Resistance risk to eliminate herbicides on irrigated farms
This can be a high risk practice unless survivors are removed after every spray application as there is no crop competition to restrict weed growth, resulting in production of large volumes of seed.
Eric Koetz, NSW DPI weeds research agronomist, said the limited options for managing weeds along irrigation infrastructure and other non-crop areas is a problem and is putting additional pressure on knock-down herbicides in irrigated systems.
Weed researcher Eric Koetz said the limited options for managing weeds along irrigation infrastructure and other non-crop areas is a problem and is putting additional pressure on knock-down herbicides in irrigated systems.
“A recent survey of cotton fields in Queensland and NSW had shown that cotton paddocks were generally relatively weed-free however the incidence of resistance to glyphosate is quite high in fleabane and windmill grass samples collected mainly from non-crop areas on cotton farms,” he said.
“Preliminary results from the samples collected across cotton farming systems in 2016–17 show 95 per cent of fleabane samples, 60 per cent of feathertop Rhodes grass samples, 80–90 per cent of windmill grass samples and 20 per cent of sowthistle samples tested as resistant to glyphosate.”
“Keep in mind that flaxleaf fleabane, feathertop Rhodes grass and windmill grass are not registered for control with Roundup Ready Herbicide with PLANTSHIELD as control of susceptible plants is generally poor, so further selection for resistance only exacerbates the problem,” he said. “Although still lower, the level of resistance emerging in sowthistle is very concerning, given that this species is listed on the herbicide label. According to a recent attitudinal study by Monsanto, growers also identify annual ryegrass and barnyard grass as showing signs of glyphosate resistance in the field.”
To date there have been no recorded cases of paraquat resistance in weeds on cotton farms, however with rising glyphosate resistance, and increased use of paraquat products, there is a high risk that paraquat resistance will also be found, leaving growers with few options to control these weeds. Mr Koetz now has additional funding available to test the seed collected in the survey for resistance to paraquat and diquat products – Gramoxone and Spray.Seed.
Mr Koetz said the lack of diversity in herbicide use in many cotton systems is likely to contribute to the increased incidence of herbicide resistance. “The attitudinal study by Monsanto indicates that less than 50 per cent of growers are applying a pre-emergent herbicide and only 25 per cent of growers apply a post-emergent herbicide in addition to their applications of glyphosate in cotton,” he said. “The label for Roundup Ready Herbicide with PLANTSHIELD states that this product must not be the only form of weed control used in Roundup Ready Flex cotton varieties.”
“In the next 5 to 10 years there will need to be a shift towards non-herbicide controls such robotic cultivation and microwave technologies, which are well suited to summer cropping on rows or beds,” he said. “Until then, optical spray technology is a good option for growers to keep weed numbers low in the fallow.”
Spray drift of Group I herbicides (e.g. 2,4-D products) late in the summer fallow to control large fleabane is causing considerable damage to cotton crops. Mr Koetz recommends growers change their fallow weed management program to target small plants earlier in the spring, before cotton emergence, using a double knock of glyphosate followed with cultivation or paraquat plus a residual herbicide as the second knock.
Protecting knock-down herbicide options
2016 Monsanto Weed Resistance Management Research – Survey report
Insecticide shown to reverse metabolic herbicide resistance
Herbicide resistance occurs at a genetic, molecular and cellular level in ways that challenge some of the most agile scientific minds. In following interesting lines of enquiry, scientists working to understand the mechanisms that drive metabolic resistance sometimes come across some unexpected findings.
One such finding is the discovery that an insecticide can reverse metabolic resistance to a herbicide, making the resistant population susceptible to the herbicide once more.
Left: Trifluralin applied to a pot containing trifluralin-resistant ryegrass seed. Centre: Trifluralin applied immediately after applying phorate insecticide granules to pots with trifluralin-resistant ryegrass seed. Right: Trifluralin applied to a pot containing trifluralin-susceptible ryegrass seed.
Metabolic resistance is the lesser known cousin to target site resistance in the world of herbicide resistance mechanisms. Target site resistance is comparatively easy to identify and study, being a more direct ‘cause and effect’ type mechanism that usually confers quite high levels of resistance.
Metabolic resistance however is more complex and more difficult to study due to many internal mechanisms involving secondary enzyme production and activity. This type of resistance is often moderate however it is also frequently effective across multiple herbicide mode of action groups. It is not uncommon for plants with metabolic resistance to be resistant to herbicides that they have never been exposed to. This has a dramatic and limiting effect on herbicide choice and makes herbicide rotation a much less powerful control tactic.
In simple terms, metabolic resistance occurs when the plant uses its metabolic pathways to produce enzymes that ‘protect’ target sites from the applied herbicide molecules. If the herbicide molecule never reaches the target site then the plant survives. The same enzyme or multiple enzymes can ‘protect’ multiple target sites, resulting in cross-resistant plants.
Weed surveys in Western Australia have revealed a high rate of multiple-resistance in annual ryegrass populations with 70 per cent of populations possessing both metabolic and target site resistance to herbicides.
Research into gaining a better understanding of one group of enzymes, known as P450s, has uncovered an unlikely synergism between an insecticide and current pre-emergent herbicides to control ryegrass. Australian Herbicide Resistance Initiative (AHRI) researcher, Roberto Busi, has shown that it is possible to reverse metabolic resistance to trifluralin in annual ryegrass using an organo-phosphate insecticide.
In conjunction with Colorado State University researcher, Todd Gaines, Dr Busi is working to better understand the genetic basis of metabolic resistance and how this knowledge can be used to control metabolic resistant weeds.
“There are just five types of pre-emergent herbicides, utilising only two modes of action, with no new modes of action in the pipeline,” says Dr Busi. “The most recent pre-emergent product, pyroxasulfone (Sakura), was commercialised in 2012 yet even before it was brought to market, research had shown its mode of action can be ‘broken’ within just three generations using low application rates to result in 10-fold resistance.”
This means that it is very important to find ways to keep current herbicides effective rather than just looking for new modes of action. In the case of trifluralin-resistant annual ryegrass, Dr Busi’s research demonstrated that inhibiting the production of P450 enzymes was the key to reversing resistance to this useful pre-emergent herbicide.
“Inhibiting the production of P450 enzymes requires the suppression of different genes in the plant that are responsible for regulating production of the enzymes at different stages of the plant’s development,” he says. “In ryegrass there are probably several different P450 enzymes that are active during the plant’s development that are offering protection against the herbicide, so there is a high level of complexity involved in trying to manipulate the genes responsible for herbicide resistance.”
“Using the insecticide phorate, applied in granular form to the soil immediately before spraying with trifluralin, we were able to prevent establishment of plants with known resistance to trifluralin,” he says, “But the effect was not as clear for plants that were resistant to Sakura. Phorate is not the solution to metabolic resistance but this proof-of-concept research confirms that it is possible to manipulate and even reverse metabolic resistance with the use of existing pesticides.”
