Herbicide Resistance FACT SHEET

Transcription

1 May 2012 Herbicide Resistance FACT SHEET western region Cropping with Herbicide Resistance Herbicide resistance is a fact of life for growers in the western grains region. Integrated weed management relies on a range of tactics, both chemical and non-chemical, and multiple attacks to keep weeds successfully in check. Key Points Resistance develops when repeated applications of the same mode-of-action (MOA) herbicide kill susceptible weeds, while resistant weeds survive and set seed. Herbicide resistance will remain as long as there is resistant seed in the soil. Integrated weed management (IWM) is a diverse approach using chemical, non-chemical and agronomic tactics that target weed seed-set. Minimising the number of weed seeds in the soil (the seedbank) by preventing weeds from setting seed or removing weed seeds at harvest before they fall to the ground should be the top priority. Multiple tactics may be required within one cropping season or year. Herbicide-resistant weeds occur naturally and can be introduced via wind, water or contamination. Consistently applying IWM will achieve a relatively weed-free system. Always use robust herbicide rates and check product labels. Monitor and detect failures early and take action to prevent weed seed blow-outs. The landscape and composition of weeds in Australia s cropping regions has changed significantly. Many weeds that were once controlled by cultivation have been favoured by Resistant populations of wild radish are a major threat to cropping in the west. Double knock, high seeding rates, croptopping in spring and harvest weed seed collection all help keep seedbanks low. the widespread adoption of reducedtillage systems. All weed populations will have a small proportion of plants that are naturally herbicide resistant. Broad-scale herbicide resistance develops when repeated applications of the same mode-of-action (MOA) herbicide lead to susceptible weeds being killed and resistant individuals surviving. These survivors then set seed, ensuring a higher proportion of resistant individuals in the next generation, leading to large populations of resistant weeds. Each type of resistance that develops onfarm means one less tool available to the grower to manage weeds. Where multiple resistance exists, effective herbicide options become severely limited. See Table 1 for known herbicide-resistant populations. Be proactive Diversity is the answer. Because herbicide resistance is now a fact of life in the west, Photo: Emma leonard growers, agronomists and researchers are now more focused on managing populations of resistant weeds. This is rarely achievable with a single weed management tactic. Integrated weed management (IWM) involves a range of chemical, non-chemical and agronomy tactics, used together to provide optimal weed control. In short, diversity in approach to weed control. By applying IWM practices, growers are aiming to reduce their weed seedbank across their farm and minimise the potential for herbicide resistance. See Useful Resources at the end of this Fact Sheet for a link to information from AHRI on surveys of herbicide resistance in WA. Major weeds of the western region Wild radish Wild radish is now the most problematic weed in WA cropping systems, especially on the sand plain soil of the north. Uncontrolled, it can cause severe yield losses, and farmers may be forced into non-crop phases to control the population. Herbicide-resistant wild radish is highly competitive, can flower at any time of year, has a large seed production potential and its seeds live longer in the soil. Growers concerned about the development of multi-resistant wild radish populations should focus on keeping wild radish populations at very low levels using all other feasible control techniques apart from herbicides. Double knock, high seeding rates, croptopping in spring, competitive crops, crop rotations and harvest weed seed Level 1, Tourism House 40 Blackall Street, Barton ACT 2600 PO Box 5367, Kingston ACT 2604 T F E grdc@grdc.com.au W

2 Table 1 Known populations of herbicide resistance in grain-producing areas of Western Australia Weeds appearing in this table have had herbicide resistance confirmed through testing. Therefore, if a weed does not appear in the table, it does not necessarily mean that it is not herbicide resistant. Weed species Herbicide group (see Table 2) Example herbicide Grass weeds Annual ryegrass A fops diclofop A dims clethodim B sulfonylureas chlorsulfuron B imidazolinones imazapic, imazapyr C triazines simazine, atrazine D dinitroanilines trifluralin M glycines glyphosate Q triazoles amitrole Barley grass B sulfonylureas sulfosulfuron, sulfometuron Brome grass A fops quizalofop Wild oat A fops diclofop A dims tralkoxydim Broadleaf weeds Wild radish B sulfonylureas chlorsulfuron B sulfonamides metosulam B imidazolinones imazapic, imazapyr C triazines simazine, atrazine C triazinones metribuzin F nicotinanalides diflufenican I phenoxys 2,4-D Indian hedge mustard B sulfonylureas chlorsulfuron B sulfonamides metosulam F nicotinanalides diflufenican Paterson s curse / Salvation Jane B