Ask an Expert

Agronomists, researchers, growers answering important questions

Article
Ask an Expert

What can I do at harvest to reduce my future weed burden?

As crops mature and harvesters begin reaping, consider the potential fate of seeds ripening on weeds that escaped in-crop control measures.
Peter Newman, WeedSmart’s western extension agronomist, says harvest time is an important opportunity to assess weed burden across the farm and be proactive about driving down the weed seed bank.
“Harvest can either be a super-spreader or a weed suppressing event,” he says. “Small patches of weeds can quickly expand when seed is blown out the back of the harvester. On the other hand, the harvester can be a powerful weed management tool if any one of the harvest weed seed control options are implemented.”
WeedSmart’s western extension agronomist, Peter Newman says efforts made to reduce the spread of weed seed at harvest will soon pay off for growers.
Australian growers have led the world in inventing and adopting harvest weed seed control tools such as impact mills, chaff carts, chaff decks and chaff lining, all of which can reliably destroy over 90 per cent of the weed seed that enters the front of the harvester.    
“In addition to harvest weed seed control there are several other actions in the WeedSmart Big 6 that growers can implement just prior to, during and immediately after harvest that will make a measurable difference to the weed burden in future growing seasons,” says Peter. “The WeedSmart Big 6 tactics are scientifically-proven to reduce the risk of herbicide resistance through diverse herbicide use and cultural control to prevent weed seed set.”
What can I do before harvest to manage late emerged weeds?
In brief: Scout for and map weedy patches. Consider sacrificing small areas of high density weeds. Swathing can be a very effective way to stop seed set of late emerged or resistant weeds. Collect weed seeds for herbicide susceptibility testing.
The details: Growers across Australia use a variety of methods to map weeds – from the simple to the sublime. ‘Dropping a pin’ using the tractor’s GPS mapping system as you travel through a weedy section when spraying or harvesting is easy and provides useful information about the distribution of weeds in the paddock. Many growers have their own drones and use them the collect images or video footage of the crop that can be viewed or analysed to identify high density weed patches.
Collect seed for herbicide susceptibility testing – knowing what still works is vital information for planning next season’s herbicide program. There are three herbicide testing facilities in Australia that are equipped to test weed seed samples – Plant Science Consulting, CSU Herbicide Resistance Testing and UWA Herbicide Resistance Testing.
Collecting weed seed before or at harvest is the most common method used. The collected seed must be mature, from green to when the seed changes colour. Before harvest, collect 30 to 40 ryegrass seedheads or several handfuls of wild oats seed. After harvest, it is common to find seedheads still in the paddock or samples of contaminated grain can be sent for analysis.
Keep samples from different locations separate and details noted on the bag. Only use paper bags (double layer) to collect and send seed samples. Ensure bags are sealed so that the samples don’t mix during transit.
Which harvest weed seed control tool is best for my situation?
In brief: There are six harvest weed seed control tools used in Australia – impact mills, chaff decks, chaff lining, chaff carts, bale direct and narrow windrow burning. Choose the one that best suits your system and budget.
The details: Impact mills are best suited to continuous cropping situations. Residues are retained and evenly spread. Chaff decks have lower capital cost and are well-suited to controlled traffic situations. Chaff carts are popular with grain producers who also run livestock. Bale direct is also expensive but has a good fit in locations where there is access to straw markets. Chaff lining is currently the best ‘entry level’ system and can be used in CTF or non-CTF systems, with best results where the harvester runs on the same track each year. Chaff lining has essentially superseded narrow windrow burning, overcoming the time required and risks involved in burning and reducing the loss of nutrients from the system.
If you haven’t used harvest weed seed control tools before, it doesn’t take long to build and fit a chaff lining chute ready for use this harvest season.

What should I be ready to do straight after harvest?
In brief: Spraying weeds immediately after harvest is fairly common practice. Weeds present may be close to maturity or fresh germinations of summer-active weed species.
The details: Some growers get in early with knockdown herbicide applied under the cutter bar when swathing barley or canola. Consider using the double knock strategy, heavy grazing pressure and possibly a soil residual herbicide that is compatible with your planned crop rotation. Pay particular attention to any weedy patches identified before or during harvest. Stopping seed set at every opportunity is the crux of an effective weed management program.
Give some thought to what might be the underlying cause of weedy patches – fixing problems such as pH and soil nutrition imbalances, waterlogging and spray practices that routinely deliver low doses of herbicide.

Article
Ask an Expert

How can I set up my summer crops to help manage weeds?

Belinda Chase, agronomist at Dalby Rural
It has been a few years since growers on the Darling Downs have been able to plan for a summer crop. With the uncertainty surrounding planting opportunities came some hesitancy to fully invest in weed control.
Belinda Chase, an agronomist at Dalby Rural Supplies, says it is a welcome change to be actively planning for summer cropping and being more proactive with weed control leading up to planting.
“Our main weeds here over summer are feathertop Rhodes grass, barnyard grass and broadleaf species like flaxleaf fleabane and peach vine,” she says. “In terms of known resistance we have some Group 1 [A] and glyphosate resistance in barnyard grass and paraquat plus glyphosate resistance in tall fleabane. Annual ryegrass seed in hay from other states has also established in small areas, potentially bringing resistance that has evolved elsewhere.”
With more soil moisture than growers have had for several years, many have been planning ahead to manage weeds in winter fallows destined from summer crops – mainly cotton and sorghum, along with some millet, corn, mungbean and sunflower.    
“Pre-emergent, or residual, herbicides have played an important role in keeping weed numbers low in winter fallows,” says Belinda. “The next rain event will most likely also be the trigger for planting, and weed control in those early weeks of crop growth will be crucial.”
The WeedSmart Big 6 tactics can be applied to all crops to reduce the risk of herbicide resistance through diverse herbicide use and cultural control to prevent weed seed set.
What are the resistance risks with pre-emergent herbicides in summer crops?
In brief: Avoid over-using the same mode of action group throughout the crop rotation.
The details: There has been widespread use of knockdown herbicides with residual activity, such as Group 14 [G] flumioxazin (e.g. Valor), in winter fallows to maintain low weed numbers. With multiple use patterns in fallow (on its own or as a tank mix ‘spike’) and post-sowing pre-emergent in a number of summer crops, there is a risk that resistance to this mode of action could develop. There is currently no known resistance to Group 14 [G] herbicides in Australia but resistance has been identified in both grass and broadleaf species in other countries.
Valor has a plant back period of one month pre-sowing for pigeon pea, maize, sorghum and navy bean, two months pre-sowing for cotton, sunflower and mungbean and up to nine months pre-sowing for canola. It can be applied after sowing and pre-emergent to protect the crop as it establishes.
Tank mixes and co-formulations are an effective way to reduce the risk of resistance to Group 14 [G] herbicides. Significant escapes must be prevented from setting seed.
Another option for pre-emergent weed control in sorghum is s-metolachlor, a Group 15 [K] herbicide such as Dual Gold. Applied as a split application before and immediately after planting sorghum gives optimal weed control, provided the seed safener (Concep II or Epivio C) has been applied at planting.
Atrazine, a Group 5 [C] herbicide, also provides a pre- and post-emergence option for broadleaf weeds and some pre-emergent control of annual grasses like barnyard, crowsfoot, summer and love grass in crops such as sorghum and maize.
How can I best incorporate imi-tolerant summer crops?
In brief: Imi-tolerant (IT) hybrids are now available for sorghum, maize, sunflower and soybean, providing for alternative in-crop control options for a range of grass and broadleaf weeds.
The details: IT sorghum seed is in short supply this year, but there are other options to consider. Growers who have Group 1 [A] resistance in barnyard grass or who are looking for alternative products to control or suppress other key summer weeds may consider growing these herbicide-tolerant crops.
With an imi-tolerant hybrid, growers will be able to safely plant into paddocks where imidazolinone (Group 2 [B]) chemistry has been recently applied, say after an IT maize, wheat, canola or pulse crop, negating the current 12+ month plant-back requirement, which otherwise restricts the choices that growers have in summer. In this situation it is advisable to avoid applying Group 2 [B] herbicides to the current crop to break the ‘imi-cycle’ that can develop where imi herbicides are used in successive crops – resulting in a high risk scenario for the evolution of herbicide resistance.
The second major benefit is that an imi-tolerant hybrid can be planted into a paddock that has a broadleaf weed burden—a situation that would not be advisable for a non-IT variety.
Imi-tolerant (IT) hybrids are now available for sorghum, maize, sunflower and soybean.
Is patch management a good tactic for feathertop Rhodes grass?
In brief: Feathertop Rhodes grass is a serious challenge, but summer cropping can help reduce the weed seed bank.
The details: Controlling FTR can be easier in a competitive mungbean crop on narrow row spacing, than in sorghum and sunflower, which are typically grown on wider row spacings and do not quickly cover the inter-row.
Controlling feathertop Rhodes grass can be easier in a competitive crop on narrow row spacing, where the crop quickly covers the inter-row.
After applying suitable pre-emergent herbicides (if FTR numbers are low) or cultivating if FTR pressure is high, do everything you can to maximise crop competition. Consider narrower row spacing, optimal plant density, avoid planting gaps and provide adequate crop nutrition.
After the crop has established, use inter-row cultivation where necessary and harvest early. If there are areas with low density escapes of this prolific seeding weed, chipping is worthwhile before the seed sets. If patches of escapes are evident it can be very beneficial to accept a ‘short-term pain for long-term gain’ by ploughing or spraying out the affected patch of crop before the FTR plants set seed.
At harvest, record any patches of FTR and return soon after harvest to remove established plants and apply a pre-emergent herbicide to manage new germinations.
Further information

