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What alternatives are there for desiccation and crop topping?

Desiccation and crop topping with pre-harvest herbicide application is a useful way to reduce seed set in late germinating weeds and is an effective harvest aid for cereal grain, pulse and oilseed crops.  Given the scrutiny that glyphosate is currently receiving Paul McIntosh, WeedSmart’s northern extension agronomist, says it may be a good time to start looking for alternative means of reducing weed seed set prior to harvest and avoiding any potential issues with market access. WeedSmart’s northern extension agronomist, Paul McIntosh has been investigating alternatives to glyphosate as a desiccant in mungbeans. “Currently, there are five herbicides registered for late season use in a variety of crops,” he says. “Glyphosate and diquat (or Reglone) are registered for use in wheat and barley in some states, canola, chickpea, lentil, faba bean, field pea, mungbean and soybean. For some of these crops, growers are also able to use paraquat, metsulfuron methyl or saflufenacil (Sharpen).” Although there are many benefits to the practice from a weed control perspective, there are also market forces at play that could curtail the future use of pre-harvest herbicides.    “It might be a good time for growers to re-visit some of the non-herbicide options for reducing seed set,” says Paul. “One possibility is to trial swathing in pulse crops like chickpeas, faba beans and mungbeans. Early commercial scale trials suggest that it could be very effective and could also have the additional benefit of hastening crop maturity, bringing harvest forward.” “In combination with harvest weed seed control, swathing is a valuable WeedSmart Big 6 tactic to manage the weed seed bank,” he says. “Swathing adds another non-herbicide tool to a diverse program, particularly for pulse crops that are often not very competitive, and for weeds that typically shed seed before the crop is ready to direct harvest.” Are there other herbicide options for crop desiccation if the current products are banned? In brief: Not really. In most instances, glyphosate is the most effective crop desiccant product. The details: Glyphosate is already a key component of cropping systems, particularly in no-till systems. In crops like mungbeans that have semi-indeterminant maturity traits that make them want to keep on growing, glyphosate applied at the label rate can give mixed results. The Australian Mungbean Association recently commissioned weeds researcher Dr Bhagirath Chauhan, QAFFI to investigate the efficacy of a range of possible alternatives to glyphosate as a desiccant, but there were no stand-out herbicide candidates. This small plot trial also included the non-herbicide option of swathing, and the results were very promising.   Has anyone trialed swathing commercial mungbean crops? In brief: Yes. A grower on the Darling Downs trialed swathing two mungbean crops in March and April 2020, the first being 0.4 ha within a larger paddock that was desiccated with herbicide, and the second was an 8 ha block. The details: These two trial paddocks were very successful and the grower was encouraged by the yield and grain quality of the swathed areas. This has generated significant interest from other growers and agronomists in the northern grains region. The crops were swathed at the standard 90 per cent physiological maturity, the same timing used for chemical desiccation in mungbeans. Harvest was delayed in the 0.4 ha block due to two falls of rain, 12 mm and then 18 mm, which meant the windrows remained in the paddock for 14 days. The crop produced 1.6 t/ha of reasonable quality grain with no evidence of dust. Picking up the mungbean windrow after a two week delay due to wet weather. The crop in the 8 ha block was shorter and sparser than the small trial block. Four days after this block was windrowed it was harvested with a Smale pea front at the correct moisture, suggesting that low yielding crops with reduced dry matter could be harvested earlier. The crop yielded just below 1 t/ha of excellent quality grain, with very few pods being left on the ground. What are the potential benefits and costs of swathing? In brief: The costs will be very similar to chemical desiccation and there could be extra benefits as the practice is fine-tuned. Swathing and windrowing costs around $35 to $40 per ha, similar to chemical desiccation, but the operation may take more time. The details: The first benefit is the avoidance of pre-harvest chemical application, removing the potential for desiccant chemical residues in the grain. The second big benefit is that it may be possible to bring harvest forward. Even if swathing is done when the crop is 90 per cent physiologically mature, the same as for chemical desiccation, the crop can be harvested within a few days and could be off the paddock nearly two weeks earlier than a desiccated crop. The clincher is the possibility of swathing before the crop reaches 90 per cent maturity. If this can be done without compromising grain size and quality, it could have very significant benefits for weed control. Many weeds in the northern cropping region set seed before traditional desiccation and harvest time and so if the crop can be cut earlier there is a chance that less weed seed will mature. Weed seed heads present in the mungbean windrow. It is early days for the revival of swathing in the northern cropping region and there are many things to be tried and tested. Early successes have also been seen in sorghum, faba beans and chickpeas.