sulfonylureas chlorsulfuron B sulfonamides metosulam collection all help to keep the seedbank at manageable levels. Cutting for silage or hay, grazing pasture, or brown manuring can also provide effective control. Sowing weed-free seed is also critical. Sowing crop seed contaminated with wild radish pods is the most common avenue of spread. This is particularly important in the southern cropping area, where wild radish is not as widespread as in the central and northern areas. Annual ryegrass Multiple resistant annual ryegrass is common across the western grains region. However, most WA growers are now managing the problem well using IWM. Recent surveys have shown that ryegrass numbers in paddocks are very low, even where herbicide resistance has been recorded. A pre-seeding double knock of glyphosate followed by paraquat or paraquat plus diquat reduces the likelihood of glyphosate resistance spreading in the paddock. The main IWM tools used for ryegrass are a combination of pre-emergent herbicide with crop-topping in spring, swathing of canola and harvest weed seed management (chaff cart or windrow burning). More recently, Roundup Ready canola has also been added to the rotation. With all weed control, target small weeds and follow-up with monitoring and eradication of any survivors. Brome grass Compiled by Andrew Storrie, 2012 Brome grass is one of the most competitive grass weeds of wheat, and yields will fall exponentially as brome grass densities increase. It is particularly prevalent on WA s non-wetting sands. Early germinations can generally be controlled with herbicides (resistance exists to Group A fops ), but this has resulted in selection for plants with delayed emergence, which then escape knock down herbicides. Seeds germinate upon shallow burial by the sowing operation, prompting a large in-crop flush. But seeds with delayed emergence also require greater chilling before germination occurs. Consequently, a staggered in-crop germination pattern is becoming more common. Similar results have been found for barley grass. Seeds can persist in the soil for up to three years, so at least two consecutive years of seed-set control are needed. page 2 A pasture phase, long fallow and brown manuring are effective management strategies. early harvesting is essential for the collection of harvest weed seed. Burn crop residues to destroy the seed. Barley grass Barley grass is characterised by rapid germination after the autumn break and can act as a green bridge for cereal root rot diseases. Barley grass should be eliminated the year before cropping by heavy grazing and inpasture selective herbicides, long fallow or brown manuring. Do not dry sow paddocks likely to have a barley grass problem. Nearly all seeds germinate in the first year after seed-set. Effectively managing the weed seedbank will see numbers fall very quickly. Wild oats Wild oats is an increasingly problematic weed of WA cropping systems. Uncontrolled, it can cause significant yield losses in wheat. Staggered germination makes longterm control difficult, with the main flush emerging in autumn and early winter. These can be controlled with pre-emergent or early post-emergent herbicides. However, wild oats will continue to germinate in small numbers through to spring, and these plants produce enough seed for the following season. Agronomic tactics such as crop choice and sequence and crop competition can play a role. If cutting for silage or hay, cut prior to wild oat grain fill and control any regrowth. A two to three-year pasture phase is an excellent option for depleting the wild oat seedbank. Although there is widespread resistance to Group A herbicides, resistance to other herbicides is still relatively low, meaning there is still a diverse range of chemical options available. The use of diverse chemical and non-chemical strategies should keep seedbank numbers low and delay the development of resistance to other MOAs. Flaxleaf fleabane Flaxleaf fleabane is a prolific seed producer (up to 110,000 seeds per plant) and will continue to flower and seed over a long period of time. These factors, combined with the dispersal of its seed by wind and

3 water over considerable distances, make it a rapidly increasing problem. Fleabane, when mature and under heat or moisture stress, is relatively unresponsive to knock down herbicides, so the best strategy is to control it at the seedling stage. Fleabane seedlings should be targeted in cereal crops in spring with a late post-emergent application of a suitable broadleaf herbicide. There are no known herbicide-resistant populations of fleabane in WA. Know which herbicides still work If a certain MOA has been used for many years (Table 2), surviving weeds should be tested for herbicide resistance. Studies have shown that resistance can vary from paddock to paddock. When a herbicide has failed or a resistance issue is suspected, it is critical to test for resistance to alternate herbicides to see what will work next time. See Useful Resources at the end of this fact sheet for contact details of herbicide resistance testing services. Glyphosate resistance Glyphosate is the most valuable