicide efficacy and the effect of other weed management practices on barnyard grass populations.” BYGUM provides growers with a robust means to evaluate five-year rotations including testing the economic value of fallows and fallow weed management, winter and summer crop sequences, cover crops, tillage, harvest weed seed control, different herbicide options and more. The computer program has a simple step-through wizard design for users to: Define the basics (prices, costs, herbicide options, base yields) Build a rotation (five years) and specify weed controls Check the results and compare with other scenarios When using BYGUM to assess a current or proposed strategy, users can enter parameters about the size of the seed bank and the cost, type and efficacy level of herbicides. They can easily test what will happen if the chosen herbicide doesn’t work or show the effect of using a different herbicide or another weed management tool. “The cost of managing resistant barnyard grass can be assessed across different strategies and under different seasonal conditions,” he says. “The user can define many different parameters or use the default settings, including commodity prices and yields.” The non-herbicide management tools included in the model are harvest weed seed management (for the grain phase of the system), cover crops and tillage. Dr Thornby says brown manuring millet and leaving it standing for improved moisture conservation is an effective tool that growers could investigate. He says tillage operations used in cotton systems for the purposes of pupae busting and bed formation generally have little effect on barnyard grass populations or the seedbank. “Tillage that is timed to control barnyard grass soon after emergence can be very effective,” he says. “A portion of the seed that is present in the soil however will remain viable for 12–18 months if buried to a depth of 10 cm and seed can germinate from a depth of 5 cm given the right conditions.” Over winter, there is significant mortality of seed in the seed bank, which means tillage following early germinations in spring can really drive down the seed bank as the majority of the seed present has germinated and very few viable seeds are buried. In most years it is safe to assume that five barnyard grass cohorts will germinate between September and March, largely in response to rainfall and temperature conditions. “In-crop, barnyard grass plants that germinate in the first cohort will have the greatest opportunity to establish using abundant moisture and nutrient resources and produce large quantities of seed before the crop is able to compete,” he says. “Later cohorts tend to be shaded out by the crop and produce far less seed.” Although BYGUM has been developed specifically for analysing control options for BYG in cotton it can also be used for other summer crops, particularly sorghum, and for summer fallows. Dr Thornby says it also has application for other summer grasses, such as feathertop Rhodes grass, but suggests that users carefully check the herbicide efficacy parameters. It is also not recommended for use on other grasses in Central Queensland farming systems. BYGUM has been developed with the financial and intellectual support of CRDC, UWA, GRDC, QAAFI, DAF and UQ. It has been tested by leading cotton growers and agronomists. Relevant links Try BYGUM for yourself Sample scenarios tested in BYGUM
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Don’t start mixing until the water quality is right

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. The careful management of spray events is highlighted in the The Big 6 for managing herbicide resistance. Spray application specialist, Craig Day of Spray Safe and Save at Cowra, NSW says water should be considered as one of the chemicals in any mix, given that water quality varies markedly depending on its source. Spray application specialist, Craig Day of Spray Safe and Save at Cowra, NSW says water should be considered as one of the chemicals in any mix, given that water quality varies markedly depending on its source. “It would be great if all herbicide sprays were applied using rainwater but that is often not practical,” he says. “The pH, hardness, electrical conductivity and dissolved solids in water all interact with the herbicides and adjuvant products in a mix. It is essential that these parameters are all addressed before any crop protection products are added to the water.” Mr Day says a water test is a cheap way to ensure that the herbicide applied will be as effective as possible in a weed control program. “Generally, water is considered hard when the calcium carbonate levels exceed 300–400 ppm,” he says. “If glyphosate is added to hard water, the calcium and glyphosate ions react, effectively reducing the uptake of the glyphosate into the plant. By adding ammonium sulfate to the water, the positively charged calcium ions bind with the negatively charged sulfate ions. When glyphosate is added to properly agitated and dissolved ammonium sulfate solution the glyphosate does not bind to calcium ions.” It is critical that no undissolved ammonium sulfate, via a handling system or within the spray tank, comes into contact with 2,4-D amine formulations. The resulting precipitate is irreversible. “Hard water will affect formulations of 2,4-DB, glyphosate, 2,4-D amine, Lontrel Advance and Tigrex. If hard water is used with these formulations, there is a greater potential for a reduction in the effectiveness of the chemical,” he says. Water with a pH over 8 supports alkaline hydrolysis, which can affect the efficacy of many herbicide and insecticide products, and is often associated with hard water. Like the example of 2,4-D in hard water, chemicals tend to separate out over time in a high pH environment and the mix become less effective over time. “The trend toward larger spray tanks means that product is in the tank longer and will be adversely affected when there is a high pH,” says Mr Day. “At a pH of 8, which is common for tap water in many areas, many products will only remain fully potent for a matter of 1.5 or 2 hours at the most. The use of an acidifying surfactant helps lower the pH to an optimal 4.5 or 6.” “Aside from herbicides, organophosphates and carbamates are particularly susceptible to alkaline hydrolysis so pH needs careful consideration when spraying aphids with a carbamate formulation.” When obtaining a water quality test, Mr Day suggests that growers ask specifically for the presence of bicarbonates in water to be included in the report. “A bicarbonate level of 75 ppm and above will lead to reductions in the efficacy of 2,4-D amine and Group A formulations,” he says. “Ammonium sulfate can be used to reduce the effect of bicarbonates on Group A herbicides, but will produce little improvement on 2,4-D amine formulations.” Both total dissolved salts and saline water cause stability issues that can result in separation and blocking of filters, especially if there is insufficient agitation during the mixing process. Also, high salt levels in water will resist manipulations of pH. Mr Day recommends growers get their water source tested regularly. “If you take two samples and retain the second jar of water you can use it to calibrate your eye using a pool and spa testing kit and the test results. This can become an ongoing monitoring tool,” he says. Another problem Mr Day commonly sees is the incorrect use of batching tanks, where products are concentrated in small tanks without sufficient water. He suggests that growers avoid having the entire tank load amount of acidifying adjuvant in direct contact with other chemistry and that 2,4-D amine is never allowed to sit on undissolved ammonium sulfate. “Similarly, high concentrations of glyphosate and 2,4-D amine in a batching tank can cause the glyphosate to lose its effectiveness,” he says. “It is very important that water is conditioned first, and that sufficient water and ample agitation is used when preparing a spray load. Time is always against growers but there is no point hurrying just to apply a load that won’t work because the chemistry is wrong in the tank.” As new products come to market, Mr Day suggests growers ask their advisors where the product fits in the mixing order and to highlight any interactions with water quality parameters. “Ask your advisor to provide the correct mixing order when writing the advice sheet,” he says. “You will then have this advice to follow and can record the mixing order on file. This information, and a water quality test, will help unravel any issues with a spray’s efficacy and, in combination with a herbicide resistance test, assist with developing a robust herbicide program.” The GRDC GrowNote Herbicide Use technical manual provides detailed information about the water quality and adjuvant requirements for each herbicide MOA group. More resources: GRDC-GrowNotes-Herbicides – provides water quality and adjuvant advice for use with each MOA group. NSW DPI water quality and herbicide interaction table (see below) in ‘Weed control in winter cropping’ publication Spray Application GrowNote video playlist From the NSW DPI ‘Weed control in winter cropping 2016’ publication. http://www.dpi.nsw.gov.au/agriculture/broadacre-crops/guides/publications/weed-control-winter-crops
