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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. 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. 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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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?

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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The ‘mix and rotate’ options for in-crop herbicides

with Jason Sabeeney, Technical Services Manager, Syngenta Australia 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?

with Maurie Street, CEO, Grain Orana Alliance (GOA) 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?

with Dr Gurjeet Gill, Associate Professor – Weed & Crop Ecology, University of Adelaide 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 seed dormancy explainer below for more information on the mechanisms at play. How can I reduce the impact of these late-emerging ryegrass and brome grass weeds? Short answer: Grow the most competitive crop possible – cultivar, seeding rate, row spacing, row orientation and time of sowing all have an impact. Longer answer: In these trials, time of sowing was by far the major contributor to weed suppression and crop yield – even when no pre-emergent herbicide was applied. Delaying sowing to wait for weeds to germinate after breaking rain is usually counter-productive, unless the delay results in better soil moisture conditions for pre-emergent herbicides. But even when there is a weed control benefit from the later sowing date there is likely to be a penalty on crop yield of at least 20 per cent. This was demonstrated at the Minnipa site in 2018, where a delay in sowing of wheat reduced in-crop ryegrass density and its seed production, but there was a yield penalty of 25 to 43 per cent. Also in 2018, the delayed sowing treatment at Marrabel, saw a large reduction in brome grass plant density in barley — however, weed seed production on these fewer plants was high. Delayed seeding also reduced barley grain yield by almost 30 per cent. In barley, the additional in-crop use of Intervix completely prevented weed seed set at both the on-time and delayed time of sowing. As resistance to Intervix is still quite rare in brome grass, use of Clearfield® crops can be a highly effective part of the management program. Field trials at various locations across South Australia clearly demonstrated that sowing a competitive crop ‘on time’ has better outcomes for both crop yield and suppressing seed production on annual ryegrass and brome grass. Photo: University of Adelaide.   What is the effect of seeding rate? Short answer: Higher crop seeding rate can greatly reduce weed seed production. Longer answer: In these trials, doubling the crop seeding rate from 100 to 200 plants per m2 usually reduced weed seed production by 30 to 40 per cent. In the barley trial, the performance of the late planted crop was improved when the higher seeding rate was used. In many other trials, very high seeding rates (such as 400 plants per m2) have been shown to vastly reduce annual ryegrass numbers. Using variable rate seeding, growers can consider sowing known weedy patches at very high seed rates (e.g. 250 to 300 plants per m2) simply to outcompete weeds. Using a sowing rate at the upper end of the recommended range for the chosen cultivar is good practice to help support the efficacy of pre-emergent herbicides early in the season and provide strong competition for weeds that emerge later in the season. Seed dormancy explainer  Seed dormancy is usually associated with a ‘dark’ requirement, where seeds can remain dormant on the surface, where they are exposed to light, but when they are ‘planted’ the dark requirement is filled and germination follows. These weeds are the target of pre-emergent herbicides in no-till farming systems. What growers have observed, and researchers have tested, is that some weeds remain dormant even after the dark requirement has been fulfilled, suggesting that some other trigger may be at play. In brome grass, for example, it has been demonstrated that some seeds do not germinate until a ‘cold requirement’ has been fulfilled. These weed seeds remain dormant in the soil until the night temperature reaches 4 degrees C, often well after any pre-emergent herbicide applied at seeding has degraded. Once the weeds with the cold dormancy trait have established and set seed they can become the dominant in-crop weed pressure to impact crop yield, and future applications of pre-emergent herbicides will have a limited effect on the population.