herbicide in Australian agriculture. It is a cheap and highly effective non-selective herbicide with a unique mode of action (MOA) that has a low environmental and toxicological risk. It is the default fallow and pre-seeding weed control option, at the expense of a more comprehensive IWM program. Glyphosate takes longer to develop resistance than many other herbicides, such as fops, dims and sulfonylureas (Table 2), but its extensive use over long periods in both agricultural and non-agricultural sectors has inevitably led to resistance. Table 2 Years of application before herbicide resistance develops Herbicide group Typical years of application before herbicide resistance develops B (SUs, IMIs) 4 A ( fops, dims ) 6 8 I (phenoxys) 10+ C (triazines) 15+ D (trifluralin, 15+ dinitroanilines DNAs) L (paraquat/diquat) >15 M (glyphosate) >12 F (nicotinanilides) N/A Source: Dr Chris Preston Growers running continuous cropping rotations are at greater risk of having weed populations develop glyphosate resistance than those with mixed cropping and grazing enterprises, that is, with some pasture in the system. Do the job right The over-reliance on glyphosate for weed control in fallows has led to the development of resistance, because a reasonably high kill rate is often considered sufficient. This leaves behind a small percentage of survivors potentially with glyphosate-resistant genes. If the survivors are not targeted with another control, they are able to set seed and increase their numbers in subsequent seasons. To preserve the effectiveness of glyphosate, it is critical that a greater level of monitoring after spraying takes place and, if necessary, any survivors eradicated. Maximise the amount of herbicide reaching the target by using the most appropriate nozzles, water rates and adjuvants, applied in optimal conditions to small and actively growing weeds, to give the highest levels of control. Always use product label rates to to maximise the number of weeds killed. See Useful Resources at the end of this Fact Sheet for a link to information from AHRI on the use of robust rates. Integrated weed management The benefits of IWM extend beyond herbicide resistance management; it has been shown to be the most effective method of long-term weed population management. Economic returns averaged over a 20-year period for IWM systems are greater than herbicide-only systems, due to the lower seedbank numbers in the IWM system and fewer weeds to manage on an ongoing basis. Rarely will a single tactic, whether chemical or non-chemical, provide 100 per cent control, but each tactic adds a degree of control. Stacking tactics within the season means the grower can keep weeds successfully in check by driving down the weed seedbank. See Useful Resources at the end of this Fact Sheet for a link to information from AHRI on the adoption of IWM. Manage the weed seedbank Most problem crop weeds in Australia rely on the establishment of a seedbank to ensure their ongoing survival. page 3 The priority for any grower considering their weed management strategy should be to drive down this weed seedbank where practical. This means targeting weed seed production and preventing viable seed from being added to the seedbank. Many problem weeds, particularly grass weeds, have seeds that are short-lived in the soil. A few years of preventing seed-set can reduce the seedbank of these weeds by up to 99 per cent. Using a range of weed-control methods in the one season and between seasons is the key to managing the seedbank and preventing the development of herbicide resistance. Weed Seed Wizard The Weed Seed Wizard is a new computer simulation tool developed by the Department of Agriculture and Food Western Australia (DAFWA) that uses farm-specific data to predict weed seed numbers and impact on yield. Users can change herbicide efficacy to mimic herbicide resistance, modify crop competition, add a harvest seed management option or other events to design a farm business weed management program. You can access the Weed Seed Wizard through the GRDC website at RIM RIM is a ryegrass seedbank model developed by the Western Australian Herbicide Resistance Initiative (now AHRI). It is being upgraded and is a useful tool in training agronomists and farmers in seedbank dynamics and the importance of using multiple weed control tactics within the one season. Crop sequences Having a diverse rotation that includes a wide range of cereal and broadleaf crops is an important tool in managing herbicide resistance as it allows for a wider range of weed management tactics and herbicide MOAs to be used as well as minimising income risk. Strategic cultivation Strategic cultivation can provide control of herbicide-resistant surface-germinating summer weeds and those that have escaped other weed control tactics. However, it will spread grass weed seeds such as wild oats, ryegrass and brome grass through the profile and prolong