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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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What’s next in controlling herbicide resistant broadleaf weeds?

PBA Kelpie XT is the fifth lentil variety released with tolerance to Group B herbicides, imazamox and imazapyr, along with one IMI-tolerant faba bean. University of Adelaide weeds researcher and PhD candidate, Alicia Merriam, says resistance to the IMI herbicides and other Group B chemistry, particularly the sulfonylureas (SU), is making control of some broadleaf weeds very difficult. University of Adelaide weeds researcher and PhD candidate, Alicia Merriam says screening has shown resistance to IMI herbicides in over 75 per cent of populations of both weeds collected in random weed surveys in South Australia. “Imi-tolerant lentils have been very popular with growers and have increased the weed control options in this important crop, but resistance in sowthistle and prickly lettuce is very widespread in the southern region,” she says. “Screening has shown resistance to IMI herbicides in over 75 per cent of populations of both weeds.” With investment from GRDC and an Australian Government Research Training Program Scholarship, Alicia conducted a trial at two sites in South Australia to investigate options to implement the best practice recommendation for lentils – to control weeds in the preceding wheat crop and again at sowing or crop emergence in the lentils. Both sowthistle and prickly lettuce are renowned for their prolific seed production when growing in non-competitive situations and wind dispersal of seed enables recruitment of resistance from crop borders, and far beyond. Consequently, eradication is not a realistic proposition. “Sequencing the gene that controls resistance to Group B herbicides has uncovered a large variety of different mutations in these species across the Mid North and Yorke Peninsula in SA,” says Alicia. “Most mutations of this type cause SU resistance, but some cause IMI resistance and the effect can vary between weed species. Crucially, we found all these mutations within a single grower paddock, which shows that they are widely distributed.” “Crop rotation and increased crop competition are essential components of the WeedSmart Big 6 to help run down the seed bank and suppress seed production by all means available,” she says. “Herbicide tolerance in pulse crops is a useful tool when coupled with strong competition and other herbicide options in as many crops as possible in the rotation.” What is the current resistance status of sowthistle and prickly lettuce in the southern region? In brief: Widespread resistance to Group B SU and IMI chemistry exists in both these broadleaf weeds. The details: In surveys conducted in the Mid North and Yorke Peninsula regions the percentage of SU-resistant populations of prickly lettuce increased from 66 per cent in 1999 to 82 per cent in 2004 and 100 per cent in 2019. The populations screened in the 2019 survey were all resistant to Group B IMI chemistry. Sowthistle surveys in the Mid North and Yorke Peninsula have found SU resistance in 89 per cent of populations and IMI resistance in 76 per cent of populations. Surveys also show that Group B resistance in sowthistle is very common across the rest of the southern cropping region. Sample populations screened with SU and IMI herbicides where Population 1 is susceptible to SU and IMI herbicides, Population 2 is moderately resistant to SU but susceptible to IMI herbicides, and Population 3 is resistant to both these Group B herbicides. Did crop competition or herbicide treatments affect weed seed production in the wheat phase or weed numbers in the following crop? In brief: The herbicide treatments used in the 2018 wheat crop had an impact on the sowthistle population in the next crop, but had little effect on prickly lettuce. Crop competition treatments did not reduce weed density in the following growing season. The details: The weed populations at both sites were confirmed resistant to Group B herbicides but susceptible to glyphosate. The three in-crop treatments were 1. no in-crop herbicide, 2. ‘conventional’ herbicide application of metsulfuron-methyl (Ally) + MCPA and 3. ‘proactive’ herbicide application of bromoxynil + picolinafen + MCPA (Flight EC). Two levels of crop competition (seeding rate 60 and 90 kg/ha) were also applied. In the 2018 wheat crop the proactive treatment gave the best control of sowthistle in that crop and this resulted in a reduction in numbers in the 2019 crop. Although the conventional treatment provided some weed control benefit in the 2018 crop, the benefit did not flow on to the next crop, probably because the sowthistle population was resistant to the residual action of the metsulfuron-methyl component of the conventional treatment. The herbicide treatments both reduced prickly lettuce density better than the untreated option but there was no additional benefit from the more expensive proactive treatment in either the year of application or the following crop. Crop competition is a well-established practice for reducing weed seed production, so it was surprising to find that increased crop competition did not reduce weed numbers in the following year. This could be due