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Spray small multi-resistant wild radish twice

Western Australian growers are regaining control over herbicide resistant weeds as a result of widespread adoption of integrated weed management systems. Australian Herbicide Resistance Initiative communications manager, Peter Newman, says many crops are cleaner than ever and growers should be acknowledged for their consistent efforts over many years to implement a wide range of herbicide and non-herbicide tactics to drive down the weed seed bank on their farms. “There was a serious problem here over the last two decades and growers have now made managing herbicide resistance in weeds a component in every farming system,” he says. “We were caught once, and had to learn the hard way. Hopefully growers in other areas will be able to take advantage of the experience and research that has helped WA growers get back in the driver’s seat with weed control.” One piece of research that the RSCN in Geraldton, WA requested that DAFWA and then Crop Circle Consulting and Landmark conduct in 2012 and 2013 has given growers additional confidence when managing herbicide resistant wild radish. “The objective of the 2-year study was to identify paddocks that had high populations of wild radish that was resistant to several herbicide groups,” says Mr Newman. “Although Velocity® (pyrasulfatole + bromoxynil, Group H/C) was working well in most situations, growers were aware that if there was over-reliance on the one product that soon enough that option would be gone.” “Some growers were using two or three shots of Velocity® in a single growing season and they had little confidence in other herbicides due to their radish being multiple resistant.” The first year trial achieved a 22 to 36 per cent yield improvement when wild radish was controlled and the researchers established that the key strategy was to spray the wild radish as early as possible while avoiding any crop damage. “Applying Velocity at the 2-leaf stage of the wheat crop is safe for the crop and kills the majority of the small – 1 to 2 leaf stage – wild radish plants present,” he says. “When this is followed with a second spray of a herbicide from another mode of action at the crop’s 5-leaf stage, when the radish is no bigger than 4-leaf stage, the level of control is very high even in multi-resistant wild radish populations.” Spraying small radish twice is a proven tactic to combat multi-resistant populations. This strategy was tested on three properties in the Geraldton area in 2013 and proved very effective. Grant Thompson from Crop Circle Consulting and Landmark led the trial work in 2013 and says the result that was the most surprising was that there were many options available for use as the second spray. The paddocks chosen for the trials all had known resistance to a wide range of herbicides from mode of action groups B, C, F and I. “Provided the paddock was sprayed early with a product that did not damage the crop, it didn’t make much difference what herbicide was used for the second spray,” he says. “This was a great result for growers as if gave them more opportunities to rotate modes of action across the whole rotation.” Even multi-resistant wild radish plants succumb to a second herbicide spray provided the first spray is applied when the radish plants are small. “For example, growers may choose to use Jaguar (Group F/C) as the first spray in cereals and avoid using Velocity twice in the one season,” he says. “Jaguar applied at 1 L/ha when the wheat is 2-leaf and the radish is 1 to 2-leaf is giving excellent control of the first cohort of weeds in most situations.” The trials also highlighted the importance of achieving excellent coverage with each spray. Mr Thompson stresses the importance of using the correct adjuvant, appropriate nozzles and not skimping on the water rate. Control improves yield and minimises costs Treating the wild radish early was very important for preserving yield. Even with one early spray, all registered treatments doubled the grain yield over the nil treatment. The second spray helps achieve optimal weed control while remaining profitable. In the two-spray system the herbicide applications cost $19 to $29 per ha and achieved a yield benefit worth an average of $90–130/ha, and as high as $260/ha in very weedy situations. Knowing the herbicide resistance profile of the weeds present helps growers to choose the mode of action herbicide most likely to give the best results. Herbicide resistance testing and an array of integrated weed management strategies are detailed on the Weedsmart website. When followed with a harvest weed seed control tactic such as narrow windrow burning or chaff cart, the contribution of new seed to the weed seed bank can realistically be zero. Consistently applied over several seasons can see the weed population driven right down and, if there are no weeds, there are no herbicide resistant weeds. Wild radish flourishing in each control patch in the trial – illustrating the multi-resistance traits in the population at this site. Relevant links Webinar with Peter Newman and Grant Thompson GRDC Update paper ‘Spray resistant radish early for best efficacy and yield’ Wild Radish factsheet
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Best seed, best establishment, fewer weeds

Crop competition is an important strategy in the Weedsmart Big 6 to manage herbicide resistance – but it’s not all about row width and seeding rate – the size of the seed sown makes a difference too. Rohan Brill, Research and Development agronomist, NSW DPI based in Wagga Wagga, along with colleagues at Trangie and Tamworth, has been teasing apart whether the improved early performance of canola hybrids over open-pollinated (OP) cultivars comes from the generally-larger seed size of hybrids or from hybrid vigour. Rohan Brill, Research and Development agronomist, NSW DPI based in Wagga Wagga, says most of the improved early growth in canola hybrids lies in the larger seed size, with heterosis providing an additional benefit. Sowing large canola seed, regardless of the cultivar, is key to strong early crop growth and the crop’s ability to compete with weeds. (Photo: Gregor Heard) “It seems that most of the improved early growth in hybrids lies in the larger seed size, with heterosis providing an additional benefit,” he says. “Our study showed that sowing large canola seed, regardless of the cultivar, is key to strong early crop growth and the crop’s ability to compete with weeds.” As a reliable rule of thumb, Mr Brill recommends cleaning and grading all farmer-retained OP canola seed to collect planting seed that is 2 mm in diameter or larger. In both glasshouse and field experiments following this rule led to improved open-pollinated TT canola emergence and early biomass production. Seed size had a greater effect on early biomass production than did cultivar type (hybrid vs OP). “A 10 per cent difference in seed diameter produces a 33 per cent difference in seed volume, so it is very important not to skimp on the 2 mm diameter rule,” he says. “There was a stark contrast between plots sown with 2 mm seed compared to 1.8 mm seed. It is very hard to tell the difference visually between these two seed sizes but the visual difference in early vigour and biomass production was very clear.” A 10 per cent difference in seed diameter produces a 33 per cent difference in seed volume, so it is very important not to skimp on the 2 mm diameter rule. There was a stark contrast between plots sown with 2 mm seed (left) compared to 1.8 mm seed (right). “This trial has shown that grading planting seed to extract the largest seeds is a far better way to increase crop competition and yield than increasing the seeding rate of smaller seed,” he says. “For example, a grower might take 30 tonne of grain from a clean paddock and screen it hard at harvest to take out 2 tonne of the best seed and sell the rest. Then store the 2 tonne of seed in a cool place before grading it (including using a gravity table) to 2 mm and above prior to planting. With the larger but still relatively cheap seed, a sowing rate of at least 3 kg/ha is recommended, resulting in a plant stand of 30 plants/m2 even at an establishment rate of 50 per cent. There is little penalty from sowing at even