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Agronomists, researchers, growers answering important questions

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How does renovating non-wetting sands help weed control?

with Peter Newman, Western Extension Agronomist, AHRI
Some weed control strategies work well in a wide range of situations and some are more targeted. This is one of the targeted strategies, but it does have potential application on some 5 million hectares, or almost one-fifth, of Australia’s cropping land.
Peter Newman, Australian Herbicide Resistance Initiative (AHRI) western extension agronomist, is an enthusiastic proponent of renovating non-wetting sands to ‘grow more crop and less weeds’.
Peter Newman, Australian Herbicide Resistance Initiative (AHRI) western extension agronomist, says weed seed burial works best if the topsoil is fully inverted using a mouldboard plough and then left undisturbed indefinitely. It relies on the grower taking the opportunity to implement a robust integrated weed management program, such as the WeedSmart Big 6.
“The package that works best is controlled traffic farming plus soil inversion plus lime,” he says. “There is very solid proof that increasing the clay content in the topsoil, through mouldboard ploughing, spading or Plozza ploughing, improving pH and burying weed seed will have an enormous effect on farm profitability.”
“Part of the overall benefit is ‘re-setting’ the weed seed bank. Seed burial works best if the topsoil is fully inverted using a mouldboard plough and then left undisturbed indefinitely. It relies on the grower taking the opportunity to implement a robust integrated weed management program, such as the WeedSmart Big 6.”
Peter says the research he and others have done over a period of more than a decade shows that for weed control, the mouldboard plough does the best job. Other claying options can be very effective and less time consuming if the weed burden is not excessive.  
“The results have consistently shown that mouldboard ploughing will bury 99 per cent of the weed seed and crops will respond with a large yield increase,” says Peter. “Some weed species remain viable at depth for only a few years while some other seeds can remain viable for five or more years – but even with these species such as wild radish, only a low percentage of the seeds actually survive that length of time.”  
Most growers who have embarked on a renovation program on non-wetting sands return to a controlled traffic, zero-tillage farming system and expect to not ever have to repeat the operation. This leaves the herbicide resistant weeds safely buried and out of harm’s way. Having re-set the weed seed bank they are then able to concentrate their efforts on keeping weed numbers low – the very best way to avoid herbicide resistance.

Is the full restoration process economically worthwhile on non-wetting sands?
Short answer: Yes.
Longer answer: The change in water use efficiency is what drives the turn-around in profitability on this soil type. Growers can expect WUE to increase by about 40 per cent, so if the non-wetting sand is achieving a WUE of 12 kg/mm then crops grown after the renovation program will achieve WUE of around 16 kg/mm. This jump in productivity makes a massive difference to profitability.
In round figures, the mouldboarding/spading operation costs $70/ha, deep ripping to 60 cm depth costs $70 and 4 t/ha of lime sand (spread) costs $100 (depending on location). Mouldboard ploughing (or spading) plus lime has been shown to achieve a return on investment of over 500 per cent over 10 years. An even higher return on investment can result if compacted soils are deep ripped.
This return on investment justifies further investment in gear for controlled traffic farming (to avoid the future need to ameliorate compaction) and to implement a weed control program that relies less on herbicides. 
Amelioration of non-wetting sands makes an enormous difference to the profitability of these sandplain soils and can also re-set the seed bank of herbicide resistant weeds to next to zero.
Does soil inversion affect herbicide use and efficacy?
Short answer: It can. Growers need to be particularly careful with pre-emergent herbicide use for a few years.
Longer answer: Following a renovation operation, additional care is needed to ensure crop safety in relation to pre-emergent herbicides. The reason for this is likely to be the increased wet-ability and the low organic matter in the ‘new’ topsoil, resulting in less binding of herbicide compared to the pre-renovation soil behaviour. Some growers avoid using pre-emergent herbicides for a year or two and re-introduce their use carefully. Trifluralin has been particularly damaging in some situations. Other growers have been able to continue using pre-emergent herbicides while paying particular attention to soil throw out of the furrow.
On the flip side, weed control can improve following soil amelioration because the herbicides are now incorporated into wettable soil and weed seeds no longer get trapped in pockets of dry soil.
Is the weed seed burial a weak link in this system?
Short answer: Only if the sub-soil is disturbed again within five or six years.
Longer answer: Non-wetting sands are prone to compaction and the alleviation of a hardpan contributes to the yield response that results from the full soil amelioration operation. In some instances, future deep ripping can bring buried weed seeds back to the soil surface. This is hard to avoid as the crop often responds favourably to deep ripping, so growers just need to be aware of the potential risk and plan their weed control accordingly.
Once the weed seed bank is ‘re-set’, how do I stop weed blow-outs after ameliorating the soil?
Short answer: Implement as many of the WeedSmart Big 6 tactics as you can.
Longer answer: We know that herbicide-only weed control is no longer an option so it is essential to use a combination of herbicide and non-herbicide tools to stop seed set and keep weed numbers low. No matter what the soil type, you need to be doing some form of harvest weed seed control right now. There are a heap of options and some are very easy and relatively cheap to adopt. Following that you need to look at ways to increase crop competition and maximise the effectiveness of the herbicide options that are still available. Herbicide resistance can be beaten!
Other resources

Crop weeds: reduce weed numbers in the soil (DAFWA)

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How can I get the most bang from crop-topping canola?

with Greg Condon, Grassroots Agronomy and AHRI
In an environment of increasing herbicide resistance, getting back in the driver’s seat with weed control relies on stacking tactics and not leaving all the heavy lifting to just one or two strategies.
Greg Condon, Grassroots Agronomy and Australian Herbicide Resistance Initiative (AHRI) southern extension agronomist, works with growers to develop and implement combinations of control measures that complement each other to drive down weed seed set.
Greg Condon, Grassroots Agronomy and Australian Herbicide Resistance Initiative (AHRI) southern extension agronomist, advocates crop-topping plus harvest weed seed control in canola as an effective way to drive down annual ryegrass and wild radish numbers.
“One of the combinations we are advocating is crop-topping plus harvest weed seed control,” he says. “Canola is a good candidate for this package, particularly to drive down annual ryegrass numbers.”
“Being able to apply the registered glyphosate products early in the crop senescence in canola gives growers a better opportunity to interrupt weed seed set. Following the herbicide with a non-herbicide tactic like harvest weed seed control helps protect the herbicide chemistry by taking another swipe at weed seed that may have evaded the crop-topping tactic.”
Greg says the over-the-top option is generally a more practical option than applying the registered glyphosate products under the cutterbar if the crop is windrowed. 
“Knowing the glyphosate resistance status of weeds in a paddock is really important,” says Greg. “If the weeds present have resistance to glyphosate then crop-topping is ineffective and should not be used.”  
On some farms canola may be grown as often as every second year, putting heavy pressure on the crop-topping tactic. Under these circumstances, adding harvest weed seed control and maximising crop competition is absolutely essential.  
What herbicides are registered for late season weed control in canola?
Short answer: Diquat and two specific glyphosate formulations*.
Longer answer: Products registered for application prior to direct heading or before/under the windrow* are weedmaster DST and Roundup Ultra Max. Weedmaster DST has a withholding period of five days. Diquat is also registered for use in canola (WHP 4 days).
The glyphosate products registered for canola can be applied to mature standing crops with a minimum of 20 per cent seed colour change to dark brown or black. Higher end label rates are recommended when crops or weeds are dense. Glyphosate must not be applied to crops intended for seed. Refer to the label prior to use and follow the instructions.
Glyphosate must not be applied to standing crops and again at windrowing.   
*Pintobi Attack is also registered for this use pattern but is not a commercial product at this time.
What other benefits do I get from crop-topping in canola?
Short answer: A head start on summer weed control.
Longer answer: Crop-topping with high water rates gives good coverage and can penetrate dense crops to kill small, germinating weeds (such as sowthistle) as well as reducing weed seed set in mature weeds (such as annual ryegrass and wild radish). This essentially provides the first summer weed control application and should reduce the use of glyphosate over summer.
Crop-topping has the added benefit of providing early control of summer growing weeds like sowthistle while they are still small.
What if I suspect glyphosate resistance is present?
Short answer: Reduce glyphosate use across the cropping program and look for ways to protect this chemistry by driving down weed numbers.
Longer answer: Collect weed seed samples and have them tested for herbicide resistance. Knowing what still works is just as important as confirming your suspicions. If immature weeds are present they can be collected and sent for a QuickTest that will give you an opportunity to take action within the season.
Research by Dr Peter Boutsalis and others has confirmed that late season applications of glyphosate on glyphosate-resistant annual ryegrass provides no control and is essentially a waste of money. This research showed that 80 per cent of the seed on glyphosate resistant annual ryegrass remains viable after crop-topping.
Two take home messages from this research are: 1. treating younger plants at lower temperatures can improve glyphosate efficacy on resistant biotypes and 2. crop-topping with glyphosate is not effective on glyphosate resistant ryegrass.
Before crop-topping canola it pays to understand the herbicide resistance profile of the weeds present. Crop-topping glyphosate resistant annual ryegrass is not effective as 80 per cent of the weed seed remains viable after crop-topping.
Which harvest weed seed control method is the best crop-topping partner?
Short answer: All harvest weed seed control (HWSC) methods provide very similar results – the key is the do everything you can to get the weed seeds into the front of the header.
Longer answer: Harvesting low and setting up your harvester to maximise weed seed capture are very important. Canola is a good crop for implementing HWSC because harvesting low does not add significantly to the amount of material that must be processed through the header. Many growers are also using canola chaff captured through HWSC tactics such as chaff carts, chaff lining or chaff decks as a high quality feed source for sheep when grazed.
More resources: 