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Get the best results using pre-emergent herbicides

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

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

with Greg Condon, WeedSmart southern agronomist The power of the WeedSmart Big 6 really ramps up when the tactics are applied in a site-specific way. The broad-brush approach of herbicide application has played a big role in the development of herbicide resistance and has effectively changed the weed spectrum on many farms in favour of weeds that are hard-to-kill using least-cost herbicide options. Greg Condon, WeedSmart agronomist based at Junee, NSW has seen the benefits that growers have won through the use of optical sprayer technology over the last 20 years and is keen to see the development of new technologies that will be commercially available in the foreseeable future. Greg Condon, WeedSmart agronomist based at Junee, NSW has seen the benefits that growers have won through the use of optical sprayer technology over the last 20 years and is keen to see the development of new technologies that will be commercially available in the foreseeable future. “Being able to identify weeds and apply a treatment that is going to be the most effective is the best way forward,” he says. “If the treatment applied kills the weed and prevents seed set then eradication of the weed – herbicide resistant or not – is within reach.” Once the location of the weeds is known it is possible to use robust herbicide rates and devise mixtures that are most effective on the weeds present, knowing that the volume of product applied will be low and therefore an economical proposition. “The mapping technologies are developing fast and many are already commercially available,” says Greg. “In some cases satellite imagery is useful to identify weedy areas and to estimate areas and in other cases drone imagery is more useful. The NDVI sensors mounted on tractors can also provide good information about the presence and density of weeds.” The site-specific weed control technology arena is pretty crowded at the moment with many companies and research bodies working hard to find the ultimate commercial products, but it won’t be long before there are paddock-ready options available to growers for both herbicide and non-herbicide control tools. Many of the leading ag-tech companies gave updates of their products and services at the 2019 WeedSmart Week events at Horsham and Emerald. What’s the best way to identify and map weeds or weedy patches? Short answer: Satellite or drone imagery. Longer answer: Satellite imagery gives a broad view of the weeds present in a paddock, particularly in a fallow situation. Recent personal experience showed that ground truthing satellite maps using NDVI filters was not very successful. Using a drone to collect the imagery is a practical option that offers more flexibility but the images collected need to be stitched together to create a single image for analysis. Some growers are effectively using their own systems but there is also a commercial option through providers such as Jerome Leray at In-Farm. Getting accurate maps that can turn your current sprayer into a spot-sprayer is the closest-to-reality technology in site-specific weed control. When combined with the rapidly moving development of software to identify individual weed species, growers expect to achieve significant reductions in product volumes and take advantage of more expensive but effective selective herbicides.      Using a drone to collect the imagery is a practical option that offers more flexibility but the images collected need to be stitched together to create a single image for analysis. What are the herbicide delivery platforms closest to commercial operation? Short answer: Robots and automated tractors are now giving a new angle to older technologies and artificial intelligence software is rapidly developing to support integrated herbicide sprayers. Longer answer: Using an automated tractor or a robot, such as a SwarmBot, to expand the use of optical sprayer technologies opens up the possibility of spraying more often and continually targeting small weeds. These platforms could also adapt to new technologies as they are developed. The reliability and practicality of green-on-green spraying is still a little way off but there is significant investment going on with ag-tech companies such as Bilberry and AgriFac teaming up to build a sprayer with wide-ranging capabilities. While the green-on-green technology is a not yet widely or economically available, an interim option is a camera-guided shielded-sprayer. Being camera-guided, sprayers like CropStalker can operate in crops like lentils that do not have the plant architecture to allow the use of conventional trailing shields. These sprayers currently apply a band of herbicide so they are not weed-detecting, but they are still able to target weeds growing in-crop and using lower volumes or more selective herbicides. There is a possibility that these sprayers could also have cameras fitted to spray only when weeds are present. While the green-on-green technology is a not yet widely or economically available, an interim option is a camera-guided shielded-sprayer like CropStalker. And the ‘old’ optical spot sprayers are also updating, WeedSeeker 2 now has on-board weed mapping capability while WEEDit have upgraded their sensor accuracy and capability to analyse data. On-board mapping capability allows the sprayer to ‘remember’ where weeds were sprayed early in the fallow, enabling the operator to apply residuals to known weedy patches while also applying a knock-down to kill existing plants prior to planting. What about delivering non-herbicide options? Short answer: Although there are a few non-herbicide tools being developed, most are likely to take a while to come to market. Longer answer: The problem with many non-herbicide options, such as microwave and laser, has been that they are high energy when applied in a broad band, but the energy required is cut by over 90 per cent when applied in a site-specific way. SwarmFarm have demonstrated the possibilities of microwave units on a robotic platform that also allows for the time required to deliver the necessary treatment. The WeedChipper is another option that uses sensors to detect weeds and activate a tyne to physically remove weeds. Once developed, this tool has great application in situations where escapes have developed into large plants where herbicide is not the best option, or to manage weeds that are difficult to kill using current herbicides. An overseas camera-guided cultivator, like the camera guided shielded sprayer, is also available and suitable for reducing weed pressure in-crop. https://www.weedsmart.org.au/app/uploads/2019/12/WeedChipper-EmeraldWSW2019.mp4 More information: AHRI Insight Thermal weed control – just hot air or site-specific reality? https://www.weedsmart.org.au/podcasts/weedseeker-2-overview-and-weed-control-findings-from-csiro/ https://www.weedsmart.org.au/whats-the-latest-in-optical-sprayer-technology/ https://www.weedsmart.org.au/is-mechanical-site-specific-weed-control-a-practical-fallow-management-option/ https://www.weedsmart.org.au/webinars/weed-control-with-automation/