Feathertop Rhodes grass and sorghum – does this work?
Avoid getting stuck in the imi-cycle
Competitive sorghum to reduce FTR and BYG pressure

Article
Ask an Expert

What strategies will rein in wild turnip in the northern region?

Although an emerging weed in the northern grain growing region, wild turnip steals crop yield and requires additional control measures that together cost over $10 million every year across Australia.
Bhagirath Chauhan, Professor in Weed Science at The University of Queensland, Gatton, says recent ecological studies have shed light on tactics that growers can use to rein in this invasive weed that could become a threat, particularly in production areas with marginal soil moisture in the northern region.
Bhagirath Chauhan, Professor in Weed Science, The University of Queensland, Gatton says if wild turnip plants are prevented from setting seed, it is possible to rapidly deplete the seedbank in a no-till system using a 6 to 12-month fallow and or competitive cropping.
“Wild turnip is considered a winter weed, but with sufficient soil moisture and mild temperatures it can also establish and set seed over summer,” he says. “Some biotypes of Brassica tournefortii have evolved resistance to chlorsulfuron, and other Group 2 [B] herbicides, first identified in South Australia in 1996.”
With investment from the GRDC, Dr Gulshan Mahajan conducted the recent studies on four biotypes collected in the northern region investigated the differences in seed dormancy, drought tolerance, effect of competition by wheat and chickpea crops, and seed persistence on the surface and at various burial depths.    
“These experiments clearly demonstrated the invasive capability of wild turnip,” says Dr Chauhan. “This species can produce vast quantities of seed with variable dormancy, meaning there can be multiple germination cohorts, mainly associated with rainfall events. However, it is a poor competitor when faced with a crop such as wheat that achieves canopy closure quickly.”
‘Grow competitive crops’ is one of the WeedSmart Big 6 tactics, providing season-long weed suppression and maximising the value of early weed control efforts.
How long does wild turnip seed persist in the seedbank?
In brief: Seed persists for up to 18 months on the soil surface and 5 per cent of seed was still viable after being buried at a depth of 2 cm for 30 months.
The details: Fresh seeds initially have high dormancy when placed on the soil surface. The seed coat extends dormancy of fresh wild turnip seed and light inhibits germination.
Once the seed coat has degraded somewhat, seedlings readily emerge from the surface after rainfall events, generating multiple cohorts between February and October. Emergence peaks from March to May, potentially challenging crops sown from the end of April to June.
Keeping the weed seed on the surface in a no-till system and minimising soil disturbance at planting, coupled with pre-planting knockdown, pre-emergent herbicide and vigorous early crop growth can reduce germination and weed seed production in-crop.
Germination rates of up to 14 per cent occurred when seed was buried at a depth of 2 cm in soil with sufficient moisture (>25 per cent off water holding capacity) and alternating day/night temperature of 25/15 °C.
Wild turnip seedlings did not emerge from a depth of 5 cm. A one-off deep tillage event could be an effective control tactic to bury the existing seedbank, provided there was no seedbank replenishment or subsequent tillage.
A single, large wild turnip plants can produce 10,000 seeds (left). Wild turnip can become a problematic weed in no-till systems because emergence of seeds in the surface layer is greater than for buried seeds (right).
How much seed does a wild turnip plant produce?
In brief: A wild turnip plant growing in a fallow or fenceline situation can produce around 10,000 seeds.
The details: Early emerged cohorts achieve greater plant height and shoot biomass, resulting in greater seed production than later emerged cohorts. The early emerged plants also enjoyed a longer growing season, reaching flowering stage after 87 days while later emerged plants reached flowering after only 70 days.
Although water stress (25% WHC) reduced the seed production to 3000 seeds per plant, this is still ample seed to establish an infestation capable of reducing crop yield.
Wide-row and slow growing crops such as chickpea do not inhibit wild turnip growth or seed production. On the other hand, a fast growing, dense wheat crop suppressed weed growth and seed production by 78 per cent for the early sown crop (15 May), 96 per cent for the crop sown on 5 June, and 65 per cent for the late sown crop (25 June). This reduction in seed production was achieved without the application of herbicide. The vigorous growth of the wheat crops sown on the latter two planting dates prevented wild turnip plants from producing enough seeds for re-infestation.

What is the best strategy to drive down wild turnip numbers?
In brief: Pre-plant knockdown, delayed sowing, pre-emergent herbicide and a fast growing, competitive crop.
The details: Wild turnip can, and has, become a problematic weed in no-till systems because emergence of seeds in the surface layer is greater than for buried seeds. The retention of stubble supports higher soil moisture at the soil surface, creating a favourable environment for germination over a long period. However, if emerged plants are prevented from setting seed, it is possible to rapidly deplete the seedbank in a no-till system during a 6 to 12-month fallow.
Shallow tillage may result in the buried seeds remaining viable for more than 2.5 years, with the potential for seed to be brought to the surface during subsequent planting operations, triggering the re-infestation of the paddock.
Avoid slower growing and wide-spaced crops such as chickpea in paddocks with a large wild turnip seedbank.

Article
Ask an Expert

How does ryegrass adapt so readily to farming practices and environmental changes?

Annual and perennial species of ryegrass (Lolium spp.) are weeds of major and global significance in cropping systems. Native to temperate regions of Europe, Asia and North America, these species have been transported, mostly as pasture plants, turf, cover crops and as contaminants in crop seed, feed grain and hay, to all grain production areas of the world.
Dr Chris Preston, Professor, Weed Management at The University of Adelaide, says perennial ryegrass, Italian ryegrass and rigid ryegrass can be difficult to distinguish and have the ability to interbreed – giving the species increased invasive powers.
Dr Chris Preston, Professor, Weed Management at The University of Adelaide, says that unlike some other weed species, ryegrass populations adapt to new environments very quickly.
“The genetic diversity of the ryegrass species has seen populations adapt very quickly to altered environments,” he says. “The most widely researched adaptations have been those associated with herbicide resistance, but we are also seeing many other examples of ryegrass evading cultural controls, adapting to new farming systems and extending its geographical and climatic range.”
Previously considered a weed of southern farming systems with Mediterranean climates and winter dominant rainfall, ryegrass is becoming increasingly common in more northernly locations with summer dominant rainfall patterns.
“Unlike some other weed species, ryegrass populations adapt to the new environment very quickly,” says Chris. “The extensive genetic diversity means populations can readily adapt to new environments and stresses. This is aided by ‘new arrivals’ that may bring new adaptations, such as seed dormancy or herbicide resistance, which have evolved elsewhere.”
Ryegrass is a dramatic example of why the WeedSmart Big 6 approach is so important – adding diversity to farming systems, both within and between seasons. There is no ‘set and forget’ integrated weed management system – every season needs to present this super-weed with a fresh challenge.
What is the best way to keep ahead of ryegrass blow-outs?
In brief: Longer and more diverse rotations.
The details: Short rotations are very easy for weeds like ryegrass to adapt to. This is seen in its ability to adapt to multiple herbicide modes of action and also to make definite shifts in the population’s phenology.
If a tight rotation has been in place for 10 or 20 years it’s definitely time to look for alternatives. Adaptive species like ryegrass will start to respond to repeated practices (herbicide and cultural) that are applied for four or five years in a row.
In a tight rotation, ryegrass can evolve resistance to early sowing in a no till system through seed dormancy, or resistance to harvest weed seed control through early shedding of seed. Each agricultural practice is in fact applying selection pressure – the only solution is to frequently alter the type of selection.
The worst thing you can do is to keep doing the same thing. If you are limited in crop choice, then consider changing other practices used regularly within each crop.
In short rotations, annual ryegrass can rapidly evolve to evade routine practices.
Why is it important to have diverse crop rotations?
In brief: To keep ahead of adaptation through seed dormancy.
The details: Pre-emergent herbicides have become an important part of a diverse herbicide program for ryegrass control. Ryegrass can and will evolve resistance to specific pre-emergent herbicide modes of action, but it can and will also adapt mechanisms to avoid pre-emergent herbicide activity, such as through altered seed dormancy.
If the pre-emergent herbicide is applied at the same time each season it will not be long before the dominant population is germinating later in the season, having not interacted with the herbicide at all.
In this situation, there is an even greater need for the crop to be highly competitive by the time the more dormant seeds germinate, to suppress weed growth and seed production.
Rotating to pasture or to crops sown later will disrupt the selection for increased dormancy.
Again, maximising the diversity in the crop rotation is the foundation of an effective integrated weed management program.
Are there things I should do every year?
In brief: All the WeedSmart Big 6 tactics need to be applied as often as possible.
The details: But there needs to be diversity within years as well. For example, harvest weed seed control is recommended for all paddocks, every year – so the diversity needs to come through other tactics, such as rotating crops and rotating herbicides.
Just as with herbicides, harvest weed seed control alone will not provide long term control of ryegrass.