Phorate is not currently registered for use in any crop except cotton and the described use is not permitted in the field. Phorate is highly toxic and it was used under carefully controlled laboratory conditions for these experiments.
It is not desirable to turn off P450 production in a crop so chemicals that inhibit P450 production are best suited to use with pre-emergent herbicides. Current research is investigating ways to design better P450 inhibitor mechanisms using gene technology and to use these mechanisms in future crop breeding programs to confer crop tolerance to certain herbicides.
Further experiments showed that ryegrass plants with metabolic resistance to Sakura use another metabolic pathway involving enzymes known as GST. In a similar manner, these mechanisms can probably confer cross-resistance for pyroxasulfone (Sakura), prosulfocarb (Boxer Gold) and triallate. To date, the fifth pre-emergent active ingredient, propyzamide, does not seem prone to metabolic resistance.
“For now, our best advice to growers and agronomists is to rotate between these three groups of pre-emergent herbicides – 1. trifluralin, 2. Sakura, Boxer Gold and triallate and 3. propyzamide – and we are researching the potential benefits of mixing herbicides from these three groups as a means of delaying metabolic resistance,” says Dr Busi. “As always, full label rates must be applied.”
For more information, check out our webinar recording with Roberto and Todd discussing metabolic resistance to herbicides.
Hybrid canola out-competes weeds
Canola production is predicted to rise by 30 per cent in 2017 with over 98 per cent of farmers growing herbicide-tolerant canola cultivars. However, Australia has been lagging behind other producing countries in the uptake of hybrid cultivars that offer higher yield and more options for effective weed suppression in crop rotations.
Researchers at Charles Sturt University compared the competitive ability of 16 genotypes – hybrid and open-pollinated cultivars – and found the most competitive cultivars reduced weed biomass at flowering by a huge 50 per cent, significantly reducing the amount of weed seed added to the seed bank. A similar trial, conducted by researchers at the University of Adelaide, found that annual ryegrass plants that survived pre-emergent herbicide produced more than twice as much seed in open-pollinated TT canola than in the hybrid Clearfield canola.
Although the triazine-tolerance (TT) trait is known to confer a yield penalty of up to 10–20 per cent, this technology is currently adopted on 65–70 per cent of the national canola area. The vast majority of TT canola cultivars are open-pollinated while most Clearfield (CL) and Roundup-Ready (RR) cultivars are hybrids.
Kevin Morthorpe, *GenTech Seeds’ Canola Business Lead and Product Stewardship Manager hopes that the new hybrids available in 2017 will encourage growers to continue to shift away from the less competitive, open-pollinated cultivars to take advantage of the more vigorous, higher biomass and higher yielding hybrids.
Kevin Morthorpe, GenTech Seeds, is concerned that canola growers are relying too heavily on the less competitive triazine-tolerant (TT) cultivars and missing out on the higher yield and better weed control benefits of hybrid canola systems.
“Crop competition is one of the best non-herbicide weed management tools available and the new hybrids are offering growers real bang for their buck in terms of extra yield and driving down the weed seed bank,” he said. “These benefits for the grower far out-weigh the additional cost of hybrid canola seed. The yield advantage achieved through hybrid vigour only occurs in the F1 generation so retaining seed for future use is counter-productive.”
Mr Morthorpe said enhanced crop competitiveness in hybrid crops is best achieved through attention to crop nutrition to optimise early growth and harvestable yield. “Research studies in canola have not shown a plateauing in crop yield response to higher nutrient input as is often observed in the ‘haying off’ of cereals at higher nutrient rates,” he said.
The suppression of weed growth is closely tied to strong root development, increased crop biomass and improved early crop vigour. Field trails also indicate that some cultivars have an allelopathetic effect on weeds. Speedy emergence, early vigour, rapid ground cover and height are all important characteristics of the most competitive hybrids.
Hybrid growers are also able to strategically integrate alternate herbicide modes of action and reduce reliance on Group A herbicide applications in the canola phase.
Hybrid canola suppresses weed biomass and seed production by about 50 percent compared to open-pollinated triazine-tolerant (TT) canola cultivars due to the speedy emergence, early vigour, rapid ground cover and height characteristics of the most competitive hybrids.
Herbicide Tolerant Canola Stewardship Guide
GenTech Seeds, an industry partner of WeedSmart, is committed to product stewardship. Their recently-published ‘Herbicide Tolerant Canola Stewardship Guide’ outlines the sustainable use pattern for all herbicide tolerant cultivars. Mr Morthorpe said the stewardship guide is a practical approach to integrated weed management within the canola phase and fully supports the The Big 6.
“In triazine-tolerant crops the main points are to carefully manage the lower crop vigour of TT canola, monitor triazine herbicide carryover risk for crop rotational planning and always adhere to mandatory application practices for triazine herbicides (atrazine, simazine) set by industry in cooperation with government,” he said.
“To protect Clearfield technology, the whole farm rotation plan needs to limit the use of ALS-inhibiting herbicides to no more than two out of four years. Avoid crop injury by using quality assured, first generation seed and monitor the carry over risk of imidazoline herbicide on future crop rotation options.”
“Planting Roundup-Ready hybrids requires a critical assessment of the use of glyphosate as a stand-alone knockdown for non-crop application such as fencelines and over-reliance across the farming system. A whole-farm strategy needs to be in place to manage glyphosate resistant weed populations and to control RR crop volunteers in the fallow phase.”
The general principles for weed control and herbicide use apply for all herbicide tolerant canola systems, including using a combination of registered pre-emergent and post-emergent herbicides, always following the label directions of use and pro-actively implementing diverse weed management tactics across the farming system.
Post-emergent herbicide applications must target small weeds (2–3 leaf ryegrass and/or 2–3 cm diameter broadleaf leaves) early in the crop’s development and selective herbicide application should occur before elongation of canola plants (BBCH 16).
“When it comes to managing herbicide resistance and protecting the plant breeding technologies available, the key is to have low weed numbers entering and leaving the canola phase,” he said. “To achieve this, growers can rotate the crop types, herbicide tolerance technologies and herbicide modes of action used across the crop rotation to broad the opportunities to implement a diverse range of weed management tactics to prevent weed seed set and to implement harvest weed seed control to collect any seed present at harvest.”
The ‘Herbicide Tolerant Canola Stewardship Guide’ includes a detailed integrated weed management (IWM) strategy that highlights herbicide and non-herbicide tactics growers can employ in the pre-planting, in-crop, harvest and post-harvest phases to control weeds and reduce weed seed set.
This Guide also promotes recommended stewardship practices along the supply chain to ensure responsible use of new technologies, predictable market access and builds on Australia’s competitive edge in international oilseed markets.
*GenTech Seeds is the exclusive producer distributor of Pioneer® brand products in Australia.
Choose highly competitive canola
Herbicide Tolerant Canola Stewardship Guide
University of Adelaide competitive canola trial (GRDC update)
Do canola biotypes have an allelopathic effect on weeds?