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Implementing the ‘mix and rotate’ strategy to combat herbicide resistance

with Tony Lockrey, consulting agronomist, AMPS Moree Mixing and rotating herbicide modes of action is a key strategy in the WeedSmart Big 6 – but it’s a herbicide response to a herbicide problem. So, while it’s critical, it must be implemented within a diverse weed management program. Tony Lockrey, senior agronomist with AMPS Agribusiness at Moree has seen herbicide resistance get out of control on some farms in northern NSW while other growers have responded early and managed to maintain a broader spectrum of effective herbicides in their program. AMPS Moree consulting agronomist Tony Lockrey has seen good results when herbicides are rotated and mixed in each phase – the fallow, pre-sowing, in-crop and for desiccation. “It has to start with herbicide resistance testing – specifically for susceptibility,” he says. “Knowing what does work is very important as you’ve probably already got a fair idea about what doesn’t.” Once all the effective actives are ‘on the table’ it’s time to look at what crops can be grown to allow the use of the widest range of herbicide groups in the rotation, and where you might be able to find synergistic mixes that can further delay resistance and potentially allow the use of actives that are no longer effective on their own. “When we sit down to plan out an integrated weed control program we want to make sure there is rotation and mixing going on in each phase – in the fallow, pre-sowing, in-crop and for desiccation, where required,” says Tony. “When this is done in conjunction with a determination to stop seed set and remove survivors then it is possible to keep weed numbers low.” With an increasing number of proprietary herbicide mixes coming onto the market and the broad spectrum of synergistic and antagonistic interactions between potential mixing partners it pays to be well-informed and to seek advice. If I already rotate modes of action why do I have to mix too? Short answer: Rotation buys you time; mixing buys you shots. Mixing and rotating buys you time and shots. Longer answer: Rotation of effective modes of action can significantly delay the onset of herbicide resistance and needs to be built into your crop rotation plan. Herbicides in Group A and Group B are particularly susceptible to multiple exposure resistance with as few as six exposures being enough to select for the resistant mutation. By mixing MOA groups, either in the same tank mix or applied separately to the same population (like a double knock), those plants that survive one MOA are often killed by the second. How does testing for susceptibility help when there’s a weed blow-out? Short answer: Knowing what will work against a resistant population helps drive down the seed bank and helps you regain control. Longer answer: One real-world example is a paddock near Moree where Group A resistant wild oats were discovered in 1998 following a history of repeated use of Topik® (Group A – fop), Verdict® (Group A – fop) and, later, Axial® (Group A – den). Testing of this population showed the wild oats was very susceptible to Group B sulfonylurea, so Atlantis was used to drive down the weed numbers. A new plan was then put in place with Groups B, A, C and M used across the winter cropping program, but there was still too much reliance on Group B. The current plan for the farm now includes pre-emergent herbicides from Groups K, J and D used individually and in mixes. How do I integrate more mixes into my herbicide program? Short answer: Look for opportunities for synergistic mixes throughout the fallow and cropping seasons. In many instances the most important mixing partner is more water. Longer answer: Many growers are looking for tank mixes to improve control of glyphosate-resistant seedlings. Knowing which mixtures are beneficial and which are antagonistic is important. In the fallow, there are often opportunities to use the mix and rotate strategy to great effect in a double-knock application, such as: Group M (glyphosate) + Group I (2,4-D or fluroxypyr or picloram) followed by Group L (paraquat) Group M (glyphosate) followed by Group L (paraquat) + Group G (Sharpen® or flumioxazin) Group M (glyphosate) followed by Group L (paraquat) + Group K (Dual® Gold) Group A (Shogun®) followed by Group L (paraquat) + Group K (Dual® Gold) The fleabane on the right was unresponsive to glyphosate on its own but mixing picloram with triclopyr or 2,4 D to the glyphosate application was effective (left). Pre-plant examples include paraquat plus a triazine herbicide (Group C) or paraquat plus an imidazalinone (Group B), which are commonly used to provide broad spectrum knockdown and residual control. Dual® Gold (Group K) is another common fallow residual option which is very compatible with glyphosate, triazines and paraquat. An example of an in-crop mix is the addition of clethodim to haloxyfop (both Group A) to improve control of fop-resistant grasses in broadleaf crops where both are registered. At the end of the season there is also some opportunity to mix desiccants for some crops. None of these mixes are provided as recommendations – seek advice for your own situation and always read and follow the label. What about application set up for mixtures? Short answer: Some herbicides require better coverage. In many instances the most important mixing partner is more water. Longer answer: Suitable product and water rates, droplet size and the right adjuvant, are critical for optimising herbicide efficacy. For example, while a fallow mix such as glyphosate plus a Group A, or a Group G (depending on the target weed), is physically compatible, the components have different requirements for optimal performance. Seek advice about the best water rate to use, the potential impact of an oil-based adjuvant (required for most Group A and Group G herbicides) on glyphosate efficacy for some summer grass weeds, and other possible risks. Factsheet – Mixing knockdown partners with Group G How do I avoid generating multiple and cross-resistance? Short answer: Implement as many different weed control strategies as possible. The WeedSmart Big 6 is a practical foundation for an integrated program of herbicide and non-herbicide tactics. Longer answer: Rotating and mixing herbicide groups can give you room to move in holding off resistance or getting more out of some marginally effective products. The only way to stave off herbicide resistance completely is to have low weed numbers and to be vigilant about preventing survivors from setting seed. Have a diverse cropping program, use herbicides to provide early weed control, set your crops up to compete strongly and monitor and remove survivor weeds.