GRDC Pre-harvest herbicide use factsheet 2017 
Does glyphosate formulation affect the control of glyphosate resistant weeds? 
Options for crop-topping canola

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Is harvest weed seed control a real option for managing northern region weeds?

with Dr Michael Walsh, The University of Sydney
Harvest weed seed control (HWSC), is a key component in the WeedSmart Big 6 tactics for managing herbicide resistance on grain farms. In the northern region there has been some scepticism surrounding how effective HWSC will be in capturing seed from common weeds in the region.
Dr Michael Walsh, Director of Weed Research at The University of Sydney is a pioneer of harvest weed seed control research and is convinced that the tactic can be effectively deployed against northern region weeds.

“The concern people usually have is that several key species in the north start shedding seed before the winter harvest,” he says. “Pre-harvest shedding does reduce the impact that HWSC can have on the seed bank of some species, but there is also solid evidence that the harvest operation can collect a significant amount of seed even if the seed shedding process has begun.”
“Even in weeds such as sowthistle where the seed is light and wind borne, not all the seed in each seed head matures at the same time. This means that while some might be lost, the immature seeds can be collected and destroyed using any of the harvest weed seed control tools.”
Dr Walsh says chickpea crops are a great place to start with HWSC because the crop requires a low harvest height, which increases the amount of weed seed that is collected, and many crops are desiccated prior to harvest, which may assist with seed retention in some weed species.
“The concept of harvest weed seed control was invented in Australia in response to herbicide resistance, and has been adopted by about 50 per cent of grain producers nation-wide,” says Dr Walsh. “The idea is now being adopted internationally and is proving beneficial in driving down the seed bank of many weed species.”
What northern weed species are the best candidates for HWSC?
Short answer: African turnip weed, turnip weed, wild oats, annual ryegrass, feathertop Rhodes grass, sowthistle and bladder ketmia.
Longer answer: These species are rated as very high to intermediate for seed retention at harvest. This means that at least 20 per cent, and up to 100 per cent, of the seed is present on the plant at harvest and able to be collected through the front of the header. Once in the header, destruction of the seed is almost guaranteed through impact, burning or breakdown, depending on the HWSC method used.
What about flaxleaf fleabane and awnless barnyard grass?
Short answer: Seed retention of these two species at harvest is highly variable.
Longer answer: Both flaxleaf fleabane and awnless barnyard grass have a wide range of seed retention at harvest. Ranging from less than 20 per cent to over 90 per cent, the impact of HWSC on the seed bank is not a sure thing, but in some years could be very effective.
Which HWSC method is best?
Short answer: All are equally effective at reducing the weed seed bank.
Longer answer: The HWSC methods currently available to growers are narrow windrow burning, impact mills, chaff lining, chaff tram-lining, chaff carts and bale direct. All except the impact mills have some level of nutrient removal or concentration. Narrow windrow burning and chaff carts usually involve a burning operation the following autumn and the bale direct system relies on a market or on-farm use for the hay.
The impact mills, bale direct and chaff cart systems require sizeable investment and harvester modification while the other systems are very cheap and relatively easy to set up.

All HWSC methods are equally effective at reducing the weed seed bank. The chaff tramlining system shown here delivers the chaff component of the crop residue onto the tramlines where it is left to decompose over time, killing much of the weed seed present.
How often does HWSC need to be done to be effective?
Short answer: The more the better but it doesn’t have to be every year in every paddock.
Longer answer: Long term focus paddocks in WA have shown that HWSC every second year, in addition to in-crop herbicide applications, will consistently drive down the weed seed bank to very low levels.
Within a crop rotation many growers choose to implement HWSC in certain crops. For many, the break crops are the preferred crops due to lower levels of stubble, and for pulses, lower harvest heights.

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How to regain control of herbicide resistant sowthistle?

with Dr Chris Preston, University of Adelaide
Having once been a fairly innocuous weed on farms around Australia, sowthistle is now a major weed in many farming systems and has become very difficult to control.
Dr Chris Preston, Associate Professor, Weed Management
 at The University of Adelaide says that changes in emergence patterns and resistance to a wide array of herbicide groups has seen sowthistle adapt well to modern farming systems.   
Dr Chris Preston, University of Adelaide says that in some paddocks, tight lentil wheat rotations has caused a complete failure in sowthistle control every second year, adding massive amounts of seed to the seed bank.
“We have seen two different scenarios unfold in the southern and northern grain growing regions of Australia,” he says. “In southern regions, sowthistle is a major problem in lentil crops where it is able to take advantage of unused soil moisture in spring to establish in gaps in the crop. Having long been resistant to the ‘SU’ herbicides in Group B, the frequent use of imi-tolerant lentil varieties has led to the rapid evolution of resistance to the ‘imi’ herbicides in Group B, leaving no in-crop options for control.”
This means the only form of herbicide control available late in the season is the use of desiccants. Desiccants for use in pulses, such as Sharpen, often do not kill the sowthistle plants and so it is able to recover and go on to set more seed.    
“In the northern cropping region the problem lies in summer fallow management,” he says. “Farmers are observing rate creep with both glyphosate and 2,4-D, where higher rates are now required to achieve the same level of control. While glyphosate resistance has been confirmed in the north, there are no confirmed cases of 2,4-D resistance in the north, but I have no doubt that this will be confirmed in the field sooner rather than later due to the amount of phenoxy herbicides being applied in fallows.”
With limited herbicide options available, finding ways to disrupt the expansion of sowthistle populations will need to be front and centre when planning a weed management program.

How can crop rotation help manage sowthistle?
Short answer: Crop competition and herbicide rotation.
Longer answer: Lentils and several other pulses have been very profitable and this has supported tight lentil wheat rotations in some areas. In some paddocks this has caused a complete failure in sowthistle control every second year, adding massive amounts of seed to the seed bank. Spreading out the less-competitive crops such as lentils and choosing highly competitive crops to add to the rotation will help drive down weed numbers. Sowthistle is well-known as a non-competitive weed when small.   
What other summer fallow methods can be used?
Short answer: Be careful with tank mix partners, look at other herbicide options and try changing the winter / summer crop mix.
Longer answer: In the northern region, the use of glyphosate + 2,4-D followed by paraquat as the double knock is unlikely to be effective long-term. When mixed together, glyphosate and 2,4-D are antagonistic on sowthistle, reducing the efficacy of both tank mix partners. Sowthistle also has the potential to evolve resistance to both glyphosate and 2,4-D, which would make the first knock ineffective. With few other herbicide options other than perhaps the use of residual chemistry, growers will need to implement other control measures.
To drive down numbers, control in spring is critical. Changing the combination of summer and winter cropping could enable better control in spring.
Glyphosate resistance in sowthistle has been confirmed in the north, and while there are no confirmed cases of 2,4-D resistance in the region, Dr Preston has no doubt that this will be confirmed in the field sooner rather than later due to the amount of phenoxy herbicides being applied in fallows.
Does size matter?
Short answer: Yes, even resistant sowthistle plants are more susceptible when they are very small.
Longer answer: The label on glyphosate products typically specifies two different rates according to plant size classes and following these instructions is very important.
If a lower rate is applied to larger weeds they can take a long time to die (if at all) and may well set seed before the plant dies, making the application a waste of money.
One of the difficulties with managing sowthistle is the fact that it can germinate at any time of the year if soil moisture is available. In favourable conditions, this weed can set seed within months and has the capacity to produce about 6000 seeds per plant.
Is mechanical weed control an option for sowthistle?
Short answer: Both harvest weed seed control and strategic cultivation could play a part in reducing seed set.
Longer answer: HWSC is not very effective in capturing sowthistle seeds, which easily disperse on the wind. The removal and destruction of immature seed heads following desiccation will still have a positive effect on weed seed numbers.
Infrequent cultivation that buries seed to a depth of 2 cm or more can help to reduce emergence however repeated use of cultivation can bring seed back to the soil surface where germination can occur. Sowthistle emergence occurs primarily from seeds close to the soil surface with up to 30% of viable seeds emerging over 5 months. Seed can emerge from a depth of up to 2 cm with approximately 4% emergence after 6 months. Seed buried below 5 cm is unable to emerge and can persist at depth.

