Herbicide resistance and integrated weed management (IWM) in crops and pasture monitoring tools
How to use these tools
Herbicide resistance is a major issue affecting crop and pasture production in Australia. The use of Integrated Weed Management (IWM) is the best approach for managing herbicide resistance and for ensuring that the herbicide options we have left will be more useful for longer.
This monitoring tool is divided into 3 sections: an introduction, tools and case studies. It provides an overview herbicide resistance – which weed species and herbicides are affected and how resistance has developed. Strategies are provided for how to prevent resistance from developing and how to control it once it presents itself on your farm (though you can never entirely eradicate it). These tools are based on the principles of IWM. The tools focus mainly on resistance in annual ryegrass, which is the most significant herbicide resistance problem to date. Also, glyphosate resistance in ryegrass is highlighted. Case studies are included to see what happens to resistant weed seedbank under different management scenarios.
The tools outlined below will allow you to manage and control herbicide resistance on your farm.
Tool A: Pre-emptive management – reducing the chances of herbicide resistance occurring on your farm
Tool B: Reactive management – controlling weed populations that already have herbicide resistance
What is herbicide resistance and how has it developed?
During the 1940s and 50s Australian agriculture relied heavily on the use of broad-spectrum pesticides to control pests. Selective herbicides began to appear in the mid 1970s and have been a fundamental tool for cropping and pasture since. However, as reliance on chemicals has grown over the years we are continuing to see more weeds that have developed resistance to herbicides. In other words, a number of chemicals that we have available to us have become less useful. Herbicide resistance was first recognised in Australia in 1981 where some annual ryegrass developed resistance to diclofop-methyl.
Herbicide resistance occurs from the repeated use of the same (or chemically similar herbicides) for weed control in crops and pastures. The potential for herbicide resistance developing is already in the paddock (it is a naturally occurring process with the natural mutation of genes). However, this process increases rapidly with the sole reliance on chemicals and practicing continuous cropping. This is because a consistent selection pressure is placed on the weed population. The few resistant individuals in a paddock survive the herbicide, enjoy having more room to grow and set a lot of seed. This resistant seed then goes on to form a resistant population. If the same herbicide is used the next time, these individuals survive to reproduce again – eventually leading to herbicide failure. It could be said that herbicide resistance is, in fact, inevitable with many herbicide groups. However, there are strategies that can delay its onset. An exception to this is glyphosate resistance, which is rare and if other practices are used (particularly the 'double-knock' strategy) resistance can be kept at bay for potentially a long time.
Recent years have seen the development of herbicide resistance in a number of weeds, most significantly in Annual Ryegrass. The development of cross resistance across these groups means that changing herbicides won't be enough though can be temporary help. Adopting Integrated Weed Management (IWM) techniques, (along with the sensible and timely use of herbicides) offers the only path forward. The word 'integrated' is used because it means that more than one control method is used. The term 'weed' refers to a plant that is present where it is not wanted. 'Management' is used to show that pests are not necessarily eradicated but numbers are controlled to an economic level. IWM is based on the planned and strategic use of pest control methods, including chemicals, not spraying simply because it is part of your usual schedule. Quite often, chemicals are applied when they are not needed or not useful – this is a waste of time and money and puts more pesticides into the ecosystem when it is not necessary.
Integrated Weed Management (IWM) is not a new concept – most farmers practice various IWM techniques as part of their general farm management. These include grazing, burning, spray and pasture topping. In the case of glyphosate resistance in Annual Ryegrass, a number of factors can be attributed: continuous reliance on it before sowing, lack of effective crop weed control, frequent glyphosate chemical fallow and frequent crop topping with glyphosate.
Which weeds have developed resistance to herbicides?
A number of weed species have developed resistance to herbicides. Of the greatest concern is Annual Ryegrass (Lolium rigidum) because it has developed cross-resistance to a number of different herbicide groups. Herbicides have been classified into a number of 'groups'. The group refers to the way a chemical works – their different chemical make-up and mode of action (see Table 1). Annual Ryegrass is one of the most significant weeds for cropping enterprises – and we are rapidly running out of chemical options to deal with it. Table 2 shows the resistance status of Annual Ryegrass to a number of Group A, B, D and M herbicides – a herbicide program made up of two to three years use of any of these can fail due to cross resistance.