to conditions in the year of the trial and the mobility of seed of these species. In less competitive situations (right) sowthistle and prickly lettuce produce vast quantities of seed whereas in competitive situations (left) seed production is considerably reduced. What’s the take-home message for using herbicide tolerant lentils in the rotation? In brief: Herbicide tolerant crops are an important tool but must complement a diverse arsenal of weed control tactics. Short rotations are a very risky option and will lead to yield-reducing numbers of these prolific seeding weeds. The details: Sowthistle and prickly lettuce can be expected to become increasingly difficult to control in the lentil phase. Neither crop competition nor proactive herbicide regimes alone are likely to provide sufficient downward pressure on these weeds in a short rotation. Building in a longer break away from lentils is likely to be a more effective strategy. The number of different resistance mutations found in the cropping regions of the Mid North and Yorke Peninsula show that Group B resistance is widespread, and here to stay. This highlights the importance of diversity in crop and herbicide groups rotations, including the strategic use of herbicide tolerant crops. The new Group G herbicide Reflex, with planned registration for IBS (knife point press wheel) application in lentils, will also be a welcome addition to improve weed control options in this crop. Further resources Common sowthistle and prickly lettuce in lentil crops of southern australia – Managing herbicide resistance and highly mobile resistance genes, GRDC Update paper Feb 2020  
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Is HWSC useful against weeds that shed seeds early?

Many weed species shed seed before the grain crop is ready to harvest, so you might expect harvest weed seed control to be fairly ineffective against such weeds. WA’s Department of Primary Industries and Regional Development weeds researcher, Dr Catherine Borger, says it might surprise many people just how much harvest weed seed control can impact the seed bank of notorious early-shedders like great brome grass and barley grass. Dr Catherine Borger, DPIRD examined great brome and barley grass emergence and shedding times in controlled conditions at Northam WA to better understand the ecology of these weeds and the value of harvest weed seed control (HWSC). Dr Catherine Borger, DPIRD examined great brome and barley grass emergence and shedding times in controlled conditions at Northam WA to better understand the ecology of these weeds and the value of harvest weed seed control (HWSC). “What we found in our studies of several populations of these two weeds in WA and SA is that firstly there is a lot of variability in how these weeds behave in different seasons, and secondly, even relatively low levels of weed seed capture at harvest can make a big difference to reducing the weed seed bank,” she says. This research is part of a national GRDC investment in better understanding the ecology of key weed species in each region. “Great brome grass and barley grass cost farmers in the Southern and Western cropping regions around $22 million and $2 million annually respectively, in control costs and lost production,” says Catherine. “Great brome grass and barley grass are problematic weeds on 1.4 million ha and 235,000 ha of farming land respectively across these two regions. Consequently, farmers are spending over $3 million a year on additional herbicide costs to manage herbicide resistant great brome grass.” The weed ecology work on these two species showed that an integrated control program can effectively run down the seed bank for both these species in three or four years and that staggered emergence, particularly in brome grass means that end of season control tactics must be included in the strategy. “The WeedSmart Big 6 is a useful planning tool because managing the weed seed bank requires a range of tactics to be implemented at different times through the year,” she says. “A combination of herbicide and non-herbicide tools used at strategic times will have the best chance of getting weed numbers down and keeping them low.” What do we know about seed retention at harvest for these two weeds? In brief: In some years it is quite low but in other years a large proportion of the seed is still in the seed heads at harvest and beyond. The details: Seed shedding is not well understood and is driven by a complex combination of genetic and environmental factors. Harvest date obviously has a large bearing on the amount of weed seed still on the plants at harvest. In paddocks with high or increasing weed numbers it may be worth harvesting as early as possible to maximise the benefit of HWSC. In 2016 to 2018 great brome seed retention at the Wongan Hills site in WA was between 40 and 70 per cent at crop maturity (around mid-November). A later harvest date in 2016 resulted in almost no seed being present on the plants at harvest. Similarly, for barley grass – in 2016 all the seed shed by harvest in December, and in 2017 and 2018 seed was still on the plants well into summer. Great brome grass retains a proportion of seed at crop maturity and even capturing 20 to 40 per cent of the weed seed through HWSC can make a big difference