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Can sheep turn weed seeds and chaff into cash?

with Ed Riggall, AgPro Management consultant, WA
Chaff carts have been doing a great job in the battle against herbicide resistant weeds and are finding a valuable niche, particularly in mixed farming enterprises in western and southern cropping regions.
AgPro Management consultant, Ed Riggall has been validating the anecdotal evidence of the value of chaff heaps as a stock feed for the past three years and has proven that there is money to be made from using chaff heaps (and probably chaff lines) as a summer feed source for sheep.
AgPro Management consultant, Ed Riggall has been validating the anecdotal evidence of the value of chaff heaps as a stock feed for the past three years and has proven that there is money to be made from using chaff heaps (and probably chaff lines) as a summer feed source for sheep.
“Using paired paddocks on farms in different regions we measured the changes in sheep weight over a period of 6 weeks and compared the performance of sheep grazing paddocks with chaff piles with that of sheep from the same flock grazing paddocks where the chaff had been spread,” he says. “Data was collected in canola, wheat, oats and barley fields in eight locations in Western Australia over three years.”
Ed analysed the feed value of the chaff heaps from different crops and while the results varied considerably, canola has proven to be a consistent performer. This is due mainly to the higher protein level compared to cereals, which enables the chaff piles to be better digested. The cereal chaff heaps were all similar in feed value with around 4% crude protein and ME over 7 MJ/kgDM, indicating that provision of additional protein, such as lupin grain, would be required for the sheep to make full use of cereal chaff piles.   
“The results showed that sheep on chaff piles gained an average of 2 kg in the first three weeks, 500 g more than sheep grazing stubbles (no chaff heaps),” he says. “At the end of six weeks grazing the sheep with access to chaff piles had gained about 100 g while sheep without access to the chaff piles had lost almost 2 kg compared to their starting weight.”

In Merino flocks this small positive net gain per head for sheep grazing chaff heaps is worth a serious amount of money, spread across the whole mob, in the form of improved condition score, increased lambing percentages and reduced summer feed costs. When you add in the value of chaff management as a weed seed control measure to combat herbicide resistant weeds, the return quickly pays for the investment in a new chaff cart.
Meat & Livestock Australia (MLA) is currently funding a 3- year demonstration site with the Gillamii group based in Cranbrook, WA to further evaluate the value of grazing chaff heaps.
Do all sheep readily graze the chaff heaps?
Short answer: No, flocks can take some time to accept the chaff heaps as a feed source.
Longer answer: Livestock usually take time to adjust to a new feed and will tend to preferentially graze familiar feeds, even though they might be scarce or of lower feed value. It helps to include some older or more experienced individuals in the flock to show the others that it is good feed. In cereal paddocks, there is a biological limit to the amount of high ME feed animals can consume if there is insufficient protein available. Providing some supplementary grain will address this problem. 
Flocks can take some time to accept the chaff heaps as a feed source. Including some more experienced animals in the flock can help.
What did the economic analysis show?
Short answer: Internal rate of return of 21% on the purchase of a $80 thousand chaff cart.
Longer answer: The analysis was done on a model farm of 2000 ha where 50 per cent of the area was cropped and the rest was pasture. Over summer, 9 DSE (7500 sheep) grazed chaff heaps on the 1000 ha cropped area. The average weight advantage of sheep with access to chaff heaps over those that did not was 3.1 kg/hd (where half the area was canola and the rest cereal). Using a feed conversion rate of 3:1, the sheep with access to chaff piles consumed $250 worth of feed for ‘free’. If the farmer invested $80 thousand in the chaff cart, spent $20 thousand in repairs and maintenance and sold it for $20 thousand in 20 years the internal rate of return on the initial investment is a huge 21%.
Adding a chaff cart to a mixed farm is a great example of a practice that is good for both the sheep and the cropping enterprises. 
Grazing sheep on chaff heaps can help to quickly repay the money invested in the purchase of a new chaff cart.
Do sheep spread weed seeds from the chaff piles across the paddocks?
Short answer: No, very few weed seeds remain viable after passing through a sheep’s gut.
Longer answer: Farmers who routinely use sheep to graze chaff piles say that there is no noticeable spreading of weeds across the paddocks. DAFWA research has validated this observation to prove that weed seed is not spread and that only 3 per cent of weed seeds remain viable after passing through a sheep’s digestive system.
Do sheep spread nutrients from the chaff piles across the paddocks?
Short answer: Probably yes. The question is how evenly? This will depend mostly on stocking rate and other variables.
Longer answer: While there is little to no documented research about nutrient redistribution in chaff pile paddocks, there is for other grazing systems so it stands to reason that some nutrients would be spread back across the paddock. A new area of research is to consider the economic benefits of sheep grazing chaff lines, which may also have a great benefit when it comes to nutrient redistribution.
More resources

Ed Riggall – Chaff as sheep feed webinar
Case study: Ben Webb, Kojonup WA

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What is the critical period for weed control?

with Graham Charles, Research Agronomist (weeds), NSW DPI
The critical period for weed control (CPWC) idea suggests that any weeds that germinate within the CPWC window should be controlled soon after germination to stop them reducing crop yield. Once the crop has passed the critical period then the crop will out-compete any new weed germinations and they will have little impact on yield.
NSW Department of Primary Industries research agronomist, Graham Charles’ work in cotton clearly demonstrated this principle for several key weed species, but he cautions that late germinations of weeds will still contribute to the weed seed bank and so must be controlled before they set seed, even though they may not affect yield of the current crop.
NSW Department of Primary Industries research agronomist, Graham Charles says early control of weeds is important to preserve yield and then growers need to turn their attention to preventing seed set in any later germinating weeds.
“To maximise crop performance and minimise the risk of herbicide resistance in farming systems growers need to make many decisions about the timing of weed control,” he says. “In Roundup Ready cotton there is an option to apply glyphosate at any stage of crop development, but once the critical period has been passed in the crop then the focus needs to switch to preventing seed set and preserving fibre quality, as these later weed emergences will not compromise yield.”
Using yield data across seven years and measuring the impact of three weed species, Graham developed a tool for cotton growers to estimate weed density and use the economic threshold to determine when to apply a weed control tactic.   
“Getting the timing right is very important when it comes to managing weeds but the fact is, there are many variables and the decisions are hard to make,” he says. “The critical period weed sampling sheet that we developed for cotton can provide a framework for other growers to use to record weed densities.”
Herbicide resistance can only be contained in a low-density weed situation. Achieving this hinges on applying herbicides when the weeds are most susceptible – small and actively growing – and preventing any escapes from setting seed. This strategy will also protect yield: less weeds, more crop.
When the weed density is low, crop competition can do most of the work provided those few weeds are controlled early, without damaging the crop. Then come back later in the season to prevent seed set of any late germinating weeds to keep the seed bank in check.
How do you measure weed density?
Short answer: Do a ‘drive-by’ survey to identify weedy and less weedy areas. Then walk a transect through the weed areas and count the weeds present.
Longer answer: Record weeds in broad categories such as ‘large broadleaf’, ‘small broadleaf’ and ‘grasses’. Every 50 m, stop and make a small quadrant with your boots – heels together and feet placed at 90 degrees. Count the weeds in the square you can imagine with your feet providing two sides. As a rule of thumb, a count of 5 weeds equates to 50 per m2, and 20 weeds equates to 200 per m2. 
Estimating weed density is difficult. One way to gather data is to do a field survey counting weeds within a small quadrant made using your boots. As a rule of thumb, a count of 5 weeds equates to 50 per m2, and 20 weeds equates to 200 per m2.
How do I time sprays to maximise the value of each herbicide application?
Short answer: Spray only young, actively growing weeds. Use a different tactic to control large or stressed weeds.
Longer answer: Delaying a spray application to wait for another germination (and try and get all weeds with one pass) may have a yield penalty and may also allow the older individuals to survive the treatment. When the weed density is low, crop competition can do most of the work provided those few weeds are controlled early, without damaging the crop. Then come back later in the season to prevent seed set of any late germinating weeds to keep the seed bank in check.
If weed density is moderate to high, then it is necessary to apply a number of tactics throughout the growing season to put downward pressure on weed numbers and seed production. In cotton, high densities (500 weeds per 10 m row) of grass weeds such as awnless barnyard grass and liverseed grass need to be controlled right through to early squaring of the cotton, just to prevent a 2% yield loss. If allowed to set seed this density of grass weed would obviously create an even greater weed problem for the following season.
How important is stopping seed set in relation to keeping glyphosate working?
Short answer: Essential! Follow the 2 + 2 & NO Survivors approach.
Longer answer: To keep glyphosate working in the cotton industry, growers are asked to use at least 2 non-glyphosate tactics in crop, 2 non-glyphosate tactics in the fallow and to ensure there are zero-survivors of any glyphosate treatment applied. This means scouting after weed control treatments and applying aggressive tactics such as spot spraying, chipping and cultivation to remove individual survivors and patches.
And don’t forget to follow the same principles to protect the other chemistry groups. Controlling survivors of a Group A application, for example, is just as important for prolonging the life of the Group A herbicides.
More resources:

Cotton BMP Weed Management 

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Diversity – it’s importance in the battle with herbicide resistance

with Mike Ashworth, Research Agronomist, AHRI
Everyone would like the answers to farming’s problems to be simple, but the fact is, all over-simplistic solutions are prone to collapse. This has been repeatedly demonstrated with different weed species adapting to challenge different farming systems or practices.
Australian Herbicide Resistance Initiative research agronomist, Dr Mike Ashworth says weeds use two mechanisms to survive weed control methods – they can either become ‘immune’ or ‘evade’ the control tactic.
Dr Mike Ashworth says the best way to keep all weed control tactics alive is to be unpredictable in your choice of crops, planting times, herbicide choices and harvest weed seed control methods.
“There is extensive research in the area of herbicide resistance showing the molecular, genetic and biological mechanisms that different weeds can use to become immune to herbicide treatments,” he says. “Just as the systematic use of herbicides will lead to resistance, the routine use of non-herbicide weed control tactics will select for individual plants that can evade that tactic.
In one study, Dr Ashworth started with wild radish plants that had not been subjected to harvest weed seed control and systematically collected seed from early flowering individuals. The seeds were planted to produce the next generation for further selection.
“Within five generations of selection the wild radish population had more than halved the time to flowering – from 60 days to just 29 days,” he says. “As a result these plants will be carrying well-matured pods at harvest, and will be more likely to drop their seed, effectively evading seed capture at harvest. This is even more likely in years where there are periods of water deficit, high temperature or high wind.”
The good news is that the early flowering wild radish populations produced less biomass prior to flowering than the later flowering individuals, likely making them more susceptible to crop competition. Unfortunately, these early flowering plants also had poor structural integrity, with more pods set below the harvest cutting height.

How can I stop weeds evading weed control tactics?
Short answer: Avoid predictable and repetitive farming systems and weed control tactics, and aim to establish highly competitive crops.
Longer answer: The more diversity the better. Look for ways to add different crops to the rotation, mix and rotate herbicide modes of action, use different row spacings, alter sowing times and implement different strategies before and at harvest.
Take advantage of weed control that comes through increased crop competition. For example, the early flowering wild radish plants are smaller, potentially making them more resistant to harvest weed seed control, but more susceptible to crop competition.
All weed control strategies are most effective when weed numbers are low so concentrate on ways to drive down weed numbers, prevent seed set and remove survivors.
What about crop competition – can weeds evade that?
Short answer: Yes, weeds will respond to increased crop competition.
Longer answer: In a highly competitive crop a few individual plants are likely to gain an advantage through increased vigour or plant height. If left unchecked these individuals can set seed and the population can increase in size. This is less likely to become a problem if the farming system includes a range of crops and pastures of varying heights for example. A trait that is an advantage in one situation is often a disadvantage in another.
The combination of highly competitive cropping with harvest weed seed control is a great example. Vigorous plants often hold their seed high in the crop canopy and so are readily collected at harvest in a competitive crop environment. On the other hand, these weeds would flourish and potentially lodge in a less competitive crop, thereby evading harvest weed seed control.
This pot trial is investigating the fitness penalty that may be associated with the early flowering trait of these specially selected wild radish plants when faced with stiff crop competition from a wheat crop compared to no competition.
How are weeds evading pre-emergent herbicides?
Short answer: Through selection for later germination.
Longer answer: Pre-emergent herbicides are being increasingly relied on to provide early weed control. Individual weed seeds that remain dormant for longer and germinate later in the season can evade pre-emergent herbicides altogether or are exposed to a low dose that can lead to resistance.
By mixing up planting dates and using different pre-emergence herbicides with different modes of action, growers can disrupt the shift in dormancy.
Weeds that emerge later, will be more susceptible to crop competition from an early seeded crop. Get the jump on weeds and make sure your crop is up and away.

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The best ways to control herbicide resistant wild oats

with Richard Daniel, CEO Northern Cropping Alliance
Wild oats takes a little longer to evolve herbicide resistance than some other grasses, but once resistance has occurred it is a challenging weed that requires a concerted effort to bring under control.
Although herbicide resistance has been known in wild oats since the late 1990s, growers have generally had some effective in-crop herbicide options that kept numbers at a manageable level. However, in recent years it has become increasingly obvious that Group A herbicides are no longer a viable option to control wild oats in many farming systems.
Richard Daniel, Northern Grower Alliance CEO says that now growers are faced with less herbicide options and few cultural practices to control wild oats, they will need to stack control tactics that work in their farming system.
Richard Daniel, CEO of the Northern Grower Alliance (NGA) says that although wild oats is self-pollinating and is a hexaploid species (comparatively more DNA than diploid species such as annual ryegrass), attributes which have delayed resistance, there are populations in Australia that are resistant to all post-emergent herbicides registered for its control.
“Faced with less herbicide options and few cultural practices to control wild oats, growers will need to stack control tactics that work in their farming system,” he says. “Canadian research has demonstrated the value of stacking tactics such as crop rotation plus high seeding rates plus a competitive cultivar to control wild oats.”
Richard says the most valuable tool growers have in the northern grains region is the ability to summer crop. He recommends a summer cropping phase to break the cycle and drive down numbers in the wild oats seed bank.
“Two consecutive winter fallows gives the best opportunity to control wild oats,” he says. “A single winter fallow and then double cropping back to a winter crop does less than half a job, particularly if the paddock is sown to chickpea.”
“Chickpea poses a higher risk because it is generally less competitive against wild oats. This risk can be reduced if the chickpea crop is sown later, which gives the grower the opportunity to apply a winter fallow weed control treatment on the first germination of wild oats to give the crop a competitive advantage.”
What is the most effective crop rotation to combat dense populations and resistant wild oats?
Short answer: A summer cropping phase.
Longer answer: For best results, implement a summer cropping phase that allows for winter fallows on both sides of the summer crop/s to control wild oats.
For this strategy to be effective for wild oats management, it is important to avoid planting the summer crop too early where there is a risk of a late flush of wild oats emerging after planting.
When returning to a winter crop, consider sowing a competitive crop such as barley first. Position chickpea carefully within the rotation, aiming to only sow chickpea into paddocks with low wild oats numbers.
Although wild oats is a prolific seed producer, about 75 per cent of seed is depleted within 12 months of entering the seed bank. This means that consistent pressure on the seed bank is a very effective control measure and large populations can be run down in 3–5 years if seed production is prevented.
A summer crop phase between two winter fallows is an effective tool to drive down wild oats numbers, allowing more options in the next winter crop season.
How can I reduce the risk of wild oats escapes in chickpea crops?
Short answer: Residuals + delayed planting + crop competition.
Longer answer: In-crop herbicides are limited to Group A in chickpea and there is widespread resistance to this mode of action in wild oats. This means that residual (pre-emergence) herbicides can be important to provide early weed control. Delayed sowing is another useful tactic that generally does not come with a yield penalty in chickpea. By delaying sowing the grower has the opportunity to apply a fallow management tactic to the first, and largest, germination of wild oats seed. Delayed sowing may have other benefits in restricting early crop biomass production and avoiding flowering during conditions when temperatures are too cold for pod set to occur.
Any effort to improve crop competition, such as narrower row spacing, will be rewarded with reduced weed biomass and seed set.
Wild oats can out-compete chickpea and deposit copious amounts of seed in the seed bank if left unchecked.
What are the best tactics to control wild oats escapes?
Short answer: Aggressively deploy patch management tactics.
Longer answer: Wild oats tends to establish in patches, which are particularly easy to see in chickpea crops. Use as many tactics as possible to keep weed seed set to a minimum, using non-Group A herbicides, harvest weed seed capture and even a longer (2 or 3 years) summer cropping phase. Each of these tactics on their own may only provide 50 to 60 per cent control but when applied in close succession they are likely to effectively control weed numbers. Swathing is one harvest weed seed control tactic that is used to good effect in cereals and although difficult to implement, wick-wiping with glyphosate in chickpeas is another option to consider to prevent seed set late in the winter crop phase. Infrequent but intense cultivation may also be an option for growers to regain control on a paddock or patch level.
Windrowing barley is an effective strategy to prevent seed set in wild oats.