Ramp up the competition in summer crops
The higher rainfall across many grain growing regions in 2016 is providing farmers with more opportunities for summer cropping. Weeds also stand to gain from the additional soil moisture, putting additional pressure on summer fallow spraying programs.
Dr Bhagirath Chauhan, Principal Research Fellow with the Queensland Alliance for Agriculture and Food Innovation (QAAFI) says agronomic trials measuring the effect of early canopy closure in summer crops are consistently resulting in lower weed biomass and higher crop yield.
“Our research in mungbean, cotton and soybean have shown that more even plant spacing across the paddock is more important for weed suppression than increased seeding rate, and this is best achieved through narrower row spacing,” he says.
“In cotton, we demonstrated a clear benefit in planting at 50 cm row spacing rather than the conventional 100 cm spacing however, the limitation for growers is the inflexibility of the current harvesting equipment.”
One configuration that has shown promise internationally but not yet fully investigated in Australia is the ultra-narrow row (UNR) concept where the beds remain at 100 cm spacing to suit the harvester but two rows of cotton are planted either side of the bed, effectively shading the inter-row earlier than a single row planting. The ultra-narrow rows are planted 19 to 38 cm apart on the bed and seeding rate is usually increased slightly.
With limited options to increase crop competition in cotton, and the widespread adoption of Roundup-Ready (RR) technology in the industry, there is now a focus on finding alternative herbicide chemistry to manage the risk of glyphosate resistant weeds in cotton systems.
Dr Chauhan says growers are achieving good weed control with the pre-emergence herbicides recently registered for use in cotton. “The biggest challenge with these herbicides is getting the application right, taking into account the effect of rainfall, irrigation type and timing and the soil type,” he says. “There are emerging weeds such as feathertop Rhodes grass, sesbania and amaranth that are challenging the Roundup-Ready cropping system and so growers need to have other weed management tactics in place early.”
The cotton industry is promoting the adoption of the 2 + 2 + 0 weed management system to protect glyphosate and the Round-Up Ready hybrids. This entails the use of two non-glyphosate herbicide options, two non-herbicide tactics and zero weed survivors. Re-introducing the use of pre-emergent herbicides in cotton farming is an important part of this weed management program. Increasing crop competition is also worth further investigation given the potential weed control and crop yield benefits to be gained if the limitations of current harvesting equipment can be addressed.
To maintain yield in cotton it is important to restrict all weed management operations to the early stages of crop growth, a distinct advantage of using pre-emergent herbicides to minimise weed growth prior to crop canopy closure.
Peter Newman, communication lead with Australian Herbicide Resistance Initiative, has long been an enthusiastic advocate of crop competition in cereals. He says the recent findings in summer crops provides growers with a valuable non-herbicide tool they can use to help suppress weeds.
“Over and over we are seeing results come from crop competition trials showing suppression of weed biomass in competitive crops, and usually a yield benefit,” he says. “This is a win win for growers and needs to become standard practice in all crops – not only regarding row width but all agronomic practices that boost early crop growth and result in early canopy closure.”
Protecting knock-down herbicide options
Most cropping systems rely fairly heavily on a small group of non-selective or ‘knockdown’ herbicides. Since the widespread adoption of zero and minimum tillage, these herbicides have provided effective control of many grass and broadleaf weeds – but these useful herbicides could be lost to the industry if steps are not taken to increase the diversity of weed management tactics used.
Mark Congreve, ICAN senior consultant, says that the highly effective double knock tactic, which combines an application of glyphosate followed by paraquat, is at risk if growers don’t remain vigilant and ensure removal of any surviving plants.
The recent discovery of flaxleaf fleabane resistance to paraquat is a clear warning to grain producers that there is no room for complacency with double knock operations.
“The double knock strategy of glyphosate, plus a Group I herbicide for weeds such as flaxleaf fleabane, followed by paraquat has provided excellent control of weeds that are difficult to kill with glyphosate alone,” he says. “Recent confirmation of a fleabane population that is resistant to paraquat, found in a New South Wales vineyard, is a clear warning to grain producers that there is no room for complacency following a double knock operation. In addition to this recent discovery, an annual ryegrass population from a West Australian vineyard was confirmed in 2013 to have resistance to both glyphosate and paraquat. This shows that a single plant can develop resistance to both of the main non-selective knockdown herbicides used in Australian grain production.”
Paraquat is a widely-used herbicide, being an active ingredient in over 100 herbicide products registered for use in broadacre cropping. It is a group L herbicide and as such is considered a ‘moderate risk’ for herbicide resistance. Having a moderate risk rating means that resistance generally takes longer to occur, not that it won’t occur.
“Paraquat resistance typically takes over 15 years of consistent use before resistant weeds are noticeable in the field,” he says. “This critical period has now elapsed on many farms where paraquat is used in cereals and broadleaf crops, and for general weed control around the farm.”
Paraquat resistance has been present and widespread in barley grass in lucerne production systems for many years in southern NSW and Victoria. While paraquat resistance is still relatively rare outside of lucerne systems, very high level resistance to paraquat was confirmed in three weed species (crowsfoot grass, blackberry nightshade and cudweed) taken from sugarcane and tomato blocks around Bundaberg in 2015.
Mark Congreve, ICAN senior consultant says growers need to be looking for survivor weeds after every herbicide application and responding to ‘rate creep’ by changing how they use herbicides across their cropping system.
In the event of widespread resistance to paraquat, Mark is concerned that there are no new modes of action likely to be commercialised within the next 10 years or more, so we need to protect what we have.
“It is essential that farmers do everything in their power to preserve the effectiveness of the herbicide groups currently available,” he said. “The key is to take a diverse approach to weed management and, most importantly, remove weeds that survive herbicide applications. This is the best way to keep weed numbers low and when numbers are low, resistant weeds can be controlled more effectively. It’s a numbers game!”
Mark suggests that growers check the results of every spray application, looking for individual plants ‘surviving’ or ‘re-growing’ after a spray application that has killed adjacent weeds. This may be a sign that the surviving plants carry the genetic mutation that ‘protects’ them from the herbicide’s mode of action.
“If this is observed, the first step is to remove those individual plants before they shed seed,” he said. “It is recommended to have the plants, or their seed, tested to confirm resistance and determine what herbicides those individuals are still susceptible to.”
A second warning sign is when a higher rate of a herbicide is needed to have the same effect as achieved on the target weed in previous years. Mark called this ‘rate, or dose, creep’ and said that it is the most common sign of resistance to herbicides like paraquat. “Paraquat resistance primarily occurs as a result of a plant having the ability to re-direct the herbicide molecules away from the chloroplasts in the cell and into the cell vacuole, where the herbicide has no effect,” he said. “If you are finding that you now need to use a higher rate of a herbicide such as paraquat, it is time to change how you manage those weeds.”
Non-crop areas around farms are often treated with paraquat annually. This can be a high risk practice unless survivors are removed after every spray application as there is no crop competition to restrict weed growth, resulting in production of large volumes of seed.