higher sowing rates, as DAFWA research found that too few plants has a greater economic penalty than too many plants.” The DAFWA canola seed size ready-reckoner is an excellent resource for growers to use and is available on the Department’s website. Simply line up retained OP canola seed along a 10 cm section of a ruler and count the number of seeds. To achieve best results there should be 50 seeds or less to take up the 10 cm length. Use this information to calculate seed rate. “Sowing larger seed is essential if planting early, in early-mid April rather than mid-May, to maximise grain yield potential in canola,” he says. “The larger seed can be safely sown slightly deeper than optimal for each soil type and still achieve reliable crop establishment.” Sowing in early-mid April into residual soil moisture gives the crop a head-start on weed germinations and the strong crop growth provides valuable support for the pre-emergent herbicides. The only catch is to make sure that the cultivar selected for sowing early will flower in the optimal window for your district. Larger seeds possess more energy resources than smaller seeds, enabling the seedling to push through more soil and stubble and to grow past insect damage and outcompete weeds. While planting larger OP cultivar seed will help close the gap in early vigour between OP and hybrid cultivars, the herbicide tolerant traits of hybrids continue to provide other benefits. Mr Brill says the extra effort to grade seed to 2 mm will see an improvement in yield provided attention is also given to good soil nutrition, timing and harvester speed. “Driving the header too fast has been shown to cause large yield losses,” he says. “This represents a direct loss of income and the volunteer canola plants suck moisture from the soil, potentially compromising the early sowing opportunity for the following season.” Crop competition is the cheapest form of weed control and it usually comes hand in hand with improved yield; a genuine win all-round. Sown side-by-side, one hour apart, these Gem canola plots were sown at 2.6 kg/ha with 80 kg/ha MAP. The only difference is that the farmer retained seed on the left was graded to 2 mm while the plot on the right was sown with purchased seed, straight from the bag. (Photo: Warwick and Di Holding, Pontara Grain, Yerong Creek) Related resources Watch a recording of Rohan’s full webinar presentation Estimating the size of retained canola seed DAFWA canola seeding rate information Competitive canola cultivars
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Multi-resistance is game changing for herbicide tolerant crops

American farmers seem to have regained the upper hand on glyphosate resistant Palmer amaranth and waterhemp, which had overwhelmed many crops from 2008 to 2012, but there are still some longer term challenges. Jason Norsworthy, professor of weed science at the University of Arkansas says that although many paddocks are cleaner than they have been for years, he doesn’t believe that the current weed management practices are sustainable. Jason Norsworthy, professor of weed science at the University of Arkansas says many growers in the States are now only one or two herbicides away from having no herbicide control options for Palmer amaranth (pigweed) in either conventional or herbicide tolerant soybean crops. In the USA, herbicide tolerant varieties and hybrids have come to dominate in the sown areas of cotton (89%), soybean (94%) and corn (89%) across all regions. The soybean and corn growing regions overlap significantly and there are some regions where all three crops are grown. Dr Norsworthy says many growers in the States are now only one or two herbicides away from having no herbicide control options for Palmer amaranth in either conventional or herbicide tolerant soybean crops. “Four modes of action are already lost on many populations of Palmer amaranth,” he says. “The multiple-resistance to these four modes of action is a real game changer for growers. It is simply not sustainable to use a herbicide or a technology until it breaks and expect to be able to switch to another option.” The practice of continuing to use a herbicide after resistance had established is one of the key reasons why many growers are in a serious predicament, according to Dr Norsworthy. He says that ALS (Group B) resistance has inevitably led to glyphosate (Group M) resistance, which has in turn led to PPO (Group G) resistance. It is only a matter of time before PPO (Group G) resistance is also widespread in weeds like Palmer amaranth. “PPOs are currently being applied both at planting and in-crop in soybean crops where there are few other herbicide options,” he says. “This high selection pressure will inevitably lead to failure of the herbicide leaving growers with dicamba [Group I] products as their last resort.” The use of low rates of dicamba in corn crops has already exposed many weed populations to low dose selection to this mode of action and researchers have demonstrated full dose dicamba survivors can evolve in just three generations in Amaranthus spp. In Australia, there are almost as many species resistant to glyphosate (13 species) as are present in the United States (15 species); however, there are not as many herbicide tolerant crop options. Dr Norsworthy suggests that Australian growers continue to adopt diverse weed management practices, including harvest weed seed control, competitive crops, strategic tillage and pasture phases rather than over-relying on herbicide tolerant hybrids as they come available. “We are encouraging American growers to look more toward non-herbicide practices to try to break the cycle of over-reliance on single herbicide modes of action,” he says. “Once multiple resistance takes hold there really are less options available and most of them will have a production penalty, but if these practices are not adopted then production systems will fail.” Australian Herbicide Resistance Initiative (AHRI) director, Stephen Powles in a soybean trial plot in America overwhelmed with herbicide resistant Palmer amaranth (pigweed), a prime candidate for harvest weed seed control. Related information Webinar – Resistance in corn, soybean and cotton crops in the US Imi-tolerant crops – Use sparingly for best effect Herbicide tolerant summer cropping options Hybrid canola outcompetes weeds
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Resistance risk to eliminate herbicides on irrigated farms

This can be a high risk practice unless survivors are removed after every spray application as there is no crop competition to restrict weed growth, resulting in production of large volumes of seed. Eric Koetz, NSW DPI weeds research agronomist, 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. 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. “A recent survey of cotton fields in Queensland and NSW had shown that cotton paddocks were generally relatively weed-free however the incidence of resistance to glyphosate is quite high in fleabane and windmill grass samples collected mainly from non-crop areas on cotton farms,” he said. “Preliminary results from the samples collected across cotton farming systems in 2016–17 show 95 per cent of fleabane samples, 60 per cent of feathertop Rhodes grass samples, 80–90 per cent of windmill grass samples and 20 per cent of sowthistle samples tested as resistant to glyphosate.” “Keep in mind that flaxleaf fleabane, feathertop Rhodes grass and windmill grass are not registered for control with Roundup Ready Herbicide with PLANTSHIELD as control of susceptible plants is generally poor, so further selection for resistance only exacerbates the problem,” he said. “Although still lower, the level of resistance emerging in sowthistle is very concerning, given that this species is listed on the herbicide label. According to a recent attitudinal study by Monsanto, growers also identify annual ryegrass and barnyard grass as showing signs of glyphosate resistance in the field.” To date there have been no recorded cases of paraquat resistance in weeds on cotton farms, however with rising glyphosate resistance, and increased use of paraquat products, there is a high risk that paraquat resistance will also be found, leaving growers with few options to control these weeds. Mr Koetz now has additional funding available to test the seed collected in the survey for resistance to paraquat and diquat products – Gramoxone and Spray.Seed. Mr Koetz said the lack of diversity in herbicide use in many cotton systems is likely to contribute to the increased incidence of herbicide resistance. “The attitudinal study by Monsanto indicates that less than 50 per cent of growers are applying a pre-emergent herbicide and only 25 per cent of growers apply a post-emergent herbicide in addition to their applications of glyphosate in cotton,” he said. “The label for Roundup Ready Herbicide with PLANTSHIELD states that this product must not be the only form of weed control used in Roundup Ready Flex cotton varieties.” “In the next 5 to 10 years there will need to be a shift towards non-herbicide controls such robotic cultivation and microwave technologies, which are well suited to summer cropping on rows or beds,” he said. “Until then, optical spray technology is a good option for growers to keep weed numbers low in the fallow.” Spray drift of Group I herbicides (e.g. 2,4-D products) late in the summer fallow to control large fleabane is causing considerable damage to cotton crops. Mr Koetz recommends growers change their fallow weed management program to target small plants earlier in the spring, before cotton emergence, using a double knock of glyphosate followed with cultivation or paraquat plus a residual herbicide as the second knock. Related links Protecting knock-down herbicide options 2016 Monsanto Weed Resistance Management Research – Survey report