The challenge of managing wild oats in northern region cropping (GRDC Update Paper)
Wild oats (DAFWA)

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Is it possible to apply additional competition to inter-row weeds?

with Hanwen Wu, Principal Research Scientist, NSW DPI
The inter-row space provides an ideal environment for weeds to grow, particularly if pre-emergent herbicides are not applied or are less effective than expected.
NSW Department of Primary Industries principal research scientist, Dr Hanwen Wu says filling the inter-row space with a productive species might be another way to suppress weed growth and reduce seed production of herbicide resistant weeds.

Dr Hanwen Wu, NSW DPI is investigating new ways to increase crop competition, particularly in the crop inter-row space.
“There is very strong evidence that narrower rows are an excellent way to increase crop competitiveness but there are some practical limitations,” he says. “We have looked at a combination approach, of planting most of the seed in rows and the rest broadcast to ‘fill in the gaps’. We have called this the ‘compound sowing technique’.”
In 2016 Hanwen set up two field trials at different locations near Wagga Wagga, NSW to assess the effect of this system on weeds and crop yield in both narrow (22.5 cm) and wider (45 cm) row spacings, with and without IBS trifluralin. Three broadcast species were evaluated – wheat, gland clover and French serradella. The two sites were assessed to have an initial annual ryegrass density of 48 and 25 plants/m2.
“We sprayed out the broadcast legumes in early September to prevent them competing with the crop for moisture,” says Hanwen. “Further trials are needed to test a range of parameters such as suitable legume species, optimal seeding rates, proportion of seed broadcast, row spacing for the conventional seeding and different pre-emergent herbicide options.”
Although the 2017 season did not allow Hanwen to replicate this trial he is keen to do more trials in 2018.
“We think this technique has merit and our initial trial suggested that weed suppression can be achieved without any yield penalty,” he says. “There even seems to be situations where a yield increase can be achieved in response to reduced weed pressure.”
Crop competition is rightfully attracting more attention from farmers and researchers in the war on herbicide resistant weeds. It is a numbers game and crop competition can play an important role in vastly reducing weed seed set.
Which was the most competitive broadcast species?
Short answer: Wheat.
Longer answer: At the weediest site the broadcast wheat treatment, without IBS trifluralin reduced annual ryegrass biomass by 71–77 per cent at both the narrow and wider row spacings. In the presence of less weeds the broadcast wheat still reduced weed biomass by 50 per cent in the narrow rows and 27 per cent in the wider row configuration.
IBS trifluralin further increased weed suppression at both sites and both row spacings. At the weedier site, annual ryegrass biomass was suppressed by 88–90 per cent. Where there were less weeds present the addition of IBS trifluralin increased biomass suppression from 27 to 70 per cent at the wider row spacing.

Compound sowing technique (conventional + broadcast sowing) dramatically increases crop competition in the inter-row compared to conventional sowing. Of the three broadcast species tested, wheat provided the strongest suppression on weed biomass.

What was the effect on yield?
Short answer: The wheat yield increased by 15–22 per cent at the weediest site when wheat was used as the broadcast species.
Longer answer: In the favourable season of 2016, only broadcast wheat generated a yield increase, and only in the presence of higher weed pressure. None of the broadcast treatments caused a yield reduction at either site. Further trials are required to evaluate the impact of site and seasonal climatic conditions on the weed control and crop yield associated with the compound sowing technique.
The broadcast legumes may provide additional soil fertility and moisture retention benefits while maintaining crop yields. More work is needed to identify more competitive legume species to have a greater impact on weed biomass and to identify the optimal timing to kill broadcast legumes to maximise weed suppression and minimise yield loss.

Using a broadcast legume that is sprayed out in September could have additional soil health and moisture retention benefits and warrants further investigation.

Have any farmers tried this idea?
Short answer: Yes.
Longer answer: Leigh Bryan at Swan Hill has tested this idea on his farm – he calls it zero-row spacing. Also in 2016, a strip-trial in barley resulted in the zero row spacing strip yielding 4.994 t/ha compared to 4.889 t/ha in the conventionally sown crop at 37.5 cm spacing. This was achieved with no pre-emergent or in-crop herbicide applied.
Leigh has noticed that the random placement of stubble is easier to sow through the next year and it still provides trellising for pulse crops and shades the soil to conserve moisture and reduce soil surface temperatures.

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Is one-time tillage a weed control option in a no-till farming system?