Herbicide resistance frequently occurs first along fencelines, roadways and irrigation channels where herbicide use tends to be the same year in year out and less attention is paid to survivor weeds or poor herbicide efficacy.
Currently there are 10 weed species with confirmed resistance to paraquat (Group L) and 13 species resistant to glyphosate (Group M) in Australia.
Table: Confirmed paraquat resistance in Australia (Source: Australian Glyphosate Sustainability Working Group Paraquat resistance factsheet)
Northern barley grass
Small square weed
tomatoes, sugar cane
tomatoes, sugar cane
tomatoes, sugar cane
Acting early pays off in the war on weeds
If nothing else, herbicide resistance is predictable, but efforts to prevent seed set do pay off, particularly if action is taken early.
NSW DPI weeds specialist Tony Cook says that growers are becoming increasingly aware of the potential impact of herbicide resistance on their farming operations and they are seeing the benefits that come from early and decisive action.
NSW DPI weeds technical specialist, Tony Cook, has documented the strategies used on four farms to successfully patch-manage resistant weeds using a variety of weed control tactics over a period of at least five years.
Many factors may influence the process but if an individual plant that is resistant to a herbicide sets seed, it is only a matter of time before a small number of resistant plants becomes an ever-increasing weedy patch.
Mr Cook says that growers can build farming systems that have a level of ‘immunity’ to herbicide resistance. “If growers concentrate on minimising or preventing weed seed set, they can win against herbicide resistance,” he says. “The trick is to use a variety of means to keep weed numbers low and to keep pressure on seed set. If resistant plants are prevented from setting seed, then the problem is contained.”
“Eight out of the twelve glyphosate resistant species in Australia are present in weed populations in NSW and Queensland, particularly near the state border,” says Mr Cook. “In this region resistant awnless barnyard grass, annual ryegrass, liverseed grass and feathertop Rhodes grass are the main problems.”
Mr Cook interviewed four northern region farmers to find out what strategies they had used to successfully manage patches of these four glyphosate-resistant weed species. The take-home message from these four farmers was that growers can drive down weed seed banks through a strong commitment to consistently preventing weed seed set for at least five years. “The costs associated with treating the patches is an additional expense but this pales in significance against the cost of doing nothing and allowing the patches to spread across paddocks and beyond,” he says.
More details about the strategies used on each case study property can be found in Mr Cook’s webinar presentation with AHRI’s Peter Newman.
While glyphosate resistance is becoming more widespread across the region, many resistant populations are still in patches up to one hectare in size. “This gives growers the opportunity to use paddock-wide tactics combined with more intense patch management in problem areas,” says Mr Cook. “Some growers have been successful in completely eradicating herbicide resistant plants from patches of one hectare or less through very focused efforts to prevent seed set.”
Strategies such as full cultivation, optical weed detection, brown manuring, double-knocking, strategic use of pre-emergent herbicides, using livestock as the second knock and close attention to the removal of survivors have all been used in different situations to treat patches of resistant weeds. While some of these strategies are often applied across the whole paddock, some can be directly applied to the patch to minimise the cost while maximising effect.
Glyphosate resistance in weeds is becoming widespread across the northern grains region, particularly around the NSW Queensland border, however these resistant weeds are often found in small patches, less than one hectare in size.
“A shift in cropping rotation and well-timed use of paraquat in place of glyphosate is another useful strategy to drive down weed numbers for these key species,” says Mr Cook. “While spot spraying can be a good option it is easy to miss the outlier plants in a small patch. The optical sprayers can give better coverage of the weeds and often involve an alternative herbicide to glyphosate. Including a second knock to remove survivors is key to success with this and all other weed control tactics.”
“Pre-emergent herbicides are being used to good effect in the fallow, provided that growers are very aware that these herbicides generally achieve only 80 to 95 per cent weed control,” he says. “To gain any benefit from these herbicides it is very important to manage the small number of weeds that are likely to survive the pre-emergent application. There are quite a few herbicide modes of action that have a residual activity and each needs to be applied correctly to achieve the best results.”
“We know that routine use of glyphosate every year in the fallow will cause glyphosate resistance in the weed population within 15 years if no follow-up action is taken to remove survivors,” he says. “This has been demonstrated in trials and is evident in the field. It may then take another five or six years for the glyphosate resistant weeds to dominate in a paddock, again if no follow-up action is taken.”
The spread of resistant weeds across a paddock or around the farm is quite easily done through movement of vehicles, machinery, people and animals as well as wind and water flow across the paddock or along irrigation channels.
Mr Cook says being aware of new weeds on the farm, such as feathertop Rhodes grass deposited on farms around Dalby during flood events over the last few years, growers can get on the front foot with effective tactics such as double knocking before the new weeds are firmly established.
“Glyphosate resistant patches are frequently associated with fencelines and other non-crop areas on the farm and can spread into cropping fields,” says Mr Cook. “Identifying alternate strategies for managing these areas needs to be a high priority on all farms, even if glyphosate resistance is not yet evident.”
Want more? Check out Tony’s webinar and GRDC update paper.
Non-herbicide weed control in the Northern region
Diversity in cropping systems and diversity in weeds in the northern GRDC grains region of NSW and Queensland calls for diversity in weed management solutions, including non-herbicide tactics.
Survey work in the region has identified over 70 different weed species that impact on grain production and over 10 per cent of these weed species have confirmed populations within Australia that are resistant to glyphosate and several other chemical modes of action (MOA).
DAF weeds researchers Annie van de Meulen and Michael Widderick.
Confirmed herbicide resistance in weed populations found in NSW and Queensland.
Source: adapted from a table prepared by M Widderick, DAF.
Mode of Action
A (fops, dims, dens)
wild oats, paradoxa grass, annual ryegrass
B (SUs, imis etc)
annual ryegrass, wild oats, paradoxa grass, Indian hedge mustard, charlock, wild radish, turnip weed, African turnip weed, common sowthistle, black bindweed
C (triazines, ureas, amides etc)
awnless barnyard grass, liverseed grass
D (DNAs, benzamides etc)
I (phenoxys, pyridines etc)
L (bipyridyls i.e. diquat, paraquat)
M (glycines i.e. glyphosate)
annual ryegrass, awnless barnyard grass, liverseed grass, windmill grass, feathertop Rhodes grass, sweet summer grass, flaxleaf fleabane, common sowthistle
Z (dicarboxylic acids etc)
A recent survey of common sowthistle determined populations as glyphosate resistant if treated seedlings were surviving and reshooting 21 days after glyphosate application. In this testing, glyphosate was applied at the upper label rate for small sized plants (up to 5 leaf).
While the majority of common sowthistle samples collected from central Queensland to central NSW were still susceptible to label rates of glyphosate applied to small seedlings, resistant populations were found throughout the study area, showing that this is not a localised problem but rather the inevitable result of over-reliance on a particular herbicide.
Most northern region weeds are self-pollinated so resistant plants will produce resistant seed. To reduce the likelihood of resistance, a key approach is to use multiple tactics to maintain low weed numbers. While weed numbers are low so too is the risk of resistance genes being present in the population.