Table 1: Herbicide groups and examples of chemical products in each group
|Group A||Hoegrass®, Nugrass®, Digrass®, Verdict®, Targa®, Fusilade®, Puma S®, Tristar®, Correct®, Sertin® Grasp®, Select®, Achieve®, Gallant®, Topik®|
|Group B||Glean®, Chlorsulfuron, Siege®, Tackle®, Ally®, Associate®, Logran®, Nugran®, Amber Post® Londax®, Spinnaker®, Broadstrike®, Eclipse®, Renovate®|
|Group C||Simazine, Atrazine, Bladex®, Igran®, Metribuzin, Diuron, Linuron, Tribunil®, Bromoxynil, Jaguar®, Tough®|
|Group D||Trifluralin, Stomp®, Yield®, Surflan®|
|Group E||Avadex®, BW, EPTC, Chlorpropham|
|Group F||Brodal®, Tigrex®, Jaguar®|
|Group I||2,4-D, MCPA, 2,4-DB, Dicamba, Tordon®, Lontrel®, Starane®, Garlon®, Baton®, Butress®, Trifolamine®|
|Group K||Dual®, Kerb®, Mataven®|
|Group L||Reglone®, Gramoxone®, Nuquat®, Spraytop®, Sprayseed®|
|Group M||Glyphosate, Glyphosate CT®, Sprayseed®, Roundup CT®, Touchdown®, Pacer®, Weedmaster®|
List of commonly used products only
List of products does not necessarily imply state registration
Check that product is registered in your state before use Groups G and J not included
Extracted from brochure "Herbicide Resistance Management Program" (NSW Agriculture - GRDC - RIRDC)
Table 2: Resistance status of a number of weeds*
|Weed species||Resistance status|
|Annual Ryegrass (Lolium rigidum)||Very high resistance to Group A (eg. Diclofop) and Group B herbicides (Sulfonylureas). Some resistance to Group D (Trifluralin) and Glyphosate (Group M herbicides).|
|Wild Oat (Avena fatua)||Diclofop-methyl (Group A herbicides) resistance Resistance to Group K (flamprop-methyl)|
|Barley grass (Hordeum leporinum)||Paraquat and Diquat resistance|
|Capeweed (Arciotheca calendula)||Paraquat and Diquat resistance|
|Barnyard Grass (Echinochloa crus-galli)||Resistance to Group C herbicides|
|Wild Radish||Resistance to chlorosulfron has increased 3 fold over last 4 years. Some resistance to Atrazine and 2, 4-D-amine.|
|Indian hedge mustard, prickly lettuce, wild turnip, sow thistle, black bindweed, silvergrass, summer grass, salvation jane.||New additions of resistant weeds with resistance to 1 or more groups of herbicides.|
* Resistance status will vary from paddock to paddock and not all populations have these characteristics.
The following tools discuss practical ways to monitor and manage herbicide resistance before, and after, it becomes an issue on your farm.
Tool A: Pre-emptive management of herbicide resistance - Reducing the chances of herbicide resistance occurring
Goal: To monitor, delay or delay further herbicide resistance developing by rotating herbicide groups, keeping good paddock records and choosing management strategies from a range of recommended methods.
Rotate herbicide groups
You should not rely on chemicals from the same group every year. Rotating groups will help delay onset of resistance. Table 1 shows the different herbicide groups.
Keep good paddock records
It is much easier to diagnose resistance if you have good paddock records. It is important to record which chemicals you have used on which paddock and what the weather conditions were like. It is also important to keep notes of how effective the spray treatment was.
Carry out regular resistance testing (see Tool B)
Because management of herbicide resistance is case specific, it is difficult to prescribe 'recipes' for how to manage a problem. Instead, you need to understand your situation and choose from a range of methods, such as those outlined below.
Choose a method from the IWM tool box. Consider how these might fit into your farming system and seek advice from your local agronomist. These methods allow you to keep weed populations under control and delay the onset of resistance.