to the weed seed bank. Great brome grass retains a proportion of seed at crop maturity and even capturing 20 to 40 per cent of the weed seed through HWSC can make a big difference to the weed seed bank. Do you recommend HWSC as a useful control tactic for great brome and barley grass? In brief: Yes, particularly for the highly competitive great brome grass. Even capturing 20 to 40 per cent of the weed seed produced can make a big difference to future weed pressure. The details: It is difficult to manage weeds that exhibit staggered germinations during the cropping season with herbicides alone. Both these weeds can be difficult to get into the harvester – great brome can bend forward and slip under the cutter bar while barley grass seed heads are often held very close to the ground. While barley grass might be almost impossible to get into the header it is also much less competitive in the crop than great brome. Modelling with the Weed Seed Wizard decision support tool showed that if the header is able to capture just 20 per cent of the great brome grass seed produced, the seed bank can be halved over a six-year rotation. Consistently collecting and destroying 60 per cent of the great brome seed each year can reduce the weed seed bank from almost 11,000 seeds to just 86 at the end of a six-year rotation. Any herbicide tactic applied early in the season that only achieved 20 per cent control would be considered a waste of time, and this highlights the value of late season weed control tactics such as HWSC. How long do great brome seeds last in the soil? In brief: Great brome grass can be brought under control in three or four years if an integrated weed management plan is implemented. The details: Under irrigation, about 40 per cent of the seed germinated in the first year and almost all the seed had germinated by the end of the third year. In field conditions a similar pattern was recorded for both great brome and barley grass. If control tactics are used to stop seed set then it is possible to reduce weed numbers within a few years. Great brome populations in SA were found to exhibit more delayed emergence traits than populations in WA. This could be due to the longer history of pre-emergent herbicide use in SA that has resulted in the evolution of delayed emergence to avoid early herbicide application. In both WA and SA, barley grass populations exhibited staggered emergence. Great brome is highly competitive and is a costly weed for growers, particularly in low crop yield seasons. However, when moisture is not a limiting factor, crops can often produce good yield even when high weed numbers are present.
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Group G herbicides & effectiveness for herbicide resistance

There has been a gap in the pre-emergent herbicide options for growers to control broadleaf weeds, particularly in pulses, but also in cereals. Dr Chris Preston, Professor, Weed Management at The University of Adelaide says a suite of new Group G herbicides will go a long way to help growers control brassica and thistle weeds and rotate away from the imi chemistry that has been heavily used in recent years. Dr Chris Preston, University of Adelaide says using the suite of new Group G herbicides in a tactical way to control early germinating broadleaf weeds could help keep pulses profitable in southern farming systems. “The new Group G herbicides will be of particular value in pulse crops,” he says. “Until recently Group G products have only been used in small quantities, predominantly as a knockdown spike ahead of planting, but several of the new products have pre-emergent herbicide use patterns.” Group G mode of action inhibits part of the process for making chlorophyll, and the leaves die as a result. Uptake of the herbicide is usually through the leaf surfaces – either through contact on emerged weeds or taken up from the soil as the weed seedlings break through the soil surface. “The introduction of Terrain (flumioxazin) as a pre-emergent broadleaf herbicide in front of faba beans offered a new and helpful use pattern in the pulse phase,” Chris says. “Terrain is also a good fenceline weed control option on heavier soils, provided all surface vegetation is removed prior to application.” Syngenta’s yet-to-be-released Group G product, Reflex (fomesafen) is expected to offer pulse growers pre-emergent control of broadleaf weeds that might be resistant to other MOA groups. This product can be used either pre-emergent incorporated by sowing (IBS) or post sowing, pre-emergent (PSPE), and will provide an alternative to imi herbicides for the control of weeds like sowthistle and prickly lettuce in pulse crops. Lentils are less tolerant than other pulse crops, so Reflex can only be used IBS in lentils. With the release of these new products Chris is reminding growers and agronomists of the importance of ensuring that herbicide product choice is always based on addressing the weeds present that are likely to cause economic loss or produce large quantities of seed. “Mixing and rotating herbicide modes of action is one of the WeedSmart Big 6 tactics, but crop competition is just as important,” he says. “Pulses are particularly sensitive to competition from weeds in the early crop stages, so using these Group G herbicides in a tactical