To keep these ‘difficult to control’ weeds in check will clearly require other, non-herbicide, tactics to reduce germination and weed seed set. Queensland Department of Agriculture and Fisheries (DAF) researchers in Queensland have been studying common weeds, particularly feathertop Rhodes grass, barnyard grass and common sowthistle, to find weaknesses in each weed’s ecology to help identify non-chemical control tactics that could be part of an effective management system.
Dr Michael Widderick and the DAF weed research team are investigating non-chemical options, including various cover crops, crop competition, strategic tillage, strategic burning and harvest weed seed control options.
He says that although growers are making good use of chemical strategies such as double knock, residual herbicides, spot spraying and weed sensing technology to preserve herbicide efficacy, there is an urgent need to investigate non-chemical options that can be added to a weed management program to target resistant weeds in the northern region, as outlined in the WeedSmart 10 Point Plan.
“Most growers are keen to preserve their zero or minimum tillage farming systems that have delivered significant benefits and so are very reluctant to re-introduce cultivation for weed control purposes,” says Dr Widderick. “We are currently researching ways to use cultivation that will have maximum effect on driving down weed numbers while having least impact on the min-till farming system.”
The aim of this research is to investigate the impact of different tillage operations in situations where the weed population has blown out and intensive patch or paddock-scale management is required.
“The key is to understand weed ecology, particularly how seed in the soil seed bank responds to different types of cultivation,” he says.
The team used small plots to determine the effect of burial at different depths on weed seed persistence (long-term viability) and emergence. They also conducted experiments to determine the displacement of seed (glass beads were used to represent the seed) throughout the cultivated zone using four different types of machine—harrows, gyral, off-set discs and one-way discs—compared to the zero till control treatment.
Sowthistle emergence occurs primarily from seeds close to the soil surface with up to 30% of viable seeds emerging over 5 months. Seed can emerge from a depth of up to 2 cm with approximately 4% emergence after 6 months. Seed buried below 5 cm is unable to emerge and can persist at depth.
Seed persistence (the percentage of viable seed after burial) in fleabane was most reduced when seed was buried to a depth of 2 cm and not disturbed for at least two years. Seed buried to a depth of 10 cm remained viable for over 3 years. Feathertop Rhodes grass seed persisted for only 12 months regardless of being left on the surface or buried to 10 cm depth. Barnyard grass however, persisted on the soil surface for up to 2 years and when buried to 10 cm depth remained viable for over three years.
The Gyral machine placed the majority of weed seed in the 0–2 cm and 2–5 cm zones while the offset discs and one-way discs achieving burial of about half the seed below 5 cm depth.
“All species responded to increased tillage intensity with reduced germinations,” says Dr Widderick. “The message here is that infrequent but intense cultivation can be a useful weed management tool within an otherwise zero tillage farming system. Generally, once a deep cultivation has been done there should be no cultivation of that area or paddock for at least four years to avoid the risk of bringing seed back to the soil surface.”
Feathertop Rhodes grass is known to colonise around mature plants and potentially spread to form distinct weedy patches. Killing the large plant at the centre of the colony is usually not possible using chemical treatments.
Strategic burning of early infestations of this weed can effectively reduce the biomass of the survivor plant and reduce the amount of viable seed present on the soil surface from 7500 seeds per m2 to less than 500 seeds per m2.
Growers have made effective use of a flame-thrower to burn large feathertop Rhodes grass plants during the fallow.
Strategic burning of early infestations of feathertop Rhodes grass in a fallow can effectively reduce the biomass of the survivor plant and reduce the amount of viable seed present on the soil surface.
Crop competition through narrower row spacing and or increased planting density provides an effective offensive against common sowthistle and flaxleaf fleabane.
“Our experiment looked at the effect of crop competition on its own, however, in commercial situations crop competition would be used in conjunction with herbicide applications,” says Dr Widderick. “Narrowing wheat rows from 50 cm to 25 cm spacing had the most marked effect on fleabane seedhead production with an additive advantage if the crop density is also increased from 50 plants per m2 to 100 plants per m2.”
Fleabane seedhead production (Source: M Widderick, DAF)
Project work is continuing to investigate the options for increasing crop competitiveness in sorghum, winter and summer pulses and wheat.
“We are particularly keen to identify ways to improve the competitiveness of sorghum crops, which are often a weak link in northern farming systems,” he says. “Highly competitive summer grasses that also have high tolerance and or resistance to herbicide can gain the upper hand in sorghum crops that often do not achieve canopy closure.”
Crop competition through narrower row spacing and or increased planting density provides an effective offensive against common sowthistle and flaxleaf fleabane.
Summer fallow periods are heavily reliant on glyphosate for summer grass control. Preliminary research has explored the potential role of cover crops in place of a chemical fallow for control of summer grass weeds. Summer cover crops such as cowpea, lablab and French millet have the potential to smother summer growing weeds, particularly barnyard grass and feathertop Rhodes grass and return large amounts of organic biomass to the soil.
French millet planted on its own, or in combination with the legumes, increased the amount of biomass produced. The higher the biomass production the greater the suppression of weeds. Cover crops will tend to use fallow stored moisture so the team investigated the effect of two termination dates on both subsequent crop yield and on weed numbers.
“Feathertop Rhodes grass germination was minimal after all the cover crop treatments, and the yield of the following wheat crop was comparable to the chemical fallow control and no yield differences were found between treatments,” says Dr Widderick. “For barnyard grass, late termination of the cover crop reduced weed emergences before and after the following wheat crop, however there was a trend toward slightly reduced wheat yield compared to the early termination treatments that tended to boost yield compared to the chemical fallow control.”
“The reduction in barnyard grass emergence and wheat yield are both likely due to reduced soil water following the late terminated cover crops,” he says. “Much more work is required to identify suitable cover crops and define the parameters for their use as a weed management tactic.”
Harvest weed seed control
Harvest weed seed control (HWSC) is known to be an effective strategy and is widely adopted in Western Australia and increasingly in South Australia, Victoria and southern NSW. The efficacy of the current tools such as narrow windrow burning, chaff carts, tramlining or chaff decks, bale direct systems and the new integrated Harrington Seed Destructor to control weeds found in the northern growing region is largely unknown.
Trial work in winter crops to date have revealed that these tools can effectively collect and concentrate brassica weeds growing in wheat and chickpea crops in the northern region. The DAF weed research team is conducting further HWSC trials this winter with a focus on collecting wild oats seed. They are also keen to hear from growers planning to plant sorghum this summer where the team will be assessing the efficacy of various HWSC tools against barnyard grass and feathertop Rhodes grass.
More information – Read the full published paper here: Non-chemical tactics for improved control of key northern region weeds and listen in to the webinar recording with Michael Widderick and Annie van de Meulen.
Kohlhagen family is keeping weed numbers low
A changed attitude to weeds has been driving brothers Malcolm and Des Kohlhagen to implement a comprehensive management program on their farm near Wagga Wagga in southern NSW.