Method 1. Autumn tickle: use light scarification to stimulate weed germination, then spray (paying attention to rotating your chemical groups) before sowing.
Method 2. Barley for weed control: barley is competitive against weeds and use of pre-emergence herbicides is effective. Its shorter growing season also allows for pre-sowing weed control.
Method 3. Catching: use a bin attachment on your harvester to collect weed seed. Then burn the seed.
Method 4. Crop-topping: Use a non-selective herbicide (paraquat based) to mature or near-mature crops to reduce weed seed set. This method is often used in lupin crops. Crop-topping in pulse crops can be very effective – wick wiping weeds in lentil crops as a prelude to crop-topping is also effective.
Method 5. Cultivation: Use cultivation to kill germinated weeds.
Method 6. Delayed sowing: Delay sowing for 2 or more weeks so that additional weeds can be killed by non-selective herbicide.
Method 7. Double-knock strategy: Use a glyphosate application followed by paraquat-based application to control weeds before sowing.
Method 8. Harvest low, no spread, burn: This method has 3 stages: Harvest the crop lower than usual, put the residue (containing weeds) into rows for burning (allows for hotter fire).
Method 9. Hay: Use crop for hay. Trials in Southern Australia showed that hay making was most effective – reducing seeds per square metre. Hay cutting alone doesn't guarantee success, you also need to: graze or spray-top after it re-shoots, and take care when feeding hay out that ryegrass seeds aren't spread to other paddocks.
Method 10. Heavy grazing: Weed seed set is reduced by timely intense grazing of paddocks not sown to crop (and seedbank is reduced).
Method 11. High crop sowing rate: Used to produce a higher crop plant density to reduce yield loss due to weeds and to suppress weed seed production.
Method 12. Manuring: Use the crop for 'green manure' before it matures to prevent weed seed set and increase organic matter.
Method 13. Mechanical pasture top: slash the pasture before weed maturity.
Method 14. Spray-topping: Use a low-rate non-selective herbicide applied to pastures to reduce weed seed set.
Method 15. Careful consideration of rotations (pasture phase instead of continuous cropping). A 2 year (or more) pasture phase treated to reduce weeds before it goes back into crop. A pasture phase longer than 2 years is very effective (1 year is not enough to reduce seedbank). It should be noted that it is not the pasture phase itself that helps to manage weeds – but it is what the phase allows you to do in addition eg. grazing and pasture topping. Crop rotations need to be managed carefully. Continuous cropping is very high risk.
Method 16. Stubble burning: Stubble is burned in autumn to reduce viable weed seeds (but reduces organic matter).
Method 17. Swathing for weed control: cutting crop near to full maturity and leave to dry in rows to reduce seed shatter (usually done in canola). Can be done in other crops but earlier than usual and lower than normal.
RIM for ryegrass management
Ryegrass Integrated Management (RIM) tool was developed in WA and is a decision support tool for evaluating ryegrass management options. The computer-based tool allows you to assess the likely biological and economic consequences of a range of ryegrass strategies, taking herbicide resistance into account. The tool tracks the changes/behaviour in ryegrass over the season. You can select and number of IWM control strategies and evaluate their effectiveness and economic implications (over 10-20 year period). It helps you to answer questions like: How much income will I lose if resistance develops? Which combination of strategies should I use once I have resistance? How long should pasture phases be? For more information see: http://ahri.uwa.edu.au/research/rim/. The use of RIM is very useful as a tool for prevention in that it helps to identify strategies to delay resistance, while maintaining the highest economic return. RIM is also useful when you already have resistance as it allows you to identify the best combination of practices.
Tool B) Reactive management of herbicide resistance – Controlling weed populations that have already developed resistance goal
Goal: This tool aims to provide an overview of how herbicide resistance can be detected, what to do if you suspect glyphosate resistance, some strategies for management where resistance has developed and how to monitor how effective these strategies are.
It is difficult to eradicate resistant weeds once they occur. The implications are that usually a whole group of chemicals is lost eg. Group A. In some cases, such as Annual Ryegrass, you might lose a number of herbicide groups.