Resources

Review: evolutionary drivers of agricultural adaptation in Lolium spp., Maor Matzrafi, Christopher Preston and Caio Augusto Brunharo, 2021, Pest Management Science

Article
Ask an Expert

What impact does dry seeding have on pre-emergent herbicide efficacy?

As is usually the case, some cropping regions across Australia have experienced excellent conditions leading up to planting while in other regions growers are still looking to skies hoping for rain.
Mark Congreve, Senior Consultant with ICAN says understanding the interaction of pre-emergent herbicides and the level of soil moisture at seeding goes a long way when it comes to deciding which herbicides to apply in dry seeding conditions.
Mark Congreve, senior consultant ICAN
“Rainfall after application assists with incorporation of most pre-emergent herbicides,” he says. “The soil moisture at the time of sowing also affects the mobility of some products into soils of different textures and the volatilisation of some products.”
When seeding needs to start prior to significant breaking rains and without the forecast of imminent rainfall, it is even more important than ever that the maximum amount of product reaches the soil surface.
“To achieve this, use large coarse droplet size, higher pressure and higher water rates – around 100 to 120 L/ha, and preferably solid over air-inducted droplets,” says Mark. “The aim is to minimise the amount of product left on stubble as you can’t rely on rain to wash it off the stubble and into the soil. Narrow fan nozzles assist in reducing stubble capture and if the rig is travelling at 20 km/hr or more consider rear facing nozzles to reduce the forward momentum of these large heavy droplets.”
Pre-emergent herbicides are an important component of many herbicide programs. In the WeedSmart Big 6 approach to weed management these herbicides are never left to do the heavy lifting alone – to do so exerts immense the selection pressure on these herbicides. Applying pre-emergent herbicides effectively is the subject of an online ‘Diversity Era’ course.
What do I need to think about in a dry seeding situation?
In brief: Getting the product onto the soil surface, incorporating by sowing and potential environmental losses.
The details: Stubble can intercept herbicides so if there is no rain forecast it is necessary to use other strategies to get the product onto the soil surface.
The seeder will be doing most of the work to incorporate the product so ensure excellent soil coverage of herbicide in the inter-row, especially for those herbicides that are more prone to breakdown or loss through UV exposure, volatility and stubble binding.
Many of the ryegrass pre-emergent products can be damaging to the emerging crop so it is important to also ensure that the seeder adequately moves treated soil out of the planting line.

What happens when it does rain?
In brief: Pre-emergent products will behave differently when it rains, according to the soil texture and soil moisture.
The details: For example, in coarse sands rainfall will readily wash the herbicide down the profile, potentially below where the weed seeds are sitting in the profile. If the soil has some moisture at planting, or is of finer texture, the movement down the profile will usually be less pronounced.
If the first rainfall event is quite heavy, there can be both vertical and sideways movement of the product into the crop row where it can cause crop damage.
Consider the mobility of the product you plan to apply. Highly mobile herbicides will move a considerable distance after the first rainfall event and have a greater risk of causing crop damage, however less mobile herbicides can still move further than predicted if the soil is porous and the first rainfall is substantial.
Where rainfall continues to be low and the herbicide treated soil remains relatively dry during weed germination, the efficacy of all pre-emergent herbicide will be reduced. Herbicides that have either higher solubility, or some vapour activity, may perform slightly better under these conditions. Low solubility herbicides that rely on the roots taking up herbicide dissolved in the soil water are likely to be the most compromised under these ongoing dry conditions.

What about mixtures and crop rotation?
In brief: Mixtures and rotation are key to managing herbicide resistance in weeds.
The details: Mixtures can broaden the activity on the weed spectrum present, and in some situations the herbicides in the mix can have an additive effect in terms of weed control.
Pairing products with different mobility may increase the chance of effective control in uncertain conditions. On the downside, there may be a higher risk of crop damage if a more mobile herbicide is included in the mix.
If using two different mode of action herbicides they are likely to breakdown independently and therefore it is unlikely that ‘mixing’ will result in increased length of residual control.
A third consideration is the frequency of use of a certain product over time, even in mixes. Resistance can still increase due to selection pressure if there are survivors allowed to set seed. There can also be a build-up of the microbes that breakdown certain products and this can speed up the degradation process if the same product is used frequently in mixes.
To avoid some of these pitfalls, try to plan a 5-year herbicide program that includes as much diversity as possible in both pre- and post-emergent herbicides and knockdowns.
Example 5-year rotation
Resources

Pre-emergent herbicide: a review of what we know and need to know (GRDC Update paper)
Understanding pre-emergent herbicides and how they interact with the environment (GRDC Factsheet)
Soil behaviour of pre-emergent herbicides in Australian farming systems (GRDC Manual)

 

Article
Ask an Expert

How can I maximise grass weed control using hybrid canola?