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Insecticide shown to reverse metabolic herbicide resistance

Herbicide resistance occurs at a genetic, molecular and cellular level in ways that challenge some of the most agile scientific minds. In following interesting lines of enquiry, scientists working to understand the mechanisms that drive metabolic resistance sometimes come across some unexpected findings. One such finding is the discovery that an insecticide can reverse metabolic resistance to a herbicide, making the resistant population susceptible to the herbicide once more. Left: Trifluralin applied to a pot containing trifluralin-resistant ryegrass seed. Centre: Trifluralin applied immediately after applying phorate insecticide granules to pots with trifluralin-resistant ryegrass seed. Right: Trifluralin applied to a pot containing trifluralin-susceptible ryegrass seed. Metabolic resistance is the lesser known cousin to target site resistance in the world of herbicide resistance mechanisms. Target site resistance is comparatively easy to identify and study, being a more direct ‘cause and effect’ type mechanism that usually confers quite high levels of resistance. Metabolic resistance however is more complex and more difficult to study due to many internal mechanisms involving secondary enzyme production and activity. This type of resistance is often moderate however it is also frequently effective across multiple herbicide mode of action groups. It is not uncommon for plants with metabolic resistance to be resistant to herbicides that they have never been exposed to. This has a dramatic and limiting effect on herbicide choice and makes herbicide rotation a much less powerful control tactic. In simple terms, metabolic resistance occurs when the plant uses its metabolic pathways to produce enzymes that ‘protect’ target sites from the applied herbicide molecules. If the herbicide molecule never reaches the target site then the plant survives. The same enzyme or multiple enzymes can ‘protect’ multiple target sites, resulting in cross-resistant plants. Weed surveys in Western Australia have revealed a high rate of multiple-resistance in annual ryegrass populations with 70 per cent of populations possessing both metabolic and target site resistance to herbicides. Research into gaining a better understanding of one group of enzymes, known as P450s, has uncovered an unlikely synergism between an insecticide and current pre-emergent herbicides to control ryegrass. Australian Herbicide Resistance Initiative (AHRI) researcher, Roberto Busi, has shown that it is possible to reverse metabolic resistance to trifluralin in annual ryegrass using an organo-phosphate insecticide. In conjunction with Colorado State University researcher, Todd Gaines, Dr Busi is working to better understand the genetic basis of metabolic resistance and how this knowledge can be used to control metabolic resistant weeds. “There are just five types of pre-emergent herbicides, utilising only two modes of action, with no new modes of action in the pipeline,” says Dr Busi. “The most recent pre-emergent product, pyroxasulfone (Sakura), was commercialised in 2012 yet even before it was brought to market, research had shown its mode of action can be ‘broken’ within just three generations using low application rates to result in 10-fold resistance.” This means that it is very important to find ways to keep current herbicides effective rather than just looking for new modes of action. In the case of trifluralin-resistant annual ryegrass, Dr Busi’s research demonstrated that inhibiting the production of P450 enzymes was the key to reversing resistance to this useful pre-emergent herbicide. “Inhibiting the production of P450 enzymes requires the suppression of different genes in the plant that are responsible for regulating production of the enzymes at different stages of the plant’s development,” he says. “In ryegrass there are probably several different P450 enzymes that are active during the plant’s development that are offering protection against the herbicide, so there is a high level of complexity involved in trying to manipulate the genes responsible for herbicide resistance.” “Using the insecticide phorate, applied in granular form to the soil immediately before spraying with trifluralin, we were able to prevent establishment of plants with known resistance to trifluralin,” he says, “But the effect was not as clear for plants that were resistant to Sakura. Phorate is not the solution to metabolic resistance but this proof-of-concept research confirms that it is possible to manipulate and even reverse metabolic resistance with the use of existing pesticides.” Phorate is not currently registered for use in any crop except cotton and the described use is not permitted in the field. Phorate is highly toxic and it was used under carefully controlled laboratory conditions for these experiments. It is not desirable to turn off P450 production in a crop so chemicals that inhibit P450 production are best suited to use with pre-emergent herbicides. Current research is investigating ways to design better P450 inhibitor mechanisms using gene technology and to use these mechanisms in future crop breeding programs to confer crop tolerance to certain herbicides. Further experiments showed that ryegrass plants with metabolic resistance to Sakura use another metabolic pathway involving enzymes known as GST. In a similar manner, these mechanisms can probably confer cross-resistance for pyroxasulfone (Sakura), prosulfocarb (Boxer Gold) and triallate. To date, the fifth pre-emergent active ingredient, propyzamide, does not seem prone to metabolic resistance. “For now, our 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 – and we are researching the potential benefits of mixing herbicides from these three groups as a means of delaying metabolic resistance,” says Dr Busi. “As always, full label rates must be applied.” For more information, check out our webinar recording with Roberto and Todd discussing metabolic resistance to herbicides.
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Hybrid canola out-competes weeds

Canola production is predicted to rise by 30 per cent in 2017 with over 98 per cent of farmers growing herbicide-tolerant canola cultivars. However, Australia has been lagging behind other producing countries in the uptake of hybrid cultivars that offer higher yield and more options for effective weed suppression in crop rotations. Researchers at Charles Sturt University compared the competitive ability of 16 genotypes – hybrid and open-pollinated cultivars – and found the most competitive cultivars reduced weed biomass at flowering by a huge 50 per cent, significantly reducing the amount of weed seed added to the seed bank. A similar trial, conducted by researchers at the University of Adelaide, found that annual ryegrass plants that survived pre-emergent herbicide produced more than twice as much seed in open-pollinated TT canola than in the hybrid Clearfield canola. Although the triazine-tolerance (TT) trait is known to confer a yield penalty of up to 10–20 per cent, this technology is currently adopted on 65–70 per cent of the national canola area. The vast majority of TT canola cultivars are open-pollinated while most Clearfield (CL) and Roundup-Ready (RR) cultivars are hybrids. Kevin Morthorpe, *GenTech Seeds’ Canola Business Lead and Product Stewardship Manager hopes that the new hybrids available in 2017 will encourage growers to continue to shift away from the less competitive, open-pollinated cultivars to take advantage of the more vigorous, higher biomass and higher yielding hybrids. Kevin Morthorpe, GenTech Seeds, is concerned that canola growers are relying too