Note: Every case of herbicide resistance has its own specific characteristics and requirements for management and you should seek expert advice from your local agronomist.
How do I know if I have herbicide resistant weeds?
You might have herbicide resistant weeds, in low numbers, for a number of years before you notice them. Eventually, you may notice patches of surviving weeds after you have sprayed. Be sure to check that it wasn't any error on your part when spraying or associated with environmental conditions (eg. temperature, soil moisture and humidity when spraying). Eventually these patches join together and you realise that resistance might be occurring.
Early detection is vital so that you can adjust your management practices and incorporate IWM strategies. The best way to detect resistant weeds is to take a sample for resistance testing. The steps are (depending on your local testing service instructions):
Step 1.Contact your local Agriculture Victoria officer or private agronomist for details of your local testing service.
Step 2. Follow the instructions provided by the testing service to collect your samples. Sampling is best done in late spring and early summer before the weed seeds are removed by grazing or weather.
Step 3. Choose which type of test you require and for which plant species. Multiple resistance tests are available which test for resistance across a number of herbicide groups. Single herbicide tests are also available if you suspect which herbicide it is.
Step 4. Sample from the areas that you suspect might be resistant. Collect mature weed seed from within each area. For your own benefit, draw a map of where you collected samples. For a single herbicide test, collect one large handful of seed. For a multiple resistance test, collect 2 large handfuls. Take the samples before stock are let in to graze.
Step 5. Expect the results to take 3 to 4 months. Once you have the results, consider your options with your local agronomist.
If you suspect glyphosate resistance
If you have glyphosate resistant ryegrass, it will begin to appear as a scattering of single plants or patches. The resistant plants may show signs of typical glyphosate damage – growth is affected for a few weeks and then the plant recovers. Using the procedure above, collect seed samples for resistance testing. Or you can contact the National Gyphosate Sustainability Working Group. This group is made up of government agencies, universities and chemical companies who offer advice on how to sample plants for testing and confirmation of resistance status.
Strategies for managing herbicide resistance once it has occurred
Herbicide resistance is difficult to manage once it occurs. Each situation is different and the farmer should seek expert advice. However, a number of principles are important:
- Know which herbicides you have resistance to (get tests done).
- Choose methods from the IWM tool box (page 6). Particularly effective methods where chemicals are less useful are: using barley crops for weed control, catching seed in a bin and burning it, cultivation, harvest low-no spread-burn, use crop for hay, heavy grazing, manuring, mechanical pasture top, stubble burning.
- Use RIM (page 7) to determine which combination of practices to use for the best economic return.
- Pay close attention to rotations – include pasture phases of adequate length (RIM is useful for this, see page 7).
- Good farm hygiene practices are vital – don't spread resistant seed around your farm or to neighbours.
Measuring the effectiveness of control of ryegrass seed bank
The ryegrass seed bank can be measured on the farm but does require a fair effort. The procedure is carried out in January or February each year (or March if you live in a very hot area) and is as follows:
Step 1. Set up a transect across the paddock by marking a post on the fence which is in line with another object (eg. tree, dam, silo).
Step 2. Take soil cores or surface soil with a shovel (but take the same surface area each time).
Step 3. Part fill a number of plastic trays with potting mix. Spread soil sample thinly over plastic trays – no more than 2 cm thick.
Step 4.Place the trays in a shady place and keep them well watered for 2 to 3 months.
Step 5. Count and remove ryegrass seedlings on a regular basis. Stir up the soil a little bit after each lot has been counted and removed. All of the seeds should have germinated by the end of April.
Step 6. Convert counts to plants per m2
Interpreting the results: This method does not give an accurate indication of ryegrass seedbank across entire paddock because this is highly variable within a paddock. However, it does give some on-going guidance about the effectiveness of various control measures. You can then use this information to guide your decisions.
The following case studies illustrate what happens to ryegrass seedbanks under a number of management scenarios (such as some of the methods listed above).
The impact of different management techniques on the resistant Annual Ryegrass population
Research in South Australia (Gill, 2003) is very useful for illustrating the impact of different management scenarios on the resistant Annual Ryegrass population. A number of case studies were carried out over 3 years.