WeedSmart Audio · How to maximise grass weed control using hybrid canola
A competitive canola crop can halve grass weed seed set. To achieve this you need a vigorous variety, robust pre-emergent herbicide and the best agronomic package.
EPAG Research agronomist, Andrew Ware, says the results of six years of trial work across the southern growing region with GRDC investment has demonstrated how important crop nutrition and time of sowing are for maximising canola vigour and yield.
EPAG Research agronomist, Andrew Ware, says crop nutrition and time of sowing are critical for maximising canola yield and vigour to out-compete weeds.
“Canola requires 80 kg nitrogen per tonne of grain produced,” he says. “Growers will be rewarded if they set a target yield and fertilise accordingly. The other key factor is to sow early – in April, through to early May for some varieties, so that start of flowering occurs in the optimal window.”
For growers in South Australia, 2021 brings the opportunity to grow many additional hybrid varieties, with an expanded range of herbicide tolerance traits, through the removal of the GM moratoria.
“Roundup Ready (RR) canola can be grown in South Australia for the first time this year,” says Andrew. “Herbicide resistance testing should be a key part of the decision to grow any herbicide tolerant crop as there is little advantage in growing a tolerant crop if the key grass weeds have evolved resistance to that herbicide.”
Canola offers growers the opportunity to tick off all the WeedSmart Big 6 tactics for reducing the impact of herbicide resistant weeds on farming systems – diversity in crops, mix and rotate chemistry, double-knock, competitive crops, stop seed set and harvest weed seed control.
What has changed in canola agronomy in the last few years?
In brief: Growing hybrid canola requires higher management input to gain the full benefits of the higher investment in seed.
The details: Hybrid canola varieties are generally very competitive against weeds, particularly when sown in the correct window and supported with a robust pre-emergent herbicide package to give the crop a head start.
Canola yield is optimised when the time of sowing results in the crop flowering and setting pods when there is minimal heat, moisture and frost stress.
Growers now have the choice of hybrid canola varieties with several new trait combinations for weed control. Genetically modified (GM) Roundup Ready (RR) canola now joins the offering in South Australia where hybrid and open pollinated imi-tolerant (Clearfield) and triazine-tolerant (TT) traits have been available for many years.
New traits and combinations available now or in the near future include stacked imi + triazine, glufosinate + triazine and glyphosate tolerance for weed control along with varieties with high omega 3 and non-shattering pod traits.
Hybrid canola is now considered a reliable option for low rainfall areas when adequate nitrogen is applied.
What are the key strategies for growing RR canola?
In brief: Use pre-emergent herbicide, sow early and apply the first glyphosate spray when the crop is at 1-2 leaf stage.
The details: If you have decided to grow RR canola this season you will already be aware of the stewardship agreement and the key requirement to a limit of two glyphosate applications per crop and applied before the crop reaches 6-leaf stage for Roundup Ready canola and first flower (three applications are permitted) for TruFlex canola. Only glyphosate formulations that are registered for use on RR / Truflex canola are permitted. Further details are available in Bayer’s Roundup Ready® Canola Technologies Grower Accreditation Workbook.
Sowing in the optimal window will often mean there is no opportunity for a pre-sowing knockdown. This means the timing of the first glyphosate application is strongly recommended at the 1-2 leaf crop stage – which is likely to occur while other paddocks are still being sown.
The RR Crop Management Plan outlines the chemical and cultural tactics applicable to this technology. Courtesy of Bayer. Click image to access the brochure.
What WeedSmart Big 6 tactics can I implement in hybrid canola crops?
In brief: All six!
The details: The value of herbicide tolerance traits in crops comes from their strategic use in a crop rotation and an understanding of the resistance profile of the weeds present. Like any tactic, herbicide tolerance traits can be ‘broken’ if they are over-used in a cropping system.
There must be a plan in place to control survivor weeds. In Roundup Ready crops this means having strategies to take the pressure off glyphosate and preventing weed seed set. Crop competition can play a crucial role here, providing season-long suppression of weeds.
Consider the other herbicide options in-crop, such as including a registered pre-emergent grass herbicide and mixing clethodim or Lontrel with the second glyphosate application. The non-shattering pod trait (Podguard) supports direct harvesting but this is going to be too late to stop weed seed set, and crop topping with glyphosate is not going to control any glyphosate resistant weeds present. Windrowing is a reliable tool for managing late germinated and resistant grass weeds, especially when coupled with a harvest weed seed control method such as an impact mill, chaff lining and the like.
Strategies also need to be in place to control herbicide tolerant canola volunteers, most of which will germinate in the following season.
More resources

The 10 key lessons from the Optimised Canola Profitability project 
Optimised canola profitability: and overview of five years of canola agronomic research in South Australia 
Roundup Ready Crop management plan 
Bayer’s Roundup Ready® Canola Technologies Grower Accreditation Workbook 

Article
Ask an Expert

How can I ensure my complex tank mix is compatible and will spray out?

The pressure for growers to get across a large area in a short period of time has led to an increased use of complex tank mixes – but the efficiency gains of this practice can easily become unstuck if taking short-cuts results in not being able to spray the brew out.
Stephen Pettenon, FMC technical services specialist.
FMC technical services specialist, Stephen Pettenon, says if there are many products in a tank mix, it becomes increasingly difficult to prevent adverse chemical reactions from occurring in the ‘brew’.
“If operators follow a few guiding principles it is possible to safely mix a complex combination of herbicides, insecticides and even crop nutrients,” he says. “But it is also quite easy to end up with a tank of sludge that can not be sprayed out, if you don’t take the time to get it right.”
With several new products, such as FMC’s Overwatch, Syngenta’s Reflex and Callisto and Bayer’s Sakura Flow, being released as suspension concentrates (SC), it is important to recognise that there is no guarantee that a desired combination can be mixed and sprayed out effectively.
“The first consideration is whether the tank mix is safe and if there are any biological antagonisms likely to arise,” says Stephen. “This is where one product impairs the efficacy of a tank mix partner or increases the risk of crop damage. These antagonisms are relatively rare in pre-emergent situations, but where they occur they can also have implications for the evolution of herbicide resistance.”
“The second, and more common, problem in tank mixes is the potential for the mix partners to be chemically incompatible,” he says. “This can result in the formation of irreversible precipitate reactions or some components settling out of suspension and potentially causing blockages.”
Tank mixing involves many products and so potential crop safety losses must also be a consideration.
The WeedSmart Big 6 tactics for reducing the impact of herbicide resistant weeds on farming systems also promotes the importance of applying herbicides in the most effective and safe manner.
*Always read the label and check with your agronomist for compatibility before mixing and applying agricultural chemicals.
What are the top tips for complex mixes?
In brief: The number one tip is to take your time. Rushing is the most common cause of tank mix failures.
The details: Products that are SC or water dispersable granules (WDG) need time to properly disperse. They also need sufficient solvent – that is water. Start by filling the tank to at least 70 per cent of its capacity with good quality water before adding any products. Each chemical must be added and dispersed fully before the next chemical is introduced to the tank. Keep water rates above 80 L/ha and ensure the agitation system is working well to improve the likelihood of keeping a complex mix in suspension.
Simplify the mix if you can. Keeping two or three products in a compatible mix is generally less challenging than achieving the same for a six or seven-way mix. Be realistic about what can be achieved in a single tank mix.
Courtesy of FMC.
Do I need to be careful when choosing between formulations?
In brief: Yes, not all products are created equal.
The details: Some products are only available as a powder formulation (suspension  concentrate – SC) and it is not possible for them to be produced as a more soluble, emulsifiable concentrate (EC).
For example, Rustler 900 WG is a formulation that requires plenty of time to absorb water and swell the granules and then to disperse into the tank water. Allow at least 5 to 10 minutes, with agitation, before adding the next product. Suspension concentrates also require significant amounts of time.
Some formulations of the same active can behave vastly differently in tank mixes. A well-known example is that potassium (K salt) loaded glyphosates are often less compatible in a tank mix than isopropylamine (IPA) and monoethanolamine (MEA) loaded glyphosate products.
K salt formulations have never been good mixers because the potassium ion has a high ionic charge and small molecular mass, so it has a high affinity to bind with other molecules.
K salt formulations are known to cause flocculation issues if mixed with SC and WDG products and such combinations should be avoided. There are some brands of potassium glyphosate formulations with complex surfactant systems that are mixing-friendly, provided agitation is maintained.
Mixing order is crucial. Start with correctly conditioned water and then add the least soluble formulation first, allowing time for each product to disperse before adding the next component.
If you are unsure of the compatibility of the desired products for the mix, conduct a jar test or ask for technical advice. The major chemical companies are involved in ongoing compatibility testing of the products that may be useful tank mix partners.
Are there things I can do with the sprayer set up to minimise potential problems?
In brief: Avoid over-filtering and be careful when using transfer systems.
The details: It is common for spray rigs to use filtration that is too fine for the nozzle size being used. Using the correct in-line and secondary filter for the selected nozzle can greatly reduce the chance of blockages.
For example, the standard 100 mesh filters on most spray rigs may not be the best choice for handling the mix. If using a single orifice nozzle that is 02 or greater in size, then using a 100 mesh filter (when a 50 mesh is adequate), will greatly reduce the area of passage and potentially increase the chance of blockages.
If transfer systems are used it is important that the small tank contains only one of the spray mix components.
Pre-mixing some or all the products in a transfer system or nursery tank can have some advantages in time efficiency for refilling the sprayer. Problems can arise if the full mixture of chemical is added to a small nurse tank. For example, if the full load of components is added to a 1000 L nurse tank destined for a 5000 L spray rig re-fill, there is unlikely to be sufficient water in the nurse tank to allow for complete dispersion of the product. If transfer systems are used it is important that the small tank contains only one of the spray mix components.
Other resources

Agricultural pesticides formulations (SmartTrain course notes)

Article
Ask an Expert

What are the best herbicide mixing strategies for winter crops?