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What’s the best way to out-compete resistant annual ryegrass in cereals?

with Chris Davey, Agriservices Agronomist, YP AG Often regarded as the ‘poor cousin’ to herbicides, crop competition is making a come-back as a simple way for growers to increase crop yield and reduce weed seed set, without breaking the bank. Chris Davey, partner and director of YP AG at Kadina has worked with growers on the Yorke Peninsula of SA for over 20 years in an on-going battle with weeds such as annual ryegrass, brome grass and bifora, and mounting resistance to herbicide. YP AG agriservices agronomist Chris Davey says several Yorke Peninsula growers have adopted east-west sowing after seeing the benefits of competitive crops combined with pre-emergent mixes in a recent trial. “Working with our grower group we have demonstrated that stacking crop competition tactics at sowing really does make a difference,” he says. “When you add an effective pre-emergent herbicide to the top of the stack to provide early weed suppression, the resulting increase in yield and reduction in weed pressure definitely puts money in the bank.” In the 2018 trial, two cultivars of wheat (Scepter and Emu Rock) and barley (Compass and Spartacus) were sown into lentil stubble. The trial compared the performance of these four cultivars when sown east-west v north-south, and with a range of pre-emergent herbicide packages. “The result was clear – when you plant a competitive crop like barley in fertile soil – such as following lentils, row orientation doesn’t make much difference, but if you plant a poorly-competitive crop like wheat, it really pays to stack as many things in its favour, including east-west sowing and an effective pre-emergent herbicide,” says Chris. “In wheat we measured a consistent 0.5 t/ha yield gain through east-west sowing across the two cultivars and pre-emergent herbicide packages. In barley, stacking a premium pre-emergent herbicide mix onto an already-competitive crop boosted yield by 1.1 t/ha and reduced ryegrass plant numbers ahead of the following seeding by over 80 per cent, compared to the least competitive, nil pre-emergent barley treatment.” With harvest weed seed control already adopted by many growers on the Yorke Peninsula, the results of Chris’ trial has prompted the adoption of even more WeedSmart Big 6 tactics to tackle herbicide resistance head-on. In the trial, the package of competitive crops and cultivars plus east-west sowing plus a premium pre-emergent herbicides mix combined to make an impressive difference in annual ryegrass numbers while also producing more grain. What is the effect of crop competition on weed seed production? Short answer: Crop competition makes a massive difference to weed seed production in-crop. Longer answer: Ahead of this trial in 2018 the weed seed potential of the site was calculated as 87,000 annual ryegrass seeds/m2. The annual ryegrass population was known to be 100 per cent resistant to trifluralin (e.g. Treflan), 50 per cent resistant to triallate (e.g. Avadex) and 30 per cent resistant to prosulfocarb + s-metolachlor (Boxer Gold). Applying crop competition plus pre-emergent herbicide drove this number down to around 4000 seeds/m2leading into the 2019 cropping season. The owner of the trial site chose to cut the surrounding crop for hay to prevent further blow-out of the annual ryegrass population. Although the crop competition plus pre-emergent herbicide package made a vast difference to weed seed production it is not a one-year fix for a ryegrass blow-out. It is important to keep the pressure on. After seeing the results of the trial, several members of Chris’ grower group immediately adopted east-west sowing on paddocks where the change was practical. It is understood that it is often necessary to sow according to land type, such as on dune swale paddocks, or other constraints, such as established CTF systems that run north-south. Where changing row orientation to east-west is not possible there are still many other ways to boost crop competition. Left: No crop competition, just solid annual ryegrass. Middle: Least competitive treatment (N/S sown non-competitive wheat variety – Emu Rock, with standard pre-emergent – trifluralin + triallate). Right: Most competitive treatment (E/W sown competitive barley variety – Compass, with premium/stacked pre-emergent – Boxer Gold + triallate). Where did the reduction in weed seed production come from? Short answer: Stacking competition tactics and pre-emergents in barley reduced ryegrass weed seed set by over 80 per cent. Longer answer: Changing from a less competitive (Spartacus) to more competitive (Compass) cultivar reduced ryegrass plant numbers at seeding in 2019 by 13 per cent on north-south