1. The impact of rotations on Annual Ryegrass seedbank (where resistance not an issue)
These graphs showed that Annual Ryegrass seedbank can be reduced significantly when a pasture phase is used. Good knowledge of your weed seed bank and resistance profile is important for making decisions on how long a to keep a paddock in the pasture phase.
2. The impact of barley and a pasture phase on the Annual Ryegrass seedbank (where resistance is not an issue)
The following figure illustrates the positive impact that 2 years of barley (a good competitor against weeds) and a pasture phase has on reducing ryegrass seedbank.
3. The impact of continuous cropping and non-selective herbicide from 1 group (Group A) on ryegrass seedbank (where resistance occurs)
This final case study shows just how quickly resistant Annual Ryegrass populations can build up in continuous cropping systems and with reliance on one herbicide group. A pasture phase or use of a barley crop would have been a better strategy. In Year 1 a wheat crop is grown – Annual Ryegrass seed numbers are low but are primed for resistance because of the continuous cropping history. In Year 2 canola is grown and a Group A herbicide used (seed levels rise to 718 seeds/m2). The next year the paddock is back in wheat – because Group A failed the previous year, Annual Ryegrass seed bank levels have exploded (over 5000/m2).
Glyphosate – the next big resistance problem?
Glyphosate is one of the most popular herbicides for treating Annual Ryegrass because of the failure of other herbicide groups because its effective and cheap. It is now the herbicide with the most rapid growing risk of developing herbicide resistance. Already, a number of cases of glyphosate resistance have occurred (40 cases in Australia to date). In recent years, the use of this chemical has doubled in most areas.
Reducing the occurrence of Glyphosate resistance in Annual Ryegrass as recommended by a Glyphosate Working Group:
- Don't use low rates: always use rates that give a high kill – low rates increase the chance of plants surviving to be resistant. Use the rates specified on the label.
- Double knock technique (Method 7, page 4): More effective if time between 1st and 2nd treatments is 1 to 3 days.
- Strategic use of alternative chemical groups
- Full cut cultivation at sowing
- Effective in-crop weed control
- Use alternative herbicide groups or tillage in fallow
- Use other IWM practices (non-herbicide)
- Crop topping with alternative herbicide groups
- Attention to farm hygiene to reduce spread of seeds
Acknowledgments: The project team gratefully acknowledge the input of the following people in development of this tool: David Panell (University of WA) and Dr Gurjeet Gill (School of Agriculture and Wine, Roseworthy Campus, University of Adelaide).
Banks, A., Broadley, R., Collinge, M., and K. Middleton (1994). Pesticide Application Manual. edition. Department of Primary Industries, Queensland.
Gill, G (2003). On-farm monitoring of weed seed-banks for optimising weed management. GRDC Research Summary, July 2003.
Heap, J (1999). Herbicide resistance in weeds. High Rainfall Advice Sheet, Southern Region, July 1999. SARDI, South Australia.
Llewellyn, R, Lindner, R, Pannell, D and Powles, S (2004). Grain grower perceptions and use of integrated weed management. Australian Journal of Experimental Agriculture, 2004, 44 (993-1001).
National Glyphosate Sustainability Working Group (2005). Keeping glyphosate resistance rare in Australian cropping. CRC for Australian Weed Management and GRDC.
Owen, M, Walsh, M and Powles, S (2005). Frequency of herbicide resistance in annual ryegrass across the WA wheat belt. Paper presented at Agribusiness Crop Updates 2005.
Powles, S and Holtum, J (2003). Herbicide resistant weeds in Australia. Farrer Centre, Riverina Outlook Conference 2003.
Preston, C (2005). Understanding herbicide resistance. GRDC Research Update for Advisers, Southern Region, 2005.
Slater, P.D. 1998. Integrated Pest Management (IPM) – Ecological Background. Proceedings of 25 years of the Riverina Outlook Conference, Wagga Wagga 1998. Farrer Centre, Charles Sturt University, Wagga Wagga.
Western Australian Herbicide Resistance Initiative (2005). See: http://wahri.agric.uwa.edu.au/dst.html