with Yash Dang, Senior Research Fellow, University of Queensland
Many would say that the widespread adoption of no-till and minimum till farming underpinned the expansion of cropping in the northern region and saved many farming families from economic hardship.
While costs and erosion damage were reduced, the heavy reliance on herbicides has resulted in a significant shift toward weeds that can thrive in this farming system. Weeds that were previously not considered a problem are now making farming unprofitable or impossible on some no-till paddocks.
Dr Yash Dang, University of Queensland research fellow, says ‘no-till’ does not have to mean ‘never till’. Occasional cultivation can be used as a weed control tactic without having a detrimental effect on the soil resource.
University of Queensland senior research fellow, Dr Yash Dang says the removal of cultivation has also led to an accumulation of certain immobile nutrients such as phosphorus, zinc and potassium in the dry surface layer of the soil where plant roots can not access them.
“Cultivation has a role in distributing nutrients, managing soil and stubble borne diseases and controlling certain weeds,” he says. “The complete removal of tillage for 15 to 20 years or more on some farms has led some farmers to the conclusion that they can not continue as no-till farmers.”
Yash undertook a 4-year project, starting in 2012, to investigate the effect of tillage on a range of soil properties in no-till paddocks throughout the northern cropping region – from Emerald, Qld to Dubbo, NSW.
“We applied tillage using disc and tyne implements and also tested the timing and frequency of tillage operations,” says Yash. “On well-structured soils there was no detrimental effect as a result of the cultivation. Even on more difficult soils, such as those with sodic subsoil or with hardsetting tendencies, one-off tillage operation at the correct soil moisture content caused only limited damage to the soil and repair was evident within two or three years.”
The research also demonstrated that cultivation is a viable weed control tactic within an otherwise no-till system, to prevent seed set of weeds such as fleabane and feathertop Rhodes grass, which flourish in a chemical-dominant control program.
“The positive effect on weed numbers is usually short-lived and has the potential to have negative effects in the years after cultivation,” says Yash. “Growers considering the re-introduction of occasional cultivation must consider all the pros and cons. Cultivation is just another tool in an integrated weed management system – not a stand-alone solution.”
How often should I use cultivation in my no-till system?
Short answer: As a last resort.
Longer answer: A move to no-till farming has provided significant benefits to the soil resource and to farmer’s profitability. This research does not suggest a return to full cultivation, however, the trials showed that one-off, occasional tillage does not have significant detrimental effects on the soil. Care is required in terms of timing, type and frequency of tillage on sodic soils and soils with hardsetting characteristics.
There is a soil moisture loss associated with cultivation so this should be taken into account when making the decision to cultivate.
The type of implement used for a one-off cultivation had little impact on the soil properties. Non-inversion tine or disc implements provided effective weed control when used before weeds flowered and set seed.
When should I use tillage?
Short answer: Before the weeds flower and set seed.
Longer answer: Cultivation should be a last resort measure to treat patches or paddocks where the weed pressure has reached unacceptable levels and where the species present do not have seed that remains viable for many years once buried.
Weeds such as fleabane and feathertop Rhodes grass are good candidates for occasional tillage as a means of preventing seed set. Numbers of these species can be driven down quickly through a dedication to preventing seed set for just a few consecutive years.
In most situations a single pass cultivation is sufficient to achieve the desired effect of reducing the target weed population. If the problem is severe then this single pass operation may be required for a few consecutive years. Timing is critical to achieve good weed control while not sacrificing a planting opportunity.
What other benefits can I expect from occasional tillage?
Short answer: A possible yield increase.
Longer answer: Cultivation will speed up mineralisation in the soil and the breakdown of organic matter. The distribution of immobile nutrients, such as phosphorus, potassium and zinc, deeper into the plant root zone. In a no-till system these nutrients tend to accumulate in the top few centimetres of soil, which is often too dry for plant roots to access. The release of nutrients may support short-term yield response.
Cultivation will also disrupt insect and disease cycles, potentially improving yield and reducing control costs. Pathogens causing diseases such as crown rot of wheat, yellow spot of wheat, ascochyta blight of chickpea and stalk rot of sorghum, can build up in the stubble and soil in a no-till farming system. Likewise soil insects such as Helicoverpa, armyworms and black field earwigs can proliferate in the surface soil.
The type of implement used for a one-off cultivation had little impact on the soil properties. Non-inversion tine or disc implements provided effective weed control when used before weeds flowered and set seed. Photo: L Wise
What are the risks associated with occasional tillage?
Short answer: Potential nutrient and soil moisture loss, and exposure to erosion.
Longer answer: Tillage is accompanied with a loss of soil moisture, however, at most of the trial sites stored moisture improved in subsequent years due to improved infiltration. If possible, cultivation should be done when there is a good chance of sufficient rainfall before the next planting opportunity. Mineralised nutrients and the soil itself is more exposed to runoff following cultivation.
In situations where weed seed has been buried previously, cultivation may bring these seeds back to the surface and initiate fresh germinations. Paddock history is an important consideration in the decision to cultivate.

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Using pasture phases to beat herbicide resistant weeds

with Tim Condon, Senior Consultant, Delta Agribusiness
Cropping a thousand hectares with a low weed seed bank is worth at least $20 thousand per year. Including a pasture or fodder phase can help achieve this while revving up the whole farming system.
Delta Agribusiness senior consultant, Tim Condon says the reason that including a pasture phase is one of the WeedSmart ‘Big 6’ tactics to manage herbicide resistance is the opportunity it provides to drive down weed seed numbers before returning to a cropping phase.
Tim Condon, Delta Agribusiness says adding a 2–3 year pasture phase provides growers with a number of additional weed control tactics to address herbicide resistance.
“The key is to always go into the crop phase with low weed numbers and also go into the pasture phase with low numbers,” he says. “The idea that pastures are an effective ‘reset’ after a weed blow out doesn’t always work.”
What does work is taking a planned approach right across the pasture phase and using a number of tactics known to be highly effective at preventing seed set. Several of the tactics available for use in a pasture phase can provide over 90 per cent control of the target weeds. The plan needs to outline how the pasture phase will fit into the crop rotation and what tactics will be used seasonally and rotationally to maximise the effect on weed numbers.
“The aim of the game is to establish dense, persistent and nodulating pastures,” says Tim. “Pastures offer the opportunity to have two to three consecutive years of no seed set. Go into the pasture phase with a clear plan for weed control and avoid the temptation in a tough winter to ‘drop the weed control program – because you need the feed’.”
Tim recommends incorporating fodder crops or short-term pastures for grazing as well as fodder conservation. The livestock options are flexible and can include running a livestock enterprise, buying in or agisting livestock for short-term grazing.
“Having livestock in the system can also influence your choice of other weed control tactics such as harvest weed seed control – where you can use the chaff dumps or chaff lines as an additional feed source,” he says. “Using heavy strategic grazing after applying glyphosate and prior to a double knock dose of gramoxone is also a very effective tool”
The weed control tactics that offer the greatest seed set prevention in pastures are hard winter clean, hay and silage. Using as many tactics as possible each year helps to target different weeds with both herbicide and non-herbicide control measures.
Two or three years of pasture also pays dividends with improvements in soil tilth, fertility and water infiltration as a result of growing persistent, nodulating, dense pastures.
What is the most effective winter clean tactic?
Short answer: A hard winter clean.
Longer answer: A hard winter clean is hard on the desirable perennials and annual legumes but very effective on weeds. If you choose to use a soft winter clean to preserve the legumes, be sure to follow up with another tactic.
For a hard winter clean to be most effective the pasture must be grazed very short prior to spraying. If there are patches of long vegetation or different weed species, consider spraying these areas separately, or twice, with the most effective chemicals. You can target both annual grasses and broadleaf weeds, but choose the herbicide mixing partners carefully.
A hard winter clean is the most effective way to end a pasture phase and to maximise the chances of re-entering the cropping phase with very low weed numbers.
What other tactics can be used?
Short answer: Spray topping.
Longer answer: Spray topping with either glyphosate or gramoxone early in the pasture phase can be used to start the process of driving down the weed seed bank. Timing is critical. For example, paraquat must be sprayed at flowering while glyphosate has a wider window of growth stages and can be more effective across a range of species at a single given application timing. Of course, a complete and timely spray fallow in the last year of the pasture phase is critical part of the process. Often a double knock with glyphosate followed by gramoxone can be employed to achieve 100% control.
What are the keys to growing dense, persistent and nodulating pastures?
Short answer: Attention to every detail!
Longer answer: A pasture phase is a good time to analyse the soil and take action to correct any acidity or other nutrient constraints. Zero or minimum tillage systems can lead to pH and nutrient stratification with sub-surface layers remaining acidic. Sampling and testing the 5–10 cm layer will identify if this is a problem. This is an ideal time to consider strategic tillage if this is an issue.
Decide on whether to establish the pasture with a cover crop or by direct seeding – both have pros and cons. Direct seeding is the best choice if the pasture mix includes a short term legume species or is grass based in drier environments.
If you choose to establish the pasture under a cover crop, the pasture must win this competition. Choose a crop that can be sown early and sow the crop in a north south direction at a low seeding rate. Remember that it is all about giving the pasture the competitive edge. Choose pasture species that are suited to your area and are weed suppressive.
For a pasture phase to be an effective weed control measure the pasture must be persistent, nodulating and dense.
What fodder conservation measures provide the best weed control?
Short answer: Both hay and silage are great for preventing seed set.
Longer answer: Using a combination of tactics works well. Silage is cut earlier in the season – so crash grazing after cutting followed by a spray fallow is highly effective. As is a pre-cutting application of glyphosate, particularly with hay making.
Other resources:

Strip grazing cattle to manage weeds
Grazing chaff heaps
Grazing strategies to reduce weeds

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How does mixing herbicide MOAs buy more shots?