way to control early germinating broadleaf weeds could be a very good way to keep pulses profitable in our southern farming systems.” The sustainable use of Group G and other new herbicides coming to market will be a key topic at WeedSmart Week 2020 in Clare, SA. The companies developing these new Group G products will be present to answer questions and provide advice. Click here to register for WeedSmart Week 2020. Should I mix and rotate these new herbicides with existing products? In brief: Yes. Read the labels as they become available and look for opportunities to mix and rotate within and between seasons and crops. The details: For example, Terrain offers broad spectrum weed control in tank mixes with TriflurX, Terbazine, Avadex Xtra, Kyte, Simagranz and Rifle. Terrain has a narrow weed spectrum for the rate registered in-crop for faba beans. With some clever planning these products can help ‘bring back’ some previously lost chemistry using the mix and rotate tactic, extending the effective life of a broader range of herbicide options. When coupled with some non-herbicide tactics the grower can regain control of herbicide resistance on their farm and operate in a low-weed situation. What new use patterns are available with the new Group G herbicides? In brief: Pre-emergent with IBS and knockdown options. The details: The first Group G with pre-emergent properties to hit the Australian market was Terrain (flumioxazin), from Nufarm with new use patterns registered for wheat and pulses. Other products with pre-emergent properties expected to come to market in the next year or two are Syngenta’s Reflex (fomesafen) and BASF’s Voraxor (saflufenacil + trifludomoxazin). Voraxor can be used pre-emergent in cereals and also as a knockdown spike. Terrador (tiafenacil) from Nufarm will have knockdown spike use and is safe for cereal or pulse planting just one hour after application. More details about the new products can be found in the Group G Tech Note. The early trials with Reflex look very promising and widespread use of this herbicide is expected in WA, where farmers have battled wild radish control in lupins for decades. Is resistance to Group G a problem? In brief: Not yet in Australia. The details: There are no recorded cases of weeds resistant to Group G herbicides in Australia. Although the Group G mode of action has been commonly used for over 15 years, these products have generally been used in small quantities. Resistance to Group G is becoming widespread in North America and we will soon have resistance problems if we do not practice the Big 6 tactics to minimise weed seed production. More resources: Podcast – What role are Group G herbicides playing in farming systems Diversity Era – Pre-emergent herbicides online course Group G Tech Sheet Nufarm Terrain – Group G pre-emergent and knockdown 500 g/kg flumioxazin New registrations for wheat and pulses in 2019. Pre-emergent weed control in lucerne, wheat, faba bean, chickpea, field pea and along fencelines. Broadleaf weed control only in pulses. Control emerged weeds with knockdown before applying Terrain in wheat and pulses or along fencelines. New chemistry for IBS use pattern. Improves brown-out and weed control of knockdown herbicides. Pre-plant knockdown spike ahead of barley, chickpea, cotton, faba beans, field peas, lentil, lupin, maize, mungbean, oats, sorghum, soybean, sunflower, wheat. Add to glyphosate and paraquat/diquat products. Requires adjuvant. Not suitable for lighter soils. Residual activity on annual ryegrass, sow thistles and wild radish. Broad spectrum weed control in tank mixes with – TriflurX, Terbazine, Avadex Xtra, Kyte, Simagranz, Rifle. Syngenta Reflex – Group G pre-emergent 240 g/L Fomesafen SL Registration expected early 2021. Suitable for a range of pulses Wide range of broadleaf weeds (including sowthistle and prickly lettuce) – check label when available. IBS or PSPE uses. IBS knife points and presswheels only in lentils. BASF Voraxor – Group G knockdown and pre-emergent herbicide 250 g/L saflufenacil plus 125 g/L trifludomoxazin Both group G actives Used in wheat, durum and barley against a wide range of grass and broadleaf weeds IBS knife points and presswheels only Registration expected in 2020 and launch in 2021 Knockdown and residual control of broadleaf weeds plus suppression of ryegrass. Broadleaf residual control for 8 to 12 weeks (at higher application rates) Partner with glyphosate as a knockdown spike or mix with paraquat for double knock applications. Nufarm Terrador – Group G knockdown spike 700 g/kg tiafenacil WG Registration and commercial launch expected in 2021. Likely use pattern, partner with glyphosate as a knockdown spike or mix with paraquat for double knock applications. Expect suppression of ryegrass as a knockdown partner with glyphosate or paraquat. 1 hr plant back for cereals and pulses, probably 7–14 days for canola, depending on rate.
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How can I maximise the weed control value of my crop rotation?

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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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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What are the ‘mix and rotate’ options for in-crop herbicides?