heavily on the less competitive triazine-tolerant (TT) cultivars and missing out on the higher yield and better weed control benefits of hybrid canola systems. “Crop competition is one of the best non-herbicide weed management tools available and the new hybrids are offering growers real bang for their buck in terms of extra yield and driving down the weed seed bank,” he said. “These benefits for the grower far out-weigh the additional cost of hybrid canola seed. The yield advantage achieved through hybrid vigour only occurs in the F1 generation so retaining seed for future use is counter-productive.” Mr Morthorpe said enhanced crop competitiveness in hybrid crops is best achieved through attention to crop nutrition to optimise early growth and harvestable yield. “Research studies in canola have not shown a plateauing in crop yield response to higher nutrient input as is often observed in the ‘haying off’ of cereals at higher nutrient rates,” he said. The suppression of weed growth is closely tied to strong root development, increased crop biomass and improved early crop vigour. Field trails also indicate that some cultivars have an allelopathetic effect on weeds. Speedy emergence, early vigour, rapid ground cover and height are all important characteristics of the most competitive hybrids. Hybrid growers are also able to strategically integrate alternate herbicide modes of action and reduce reliance on Group A herbicide applications in the canola phase. Hybrid canola suppresses weed biomass and seed production by about 50 percent compared to open-pollinated triazine-tolerant (TT) canola cultivars due to the speedy emergence, early vigour, rapid ground cover and height characteristics of the most competitive hybrids. Herbicide Tolerant Canola Stewardship Guide GenTech Seeds, an industry partner of WeedSmart, is committed to product stewardship. Their recently-published ‘Herbicide Tolerant Canola Stewardship Guide’ outlines the sustainable use pattern for all herbicide tolerant cultivars. Mr Morthorpe said the stewardship guide is a practical approach to integrated weed management within the canola phase and fully supports the The Big 6. “In triazine-tolerant crops the main points are to carefully manage the lower crop vigour of TT canola, monitor triazine herbicide carryover risk for crop rotational planning and always adhere to mandatory application practices for triazine herbicides (atrazine, simazine) set by industry in cooperation with government,” he said. “To protect Clearfield technology, the whole farm rotation plan needs to limit the use of ALS-inhibiting herbicides to no more than two out of four years. Avoid crop injury by using quality assured, first generation seed and monitor the carry over risk of imidazoline herbicide on future crop rotation options.” “Planting Roundup-Ready hybrids requires a critical assessment of the use of glyphosate as a stand-alone knockdown for non-crop application such as fencelines and over-reliance across the farming system. A whole-farm strategy needs to be in place to manage glyphosate resistant weed populations and to control RR crop volunteers in the fallow phase.” The general principles for weed control and herbicide use apply for all herbicide tolerant canola systems, including using a combination of registered pre-emergent and post-emergent herbicides, always following the label directions of use and pro-actively implementing diverse weed management tactics across the farming system. Post-emergent herbicide applications must target small weeds (2–3 leaf ryegrass and/or 2–3 cm diameter broadleaf leaves) early in the crop’s development and selective herbicide application should occur before elongation of canola plants (BBCH 16). “When it comes to managing herbicide resistance and protecting the plant breeding technologies available, the key is to have low weed numbers entering and leaving the canola phase,” he said. “To achieve this, growers can rotate the crop types, herbicide tolerance technologies and herbicide modes of action used across the crop rotation to broad the opportunities to implement a diverse range of weed management tactics to prevent weed seed set and to implement harvest weed seed control to collect any seed present at harvest.” The ‘Herbicide Tolerant Canola Stewardship Guide’ includes a detailed integrated weed management (IWM) strategy that highlights herbicide and non-herbicide tactics growers can employ in the pre-planting, in-crop, harvest and post-harvest phases to control weeds and reduce weed seed set. This Guide also promotes recommended stewardship practices along the supply chain to ensure responsible use of new technologies, predictable market access and builds on Australia’s competitive edge in international oilseed markets. *GenTech Seeds is the exclusive producer distributor of Pioneer® brand products in Australia. Related topics: Choose highly competitive canola Herbicide Tolerant Canola Stewardship Guide University of Adelaide competitive canola trial (GRDC update) Do canola biotypes have an allelopathic effect on weeds?
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Ramp up the competition in summer crops

The higher rainfall across many grain growing regions in 2016 is providing farmers with more opportunities for summer cropping. Weeds also stand to gain from the additional soil moisture, putting additional pressure on summer fallow spraying programs. Dr Bhagirath Chauhan, Principal Research Fellow with the Queensland Alliance for Agriculture and Food Innovation (QAAFI) says agronomic trials measuring the effect of early canopy closure in summer crops are consistently resulting in lower weed biomass and higher crop yield. “Our research in mungbean, cotton and soybean have shown that more even plant spacing across the paddock is more important for weed suppression than increased seeding rate, and this is best achieved through narrower row spacing,” he says. “In cotton, we demonstrated a clear benefit in planting at 50 cm row spacing rather than the conventional 100 cm spacing however, the limitation for growers is the inflexibility of the current harvesting equipment.” One configuration that has shown promise internationally but not yet fully investigated in Australia is the ultra-narrow row (UNR) concept where the beds remain at 100 cm spacing to suit the harvester but two rows of cotton are planted either side of the bed, effectively shading the inter-row earlier than a single row planting. The ultra-narrow rows are planted 19 to 38 cm apart on the bed and seeding rate is usually increased slightly. With limited options to increase crop competition in cotton, and the widespread adoption of Roundup-Ready (RR) technology in the industry, there is now a focus on finding alternative herbicide chemistry to manage the risk of glyphosate resistant weeds in cotton systems. Dr Chauhan says growers are achieving good weed control with the pre-emergence herbicides recently registered for use in cotton. “The biggest challenge with these herbicides is getting the application right, taking into account the effect of rainfall, irrigation type and timing and the soil type,” he says. “There are emerging weeds such as feathertop Rhodes grass, sesbania and amaranth that are challenging the Roundup-Ready cropping system and so growers need to have other weed management tactics in place early.” The cotton industry is promoting the adoption of the 2 + 2 + 0 weed management system to protect glyphosate and the Round-Up Ready hybrids. This entails the use of two non-glyphosate herbicide options, two non-herbicide tactics and zero weed survivors. Re-introducing the use of pre-emergent herbicides in cotton farming is an important part of this weed management program. Increasing crop competition is also worth further investigation given the potential weed control and crop yield benefits to be gained if the limitations of current harvesting equipment can be addressed. To maintain yield in cotton it is important to restrict all weed management operations to the early stages of crop growth, a distinct advantage of using pre-emergent herbicides to minimise weed growth prior to crop canopy closure. Peter Newman, communication lead with Australian Herbicide Resistance Initiative, has long been an enthusiastic advocate of crop competition in cereals. He says the recent findings in summer crops provides growers with a valuable non-herbicide tool they can use to help suppress weeds. “Over and over we are seeing results come from crop competition trials showing suppression of weed biomass in competitive crops, and usually a yield benefit,” he says. “This is a win win for growers and needs to become standard practice in all crops – not only regarding row width but all agronomic practices that boost early crop growth and result in early canopy closure.”