When a herbicide is released on the market it has a fairly predictable number of uses or ‘shots’ before resistance to that mode of action begins to be evident in the weed population. For example, in-field experience and computer modelling both show that repeated use of glyphosate on a weed population will evolve full-blown glyphosate resistance in approximately 15 years. This effective lifespan ranges from around four years to over 20 years for the most commonly used herbicides, before resistance is evident.
Rick Rundell-Gordon, consultant agronomist with Grounded Agronomy, Swan Hill, says the widely-promoted and well-adopted practice of rotating between herbicide modes of action has the beneficial effect of ‘buying time’ because if a MOA is used once every two years the lifespan of the herbicide effectively doubles.
Rick Rundell-Gordon, Grounded Agronomy speaking at WeedSmart Week 2017 in Wagga Wagga about the science behind the ‘mix and rotate’ tactic to target multiple resistance mutations in weed populations.
“This means the herbicide remains a viable option for weed control for longer and the more diversity in herbicide MOAs applied, the longer the effective use is for all the herbicides in the program,” he says. “If a nil-tolerance approach is taken to weeds that survive a herbicide application then herbicide resistance is much less likely to evolve.”
“We now know that mixing two or more compatible herbicides with different modes of action can also increase the number of times an individual herbicide can be used within the herbicide program,” says Rick. “Mixing works by targeting different mutations within the weed population with the one spray application.”
For example, mixing trifluralin with another pre-emergent herbicide, both at full label rates, can increase the number of ‘shots’ of both herbicides across the cropping rotation. An important proviso is that the weed population must still have some susceptibility to all of the tank mix partners.
‘Mix and rotate MOA’ is so important it is one of the ‘Big 6’ WeedSmart strategies to manage herbicide resistance.
What evidence is there that mixing herbicide MOAs is effective in delaying herbicide resistance?
Short answer: Glyphosate resistance in waterhemp in the USA was less likely to occur when farmers used tank mixes.
Longer answer: Researchers from the University of Illinois, the USDA-ARS Global Change and Photosynthesis Research Unit, and the State University, New Mexico used spray records from a local spray contractor to compare 50 fields with glyphosate resistant waterhemp and 50 fields without. They looked at a total of 61 management and environmental variables and found that mixing herbicides was the single management strategy that made the most difference to whether or not glyphosate resistant waterhemp became a problem in any field.
In a review of herbicide application records from 2004 to 2006 and glyphosate resistance tests in 2010, the researchers found that adding more products to the tank at full rates for a single application causes the probability of resistance in these fields to decline sharply.
Peter Newman, AHRI says the evidence is mounting that ‘mixing and rotating herbicide MOAs buys you time and shots’.

When mixing herbicide modes of action, always use full label rates and ensure all products are compatible.
Is there any research to suggest that mixing pre-emergent herbicides could be effective?
Short answer: Computer modelling has demonstrated that the onset of herbicide resistance can be delayed when a mix and rotate strategy is used with pre-emergent herbicides to control annual ryegrass.
Longer answer: Based on research from the Australian Herbicide Resistance Initiative, the best advice to growers and agronomists is to rotate between these three groups of pre-emergent herbicides – 1. trifluralin, 2. Sakura, Boxer Gold and triallate and 3. propyzamide. Full label rates must be applied.
Assuming four herbicides available (trifluralin, prosulfocarb, pyroxasulfone and propyzamide) AHRI researcher Dr Roberto Busi has also simulated three different scenarios 1. use the same herbicide continuously (trifluralin or any other herbicide if the crop rotation permits), 2. follow a simple herbicide rotation pattern or 3. mix and rotate using two herbicides in each mix. The results show that mixtures are more effective than just rotating MOA in delaying resistance as mixes generally achieve a greater kill rate.
Rotate between the boxes, and avoid rotation between the blue boxes from year to year if possible. Mixing is also a good idea.
Is mixing and rotating herbicides all I need to do?
Short answer: No. Mixing and rotating herbicide modes of action can effectively lengthen the ‘life’ of a herbicide MOA on your farm but it will not prevent resistance on its own.
Longer answer: In addition to carefully selecting and managing herbicides it is necessary to also implement as many cultural (non-herbicide) tactics into your weed control program as possible. The overall aim of a sustainable weed management program is to use as many tactics as possible to keep weed numbers low, prevent weed seed set and remove all survivors.
Other resources:

Increase pre-em efficacy through a mix and rotate strategy
Using tank mixes to extend herbicide ‘life’

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Stewardship of synthetic auxin herbicides

with Roberto Busi, Research Fellow, AHRI
Globally, resistance to the world’s oldest herbicide is relatively rare. Unfortunately, one of the economically significant examples of auxin (2,4-D, dicamba and MCPA) resistance is in wild radish populations in Western Australia.
Roberto Busi, research fellow with the Australian Herbicide Resistance Initiative (AHRI), says the recent and future release of new crops with resistance to synthetic auxins (USA only) will heighten the risk unless stewardship measures are followed.
Dr Roberto Busi, AHRI research fellow says researchers around the world are working to better understand the way auxin herbicides work and the mechanisms plants use to evolve resistance.
“The synthetic auxin herbicides have been in wide-spread use since their discovery in the 1940s. According to a 2014 Dow AgroSciences report, herbicides such as 2,4-D and dicamba are used on about 200 million ha world-wide. They mimic the activity of natural plant hormones and seem to have multiple sites of action along with both physiological and biochemical effects that lead to impaired plant growth and death,” he says. “The rate of resistance evolution has been lower than other herbicide modes of action.”
“2,4-D resistant wild radish was first documented in Western Australia in 1999,” says Dr Busi. “Since then surveys have revealed some populations with high-level 2,4-D resistance and a total of 60 per cent of randomly surveyed samples in 2003 that has levelled out at around 74 per cent in both 2010 and 2015.”
Multiple resistance to at least two herbicides is quite common in wild radish populations in Western Australia, generally to chlorsulfuron and 2,4-D, however the last survey also showed alarming levels of resistance to diflufenican. However, resistance to atrazine, glyphosate or Group H herbicides remains rare or is not reported.
“Western Australian farmers have been able to find other ways to reduce the overall number of wild radish plants in their crops by running down the soil seed-bank and this is the correct way to overcome resistance,” says Dr Busi. “If auxin herbicide use were to increase or be used more frequently in Australian cropping rotations growers would need to implement non-herbicidal strategies that reduce the risk of herbicide resistance.”
Group I resistance has also been recorded in wild radish in South Australia, Victoria and New South Wales and in other species including Indian hedge mustard, sowthistle and capeweed.
Why did wild radish in WA evolve 2,4-D resistance when it is so rare globally?
Short answer: Wild radish in the northern WA wheatbelt infested paddocks at high density and the repeated use of Group B and 2,4-D rapidly selected for high levels of resistance.
Longer answer: Growers and agronomists are now well aware that simply swapping from one herbicide to another does not help manage herbicide resistance. This lesson was learnt the hard way in WA where 2,4-D initially provided excellent control of Group B-resistant wild radish. Failure ensued because 2,4-D was applied frequently to dense populations of wild radish, a species that is a prolific seeder and the seed can persist in the soil for many years. In two study populations we found that 2,4-D resistance was clearly inherited as a single, nuclear (pollen transmitted) dominant or near-dominant gene trait. This contributed to the speed of resistance evolution to 2,4-D in wild radish. In other species the traits conferring 2,4-D resistance may be less frequent or be less genetically dominant, and that slows the evolution of resistance.
Are 2,4-D resistant plants less fit than susceptible plants?
Short answer: Following a 2,4-D application, surviving weeds are less competitive.
Longer answer: True fitness penalty has not been established for 2,4-D resistant weeds. A ‘fitness penalty’ is when a resistant plant, growing in the absence of the herbicide is less fit than a susceptible plant.
AHRI research has demonstrated that when 2,4-D resistant plants are sprayed with 2,4-D they are suppressed, show strong symptoms of herbicide damage, but do not die. However, if these plants are sprayed when they are small and grow in competition with a crop, the resistant weeds may die or may produce far less seed than it would in a non-competitive situation.
Crop competition can therefore be used as a weapon against 2,4-D resistant weeds while continuing to use 2,4-D or MCPA as part of a tank mix.
The release of new auxin herbicide formulations in Australia means farmers have more options for broadleaf weed control but researchers emphasise the heightened need for stewardship to reduce the risk of resistance and the importance of crop competition in managing resistance.
How can tank mixes extend the use of herbicides that weeds are resistant to?
Short answer: Multiple resistance to auxin herbicides and other MOA is rare in individual plants. Focus on low seed-banks, use herbicide mixes and mix it up with non-herbicide strategies to keep ahead of resistance.
Longer answer: The most important objective is to aim for low weed numbers. In this situation, while a population of wild radish in a paddock may be resistant to a number of herbicide modes of action the chance of an individual plant being resistant to more than one MOA is very low. In plants such as wild radish that are cross-pollinated there is a lot of mixing of the genetic material so stacking of resistant traits is initially uncommon, particularly for resistance traits that are recessive. Consequently, if a tank mix of three compatible herbicides with different modes of action is applied, with each herbicide at robust label rates, an individual plant is highly likely to be susceptible to at least one of the herbicides applied and so overall plant numbers are reduced through the use of the tank mix.
Relevant links