4 their seedbank dormancy, so assess your own weed situation carefully before implementing strategic cultivation. Autumn tickle Autumn tickling stimulates weed seed germination by placing seed in a better physical position in the soil. An autumn tickle can be conducted with tined implements, skim ploughing, heavy harrows, pinwheel (stubble) rakes, dump rakes or disc chains. Weeds that germinate after an autumn tickle can be controlled and the process can assist in depleting the weed seedbank. The Harrington Seed Destructor destroys the weed seed present in the chaff fraction exiting the harvester. It is a one-pass operation that is effective on those weeds that set a large amount of seed at harvest. The closer the harvest is to weed maturity, the greater the proportion of weed seeds is collected. Photo: Chris Saunders Pre-emergent herbicides Reduced tillage systems have led to an over-reliance on post-emergent herbicides. Residual herbicides have rapidly increased in use with the widespread adoption of minimum-till crop establishment and wider row spacings. Pre-emergent herbicide effectiveness is dependent on post-application rainfall, soil properties and the right herbicide selection. The judicious use of pre-emergents takes pressure off post-emergence options. Pre-emergent herbicides will not provide full control. Like any other weed control measure, they should be used with other tactics such as crop-topping or weed seed collection at harvest. The recent release of pyroxasulfone (Sakura ) offers another pre-emergent option in wheat. Growers should consider using trifluralin, prosulfocarb plus S-metolachlor (Boxer Gold ) and pyroxasulfone in rotation. Sow weed-free seed Growers may be unwittingly introducing herbicide-resistant weeds back into their system. Post-harvest seed cleaning may be only partially effective in removing weed seeds. A significant proportion of weed seeds, some of which can be herbicide resistant, can remain in the seed that is subsequently sown into the paddock. The main contaminants are annual ryegrass followed by wild radish. They often prove to be resistant to some herbicides upon testing. With the right strategies, it is possible to achieve weed-free seed for sowing. Select seed only from the cleanest paddocks, use the most effective cleaning methods (gravity table, rotary screens, sieves or a combination) and control weeds in non-crop areas, movement of stock and machinery. See Useful Resources at the end of this Fact Sheet for a link to information from AHRI on clean crop seed. Double knock Double knock refers to any two different weed control tactics practised in close succession, where the second measure controls survivors of the first; for example, any sequence that includes a herbicide and a non-chemical measure, such as heavy grazing or cultivation. It most commonly refers to two herbicide applications from different MOA groups, between two and 10 days apart. This is often glyphosate (Group M) or glyphosate plus 2,4-D (Group I) followed by paraquat or paraquat plus diquat (both Group L), known as double knock down. Double knock is an extremely effective tactic when done properly, but it requires attention to detail and can be expensive. It has been proven to delay the onset of herbicide resistance. Target small weeds Most weeds are less responsive to chemical control once they have grown beyond seedling stage. It is crucial to target small (pre to early tillering/small rosette) actively growing weeds when spraying. Monitor paddocks post-spraying and clean up any survivors. However, spraying grass between the half-leaf and two-leaf stage can give poor results. There are limitations with registration of some of the common herbicides. Some labels state only a particular growth stage and not beyond. Some labels suggest higher rates for larger weeds. Always read the product label for recommendations for weed size, adjuvants and rates. page 4 Crop competition Crop competition is driven by crop type and variety choice and works by using crop uniformity, density, vigour and good agronomy to outcompete weeds. It works best with cereal crops. The weeds that emerge before or with the crop are the ones that are the most competitive. Before crop competition kicks in, aim to control with herbicides to gain a head start. Late season herbicide use Crop-topping is the application of a nonselective herbicide such as glyphosate (wheat and pulses) or paraquat (pulses only) prior to harvest when the target weed is at flowering/early grain fill. The selectivity of the crop-topping process is dependent on a sufficient gap in maturity between crop and weed and will be determined by crop type and seasonal conditions. Selective spray-topping is the technique of using selective herbicides to control seed production of a weed within a crop prior to head emergence and thus reduce future weed populations. Selective spray-topping is the technique of using selective herbicides to control seed production of a weed within a crop prior to head emergence and thus reduce future weed populations. Correctly executed, selective spray-topping can result in a 90 per cent reduction in weed seed-set, provided there is no resistance to the MOA. It is not a substitute for early weed control and occurs too late for significant yield protection. See Useful Resources for more information on late season herbicide use.