Mixing pre-emergent herbicide groups is known to extend the effective life of all the mixing partners, and is even more important than simply rotating herbicide modes of action. Mixing can even breathe new life into herbicides that appear to have ‘run out of puff’.
Agrivision agronomist, Tim Pohlner, says it is well worth the effort to review and fine-tune your pre-emergent and in-crop herbicide plan for the coming season and beyond to make sure you get the best bang for buck.
Tim Pohlner, Agrivision says it is important to keep as many options as possible ‘alive’ as viable tactics to use in a diverse program.
“Effective weed control underpins profitability and while doing a good job may cost more there are rewards in productivity and profitability in keeping weed numbers low,” he says. “A robust pre-emergent mix has a two-fold benefit in providing early weed control while the crop is small, as well as reducing the pressure on in-crop herbicides.”
Some pre-emergent herbicides, such as trifluralin, are no longer an option as a standalone herbicide for ryegrass control, but can be a useful mix partner with other pre-emergent herbicides.
“It is important to keep as many options as possible ‘alive’ as viable tactics to use in a diverse program,” says Tim. “Herbicides can’t do all the work on their own and need the support of cultural practices as well.”
“Mixing and rotating herbicide groups is one of the WeedSmart Big 6 tactics to reduce weed pressure and prolong the useful life of current chemistry,” he says. “There is strong scientific evidence for the value of mixing pre-emergent herbicides whenever possible, provided all the necessary precautions are met.”
*Always read the label and check with your agronomist for compatibility before mixing and applying agricultural chemicals.
What’s a good pre-emergent strategy for wheat and barley?
In brief: Mix trifluralin with a partner for better weed control.
The details: Trifluralin is no longer an option as a stand-alone herbicide against ryegrass. Application rates increased over time in response to increasing resistance but the release of Boxer Gold and Sakura have offered alternative chemistry that partner well with trifluralin. Rotating compatible mixes prolongs the life of all the applied chemistries.
When trifluralin is applied on its own each year resistance is expected within 10 years. When trifluralin is rotated with other single-shot herbicides, the onset of resistance is delayed by another two or three years, but when trifluralin is mixed with other pre-ems, and the mixes are rotated, it takes 25 years for resistance to evolve, even though trifluralin is applied in two years out of every three.
Including trifluralin in a herbicide mix with Boxer Gold, Sakura or prosulfocarb broadens activity on more weed species and extends the length of control into the growing season.
Some tried and true pre-em mixes for cereals are:

Boxer Gold + trifluralin IBS
Prosulfocarb + trifluralin IBS
Prosulfocarb + Sentry PSPE
Prosulfocarb + Avadex IBS
Sakura + trifluralin IBS
Sakura + Avadex IBS
Sakura + Sentry PSPE, disc system

(Diuron can be added to all of the above)
Recently, several new pre-emergent grass herbicides have been released into Australia, providing additional rotate and mix options. Luximax (Group T / Group 33) was a new herbicide group to broadacre agriculture in 2020. Luximax is only registered in wheat and should be applied IBS in front of a knife point press wheel. BASF strongly recommends that the wheat seed has 3 cm of soil covering to minimise crop damage.
Overwatch is a Group Q (Group 30) bleacher being released this year. It has a very unique use pattern where it controls annual ryegrass and suppresses brome and wild oats, provides control for some of our hard to control broadleaf weeds and suppresses many others. Key broadleaf weeds are bifora, hog weed and sowthistle, and suppression of bedstraw capeweed, prickly lettuce and wild radish.
FMC promotes Avadex as an excellent partner mix. In extreme ryegrass populations, Overwatch + Sakura is very effective although pricey. Trifluralin is a good, cost-effective partner mix.
What is the best mixing strategy for break crops?
In brief: In break crops there are opportunities to mix pre-ems and then to use a mix of in-crop grass selectives.
The details: Widespread resistance to Group A (Group 1) herbicides has changed the way break crops are grown and made it essential to have a strategy in place for ryegrass control. In some situations, it may be worth considering growing a legume that allows a substantial knockdown e.g. field peas, chickpeas.
Mixing trifluralin with propyzamide improves overall grass control when applied pre-emergent and incorporated by sowing. Propyzamide can also be mixed with Simazine or Terbyne at robust rates.
Ultro is a new pre-emergent grass herbicide for pulses for 2021. Ultro is a Group E (Group 23) and will give a new option for ryegrass control and provides better control of brome grass and barley grass than many other pre-emergent options. Ultro has good water solubility, enabling good weed control even in marginal breaks to the season. Ultro can be mixed with most other pre-emergent herbicides.
In-crop, clethodim is still a cornerstone herbicide for grass control in break crops. To maximise effectiveness, avoid applying when the weather is cold or frosty, establish dense, competitive crops, use pre-ems to reduce weed pressure, apply robust rates and mix clethodim with Factor (butroxydim) or Intervix / Intercept over IT canola and XT lentils and beans. Implement crop topping prior to harvest to stop weed seed set in late germinating weeds.
What makes a good herbicide mix?
In brief: Two or more compatible herbicides, each at full label rate for the target weed.
The details: Additionally, there should be no (or low) resistance to the individual herbicides in the mix, no antagonism between the herbicides, the products must be chemically compatible when mixed and the mix must be safe to the crop and cost effective.
The aim of the weed control strategy should be to target zero weeds. Make the most of the rotational options available in your growing region and use cultural practices as well as herbicides to reduce weed numbers.
Avoid rotating to an inferior product because that will inevitably result in a weed blow-out. However, don’t discount a herbicide even if you know the weeds present have a level of resistance. Mixing this less effective herbicide with another mode of action will often improve the outcome.
Mixing herbicides may increase production costs but you can be confident that the reduced weed burden will increase production and profitability. With a little forward planning it is usually possible to solve the majority of weed issues that are encountered.

More mix ‘n’ rotate resources:

Mix and rotate in the Big 6
AHRI Insight : The herbicide mixture is greater than the sum of herbicides in the mix
AHRI Insight : Mixing herbicides wins again
The global classification of herbicide modes of action is changing. You can find out more on the Herbicide Resistance Action Committee website. 

Watch Tim Pohlner’s presentation at WeedSmart Week 2019 (please note that new products have since been released):

Article
Ask an Expert

What alternatives are there for desiccation and crop topping?

Desiccation and crop topping with pre-harvest herbicide application is a useful way to reduce seed set in late germinating weeds and is an effective harvest aid for cereal grain, pulse and oilseed crops. 
Given the scrutiny that glyphosate is currently receiving Paul McIntosh, WeedSmart’s northern extension agronomist, says it may be a good time to start looking for alternative means of reducing weed seed set prior to harvest and avoiding any potential issues with market access.
WeedSmart’s northern extension agronomist, Paul McIntosh has been investigating alternatives to glyphosate as a desiccant in mungbeans.
“Currently, there are five herbicides registered for late season use in a variety of crops,” he says. “Glyphosate and diquat (or Reglone) are registered for use in wheat and barley in some states, canola, chickpea, lentil, faba bean, field pea, mungbean and soybean. For some of these crops, growers are also able to use paraquat, metsulfuron methyl or saflufenacil (Sharpen).”
Although there are many benefits to the practice from a weed control perspective, there are also market forces at play that could curtail the future use of pre-harvest herbicides.   
“It might be a good time for growers to re-visit some of the non-herbicide options for reducing seed set,” says Paul. “One possibility is to trial swathing in pulse crops like chickpeas, faba beans and mungbeans. Early commercial scale trials suggest that it could be very effective and could also have the additional benefit of hastening crop maturity, bringing harvest forward.”
“In combination with harvest weed seed control, swathing is a valuable WeedSmart Big 6 tactic to manage the weed seed bank,” he says. “Swathing adds another non-herbicide tool to a diverse program, particularly for pulse crops that are often not very competitive, and for weeds that typically shed seed before the crop is ready to direct harvest.”