Chances are you have a suite of herbicides that are your ‘go-to’ products for weeds growing in-crop. Chances are you use them regularly because they work. Jason Sabeeney, Technical Services Manager at Syngenta Australia is challenging growers and advisors to consider breaking established patterns and to start using as many herbicide modes of action as possible in their weed control program, at least on part of the farm.   Jason Sabeeney, Technical Services Manager at Syngenta Australia is challenging growers and advisors to consider breaking established patterns and to start using as many herbicide modes of action as possible in their weed control program, at least on part of the farm. “There are many options available and many considerations when it comes to planning an effective in-crop herbicide program,” he says. “The heavy reliance on mainly Group B and I herbicides for broadleaf weed control in cereals, and on Group A and B grass selectives, is putting great pressure on high resistance risk chemistry.” There are currently 46 approved herbicide active ingredients or mixes for broadleaf weed control in cereals, spanning seven herbicide modes of action (MOA). Almost three-quarters (34 out of 46) of these actives are from herbicide Groups B (e.g. SU herbicides) or I (e.g. phenoxy herbicides). Instead of one application with one herbicide mode of action, Jason is urging growers and agronomists to employ multiple modes of action in-crop, apply herbicides at the optimal times and integrate non-chemical weed control methods that prevent weed seed set. “Unfortunately, in-crop herbicide application frequently involves making compromises when it comes to efficacy and logistics,” Jason says. “With large spray programs the temptation is to use wider gear and travel faster to get across the area as quickly as possible.” “There is also the temptation to minimise the number of passes by adding multiple products into each tank mix and to wait for a second or even third flush of weeds to emerge before spraying. These can be high risk practices that can compromise the efficacy of the herbicide treatments.” While understanding the constraints, Jason is recommending that growers and agronomists focus on maximum product efficacy and reducing the weed seedbank when making spray decisions. He says this approach will reduce the risk of weed blow-outs and slow the pace of herbicide resistance. “The WeedSmart Big 6 includes a range of herbicide and non-herbicide tactics that can be implemented in-crop to minimise weed seed set. Mixing and rotating in-crop herbicides is just part of the bigger strategy,” he says. Can I just add more modes of action to the tank mix? In brief: Not necessarily; but some herbicide mixes could be a very good option. The details: Herbicide mixes of multiple modes of action can be useful to broaden the spectrum of weeds controlled and to assist with resistance management. If used in rotation with other herbicide options these products add to the diversity, increase overall weed control and reduce weed seed set. The compatibility of herbicides, and other agricultural chemicals that might be added to the tank, such as insecticides, fungicides and trace elements, is not just about their physical compatibility. Conducting a jar test will show if the mix can be made without forming a glug or precipitate, but it doesn’t tell you if the efficacy of the individual products is maintained, or if it is safe to the crop. Using a proprietary mix or one that is recommended on label takes out the guesswork because the mix has usually been thoroughly tested for biological compatibility, crop safety and weed efficacy within the prescribed use pattern. Some mixes have an additive effect where each component improves the overall weed control, compared to using the products individually. Some are antagonistic, and when combined they reduce overall weed control compared to using the products individually. In rare cases, the combination produces a synergistic mix, where the combination delivers a result greater than the sum of their parts. Where this occurs, it should be exploited. A good example is a mix of Group H + Group C products. When combined these two modes of action deliver efficacy greater than the sum of their parts (true synergy) and are highly effective. Mixing and rotating herbicide modes of action in-crop is not as simple as just adding multiple herbicide modes of action to a tank mix. The products often need to be applied separately and with the aim of maximising weed control efficacy. Do I need to treat broadleaf herbicides products differently? In brief: Yes, understand the mode of action. Consider how each product works and the conditions and application parameters that maximise performance. The details: Contact herbicides, for example mixes containing bromoxynil (Group C), carfentrazone (Group G) and diflufenican or picolinofen (Group F) are most effective when applied early in the season and onto small weeds (2 to 6 leaf stage). Contact herbicides rely on good weed coverage, which is best achieved before the crop canopy begins to shade the weeds. Light and temperature also play a significant role in activity of these products. These products are often combined with another mode of action like Group H, C, F or I to broaden the spectrum and or assist with coverage. On the other hand, herbicides in Groups I and B are systemic, so whilst it is always best to target smaller weeds with good coverage, some of these products perform well even if sprayed later in the season and they are generally effective even on larger weeds. What about using tank mixes for grass control? In brief: The registered options for grass control post-emergent are primarily Group A and B herbicides, along with early post-emergent group J. The details: Group A and B herbicides are generally very effective where there are still susceptible grass populations, but both have a high risk of evolving herbicide resistance. As a rule, Group A herbicides perform best when applied alone rather than mixed with broadleaf herbicide options. For example, a tank mix of Group A + some Group I herbicides is often antagonistic, resulting in a 10 to 30 per cent decrease in grass weed control compared to applying the Group A herbicide on its own. For early post-emergent application, Boxer Gold (Groups J + K) can be tank mixed with broadleaf herbicides but it is generally accepted as suppressing, rather than controlling, grass weeds such as annual ryegrass. The grass selective herbicides are very responsive to adjuvants and environmental factors. The recommended adjuvants will be listed on the label and it is important to follow these instructions to optimise efficacy. Whilst resistance is widespread to Group A and B chemistry in grass weeds, environmental conditions such as frost, waterlogging and drought can have a significant impact on performance of these herbicides, and resistance is sometimes mis-diagnosed as the cause of product failure. Integrated weed management practices including non-herbicide tools, such as crop competition, harvest weed seed control and cutting for hay, are essential components in the grass weed control program, particularly in seasons where the pre-emergent herbicides don’t perform to their full potential. Other resources Big 6 – Mix and rotate tactic
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Is poor weed control due to herbicide resistance?