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Protecting knock-down herbicide options

Most cropping systems rely fairly heavily on a small group of non-selective or ‘knockdown’ herbicides. Since the widespread adoption of zero and minimum tillage, these herbicides have provided effective control of many grass and broadleaf weeds – but these useful herbicides could be lost to the industry if steps are not taken to increase the diversity of weed management tactics used. Mark Congreve, ICAN senior consultant, says that the highly effective double knock tactic, which combines an application of glyphosate followed by paraquat, is at risk if growers don’t remain vigilant and ensure removal of any surviving plants. The recent discovery of flaxleaf fleabane resistance to paraquat is a clear warning to grain producers that there is no room for complacency with double knock operations. “The double knock strategy of glyphosate, plus a Group I herbicide for weeds such as flaxleaf fleabane, followed by paraquat has provided excellent control of weeds that are difficult to kill with glyphosate alone,” he says. “Recent confirmation of a fleabane population that is resistant to paraquat, found in a New South Wales vineyard, is a clear warning to grain producers that there is no room for complacency following a double knock operation. In addition to this recent discovery, an annual ryegrass population from a West Australian vineyard was confirmed in 2013 to have resistance to both glyphosate and paraquat. This shows that a single plant can develop resistance to both of the main non-selective knockdown herbicides used in Australian grain production.” Paraquat is a widely-used herbicide, being an active ingredient in over 100 herbicide products registered for use in broadacre cropping. It is a group L herbicide and as such is considered a ‘moderate risk’ for herbicide resistance. Having a moderate risk rating means that resistance generally takes longer to occur, not that it won’t occur. “Paraquat resistance typically takes over 15 years of consistent use before resistant weeds are noticeable in the field,” he says. “This critical period has now elapsed on many farms where paraquat is used in cereals and broadleaf crops, and for general weed control around the farm.” Paraquat resistance has been present and widespread in barley grass in lucerne production systems for many years in southern NSW and Victoria. While paraquat resistance is still relatively rare outside of lucerne systems, very high level resistance to paraquat was confirmed in three weed species (crowsfoot grass, blackberry nightshade and cudweed) taken from sugarcane and tomato blocks around Bundaberg in 2015. Mark Congreve, ICAN senior consultant says growers need to be looking for survivor weeds after every herbicide application and responding to ‘rate creep’ by changing how they use herbicides across their cropping system. In the event of widespread resistance to paraquat, Mark is concerned that there are no new modes of action likely to be commercialised within the next 10 years or more, so we need to protect what we have. “It is essential that farmers do everything in their power to preserve the effectiveness of the herbicide groups currently available,” he said. “The key is to take a diverse approach to weed management and, most importantly, remove weeds that survive herbicide applications. This is the best way to keep weed numbers low and when numbers are low, resistant weeds can be controlled more effectively. It’s a numbers game!” Mark suggests that growers check the results of every spray application, looking for individual plants ‘surviving’ or ‘re-growing’ after a spray application that has killed adjacent weeds. This may be a sign that the surviving plants carry the genetic mutation that ‘protects’ them from the herbicide’s mode of action. “If this is observed, the first step is to remove those individual plants before they shed seed,” he said. “It is recommended to have the plants, or their seed, tested to confirm resistance and determine what herbicides those individuals are still susceptible to.” A second warning sign is when a higher rate of a herbicide is needed to have the same effect as achieved on the target weed in previous years. Mark called this ‘rate, or dose, creep’ and said that it is the most common sign of resistance to herbicides like paraquat. “Paraquat resistance primarily occurs as a result of a plant having the ability to re-direct the herbicide molecules away from the chloroplasts in the cell and into the cell vacuole, where the herbicide has no effect,” he said. “If you are finding that you now need to use a higher rate of a herbicide such as paraquat, it is time to change how you manage those weeds.” Non-crop areas around farms are often treated with paraquat annually. This can be a high risk practice unless survivors are removed after every spray application as there is no crop competition to restrict weed growth, resulting in production of large volumes of seed. Herbicide resistance frequently occurs first along fencelines, roadways and irrigation channels where herbicide use tends to be the same year in year out and less attention is paid to survivor weeds or poor herbicide efficacy. Currently there are 10 weed species with confirmed resistance to paraquat (Group L) and 13 species resistant to glyphosate (Group M) in Australia.   Table: Confirmed paraquat resistance in Australia (Source: Australian Glyphosate Sustainability Working Group Paraquat resistance factsheet) Species Common name Year confirmed State Crop Hordeum glaucum Northern barley grass 1983 Victoria lucerne Arctotheca calendula Capeweed 1984 Victoria lucerne Hordeum leporinum Barley grass 1988 Victoria lucerne Vulpia bromoides Silver grass 1990 Victoria lucerne Mitracarpus hirtus Small square weed 2007 Queensland mangoes Lolium rigidum Annual ryegrass 2010 South Australia pasture seed Gamochaeta pensylvanica Cudweed 2015 Queensland tomatoes, sugar cane Solanum nigrum Blackberry nightshade 2015 Queensland tomatoes, sugar cane Eleusine indica Crowsfoot grass 2015 Queensland tomatoes, sugar cane Conyza bonariensis Flaxleaf fleabane 2016 NSW grape vines    
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Acting early pays off in the war on weeds

If nothing else, herbicide resistance is predictable, but efforts to prevent seed set do pay off, particularly if action is taken early. NSW DPI weeds specialist Tony Cook says that growers are becoming increasingly aware of the potential impact of herbicide resistance on their farming operations and they are seeing the benefits that come from early and decisive action. NSW DPI weeds technical specialist, Tony Cook, has documented the strategies used on four farms to successfully patch-manage resistant weeds using a variety of weed control tactics over a period of at least five years. Many factors may influence the process but if an individual plant that is resistant to a herbicide sets seed, it is only a matter of time before a small number of resistant plants becomes an ever-increasing weedy patch. Mr Cook says that growers can build farming systems that have a level of ‘immunity’ to herbicide resistance. “If growers concentrate on minimising or preventing weed seed set, they can win against herbicide resistance,” he says. “The trick is to use a variety of means to keep weed numbers low and to keep pressure on seed set. If resistant plants are prevented from setting seed, then the problem is contained.” “Eight out of the twelve glyphosate resistant species in Australia are present in weed populations in NSW and Queensland, particularly near the state border,” says Mr Cook. “In this region resistant awnless barnyard grass, annual ryegrass, liverseed grass and feathertop Rhodes grass are the main problems.” Mr Cook interviewed four northern region farmers to find out what strategies they had used to successfully manage patches of these four glyphosate-resistant weed species. The take-home message from these four farmers was that growers can drive down weed seed banks through a strong commitment to consistently preventing weed seed set for at least five years. “The costs associated with treating the patches is an additional expense but this pales in significance against the cost of doing nothing and allowing the patches to spread across paddocks and beyond,” he says. More details about the strategies used on each case study property can be found in Mr Cook’s webinar presentation with AHRI’s Peter Newman. While glyphosate resistance is becoming more widespread across the region, many resistant populations are still in patches up to one hectare in size. “This gives growers the opportunity to use paddock-wide tactics combined with more intense patch management in problem areas,” says Mr Cook. “Some growers have been successful in completely eradicating herbicide resistant plants from patches of one hectare or less through very focused efforts to prevent seed set.” Strategies such as full cultivation, optical weed detection, brown manuring, double-knocking, strategic use of pre-emergent herbicides, using livestock as the second knock and close attention to the removal of survivors have all been used in different situations to treat patches of resistant weeds. While some of these strategies are often applied across the whole paddock, some can be directly applied to the patch to minimise the cost while maximising effect. Glyphosate resistance in weeds is becoming widespread across the northern grains region, particularly around the NSW Queensland border, however these resistant weeds are often found in small patches, less than one hectare in size. “A shift in cropping rotation and well-timed use of paraquat in place of glyphosate is another useful strategy to drive down weed numbers for these key species,” says Mr Cook. “While spot spraying can be a good option it is easy to miss the outlier plants in a small patch. The optical sprayers can give better coverage of the weeds and often involve an alternative herbicide to glyphosate. Including a second knock to remove survivors is key to success with this and all other weed control tactics.” “Pre-emergent herbicides are being used to good effect in the fallow, provided that growers are very aware that these herbicides generally achieve only 80 to 95 per cent weed control,” he says. “To gain any benefit from these herbicides it is very important to manage the small number of weeds that are likely to survive the pre-emergent application. There are quite a few herbicide modes of action that have a residual activity and each needs to be applied correctly to achieve the best results.” “We know that routine use of glyphosate every year in the fallow will cause glyphosate resistance in the weed population within 15 years if no follow-up action is taken to remove survivors,” he says. “This has been demonstrated in trials and is evident in the field. It may then take another five or six years for the glyphosate resistant weeds to dominate in a paddock, again if no follow-up action is taken.” The spread of resistant weeds across a paddock or around the farm is quite easily done through movement of vehicles, machinery, people and animals as well as wind and water flow across the paddock or along irrigation channels. Mr Cook says being aware of new weeds on the farm, such as feathertop Rhodes grass deposited on farms around Dalby during flood events over the last few years, growers can get on the front foot with effective tactics such as double knocking before the new weeds are firmly established. “Glyphosate resistant patches are frequently associated with fencelines and other non-crop areas on the farm and can spread into cropping fields,” says Mr Cook. “Identifying alternate strategies for managing these areas needs to be a high priority on all farms, even if glyphosate resistance is not yet evident.”     Want more? Check out Tony’s webinar and GRDC update paper.