Are there other herbicide options for crop desiccation if the current products are banned?
In brief: Not really. In most instances, glyphosate is the most effective crop desiccant product.
The details: Glyphosate is already a key component of cropping systems, particularly in no-till systems. In crops like mungbeans that have semi-indeterminant maturity traits that make them want to keep on growing, glyphosate applied at the label rate can give mixed results.
The Australian Mungbean Association recently commissioned weeds researcher Dr Bhagirath Chauhan, QAFFI to investigate the efficacy of a range of possible alternatives to glyphosate as a desiccant, but there were no stand-out herbicide candidates. This small plot trial also included the non-herbicide option of swathing, and the results were very promising.  
Has anyone trialed swathing commercial mungbean crops?
In brief: Yes. A grower on the Darling Downs trialed swathing two mungbean crops in March and April 2020, the first being 0.4 ha within a larger paddock that was desiccated with herbicide, and the second was an 8 ha block.
The details: These two trial paddocks were very successful and the grower was encouraged by the yield and grain quality of the swathed areas. This has generated significant interest from other growers and agronomists in the northern grains region.
The crops were swathed at the standard 90 per cent physiological maturity, the same timing used for chemical desiccation in mungbeans. Harvest was delayed in the 0.4 ha block due to two falls of rain, 12 mm and then 18 mm, which meant the windrows remained in the paddock for 14 days. The crop produced 1.6 t/ha of reasonable quality grain with no evidence of dust.
Picking up the mungbean windrow after a two week delay due to wet weather.
The crop in the 8 ha block was shorter and sparser than the small trial block. Four days after this block was windrowed it was harvested with a Smale pea front at the correct moisture, suggesting that low yielding crops with reduced dry matter could be harvested earlier. The crop yielded just below 1 t/ha of excellent quality grain, with very few pods being left on the ground.
What are the potential benefits and costs of swathing?
In brief: The costs will be very similar to chemical desiccation and there could be extra benefits as the practice is fine-tuned. Swathing and windrowing costs around $35 to $40 per ha, similar to chemical desiccation, but the operation may take more time.
The details: The first benefit is the avoidance of pre-harvest chemical application, removing the potential for desiccant chemical residues in the grain. The second big benefit is that it may be possible to bring harvest forward. Even if swathing is done when the crop is 90 per cent physiologically mature, the same as for chemical desiccation, the crop can be harvested within a few days and could be off the paddock nearly two weeks earlier than a desiccated crop. The clincher is the possibility of swathing before the crop reaches 90 per cent maturity. If this can be done without compromising grain size and quality, it could have very significant benefits for weed control. Many weeds in the northern cropping region set seed before traditional desiccation and harvest time and so if the crop can be cut earlier there is a chance that less weed seed will mature.
Weed seed heads present in the mungbean windrow.
It is early days for the revival of swathing in the northern cropping region and there are many things to be tried and tested. Early successes have also been seen in sorghum, faba beans and chickpeas.

Article
Ask an Expert

How can I be certain that herbicide residues in the soil have fully degraded at planting?

In the course of a chemical fallow there are often several applications of herbicide and some residues may still be present on or near the soil surface when it is time to plant the next crop. In particularly dry years, residues may even carryover from the crop prior to the fallow.
NSW Department of Primary Industries soil scientist, Dr Mick Rose, says there has been concern in recent years about the effect these residues may have on soil microbial activity and on the establishment and growth of crops following the fallow, even after the plant back period.

Dr Mick Rose, DPI NSW soils researcher, is developing tests and predictive models to support growers in their decisions about crop choice after using residual herbicides. (Photo: GRDC).
“Glyphosate has been the most commonly used knockdown herbicide in northern fallows for several decades and more recently growers have been looking to use more diverse programs that include chemicals with residual activity on weeds,” he says. “The increased use of imazapic and diuron have been of most concern to growers when choosing the next crop, particularly after a low rainfall fallow period.”
With investment from GRDC, Mick has been working on a project led by Dr Michael Widderick from the Department of Agriculture and Fisheries, Queensland to develop a soil test for imazapic and diuron residues that will indicate damaging residue levels and help growers to decide which crops would be safe to plant in a paddock.
“We are determining the threshold levels of residues of these two herbicides at which crop damage is likely for six crops, both winter and summer growing, in a range of soil types,” he says.
In earlier work he also looked at the level of glyphosate residue in soils around the country at planting time and the impact these residues have on soil biological processes.
“We found that residues of glyphosate were commonly detected in the soil at planting but there was no indication that the herbicide was adversely affecting soil biological activity,” says Mick. “This suggests that the label recommendations are suitable and the proper application of glyphosate in Australia is not posing a threat to soil health.”
“For growers to be able to keep using glyphosate they need to implement the WeedSmart Big 6 strategy, including using diverse chemistry in fallows,” he says. “Residual herbicides are a useful tool for growers but there are some gaps in our knowledge about how these herbicides break down in different soils and under different seasonal conditions.”

Why not just follow the plant back recommendations on the label?

In brief: The label provides the minimum plant back period provided certain environmental conditions are met. There is a possibility of crop injury even though plant back periods are observed.
The details: Many factors affect the bioavailability of a herbicide in the soil. For example, even though a clay soil and a sandy soil might have similar residue levels, more herbicide will be available for uptake in the sandy soil. More rain will increase the rate of breakdown, but it is not known exactly how much rain will ensure the specific soil is ‘safe’ to plant into.
Another important factor is that many things can contribute to a germination failure. In some situations, residual herbicide may be suspected as the culprit, but can be difficult to either rule it in or out with certainty when diagnosing the reason for a problem at planting. If herbicide residues in the plant tissue can be shown to be phytotoxic, then another, less susceptible crop could be sown into the paddock.
Dr Annie Ruttledge, DAF Qld weeds researcher, inspecting chickpea plants growing in soils containing different levels of imazapic and diuron herbicide residue.

What effects can herbicide residues have on emerging crops?

In brief: The herbicide itself can inhibit germination and growth, or it can exacerbate other factors, such as root disease.
The details: At different levels of bioavailability, herbicide residues will have different effects on crop plants. If the herbicide is readily available to the plant, then susceptible crops will take it up from the soil and it can have phytotoxic effects ranging from suppressed vigour to yellowing and potentially plant death.
Testing the plant tissue of a struggling crop can show if the leaves contain sufficient herbicide to have caused the observed symptoms.
Some herbicide residues in soil can also ‘prune’ plant roots, particularly the fine roots that help access moisture and nutrients. Obviously, if the young plants are struggling to access resources then they will be less vigorous and possibly die. Damaged roots are also more susceptible to water stress, disease and poor nodulation in legumes, making it difficult to determine the initial cause of the problem in the field.
If herbicide residues are shown to be the problem then a more tolerant crop can be sown, speeding up the breakdown of the residue and there will be more rainfall events before the next cropping season comes around.
Seedling emergence and establishment is being measured for six crops (winter and summer) in the presence of different levels of herbicide.

What pre-planting soil tests are being developed to give growers confidence to plant?

In brief: The current project is establishing phytotoxicity thresholds for six summer and winter crops in a range of soil types, for two herbicides – imazapic and diuron.
The details: By mid-2021 the aim is to have established the thresholds so that soil could be tested pre-plant to determine what crops would be safe to plant.
This will give growers confidence to use these herbicides in a diverse strategy to manage weeds like feathertop Rhodes grass in the fallow, while avoiding germination or establishment failures in the following crop.
Spray records play an important role in the management of these herbicides and mistakes can easily be made if the spray history for the past several years is not taken into account.
In time, growers and their agronomists will gain a better understanding of how these herbicide residues behave in the soils on a particular property and will be able to make herbicide application and crop rotation decisions with more confidence.
In another project with the Soil CRC, Mick is developing a predictive model for herbicide breakdown for a wider range of herbicides used in southern and western cropping systems.
Until these tests and models become available, the use of an in-field or pot bioassay with a susceptible crop can be helpful in determining potential plant back issues.
 

Related resources

Herbicide residues in soil – the scale and significance (GRDC Update paper)
Herbicide residues in soil (GRDC Podcast)

Article
Ask an Expert

What’s next in controlling herbicide resistant broadleaf weeds?

PBA Kelpie XT is the fifth lentil variety released with tolerance to Group B herbicides, imazamox and imazapyr, along with one IMI-tolerant faba bean.

University of Adelaide weeds researcher and PhD candidate, Alicia Merriam, says resistance to the IMI herbicides and other Group B chemistry, particularly the sulfonylureas (SU), is making control of some broadleaf weeds very difficult.

University of Adelaide weeds researcher and PhD candidate, Alicia Merriam says screening has shown resistance to IMI herbicides in over 75 per cent of populations of both weeds collected in random weed surveys in South Australia.
“Imi-tolerant lentils have been very popular with growers and have increased the weed control options in this important crop, but resistance in sowthistle and prickly lettuce is very widespread in the southern region,” she says. “Screening has shown resistance to IMI herbicides in over 75 per cent of populations of both weeds.”

With investment from GRDC and an Australian Government Research Training Program Scholarship, Alicia conducted a trial at two sites in South Australia to investigate options to implement the best practice recommendation for lentils – to control weeds in the preceding wheat crop and again at sowing or crop emergence in the lentils.