If a grower experiences a poor spray outcome on annual ryegrass with glyphosate, or seemingly needs to continually increase rates to achieve the same level of control, it might seem obvious that herbicide resistance is the most likely problem. On the contrary, Maurie Street, CEO at Grain Orana Alliance says that this is not always the case, having investigated ways to regain control of problematic ryegrass populations. Grain Orana Alliance CEO Maurie Street says growers and agronomists need to critically assess any spray job that does not achieve the expected results. Photo: GRDC “Annual ryegrass has always been present on most farms in the Central NSW cropping region and many populations are resistant to Group A and B herbicides,” says Maurie. “So, when growers started to have trouble controlling this weed with glyphosate it seemed natural to assume that populations were becoming resistant to our most relied on knockdown herbicide.” In an effort to finesse the available control options, GOA established trials over three years with GRDC investment on seven sites where poor control of annual ryegrass over previous years had resulted in increased weed seed banks. “The first thing we did was collect samples from each site and have them tested using the Quick Test method for glyphosate resistance,” says Maurie, “We were surprised to discover that five of the populations were in fact susceptible, even at lower label rates of glyphosate, and the other two populations were only moderately resistant to the lower rate and 100 per cent susceptible to higher label rates.” What this suggests is that there can be something other than resistance contributing to herbicide failures. Testing will reveal if herbicide resistance is at play and identify herbicide products and rates that can be expected to provide acceptable control. Next, critically assess the spray operation and identify factors that could have affected the efficacy of the spray job. Finally, look for ways to implement the WeedSmart Big 6 tactics in your weed control program to keep weed numbers low.   Trials that ran at 7 sites for 3 years revealed that many things can contribute to a spray failure and growers should not rush to the conclusion that herbicide resistance is always the cause. So, if the ryegrass was susceptible to glyphosate, why was control poor? Short answer: Most likely a combination of reasons. Longer answer: Instances of poor weed control after a herbicide application could be associated with one or more factors such as poor spray water quality, incorrect spray timing, inappropriate sprayer set up delivering less than optimal spray droplet size and/or water rates, products with sub-optimal surfactant loadings, or environmental stress affecting the plant and or the herbicide activity. Or in some cases, the herbicide rate is too low for the job at hand. In the event of an apparent herbicide failure, review and investigate all aspects of the application – including Stress, Timing, Application and Rate (STAR). Don’t just assume that herbicide resistance is to blame. In the paddocks used for this trial, early testing for herbicide susceptibility would have revealed that glyphosate was still effective, although some higher application rates would be needed in some fields to achieve adequate levels of control. Attention could then be turned to other factors that might have caused the poor control of annual ryegrass in these paddocks. Is it safe to use higher rates of glyphosate and how does it help? Short answer: Using a rate at or near the upper end of the allowed range for glyphosate can improve efficacy in both resistant and susceptible populations. Always stay within the label rate range when applying herbicide. Longer answer: In the case of glyphosate resistant plants, the resistance is often still rate related. Consequently, increasing product rates will effectively control plants with lower level of resistance. In the case of both resistant and non-resistant plants, increasing glyphosate rates may contribute to more effective control by counteracting poor application, improving control of older or stressed plants, overcoming reduced efficacy due to using poor quality water and when treating plants covered by dust. Higher label rates can also improve glyphosate activity on plants exposed to the higher temperatures that can arise in early autumn or late spring (Boutsalis et al. 2015). In these GOA trials the use of higher glyphosate rates gave acceptable control in all but one paddock. The population in question was confirmed susceptible and although the application conditions were noted as ‘very dry and the weeds somewhat stressed with warm spray conditions’, another trial site sprayed under similar conditions, using the same water source, spray set up and products achieved acceptable control. The reason for the