5 Annual ryegrass and brome grass present in a crop at harvest. Early harvesting is essential for destruction of seed via harvest weed seed collection, followed by burning of crop residues. Brown manuring Brown manuring is the growing of a crop, pasture or weeds that are then sprayed out before weeds set seed. Brown manuring has the advantages of competition and high weed seed-set control, maintaining ground cover, reducing the number of applications of herbicide, particularly when compared with long fallow, and improving the yield of following crops through increased nitrogen availability, storage of soil moisture and reduction in cereal root disease. It can be used to patch-treat problem areas and prevent weed seed being spread by the header, but, it is better as a planned option for a whole paddock. Harvest weed seed control Weed seed control at harvest will not fix all weed problems growers face, but it is the most important non-herbicide tool for grain growers. Harvest weed seed control includes narrow windrow burning, chaff carts and seed destruction. All methods rely on weed seeds entering the harvester, therefore the closer the harvest is to weed maturity, the greater the proportion of weed seeds collected. See Useful Resources at the end of this Fact Sheet for a link to information from AHRI on harvest weed seed control. Photo: Neree Martinez Narrow windrow burning More WA growers are now practising narrow windrow burning, particularly in broadleaf crops such as lupins and canola, to reduce the proportion of wild radish and ryegrass seed that is carried over into the following season. If using an autosteer system, growers should move their windrows by about one metre each year to avoid nutrient losses. Windrow burning is difficult to achieve in high-yielding wheat crops as the whole paddock is likely to burn. Harvest very low and do not graze the paddock livestock tend to create breaks in the windrows and spread them around, reducing the trash level available for a hot, clean burn. Harrington Seed Destructor The Harrington Seed Destructor, which is being trialled around the country, processes the chaff material exiting the harvester and has been shown to destroy a high percentage of weed seed present in the chaff fraction. The Harrington Seed Destructor destroys the weed seed in one pass without the need to come back and burn and leaves the residue (containing nutrients) spread in the paddock. This will make it an attractive option for some growers. Inversion ploughing Inversion ploughing involves the use of a mouldboard plough with skimmers to bury weed seed at least 30cm deep in the soil profile. This is too deep for any weeds to emerge. Because of the extent of the soil disturbance, it should be undertaken only once every 10 to 15 years. Inversion ploughing will prolong the life of the seedbank, therefore this deep-buried seed should not be disturbed for at least 10 years. Weed-detection technologies Weed-detection technologies such as WeedSeeker and WEEDit use optical infra-red sensors connected to solenoid switches that turn boom-mounted nozzles on and off when a green weed is detected in a fallow. There is potential for huge savings in summer fallow weed management as only five to 15 per cent of the field will normally be sprayed. The increased area of chemical fallow in WA will inevitably reveal more populations of glyphosate-resistant ryegrass. This technology has the ability to delay or prevent the onset of glyphosate resistance. For fallow weeds it allows the use of alternative herbicides to glyphosate at higher rates that may not be economically viable for a blanket application. Monitor With any weed-control measure, always scout for surviving weeds that could indicate potential resistance and follow-up with spot spraying and/or other control methods. Thorough paddock monitoring soon after spraying will ensure that any spray failures are detected early before a weed seed blow-out has occurred. Not detecting a spray failure until harvest can have potentially devasting economic consequences. Years of hard work and expense will be required to bring the seedbank back under control. Knowing the status of weed populations and how well control measures are working are the key to ensuring optimal weed management decisions. Photo: Michael Walsh Chaff carts Chaff carts remove weed seed in the chaff fraction at harvest. The contents are then burned in autumn to destroy the weed seed. Chaff carts fell out of favour but are now enjoying renewed popularity as growers look to expand their weed control toolkit. More WA growers are now practising narrow windrow burning, particularly in broadleaf crops such as lupins and canola, to reduce the proportion of wild radish and ryegrass seed that is carried over into the following season. If using an autosteer system, growers should move their windrows by about one metre each year to avoid nutrient losses. page 5