Fifteen years ago the Kohlhagens assessed their weeds in terms of their likely economic impact but their mindset has changed to a much lower tolerance of weeds and they aim to keep numbers low year in, year out.
Brothers Malcolm (left) and Des (rear) Kohlhagen along with Malcolm’s son, Adrian (centre), have developed and implemented a comprehensive weed management program for their 100 per cent cropping operation in southern NSW.
The sheep have all gone from their 1600 ha operation and the family has expanded their cropping program to include a wider range of crops. The winter program now includes wheat, barley, canola, albus lupin and, most recently, faba bean.
Malcolm and Des use break crops to introduce a different range of herbicides into the rotation and a double break of a pulse followed with canola provides two years of grass control so the cereals are sown into clean paddocks. The Kohlhagens have stuck to their crop rotation even when many other growers in the district reduced their canola hectares during the 2000s.
They grew field peas many years ago but gave them away due to harvesting difficulties, and now find lupin and faba bean are a better fit in the rotation, providing a definite nitrogen boost for the following canola crop.
Triazine tolerant (TT) and Clearfield canola are used to rotate chemical modes of action. The Kohlhagens don’t currently consider RoundUp Ready canola an option for them due to delivery point and marketing issues.
Harvesting weed seed
Canola crops are windrowed to aid in harvest management and, from this season, will be crop topped under the cutter bar to capture any late escaping weeds. The Kohlhagens also plan to crop top their pulse crops in years where late escapes are a problem.
A narrow windrow chute is used on the harvester to collect weed seed in the pulse and canola crops and the narrow windrows are burnt to kill any seed present. This means over 40 per cent of their cropping area is subject to this very effective non-herbicide weed control method, particularly for annual ryegrass.
A double break crop of a pulse crop followed with canola provides excellent grass weed control, including narrow windrow burning, before returning to cereals in the rotation.
The brothers do not narrow windrow burn their cereal crops because of the high stubble load from barley and wheat crops yielding up to 6.5 t/ha and 5 t/ha respectively. When grown back to back these cereal crops generate too much stubble to effectively confine the fire to the narrow windrows.
Getting the right conditions for burning is not always easy but the Kohlhagens believe it is worth doing and are looking forward to when they can justify investing in an integrated Harrington Seed Destructor so they can avoid burning, a practice that is not popular in town!
In years where weed populations increase for any reason, haymaking is an effective method to stop weed seed set. The Kohlhagens find their heavier soil types are more likely to be challenged with weed blow-outs so they target these areas for haymaking as a salvage operation when necessary, giving great weed management benefits in poorer seasons.
Malcolm and Des currently contend with ryegrass that is resistant to Hoegrass (Group A, fop) and are aware of similar resistance arising in wild oats and possibly wild radish on the farm. To keep wild radish numbers as low as possible they hand rogue plants in spring, to avoid any seed going through the header.
Further narrowing their row spacing to create greater crop competition is currently on the table for the family but until they have decided on the best option they are using high seeding rates, especially in the already-competitive barley crops. Blockages in the seeder and slug and slater damage to seedlings can create gaps in the rows, which provides the opportunity for weeds to flourish, and wider rows also allow more weeds to grow between rows.
Changing the seeding setup is quite an expense so the family is considering whether to reduce the spacing on their current tined seeder to 250 mm or to change to a disc seeder.
Even at 300 mm they are having trouble managing the stubble load so are hesitant to narrow the spacing much further, however the tined seeder allows them more herbicide options than can be safely used with disc seeders. On the other hand, they have been impressed with a 150 mm disc seeder they have seen operating in high stubble environments in South Australia.
When the Kohlhagens first moved to controlled traffic, wheel tracks were left bare. However, the gap left by the wheel tracks in the controlled traffic system lets more light into the adjacent rows, allowing more weeds to establish. To reduce this effect the Kohlhagens now seed their wheel tracks using a mid-row banding disc to provide increased crop competition and reduce weeds.
Pre-emergent herbicides are applied in all crops for grass control. While clethodim is still providing effective control in canola the Kohlhagens are well aware that it may not continue to be an option in the future. To support the pre-emergent herbicides the Kohlhagens are sowing their canola and pulse crops early to encourage better establishment and more rapid canopy closure, reducing the opportunity for in-crop weed germinations after the residual effect has diminished.
Changed farming system; changed weed spectrum
Since moving from a mixed farming operation to 100 per cent cropping, Malcolm and Des have seen a change in the weed spectrum present on the farm, with less capeweed present and less movement of weed seed around the farm.
Not having livestock however has increased the need for more vigilance over summer to prevent weeds using precious soil moisture that may be the difference between finishing a crop and crop failure as the October rainfall is now less reliable. Fleabane is of particular concern in summers with higher rainfall seeing explosions in fleabane populations, which may require double knock treatments.
Hairy panic is another persistent weed that must be sprayed when small to achieve effective and economical control. To manage broadleaf weeds the Kohlhagens use a low volatile 2,4D ester spike in glyphosate sprays applied over summer. Milk thistle is another emerging weed that is taking advantage of the no-till farming system.
Clean seed a priority
Prior to harvest Malcolm and Des inspect their paddocks to identify the cleanest areas of each variety suitable to harvest for retained seed. They then use a low capacity seed grader to remove small or damaged seed and as much weed seed as possible from their seed before storing on-farm.
A mobile grader is contracted to grade the pulse grain for marketing and to thoroughly clean the pulse seed retained for sowing.
Malcolm and Des are clearly on top of their game with their weed management program but they are also full of praise for their agronomist, Greg Condon from Grassroots Agronomy, who provides excellent agronomic advice and keeps their herbicide program up to date.
IMI-tolerant crops – use sparingly and to best effect
The much-anticipated release of imi-tolerant sunflower and sorghum hybrids over the next few years has the potential to increase the options open to growers to better utilise summer rainfall.
This new technology also increases the potential for over-use of the broad-spectrum imidazolinone (or imi) herbicides, along with an increased risk of escalating weed resistance. To manage this risk, it is essential that growers use a calculated approach to crop and herbicide rotation, and use multiple weed control tools in the year of imi herbicide application to prevent seed set – as recommended in the Weedsmart 10-Point Plan.
Since the Clearfield technology was commercialised in 1992, plant breeders in Australia have developed the range of imi-tolerant summer (maize, and soon sorghum and sunflower) and winter (wheat, barley and canola) crops, along with lentils with improved tolerance of imi herbicides.
Imi-tolerant sorghum hybrids (centre) currently in the development phase are showing promise for improved control of grass weeds in-crop. (Photo: Rob Crothers, DuPont Pioneer)
Understanding the risk
Plant breeders use conventional methods to breed the single gene imi-tolerance trait into a range of crops. Unfortunately, weeds can also naturally evolve resistance to Group B herbicides, all of which work by targeting the function of the ALS enzyme required to produce essential plant proteins.