failure on a susceptible population was not confirmed in the trial but highlights the importance of susceptibility testing both to determine if resistance is a contributing factor and if increasing the application rate is likely to be an effective strategy. In these GOA trials the use of higher glyphosate rates gave acceptable control in all but one of the problematic paddocks. What’s more important, adjuvants and surfactants or product rate? Short answer: Generally speaking, increasing the product rate gives the most consistent improvement in control. Longer answer: In the trial, a range of glyphosate formulations, adjuvants and surfactants were tested for efficacy at the seven sites. There was no consistent difference in performance of Roundup® CT®, Roundup ULTRA® MAX (a premium, fully loaded product) and a low priced- generic brand, if robust rates were used. At the lower rates tested, Roundup ULTRA® MAX often performed better than the other two products tested. Similarly, when surfactants or additives were applied with robust rates of glyphosate there was no consistent advantage to the addition of BS1000®, LI700®, Liase®, Wetter TX® or Activator®. At lower glyphosate rates, the addition of these surfactants sometimes improved control, but often not to the levels achieved with higher rates of glyphosate, and the response was inconsistent. Having said this, if surfactants or additives are required to improve water quality they should always be used. More information GRDC Update Paper : Is our ryegrass really getting harder to kill through our over reliance on glyphosate? Herbicide resistance and susceptibility testing
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Does delayed sowing help manage weed populations?

The answer to this question is a very simple ‘no’. Waiting for weed seeds with longer dormancy to germinate before sowing costs yield and weeds often set more seed in late sown crops.   Dr Gurjeet Gill, Associate Professor of Weed and Crop Ecology at The University of Adelaide says sowing a competitive crop ‘on time’ has better outcomes for both crop yield and suppressing weed seed production. Dr Gurjeet Gill, Associate Professor of Weed and Crop Ecology at The University of Adelaide is one of the four experts presenting the new WeedSmart Crop Competition 101 online course. “The lack of effective in-crop herbicides leaves growers with few chemical options when it comes to controlling weeds like annual ryegrass and brome grass that are emerging later in the crop,” says Gurjeet. “Our field trials in South Australia demonstrated that sowing ‘on time’ is the best way to maximise crop yield and suppress weeds that germinate in-crop, both with and without the use of pre-emergent herbicides.” The time of sowing x seeding rate x herbicide field trials were conducted at several sites in South Australia in 2018 and 2019 with GRDC investment. “The other aspect of these trials was investigating the effect of seeding rate on weed density and seedhead production,” says Gurjeet, “Higher seeding rate increased the yield in wheat at Minnipa at both times of sowing, and did not increase screenings.” Early sown crops consistently produce more crop and less weeds. Dr Gill is one of the presenters in WeedSmart’s new Diversity Era ‘Crop Competition 101’ course, which can be completed online in less than 10 hours, giving you a solid grounding in ways to effectively implement tactics that give crops a competitive advantage over weeds. This free online course can be found at www.diversityera.com/courses/crop-competition-101 Why are weeds in cropping systems becoming more dormant? Short answer: Increased cropping intensity and routine use of pre-emergent herbicides selects for the longer dormancy trait in annual ryegrass and brome grass. Longer answer: Weed populations have a mix of individuals with different levels of seed dormancy. In self-regenerating pastures, there is likely to be a penalty for high seed dormancy and germinating later than the neighbouring plants. Therefore, high dormancy late germinating plants remain a minority in the population. However, the situation changes when growers switch to intensive cropping where knockdown herbicides routinely kill the very early germinating plants. In such systems, weeds that emerge with or soon after the crop have a greater survival because they escape the effects of the knockdown herbicides. After several years of cropping, weed populations change from being early germinating to later germinating. These later germinating weed populations are less responsive to delayed sowing and are now common in southern farming systems. See the
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Get the best results using pre-emergent herbicides

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

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