Both sowthistle and prickly lettuce are renowned for their prolific seed production when growing in non-competitive situations and wind dispersal of seed enables recruitment of resistance from crop borders, and far beyond. Consequently, eradication is not a realistic proposition.

“Sequencing the gene that controls resistance to Group B herbicides has uncovered a large variety of different mutations in these species across the Mid North and Yorke Peninsula in SA,” says Alicia. “Most mutations of this type cause SU resistance, but some cause IMI resistance and the effect can vary between weed species. Crucially, we found all these mutations within a single grower paddock, which shows that they are widely distributed.”

“Crop rotation and increased crop competition are essential components of the WeedSmart Big 6 to help run down the seed bank and suppress seed production by all means available,” she says. “Herbicide tolerance in pulse crops is a useful tool when coupled with strong competition and other herbicide options in as many crops as possible in the rotation.”

What is the current resistance status of sowthistle and prickly lettuce in the southern region?

In brief: Widespread resistance to Group B SU and IMI chemistry exists in both these broadleaf weeds.

The details: In surveys conducted in the Mid North and Yorke Peninsula regions the percentage of SU-resistant populations of prickly lettuce increased from 66 per cent in 1999 to 82 per cent in 2004 and 100 per cent in 2019. The populations screened in the 2019 survey were all resistant to Group B IMI chemistry.

Sowthistle surveys in the Mid North and Yorke Peninsula have found SU resistance in 89 per cent of populations and IMI resistance in 76 per cent of populations. Surveys also show that Group B resistance in sowthistle is very common across the rest of the southern cropping region.
Sample populations screened with SU and IMI herbicides where Population 1 is susceptible to SU and IMI herbicides, Population 2 is moderately resistant to SU but susceptible to IMI herbicides, and Population 3 is resistant to both these Group B herbicides.

Did crop competition or herbicide treatments affect weed seed production in the wheat phase or weed numbers in the following crop?

In brief: The herbicide treatments used in the 2018 wheat crop had an impact on the sowthistle population in the next crop, but had little effect on prickly lettuce. Crop competition treatments did not reduce weed density in the following growing season.

The details: The weed populations at both sites were confirmed resistant to Group B herbicides but susceptible to glyphosate. The three in-crop treatments were 1. no in-crop herbicide, 2. ‘conventional’ herbicide application of metsulfuron-methyl (Ally) + MCPA and 3. ‘proactive’ herbicide application of bromoxynil + picolinafen + MCPA (Flight EC). Two levels of crop competition (seeding rate 60 and 90 kg/ha) were also applied.

In the 2018 wheat crop the proactive treatment gave the best control of sowthistle in that crop and this resulted in a reduction in numbers in the 2019 crop. Although the conventional treatment provided some weed control benefit in the 2018 crop, the benefit did not flow on to the next crop, probably because the sowthistle population was resistant to the residual action of the metsulfuron-methyl component of the conventional treatment.

The herbicide treatments both reduced prickly lettuce density better than the untreated option but there was no additional benefit from the more expensive proactive treatment in either the year of application or the following crop.

Crop competition is a well-established practice for reducing weed seed production, so it was surprising to find that increased crop competition did not reduce weed numbers in the following year. This could be due to conditions in the year of the trial and the mobility of seed of these species.
In less competitive situations (right) sowthistle and prickly lettuce produce vast quantities of seed whereas in competitive situations (left) seed production is considerably reduced.

What’s the take-home message for using herbicide tolerant lentils in the rotation?

In brief: Herbicide tolerant crops are an important tool but must complement a diverse arsenal of weed control tactics. Short rotations are a very risky option and will lead to yield-reducing numbers of these prolific seeding weeds.

The details: Sowthistle and prickly lettuce can be expected to become increasingly difficult to control in the lentil phase. Neither crop competition nor proactive herbicide regimes alone are likely to provide sufficient downward pressure on these weeds in a short rotation. Building in a longer break away from lentils is likely to be a more effective strategy.

The number of different resistance mutations found in the cropping regions of the Mid North and Yorke Peninsula show that Group B resistance is widespread, and here to stay. This highlights the importance of diversity in crop and herbicide groups rotations, including the strategic use of herbicide tolerant crops.

The new Group G herbicide Reflex, with planned registration for IBS (knife point press wheel) application in lentils, will also be a welcome addition to improve weed control options in this crop.
Further resources

Common sowthistle and prickly lettuce in lentil crops of southern australia – Managing herbicide resistance and highly mobile resistance genes, GRDC Update paper Feb 2020  

Article
Ask an Expert

Is HWSC useful against weeds that shed seeds early?

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

Article
Ask an Expert

Group G herbicides & effectiveness for herbicide resistance

There has been a gap in the pre-emergent herbicide options for growers to control broadleaf weeds, particularly in pulses, but also in cereals.
Dr Chris Preston, Professor, Weed Management at The University of Adelaide says a suite of new Group G herbicides will go a long way to help growers control brassica and thistle weeds and rotate away from the imi chemistry that has been heavily used in recent years.
Dr Chris Preston, University of Adelaide says using the suite of new Group G herbicides in a tactical way to control early germinating broadleaf weeds could help keep pulses profitable in southern farming systems.
“The new Group G herbicides will be of particular value in pulse crops,” he says. “Until recently Group G products have only been used in small quantities, predominantly as a knockdown spike ahead of planting, but several of the new products have pre-emergent herbicide use patterns.”
Group G mode of action inhibits part of the process for making chlorophyll, and the leaves die as a result. Uptake of the herbicide is usually through the leaf surfaces – either through contact on emerged weeds or taken up from the soil as the weed seedlings break through the soil surface.
“The introduction of Terrain (flumioxazin) as a pre-emergent broadleaf herbicide in front of faba beans offered a new and helpful use pattern in the pulse phase,” Chris says. “Terrain is also a good fenceline weed control option on heavier soils, provided all surface vegetation is removed prior to application.”
Syngenta’s yet-to-be-released Group G product, Reflex (fomesafen) is expected to offer pulse growers pre-emergent control of broadleaf weeds that might be resistant to other MOA groups. This product can be used either pre-emergent incorporated by sowing (IBS) or post sowing, pre-emergent (PSPE), and will provide an alternative to imi herbicides for the control of weeds like sowthistle and prickly lettuce in pulse crops. Lentils are less tolerant than other pulse crops, so Reflex can only be used IBS in lentils.
With the release of these new products Chris is reminding growers and agronomists of the importance of ensuring that herbicide product choice is always based on addressing the weeds present that are likely to cause economic loss or produce large quantities of seed.
“Mixing and rotating herbicide modes of action is one of the WeedSmart Big 6 tactics, but crop competition is just as important,” he says. “Pulses are particularly sensitive to competition from weeds in the early crop stages, so using these Group G herbicides in a tactical way to control early germinating broadleaf weeds could be a very good way to keep pulses profitable in our southern farming systems.”
The sustainable use of Group G and other new herbicides coming to market will be a key topic at WeedSmart Week 2020 in Clare, SA. The companies developing these new Group G products will be present to answer questions and provide advice. Click here to register for WeedSmart Week 2020.
Should I mix and rotate these new herbicides with existing products?
In brief: Yes. Read the labels as they become available and look for opportunities to mix and rotate within and between seasons and crops.
The details: For example, Terrain offers broad spectrum weed control in tank mixes with TriflurX, Terbazine, Avadex Xtra, Kyte, Simagranz and Rifle. Terrain has a narrow weed spectrum for the rate registered in-crop for faba beans.
With some clever planning these products can help ‘bring back’ some previously lost chemistry using the mix and rotate tactic, extending the effective life of a broader range of herbicide options. When coupled with some non-herbicide tactics the grower can regain control of herbicide resistance on their farm and operate in a low-weed situation.
What new use patterns are available with the new Group G herbicides?
In brief: Pre-emergent with IBS and knockdown options.
The details: The first Group G with pre-emergent properties to hit the Australian market was Terrain (flumioxazin), from Nufarm with new use patterns registered for wheat and pulses. Other products with pre-emergent properties expected to come to market in the next year or two are Syngenta’s Reflex (fomesafen) and BASF’s Voraxor (saflufenacil + trifludomoxazin). Voraxor can be used pre-emergent in cereals and also as a knockdown spike. Terrador (tiafenacil) from Nufarm will have knockdown spike use and is safe for cereal or pulse planting just one hour after application.
More details about the new products can be found in the Group G Tech Note.
The early trials with Reflex look very promising and widespread use of this herbicide is expected in WA, where farmers have battled wild radish control in lupins for decades.
Is resistance to Group G a problem?
In brief: Not yet in Australia.
The details: There are no recorded cases of weeds resistant to Group G herbicides in Australia. Although the Group G mode of action has been commonly used for over 15 years, these products have generally been used in small quantities.
Resistance to Group G is becoming widespread in North America and we will soon have resistance problems if we do not practice the Big 6 tactics to minimise weed seed production.
More resources:

Podcast – What role are Group G herbicides playing in farming systems
Diversity Era – Pre-emergent herbicides online course

Group G Tech Sheet
Nufarm Terrain – Group G pre-emergent and knockdown

500 g/kg flumioxazin
New registrations for wheat and pulses in 2019.
Pre-emergent weed control in lucerne, wheat, faba bean, chickpea, field pea and along fencelines. Broadleaf weed control only in pulses.
Control emerged weeds with knockdown before applying Terrain in wheat and pulses or along fencelines.
New chemistry for IBS use pattern.
Improves brown-out and weed control of knockdown herbicides. Pre-plant knockdown spike ahead of barley, chickpea, cotton, faba beans, field peas, lentil, lupin, maize, mungbean, oats, sorghum, soybean, sunflower, wheat. Add to glyphosate and paraquat/diquat products. Requires adjuvant.
Not suitable for lighter soils.
Residual activity on annual ryegrass, sow thistles and wild radish.
Broad spectrum weed control in tank mixes with – TriflurX, Terbazine, Avadex Xtra, Kyte, Simagranz, Rifle.

Syngenta Reflex – Group G pre-emergent

240 g/L Fomesafen SL
Registration expected early 2021.
Suitable for a range of pulses
Wide range of broadleaf weeds (including sowthistle and prickly lettuce) – check label when available.
IBS or PSPE uses. IBS knife points and presswheels only in lentils.

BASF Voraxor – Group G knockdown and pre-emergent herbicide

250 g/L saflufenacil plus 125 g/L trifludomoxazin
Both group G actives
Used in wheat, durum and barley against a wide range of grass and broadleaf weeds
IBS knife points and presswheels only
Registration expected in 2020 and launch in 2021
Knockdown and residual control of broadleaf weeds plus suppression of ryegrass.
Broadleaf residual control for 8 to 12 weeks (at higher application rates)
Partner with glyphosate as a knockdown spike or mix with paraquat for double knock applications.

Nufarm Terrador – Group G knockdown spike

700 g/kg tiafenacil WG
Registration and commercial launch expected in 2021.
Likely use pattern, partner with glyphosate as a knockdown spike or mix with paraquat for double knock applications. Expect suppression of ryegrass as a knockdown partner with glyphosate or paraquat. 1 hr plant back for cereals and pulses, probably 7–14 days for canola, depending on rate.

Article
Ask an Expert

How can I maximise the weed control value of my crop rotation?

A diverse crop rotation is the twine that holds a good farming system together and underpins an effective weed management program.
Kevin Morthorpe, Pioneer Seeds’ Trait & Seed Technology Stewardship Manager says herbicide tolerance traits in crop hybrids can be used to maximise competition against weeds and increase the herbicide options available to growers while optimising yield and profitability of the crop sequence in rotations.
Kevin Morthorpe (left) – Pioneer Seeds’ Trait & Seed Technology Stewardship Manager, with Dr Ray Cowley – Canola Research Scientist, Corteva Agriscience and Pioneer Seeds’ Rob Wilson – Strategic Customer & Market Development Manager and Clint Rogers – Western Regional Sales Manager & Canola Product Lead at a canola research trial near Jindera in southern NSW.
Plant breeders continue to introduce herbicide tolerance traits in a number of crops in Australia, including corn, canola, pulses, cereals, grain sorghum, summer forages and cotton.
“For example, in canola there are several herbicide tolerance traits and they are primarily available in hybrids,” he says. “This means growers get both improved crop performance due to hybrid vigour and more flexibility in herbicide use patterns.”
The increased vigour of canola hybrids also generates greater biomass production and early canopy closure that suppresses growth and seed set of weeds that germinate in-crop, complementing the use of pre-emergent herbicides.
“Hybrids super-charge crop competition through a strong root system and vigorous growth,” Kevin says. “From an economic angle, hybrids optimise yield in both high input and tough environments. In fact, we see more growers selecting hybrids when producing canola in tough conditions.”
Since the release of the first herbicide tolerant canola in 1991, the popularity of herbicide tolerance has seen a 98 per cent adoption of canola varieties with tolerance to imidazolinone (Clearfield), triazine (TT) or glyphosate (RR). In the last 15 years, the area sown to hybrid canola has risen to an impressive 47 per cent in Australia. With glyphosate tolerant canola hybrids entering South Australia in 2021 and new hybrid releases, the hybrid percentage will increase further over coming years.
With glyphosate tolerant canola hybrids entering South Australia in 2021 and new hybrid releases, the hybrid percentage will increase further over coming years.
Kevin says that Pioneer Seeds have seen increasing demand for Clearfield canola in recent years following a dip in popularity. Through strategic application of herbicide tolerant traits in diverse crop rotations it seems that farmers are overcoming the resistance problems that were prevalent with the Clearfield technology and can now re-introduce these varieties and take advantage of the weed control benefits and high yields they offer, and manage herbicide residues in the soil.
“A diverse rotation of crops and pastures is one of the WeedSmart Big 6 tactics, which Pioneer Seeds endorses wholeheartedly to protect the longevity and effectiveness of herbicide tolerance traits,” he says. “Through an effective crop rotation you can tick off all the herbicide and non-herbicide tactics needed to drive down weed numbers.”
How do I make the most of a hybrid crop?
In brief: Employ best practice agronomy.
The details: Grain hybrids are vigorous plants that produce increased biomass and grain yield. To do this, they must be supported with adequate crop nutrition. When properly fed, hybrids will provide increased crop competition and achieve greater water use efficiency compared to their conventional counterparts.
Growing a hybrid crop with herbicide tolerance traits does not equate to a full weed control program. These crops must be used within the WeedSmart Big 6 framework, within a diverse crop rotation and using herbicide tactics such as double knocking alongside cultural practices such as harvest weed seed control and crop competition to reduce seed set. They also combine well with pre-emergent herbicides to achieve excellent early weed control and suppress seed set in any late germinating weeds.
Can I use hybrid crops with herbicide tolerance to fix a weed blow-out?
In brief: No. This technology is not suitable for salvage operations.
The details: When Roundup Ready canola varieties were first released there was an expectation that these traits could be used to reverse a weed infestation. This proved not to be the case.
Hybrid crops are best used in low weed density situations where they can effectively drive down the weed seedbank. They should be grown in rotations that include an effective double-break, brown manure crop or a pasture phase.
Having hybrid crop options for both summer and winter growing seasons increases the opportunities to tackle weeds throughout the year or to use different fallow herbicides while maintaining the ability to safely grow crops in the following season.
New glyphosate tolerance traits (Truflex® and Optimum GLY®) and the stacking of herbicide tolerance traits of triazine tolerant and Clearfield® (TT+CL) have expanded the safe window for herbicide application in canola.
Are residues in grain a concern when using stacked trait herbicide tolerant hybrids?
In brief: Not if the stewardship program is followed.
The details: New glyphosate tolerance traits (Truflex® and Optimum GLY®) and the stacking of herbicide tolerance traits of triazine tolerant and Clearfield® (TT+CL) have expanded the safe window for herbicide application in canola. This gives more options, more flexibility and more crop safety through the rotation.
The stewardship program for the herbicide tolerant trait hybrids describe herbicide use patterns that growers must follow to confidently avoid the accumulation of herbicide residue in the grain and ensure that Australian maximum residue limits (MRLs) will not be exceeded. [Note that MRLs in other countries may be different to the Australian MRL. Find out more at Grain Trade Australia]
To avoid problems with crop safety within the rotation it is important to maintain accurate paddock records to avoid applying herbicide to the wrong crop variety and ensure susceptible crops are not sown into paddocks with herbicide residues in the soil.
On the flip-side, herbicide tolerance in crops increases the options for crop selection within the rotation.
Also, keep in mind the importance of controlling any volunteers from a herbicide tolerant crop in the summer fallow or following crop.

Article
Ask an Expert

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

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

Other resources

Big 6 – Mix and rotate tactic