Weeds that are resistant to one Group B herbicide will often be resistant to others in the group, even if there has been no exposure to the other sub-groups. This propensity for cross-resistance is a considerable risk and strategies must be in place to minimise exposure across the whole Group B MOA, not only the imidazolinone sub-group.
To protect the imi-tolerant crop technology it is essential to implement the stewardship guidelines for these crops. Richard Holzknecht, BASF technical services manager for Queensland and NSW, says growers will see optimal results in imi-tolerant summer crops when they use post-emergent imi herbicide products to control weeds.
“Pre-emergent imi products are generally less effective in controlling germinating weeds compared with well-timed post-emergent products,” he says. “Some pre-emergent imi herbicides, such as imazapic, have longer soil half-life and can restrict crop rotational choice or cause adverse plant-back effects.”
“The residual use of imazapic products also exposes new weed germinations to ever-reducing sub-lethal doses of product, which increases the risk of weed resistance.”
There are limited post-emergent grass weed options available in summer cropping due to increasing incidence of weed resistance to Group A and B (SU) and potential crop damage. One solution is the use of the post-emergent imi herbicide, imazamox (e.g. Intervix®) to provide broadleaf and grass weed control in the one product.
“Post-emergent application targets a known weed population and takes advantage of crop canopy closure to suppress late germinations,” says Mr Holzknecht. “Growers also have the option to use an alternate MOA for their pre-emergent and fallow applications to layer control measures to achieve better season-long weed control.”
Imi herbicides can then be used to control a broad spectrum of grass and broadleaf weeds, including weeds that are closely related to the imi-tolerant crop. Crops with imi-tolerance are also a useful option if residues are present in the soil from previous applications that would otherwise prevent the production of conventional varieties or hybrids.
The over-arching recommendation for imidazolinone resistance management is to limit using Group B herbicides to twice in every four-year period per paddock and to thoroughly control any surviving weeds.
Mr Holzknecht says it is necessary to understand and utilise each herbicide’s strengths and to use each product to its best advantage. “Weed species biology, crop rotation and herbicide choice all need to be taken into account when planning an integrated weed management program,” he says. “Growers must avoid over-use of imidazolinone products in cropping systems, as the risk of resistance to these herbicides is high.”
“Across the rotation, look for ways to layer control measures by using alternate modes of action for fallow, pre-emergent and post-emergent applications in different crops,” he says. “And consider a tank mix with a non-ALS mode of action herbicide at full label rates for in-crop weed control.”
Imi-tolerant sorghum and sunflower hybrid releases
Nuseed’s program manager for summer crops, Chris Haire has been involved in the development of imidazole (imi) tolerant sunflower hybrids that will increase the opportunities to safely plant sunflower into situations that would be untenable with current hybrids.
“With an imi-tolerant hybrid, growers will be able to safely plant into paddocks where imidazole (Group B) chemistry has been recently applied, say after an IT-maize, IT-wheat or pulse crop, negating the current 12+ month plant-back requirement, which restricts the choices that growers have in summer,” he says.
“The second major benefit will be that an imi-tolerant sunflower hybrid could be planted into a paddock that had a broadleaf weed burden—a situation that would currently not be advisable.”
Nuseed expect to be conducting in-country trials required to gain chemical registrations for the new use pattern in 2017–18. Mr Haire is hopeful that a small amount of seed may be commercially available in 2018–19, opening up more cropping options for growers in Central Queensland and the western Darling Downs particularly.
Imi-tolerant sunflower trial – 28 days after application after planting into a weedy situation (0 days after application inset).
Similarly, Pioneer Seeds have sorghum hybrids with Group B tolerance coming through their plant breeding program, adding to their suite of IT canola and corn hybrids. Rob Crothers, Australian sorghum and northern corn product manager said that INZEN hybrids will be evaluated in field trials again this coming summer.
“When we are successful in bringing this new technology to the market place it will enable growers to spray grass weeds with a post-emergent herbicide in crop for the first time,” he says.
Windrow to collect early shedding weed seed
Looking toward harvest, the pressure is on to get the timing right to maximise yield and grain quality. Delta Ag senior farm consultant, Tim Condon, says it is also a great time to be capturing weed seeds, particularly from wild oats plants that have escaped in-crop herbicides, using a non-herbicide tool to manage herbicide resistance.
“Windrowing, or swathing, barley has been traditionally used as a harvest management tool when growers have a number of different crops to harvest in a short time,” he says. “The really good thing about this technique is that it also gives growers another shot at collecting and destroying weed seed.”
Successful windrowing begins with cutting the crop low and, where possible at an angle to the seeding direction. When followed with any harvest weed seed management tactic, even early shedding weeds such as wild oats can be effectively controlled.
Swathing barley at the earliest opportunity maintains the crop’s yield, and if done correctly, the swaths protect the grain from weathering. Mr Condon recommends checking the crop away from the paddock edges and looking for signs of physiological maturity in the barley plant. “Barley, and wheat, can be swathed when the grain is at 30 to 35 per cent moisture,” he says. “This is indicated by the peduncle [the little stem just below the head] of the grain head turning from green to brown and if you press your thumbnail into the grain there should be no dent created in the grain.”
“Once the crop is mature, the key to effective swathing – and also for the best results with weed seed collection – is to cut the crop low,” he says. “Cutting at boot or beer can height provides sturdy support for the swath, keeping the grain and the weed seed off the ground and making harvest more efficient.”
In some situations, it can also be beneficial to swath at an angle to the seeding direction to provide extra support under the swath. Good results have been seen in Western Australian crops swathed 15, 45 and 90 degrees to the seeding direction. In controlled traffic systems, using a mixer belt, or setting the belts at different speeds, is an alternative to swathing at an angle to achieve the necessary cross thatching to keep the swathed crop off the ground, maintain grain quality and allow easy pick up.
“Crops sown on narrow row spacing will also swath better, and there is good evidence that narrow row spacing also suppresses weed seed production,” says Mr Condon. “In southern NSW the main weed that we can target with swathing is wild oats. Wild oats is renowned for shedding seed before crop harvest so swathing can reduce the amount of seed shed onto the ground. Even with good control of wild oats germinations around sowing, there are often later germinated weeds that grow with the crop but are less mature at harvest than earlier germinations would have been.”
Wild oats is notorious for shedding its seed early. Windrowing is an excellent way to collect the seed from wild oats plants that have escaped in-crop herbicide treatment.
Swathing, followed with any of the harvest weed seed control tactics, becomes a very useful non-herbicide tactic to use with weed species that shed their seed early. Other potential targets with this strategy are wild radish, brome grass and annual ryegrass.
Once the swathing is done, the barley can be left for several weeks until there is an opportunity to harvest. However, there is always a risk of the windrow not drying quickly if wet weather persists, which may cause grain quality to deteriorate. Options for destroying the weed seed captured in the swath include narrow windrow burning, chaff carting, Harrington seed destructor, hay baling or chaff lining (in controlled traffic systems), all of which are detailed on the Weedsmart website.