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Non-herbicide weed control in the Northern region

Diversity in cropping systems and diversity in weeds in the northern GRDC grains region of NSW and Queensland calls for diversity in weed management solutions, including non-herbicide tactics. Survey work in the region has identified over 70 different weed species that impact on grain production and over 10 per cent of these weed species have confirmed populations within Australia that are resistant to glyphosate and several other chemical modes of action (MOA). DAF weeds researchers Annie van de Meulen and Michael Widderick. Confirmed herbicide resistance in weed populations found in NSW and Queensland. Source: adapted from a table prepared by M Widderick, DAF. Mode of Action Resistant weeds A (fops, dims, dens) wild oats, paradoxa grass, annual ryegrass B (SUs, imis etc) annual ryegrass, wild oats, paradoxa grass, Indian hedge mustard, charlock, wild radish, turnip weed, African turnip weed, common sowthistle, black bindweed C (triazines, ureas, amides etc) awnless barnyard grass, liverseed grass D (DNAs, benzamides etc) annual ryegrass I (phenoxys, pyridines etc) wild radish L (bipyridyls i.e. diquat, paraquat) flaxleaf fleabane M (glycines i.e. glyphosate) annual ryegrass, awnless barnyard grass, liverseed grass, windmill grass, feathertop Rhodes grass, sweet summer grass, flaxleaf fleabane, common sowthistle Z (dicarboxylic acids etc) wild oats A recent survey of common sowthistle determined populations as glyphosate resistant if treated seedlings were surviving and reshooting 21 days after glyphosate application. In this testing, glyphosate was applied at the upper label rate for small sized plants (up to 5 leaf). While the majority of common sowthistle samples collected from central Queensland to central NSW were still susceptible to label rates of glyphosate applied to small seedlings, resistant populations were found throughout the study area, showing that this is not a localised problem but rather the inevitable result of over-reliance on a particular herbicide. Most northern region weeds are self-pollinated so resistant plants will produce resistant seed. To reduce the likelihood of resistance, a key approach is to use multiple tactics to maintain low weed numbers. While weed numbers are low so too is the risk of resistance genes being present in the population. To keep these ‘difficult to control’ weeds in check will clearly require other, non-herbicide, tactics to reduce germination and weed seed set. Queensland Department of Agriculture and Fisheries (DAF) researchers in Queensland have been studying common weeds, particularly feathertop Rhodes grass, barnyard grass and common sowthistle, to find weaknesses in each weed’s ecology to help identify non-chemical control tactics that could be part of an effective management system. Dr Michael Widderick and the DAF weed research team are investigating non-chemical options, including various cover crops, crop competition, strategic tillage, strategic burning and harvest weed seed control options. He says that although growers are making good use of chemical strategies such as double knock, residual herbicides, spot spraying and weed sensing technology to preserve herbicide efficacy, there is an urgent need to investigate non-chemical options that can be added to a weed management program to target resistant weeds in the northern region, as outlined in the WeedSmart 10 Point Plan. Strategic tillage “Most growers are keen to preserve their zero or minimum tillage farming systems that have delivered significant benefits and so are very reluctant to re-introduce cultivation for weed control purposes,” says Dr Widderick. “We are currently researching ways to use cultivation that will have maximum effect on driving down weed numbers while having least impact on the min-till farming system.” The aim of this research is to investigate the impact of different tillage operations in situations where the weed population has blown out and intensive patch or paddock-scale management is required. “The key is to understand weed ecology, particularly how seed in the soil seed bank responds to different types of cultivation,” he says. The team used small plots to determine the effect of burial at different depths on weed seed persistence (long-term viability) and emergence. They also conducted experiments to determine the displacement of seed (glass beads were used to represent the seed) throughout the cultivated zone using four different types of machine—harrows, gyral, off-set discs and one-way discs—compared to the zero till control treatment. Sowthistle emergence occurs primarily from seeds close to the soil surface with up to 30% of viable seeds emerging over 5 months. Seed can emerge from a depth of up to 2 cm with approximately 4% emergence after 6 months. Seed buried below 5 cm is unable to emerge and can persist at depth. Seed persistence (the percentage of viable seed after burial) in fleabane was most reduced when seed was buried to a depth of 2 cm and not disturbed for at least two years. Seed buried to a depth of 10 cm remained viable for over 3 years. Feathertop Rhodes grass seed persisted for only 12 months regardless of being left on the surface or buried to 10 cm depth. Barnyard grass however, persisted on the soil surface for up to 2 years and when buried to 10 cm depth remained viable for over three years. The Gyral machine placed the majority of weed seed in the 0–2 cm and 2–5 cm zones while the offset discs and one-way discs achieving burial of about half the seed below 5 cm depth. “All species responded to increased tillage intensity with reduced germinations,” says Dr Widderick. “The message here is that infrequent but intense cultivation can be a useful weed management tool within an otherwise zero tillage farming system. Generally, once a deep cultivation has been done there should be no cultivation of that area or paddock for at least four years to avoid the risk of bringing seed back to the soil surface.” Strategic burning Feathertop Rhodes grass is known to colonise around mature plants and potentially spread to form distinct weedy patches. Killing the large plant at the centre of the colony is usually not possible using chemical treatments. Strategic burning of early infestations of this weed can effectively reduce the biomass of the survivor plant and reduce the amount of viable seed present on the soil surface from 7500 seeds per m2 to less than 500 seeds per m2. Growers have made effective use of a flame-thrower to burn large feathertop Rhodes grass plants during the fallow. Strategic burning of early infestations of feathertop Rhodes grass in a fallow can effectively reduce the biomass of the survivor plant and reduce the amount of viable seed present on the soil surface. Crop competition Crop competition through narrower row spacing and or increased planting density provides an effective offensive against common sowthistle and flaxleaf fleabane. “Our experiment looked at the effect of crop competition on its own, however, in commercial situations crop competition would be used in conjunction with herbicide applications,” says Dr Widderick. “Narrowing wheat rows from 50 cm to 25 cm spacing had the most marked effect on fleabane seedhead production with an additive advantage if the crop density is also increased from 50 plants per m2 to 100 plants per m2.” Fleabane seedhead production (Source: M Widderick, DAF) Project work is continuing to investigate the options for increasing crop competitiveness in sorghum, winter and summer pulses and wheat. “We are particularly keen to identify ways to improve the competitiveness of sorghum crops, which are often a weak link in northern farming systems,” he says. “Highly competitive summer grasses that also have high tolerance and or resistance to herbicide can gain the upper hand in sorghum crops that often do not achieve canopy closure.” Crop competition through narrower row spacing and or increased planting density provides an effective offensive against common sowthistle and flaxleaf fleabane. Cover crops Summer fallow periods are heavily reliant on glyphosate for summer grass control. Preliminary research has explored the potential role of cover crops in place of a chemical fallow for control of summer grass weeds. Summer cover crops such as cowpea, lablab and French millet have the potential to smother summer growing weeds, particularly barnyard grass and feathertop Rhodes grass and return large amounts of organic biomass to the soil. French millet planted on its own, or in combination with the legumes, increased the amount of biomass produced. The higher the biomass production the greater the suppression of weeds. Cover crops will tend to use fallow stored moisture so the team investigated the effect of two termination dates on both subsequent crop yield and on weed numbers. “Feathertop Rhodes grass germination was minimal after all the cover crop treatments, and the yield of the following wheat crop was comparable to the chemical fallow control and no yield differences were found between treatments,” says Dr Widderick. “For barnyard grass, late termination of the cover crop reduced weed emergences before and after the following wheat crop, however there was a trend toward slightly reduced wheat yield compared to the early termination treatments that tended to boost yield compared to the chemical fallow control.” “The reduction in barnyard grass emergence and wheat yield are both likely due to reduced soil water following the late terminated cover crops,” he says. “Much more work is required to identify suitable cover crops and define the parameters for their use as a weed management tactic.” Harvest weed seed control Harvest weed seed control (HWSC) is known to be an effective strategy and is widely adopted in Western Australia and increasingly in South Australia, Victoria and southern NSW. The efficacy of the current tools such as narrow windrow burning, chaff carts, tramlining or chaff decks, bale direct systems and the new integrated Harrington Seed Destructor to control weeds found in the northern growing region is largely unknown. Trial work in winter crops to date have revealed that these tools can effectively collect and concentrate brassica weeds growing in wheat and chickpea crops in the northern region. The DAF weed research team is conducting further HWSC trials this winter with a focus on collecting wild oats seed. They are also keen to hear from growers planning to plant sorghum this summer where the team will be assessing the efficacy of various HWSC tools against barnyard grass and feathertop Rhodes grass. More information – Read the full published paper here: Non-chemical tactics for improved control of key northern region weeds and listen in to the webinar recording with Michael Widderick and Annie van de Meulen.