orientation and 24 per cent on east-west orientation, with no pre-emergent applied. Keeping the cultivar the same and changing row direction reduced weed numbers by 26 per cent in Compass and 16 per cent in Spartacus. This suggests that changing to east-west sowing will not achieve much in barley unless a more competitive cultivar is chosen. Changing both the competitiveness of the cultivar and the row orientation achieved a very useful reduction in weed numbers of 34 per cent (without using any pre-emergent). The result may be even greater in a more common rotation where barley is planted after wheat and the initial soil nutrient status is less than after lentils. When you add a standard pre-emergent mix (trifluralin plus triallate) to the east-west sown Compass, weed numbers are driven down by 50 per cent. Using a premium mix of (Boxer Gold plus triallate) achieved an 82 per cent reduction in annual ryegrass plants going into the 2019 season. This is particularly impressive given the field’s known resistance to the applied pre-emergent chemistry and highlights the value of stacking pre-emergents together and growing a competitive crop. What impact did the treatments have on yield? Short answer: East-west sowing increased wheat yield in this trial, probably due to extra competition at a very weedy site. Longer answer: In wheat there was a consistent 0.5 t/ha yield gain through east-west sowing across the two cultivars and pre-emergent herbicide packages. In barley, stacking a premium pre-emergent herbicide mix onto an already-competitive crop boosted yield by 1.1 t/ha compared to the nil pre-emergent, north-south treatment. E/W sown barley (left) v N/S sown wheat (right). Why worry about crop competition if there are new pre-emergent herbicides coming to market? Short answer: The new herbicides will provide another useful tool for growers but are not the answer on their own. Longer answer: The choice of pre-emergent herbicide should be the final decision after you have stacked as many crop competition tactics as possible. Look for the most competitive combination of crop species/cultivar, row spacing, seeding rate, row orientation, sowing time for early vigour and healthy soil, then add a pre-emergent that is known to be effective. If the crop competition is strong then the pre-emergent just needs to provide the early weed suppression that gives the crop a head start. Strong competition plus a current premium pre-emergent package (Boxer Gold plus triallate) performed as well as the ‘experimental’ pre-emergent products in this trial.
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How can I avoid getting stuck in an imi herbicide cycle?

with Sarah Wilson, Senior Technical Services Specialist, BASF Group B herbicides are very handy tools in the weed control toolkit, but weeds can evolve resistance relatively quickly to their mode of action. To keep these herbicides as an option, and to maximise the benefits of imi-tolerant crops, it is essential that they are used correctly within farming systems. Senior Technical Services Specialist with BASF, Sarah Wilson says it is easy for growers to get caught up in the imi-cycle of using imi-tolerant crops to avoid plant-back issues with imi residuals in the soil. Senior Technical Services Specialist with BASF, Sarah Wilson says it is easy for growers to get caught up in the imi-cycle of using imi-tolerant crops to avoid plant-back issues with imi residuals in the soil. “The problem arises when an imi-tolerant crop is sown to avoid imi-residues from the previous crop or fallow, but then the grower also wants to use the imi chemistry in the crop,” she says. “This leads very quickly to over-use of the imi herbicides, and research has shown that as few as four applications of group B herbicides (to which imi herbicides belong) to the same population of weeds can result in the selection of resistant individuals, so resistance can evolve within a very short period of time.” In Australia there are four imidazolinone or ‘imi-type’ active ingredients registered to control a variety of grass and broadleaf weeds in crops and fallow. These actives are imazamox (e.g. Intervix®*, Raptor®), imazapic (e.g. Bobcat I-Maxx®*, Flame®, Midas®*, OnDuty®*), imazapyr (Arsenal Xpress®*, Intervix®*, Lightning®*, Midas®*, OnDuty®*) and imazethapyr (Lightning®*, Spinnaker®). The other types of herbicides in Group B are the pyrimidinylthiobenzoates, sulfonylureas (SUs) and triazolopyrimidines herbicides. They all inhibit the plant’s production of specific essential proteins. Use the WeedSmart Big 6 to develop an integrated weed control program that keeps Group B herbicides as a viable option well into the future. *products that contain more than one active. How do Group B herbicides work? Short answer: The