6 Frequently Asked Questions How do I know if I have a problem with resistance? Normally resistance is first noticed as patches of uncontrolled weeds following a herbicide application. Take samples from these patches for testing. See Useful Resources for contact details for testing services. What herbicides should I test? The aim of testing is to find out what still works. Discuss with the test provider which herbicides you are likely to use in the future. What do I put in for first and second knock in a double knock treatment? Double knock refers to any two different weed control tactics, chemical or non-chemical, practised in close succession, where the second measure controls survivors of the first. Although it most commonly refers to two herbicide applications from different MOA groups, you need to consider the target weed and any existing resistance issues before deciding on which measure to apply for each knock. Most commonly, glyphosate is the primary first herbicide as it controls a broad spectrum of weeds. What timing should I apply between knocks in a double knock treatment? Generally between two and 10 days, although the interval can be anything from one to 14 days depending on the mix of herbicides to be used, conditions and weed species. Will using tank mixes reduce the chance of glyphosate resistance developing? To be effective in reducing the risk of glyphosate resistance, the tank mix partner must be fully compatible with glyphosate and must be applied at a rate that will kill any glyphosate-resistant plants in the treated population. Can tillage lower the risk of resistant weeds? Yes, but only if the survivors are killed by the cultivation. For this to happen, the cultivation needs to be well timed and executed. How do I control mature plants? Double knock with higher rates of herbicide, for example spot spraying, may provide control, but in some instances cultivation may be the only option remaining. more information Peter Newman, DAFWA ; peter.newman@agric.wa.gov.au Andrew Storrie, Agronomo Consulting ; andrew@agronomo.com.au Michael Walsh, research associate professor, UWA ; michael.walsh@uwa.edu.au Acknowledgments: Peter Newman, Sally Peltzer, Chris Preston, Stephen Powles, Andrew Storrie, Michael Walsh. Useful Resources For testing suspected resistant samples: Charles Sturt University Herbicide Resistance Testing, School of Agricultural and Wine Sciences Charles Sturt University Locked Bag 588 Wagga Wagga NSW grahamcentre/producers/ herbicideresistancetesting.htm Plant Science Consulting 22 Linley Ave Prospect SA info@plantscienceconsulting.com.au Australian Herbicide Resistance Initiative (AHRI) Current Herbicide Resistance Status (Western Australia) Current Herbicide Resistance Status (International) Glyphosate resistance Australian Glyphosate Sustainability Working Group GRDC Weedlinks Integrated Weed Management in Australian Cropping Systems manual.html Weather Essentials for Pesticide Application protection Weed Seed Wizard html?s=1001 GRDC Project code ICN0009, UA00124, UWA00146 Disclaimer Any recommendations, suggestions or opinions contained in this publication do not necessarily represent the policy or views of the Grains Research and Development Corporation. No person should act on the basis of the contents of this publication without first obtaining specific, independent professional advice. The Corporation and contributors to this Fact Sheet may identify products by proprietary or trade names to help readers identify particular types of products. We do not endorse or recommend the products of any manufacturer referred to. Other products may perform as well as or better than those specifically referred to. The GRDC will not be liable for any loss, damage, cost or expense incurred or arising by reason of any person using or relying on the information in this publication. CAUTION: RESEARCH ON UNREGISTERED PESTICIDE USE Any research with unregistered pesticides or of unregistered products reported in this document does not constitute a recommendation for that particular use by the authors or the authors organisations. All pesticide applications must accord with the currently registered label for that particular pesticide, crop, pest and region. Copyright All material published in this Fact Sheet is copyright protected and may not be reproduced in any form without written permission from the GRDC. produced by page 6