AHRI Insight – Left jab, right hook
Group I resistance in Australia
Spray small multi-resistant radish twice

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Taking the pressure off glyphosate across crop rotation

Now is the time to take pre-emptive action to reduce the incidence of glyphosate resistance in weeds.
Eric Koetz, NSW DPI weeds research agronomist, says the majority of agronomists and a growing number of farmers recognise the need to implement management practices that help protect the efficacy of glyphosate in farming systems.
“We know that it takes between 14 and 19 years of routine use of glyphosate to evolve resistance,” he says. “We have now had Roundup Ready cotton for 15 years and there are many instances of glyphosate resistant populations of weeds such as fleabane, windmill grass and barnyard grass.”
“The 2 + 2 + 0 strategy developed in the cotton industry to protect glyphosate and Roundup Ready technology is also applicable to other farming systems,” says Mr Koetz. “When planning an integrated weed management program, the Cotton RDC recommends including two non-glyphosate tactics in-crop, two non-glyphosate tactics in the fallow and ensuring zero survivors.”
This is where the use of residual herbicides can play a part, along with non-herbicide tactics, such as strategic tillage in both summer and winter crops and in fallow situations. Including residual herbicides in both the fallow and crop phases increases the diversity of herbicide modes of action and places downward pressure on the weed seed bank.

Eric Koetz, NSW DPI weeds research agronomist says residual herbicides need to play a part in the control of glyphosate resistant weeds in crop and in the fallow.
“Roundup Ready technology has been of great benefit to the cotton industry, and has a fit in other farming systems too, but it can not stand alone. It must be supported and protected through an integrated weed management strategy,” says Mr Koetz. “To preserve glyphosate it is necessary to reduce the total number of applications across the crop rotation.”
What are the non-glyphosate options for in-crop weed control?
Short answer: Residual herbicides applied at sowing, inter-row cultivation, crop rotation, maintaining ground cover and increasing crop competition.
Longer answer: Residual herbicides that require incorporation are best applied at sowing. There are some older herbicides that have not been used for several years that are still quite effective and could make a come-back to farming systems that have come to over-rely on glyphosate. There are also new use patterns being registered for a number of different products that can be used to add diversity to control weeds at different growth stages or to control later in-crop germinations. Increasing crop competition and promoting rapid canopy closure has a significant impact on in-crop germination. Diversifying the crops grown automatically makes more herbicide options available to use against weed populations that may be evolving glyphosate resistance.

Residual herbicides applied at seeding or soon after harvest help reduce the number of glyphosate applications across the cropping cycle.
What are the non-glyphosate options for controlling weeds in the fallow?
Short answer: Residual herbicides applied soon after harvest, strategic tillage, double knock, optical sprayer, cover cropping.
Longer answer: Choose residual herbicides carefully. Some products have long plant back periods and will reduce the grower’s options for the following season.
Tillage is widely practiced in irrigated cotton and is an effective way to eliminate weeds. In dryland systems tillage operations may be best suited to low crop residue situations such as following a chickpea crop. The cultivation operation may be done across the whole paddock or in small patches and can be coupled with paddock renovation, the incorporation of soil ameliorants or deep banding of nutrients. Several research projects are currently investigating the usefulness of cover cropping and brown manuring on weed numbers.
What can be done to ensure there are no survivors?
Short answer: Scouting and chipping, optical sprayer, patch tillage.
Longer answer: Actively looking for survivors must become a key management practice. A few large plants that have survived all control tactics can generate a huge number of seeds that are very likely to carry some level of herbicide resistance. If these plants are physically removed before they set seed they will not contribute to the seed bank for future germinations.

Ask an Expert

Can I retain stubble and keep weeds under control?

Tony Swan, Senior Experimental Scientist with CSIRO Agriculture and Food says growers can manage stubble without compromising weed, disease and pest management or the timeliness of the seeding operation.
Tony Swan, Senior Experimental Scientist with CSIRO Agriculture and Food says crop sequence, including a double-break, is an effective way to manage weeds, improve profit and manage stubble residues in southern grain farming systems, with or without livestock.
“Decisions at harvest, post-harvest/pre-sowing and at sowing influence the success of a seeding operation into stubble,” he says. “With a flexible approach, growers can manage stubble loads to suit their seeding operations, weed control tactics, herbicide choice and profit.”
“As a rule of thumb, the stubble load after harvest is 1.5 to 2 times the grain yield for wheat and 2 to 3 times the grain yield for canola,” says Mr Swan. “A high stubble load can create issues for all types of seeding systems by restricting herbicide choice, effectiveness, contact on the soil or weed target and even reduce crop emergence.”
In 2014 an experiment was established at Temora, NSW in a paddock with high levels of Group B resistance in annual ryegrass. The trial compared the yield, profit and annual ryegrass (ARG) population status in three management strategies in a no-till (tine opener) or zero tilled (disc opener) farming system.
Introducing more crop and herbicide diversity into the farming system generated a higher average net margin over three years while reducing the seedbank from 1864 plants per m2 to 351 plants per m2 in just 24 months. Following the wet 2016 season, with a soft late finish, three years of the diverse strategy (which included a double break) reduced the ryegrass seed bank by 70% compared to the aggressive (high input) strategy (145 cf 573 seeds/m2), while the conservative (low input) strategy increased the seedbank by 600% to above 4000 seeds/m2.
“Crop sequence is an effective way to manage weeds, improve profit and manage stubble residues in southern grain farming systems, with or without livestock,” says Mr Swan.
Mr Swan will be presenting his research into double-break cropping at the WeedSmart Forum on 21 August, in Wagga Wagga. Go to the website to register.
Does choice of seeder type influence weed control?
Short answer: In our experiment, after 3 years ARG seed numbers were generally lower when crops were sown with a tine seeder.
Longer answer: Averaged across the three management strategies, ARG seedbank populations were lower in tine seeded plots in 2016 (650 seeds/m2 in tine cf 1080 seeds/m2 in disc) and 2017 (384 seeds/m2 in tine cf 944 seeds/m2 in disc).
However, the use of expensive pre-emergent herbicides eliminated any significant difference in ARG seedbank populations in crops sown with a disc or tined seeder. In the conservative strategy where trifluralin can not be used with a disc seeder (not on label), the result was a higher ARG seedbank population (4045 seeds/m2) with a disc seeder compared to 1840 seeds/m2 with a tine seeder.
By February 2017, following the 2016 decile 9 season the lowest average ARG seedbank population was found in the diverse cropping strategy sown with a tine seeder (82 seeds/m2). In the conservative strategy weed populations continued to increase when sown with a tine seeder (2322 seeds/m2) and with a disc seeder (7631 seeds/m2).
Tony Swan inspecting the stubble management trial plots, looking at the ‘Conservative (lower input)’ second year wheat treatment sown with a tine seeder (left) compared to the ‘Conservative’ second year wheat treatment sown with a disc seeder where trifluralin could not be applied (not on label for disc seeders).
What options do I have if stubble is too thick to sow through?
Short answer: Reduce stubble load by grazing, baling, mulching, incorporating with nutrients, or use a strategic late burn.
Longer answer: Tine seeding systems will struggle to establish crops into large stubble loads >6 t/ha. While disc seeders may penetrate the stubble more easily and plant crop seeds into high stubble loads, patchy crop establishment and ineffective herbicide application can result in any seeding system sown into high stubble loads. Retaining stubble, with all its benefits, should not be allowed to compromise effective weed control. Mulching, incorporation and grazing all potentially retain more nutrients in situ however mulching or incorporation are likely to lead to nutrient tie-up in the soil unless additional nutrients are added.
Grazing and late burning both improve in-crop nitrogen availability, and often the yield of the following crop. It is not recommended to grow wheat after wheat unless the stubble from the first wheat crop is burnt or grazed, or supplementary nitrogen is applied to offset N immobilisation. To try and avoid burning at all costs, the best option is to sow a diverse crop sequence and use the legume crop to reduce the cereal stubble.