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Kohlhagen family is keeping weed numbers low

A changed attitude to weeds has been driving brothers Malcolm and Des Kohlhagen to implement a comprehensive management program on their farm near Wagga Wagga in southern NSW. Fifteen years ago the Kohlhagens assessed their weeds in terms of their likely economic impact but their mindset has changed to a much lower tolerance of weeds and they aim to keep numbers low year in, year out. Brothers Malcolm (left) and Des (rear) Kohlhagen along with Malcolm’s son, Adrian (centre), have developed and implemented a comprehensive weed management program for their 100 per cent cropping operation in southern NSW. The sheep have all gone from their 1600 ha operation and the family has expanded their cropping program to include a wider range of crops. The winter program now includes wheat, barley, canola, albus lupin and, most recently, faba bean. Malcolm and Des use break crops to introduce a different range of herbicides into the rotation and a double break of a pulse followed with canola provides two years of grass control so the cereals are sown into clean paddocks. The Kohlhagens have stuck to their crop rotation even when many other growers in the district reduced their canola hectares during the 2000s. They grew field peas many years ago but gave them away due to harvesting difficulties, and now find lupin and faba bean are a better fit in the rotation, providing a definite nitrogen boost for the following canola crop. Triazine tolerant (TT) and Clearfield canola are used to rotate chemical modes of action. The Kohlhagens don’t currently consider RoundUp Ready canola an option for them due to delivery point and marketing issues. Harvesting weed seed Canola crops are windrowed to aid in harvest management and, from this season, will be crop topped under the cutter bar to capture any late escaping weeds. The Kohlhagens also plan to crop top their pulse crops in years where late escapes are a problem. A narrow windrow chute is used on the harvester to collect weed seed in the pulse and canola crops and the narrow windrows are burnt to kill any seed present. This means over 40 per cent of their cropping area is subject to this very effective non-herbicide weed control method, particularly for annual ryegrass. A double break crop of a pulse crop followed with canola provides excellent grass weed control, including narrow windrow burning, before returning to cereals in the rotation. The brothers do not narrow windrow burn their cereal crops because of the high stubble load from barley and wheat crops yielding up to 6.5 t/ha and 5 t/ha respectively. When grown back to back these cereal crops generate too much stubble to effectively confine the fire to the narrow windrows. Getting the right conditions for burning is not always easy but the Kohlhagens believe it is worth doing and are looking forward to when they can justify investing in an integrated Harrington Seed Destructor so they can avoid burning, a practice that is not popular in town! In years where weed populations increase for any reason, haymaking is an effective method to stop weed seed set. The Kohlhagens find their heavier soil types are more likely to be challenged with weed blow-outs so they target these areas for haymaking as a salvage operation when necessary, giving great weed management benefits in poorer seasons. Malcolm and Des currently contend with ryegrass that is resistant to Hoegrass (Group A, fop) and are aware of similar resistance arising in wild oats and possibly wild radish on the farm. To keep wild radish numbers as low as possible they hand rogue plants in spring, to avoid any seed going through the header. Competitive cropping Further narrowing their row spacing to create greater crop competition is currently on the table for the family but until they have decided on the best option they are using high seeding rates, especially in the already-competitive barley crops. Blockages in the seeder and slug and slater damage to seedlings can create gaps in the rows, which provides the opportunity for weeds to flourish, and wider rows also allow more weeds to grow between rows. Changing the seeding setup is quite an expense so the family is considering whether to reduce the spacing on their current tined seeder to 250 mm or to change to a disc seeder. Even at 300 mm they are having trouble managing the stubble load so are hesitant to narrow the spacing much further, however the tined seeder allows them more herbicide options than can be safely used with disc seeders. On the other hand, they have been impressed with a 150 mm disc seeder they have seen operating in high stubble environments in South Australia. When the Kohlhagens first moved to controlled traffic, wheel tracks were left bare.  However, the gap left by the wheel tracks in the controlled traffic system lets more light into the adjacent rows, allowing more weeds to establish. To reduce this effect the Kohlhagens now seed their wheel tracks using a mid-row banding disc to provide increased crop competition and reduce weeds. Residual herbicides Pre-emergent herbicides are applied in all crops for grass control. While clethodim is still providing effective control in canola the Kohlhagens are well aware that it may not continue to be an option in the future. To support the pre-emergent herbicides the Kohlhagens are sowing their canola and pulse crops early to encourage better establishment and more rapid canopy closure, reducing the opportunity for in-crop weed germinations after the residual effect has diminished. Changed farming system; changed weed spectrum Since moving from a mixed farming operation to 100 per cent cropping, Malcolm and Des have seen a change in the weed spectrum present on the farm, with less capeweed present and less movement of weed seed around the farm. Not having livestock however has increased the need for more vigilance over summer to prevent weeds using precious soil moisture that may be the difference between finishing a crop and crop failure as the October rainfall is now less reliable. Fleabane is of particular concern in summers with higher rainfall seeing explosions in fleabane populations, which may require double knock treatments. Hairy panic is another persistent weed that must be sprayed when small to achieve effective and economical control. To manage broadleaf weeds the Kohlhagens use a low volatile 2,4D ester spike in glyphosate sprays applied over summer. Milk thistle is another emerging weed that is taking advantage of the no-till farming system. Clean seed a priority Prior to harvest Malcolm and Des inspect their paddocks to identify the cleanest areas of each variety suitable to harvest for retained seed. They then use a low capacity seed grader to remove small or damaged seed and as much weed seed as possible from their seed before storing on-farm. A mobile grader is contracted to grade the pulse grain for marketing and to thoroughly clean the pulse seed retained for sowing. Malcolm and Des are clearly on top of their game with their weed management program but they are also full of praise for their agronomist, Greg Condon from Grassroots Agronomy, who provides excellent agronomic advice and keeps their herbicide program up to date.