Group B herbicides, including the imis, interfere with the activity of the ALS enzyme that is used in the production of certain essential plant proteins. Longer answer: The Group B mode of action is to inhibit the production of the acetolactate synthase (ALS) enzyme in the plant cells. This enzyme is needed to produce essential plant proteins. By inhibiting ALS production, a foliar herbicide application causes the plant to deplete its supplies of the essential proteins and the plant will slowly die, often taking about three weeks. Group B herbicides with residual activity inhibit the production of amino acids so the plant uses up the reserves in the seed as it germinates and is exhausted before it breaks through the surface of the soil. Whether using Group B herbicides as a pre-emergent, or post-emergent application; consider the use of registered tank mixes with herbicides from other modes of action. What conditions do Group B herbicides need to work best? Short answer: Small weed size is critical for effective foliar application. Imis will not kill older weeds, so applying these herbicides to large weeds is a waste of money. Longer answer: ALS concentration is highest in young plant tissue and so foliar Group B herbicides are most effective when plants are small and actively growing. When plants are moisture-stressed there will be reduced uptake and translocation of foliar-applied imis. Foliar uptake of imis is very sensitive to high temperatures. In summer, temperatures in the 30s will require much more active ingredient for the same level of control achieved at lower temperatures. Follow the label instructions. There is a wide range of soil characteristics and environmental conditions that affect the efficacy of soil-applied Group B herbicides. What are recommended Group B use patterns? Short answer: Apply no more than two (2) Group B herbicides in any four (4) year period on the same paddock, and choose the right product for the situation. Longer answer: A Group B herbicide application in either a summer crop or summer fallow is equivalent to a winter crop pre-emergent application, so no further Group B applications should be made in that paddock, that year. Use Group B herbicides strategically, if you use imazapic (Flame) in the summer fallow and Ally, Logran, Atlantis or Intervix (for imi-tolerant cereals) over the top of your cereal crop in the winter, you then need to wait three years before using any other Group B chemistry in that paddock. If you are planting imi-tolerant varieties to get around an imi residue problem, do not use imi chemistry over the top – it’s not good practice for resistance management and you will get stuck in the imi-cycle! ALWAYS READ AND FOLLOW LABEL INSTRUCTIONS. What are my options if there’s sufficient planting rain but the plant-back requirements for the Group B herbicide haven’t been met? Short answer: Consider planting a Clearfield or imi-tolerant crop, but try to avoid using imis or other Group Bs in-crop. Imazapic (e.g. Flame) applied in a summer fallow is cheap and effective, but it will have implications for crop rotation flexibility. Longer answer: Imis have a broad range of soil binding characteristics and the period of residual decay varies markedly. Microbial activity is the primary mechanism for breakdown of soil-applied imis. Consequently, soil moisture and temperature play an extremely important role in how long the herbicide remains effective in the soil and when it is safe to plant a sensitive crop. Source: Soil behaviour of pre-emergent herbicides in Australian farming systems (GRDC) Even if the residual has not broken down sufficiently to safely plant sensitive crops, there may be poor weed control due to sub-lethal amounts of herbicide remaining in the soil. This scenario represents a serious risk of partially-resistant weeds setting seed. Other weed control options must be set in place to control weed escapes. While Clearfield and imi-tolerant crops are the most tolerant crops available, there are several non-Clearfield crops, such as chickpea, field pea, mungbean, peanut and soybean that have a degree of natural tolerance to imi herbicides. Look for a safe option that also enables the use of non-Group B herbicides and or cultural methods to manage weeds in-crop. If you need to use a pre-emergent, be sure to choose from an alternative herbicide MOA group. Other resources: Imi-tolerant crops – use sparingly and to best effect Growers and agronomists interested in learning more about the benefits and risks of pre-emergent herbicides can access a free online course at www.diversityera.com, presented by Mr Congreve and Dr Chris Preston. Clearfield herbicide plantback guide Clearfield stewardship Soil behaviour of pre-emergent herbicides in Australian farming systems