“Narrow windrow burning can be risky in leafy crops like barley, especially in high yielding districts,” says Mr Condon. “A new technique being trialed in these situations is chaff lining, where the chaff component is dropped into a narrow band and left to rot down. Early observations suggest that weed seeds don’t survive, especially when the chaff stream is directed into the wheel tracks in a controlled traffic system.”
Avoid confusion: The terms ‘swathing’ and ‘windrowing’ refer to the same practice. In NSW and Vic ‘windrowing’ is the more commonly used term while ‘swathing’ is the more commonly used term in WA and SA. However, ‘narrow windrow burning’ is a completely different practice.
Webinar – Windrowing and crop topping to stop weed seed set
AHRI Insight – Make time stand still
Why consider sowing east-west?
In winter, when the sun is travelling at a lower angle the additional shading of the inter-row that occurs when crops are sown in rows running east-west can help suppress weeds growing in-crop. This effect is most noticeable in southern latitudes.
In Western Australia, Department of Agriculture and Food researcher Dr Catherine Borger demonstrated in six trials that east-west sowing can halve annual ryegrass weed seed set, even though weed biomass may not be reduced. This is a rare opportunity for free weed control that could be worth implementing in suitable paddocks.
Weeds researcher Dr Catherine Borger measured the difference in light penetration through the crop and weed growth when the crop was sown east-west compared to north-south. Photo: DAFWA
cent weed biomass reduction when crop rows ran east–west compared to north–south. At Trangie the barley varieties Hindmarsh and Granger had higher yield and were more competitive against weeds when sown east–west. In these trials weed biomass was measured rather than weed seed production, however both are good indicators of the weed response to the treatments.
NSW DPI research and development agronomist, Greg Brooke says that in many cases there are other, more practical, ways to suppress weeds but that row orientation is well worth considering and implementing if it can be easily done.
“The more non-herbicide tactics a grower can use to support their herbicide program the better,” he says. “Crop yield is not compromised under east–west sowing.”
Researchers in NSW also noticed a stark difference in the growth of the weed fumitory, with the weed being prolific in the north–south rows and absent in the east–west rows.
Growing crops in rows at right-angles to the sun decreases the amount of solar radiation that the weed plants can intercept and use for photosynthesis. This translates into lower biomass production and potentially decreased seed production.
Importance of early weed control
A weed seedling can impact crop yield potential well before the weeds are large enough to take moisture, light and nutrients away from the crop.
Canadian researchers Dr Clarence Swanton and Dr Jessica Gal observed this effect in both corn and soybean. The weeds emerging alongside or soon after the crop seedlings reduced the crop yield potential, with the effect occurring at the seedling stage, long before the weeds could be competing directly for resources such as moisture, nutrients and light. The differences in light wavelengths bouncing off bare soil compared to growing weeds triggers a reaction in the crop plants to put a disproportionate amount of resources into growing taller, with more leaf area—resulting in a comparatively smaller root system.
This response can pre-condition the crop to a lower yield, particularly in years where the crop may encounter moisture stress and have limited ability to seek deeper stored moisture.
DAFWA researcher Dr Catherine Borger says that crop plants adopt this ‘shade avoidance growth characteristic’ to allow them to more effectively shade weeds as the plants mature and begin directly competing for light.
“Since crop plants in an east-west orientation physically shade weeds at an earlier growth stage, weeds have less opportunity to trigger the shade avoidance growth characteristics,” she says.
The interrow area is shaded earlier in the season when the crop is sown east-west, suppressing weed germination and growth early.
Choosing competitive crops and varieties
“The row orientation is one part of increasing crop competition,” says Mr Brooke. “The other factors to consider are row width, seeding rate and crop and variety selection.”
“The difference in competitiveness between crop types is well known with general rules of thumb such as triticale, barley and rye being more competitive than wheat; and cereals are generally more competitive than pulses and canola,” he says. “What is less well understood are the differences between varieties. In one trial Skipper barley achieved a 30–40 per cent greater weed biomass reduction compared to Hindmarsh barley, however the competitive ability of different varieties varies widely between sites and seasons.”
Generally, varieties that exhibit rapid early growth and early canopy closure will provide better weed suppression than less vigorous varieties of the same crop type. Seeding rate and row spacing also play a part in increasing the crop’s competitiveness.
Choosing a highly competitive cultivar in one year of a three to five year program can have a significant and lasting effect on the weed seed bank. If this is coupled with harvest weed seed control, and possibly east–west sowing, the result could be impressive and achieved at a very low cost.
“Introducing more non-herbicide tactics into the farming program gives crops the advantage over weeds and reduces the pressure on the herbicide,” says Mr Brooke. “Farmers have started using tactics like double-sowing known weed areas such as along irrigation channels. With GPS, double pass seeding automatically gives narrow rows, doubles the seeding rate and makes those key management areas much more competitive.”
Dry sow clean paddocks first
More and more growers are dry seeding wheat to avoid terminal drought at the end of the season. Research undertaken by CSIRO and WANTFA has shown that this can result in significant yield gains and larger whole farm grain production albeit with a small increase in frost risk.
However, WANTFA executive director Dr David Minkey said that there is a greater risk of weeds germinating alongside the crop. “Unless the weed seed bank is very low, the heavy reliance on in-crop selective herbicides can quickly lead to herbicide resistance,” he said.
The breakdown patterns of pre-emergent herbicides under a variety of seasonal conditions can affect the efficacy of early and staggered germinations of annual ryegrass. WANTFA executive director Dr David Minkey recommends dry sowing the cleanest paddocks first to minimise the risk of poor weed control results.
With this risk in mind WANTFA has been investigating the behaviour of pre-emergent herbicides under early dry sowing conditions to determine the rate of breakdown under various seasonal conditions.
“While the first step is to sow the cleanest paddocks, the next important strategy is to lay down pre-emergent herbicides to take care of any early germinating weeds,” he said. “We need to determine the rate of decay of the pre-emergent herbicides so growers will have a good idea whether the pre-emergent herbicide will still be effective when the rain comes, and can take action early if necessary.”
“Our research is focusing on overlaying emergence patterns for annual ryegrass and the efficacy of different pre-emergent products in different seasonal scenarios,” said Dr Minkey. “When the pre-emergent herbicides are applied to dry soil there is usually very little decay of the product. The biggest risk of decay is in years where a small rain event occurs that is enough to activate the herbicide but is insufficient to initiate crop germination. Under these conditions the pre-emergent product will have reduced efficacy when good rains arrive.”
“Some annual ryegrass seeds can remain dormant through April and May, creating a situation where the early germinating ryegrass is controlled but later, staggered flushes occur after all the pre-emergent herbicide has decayed. In these situations the need for effective harvest weed seed control is important,” he said.
With the 2016 season being off to a hot and wet start in Western Australia Dr Minkey expects ryegrass dormancy to be broken down early, suggesting that the main flush of ryegrass germination could be early in the season when the pre-emergent herbicide is active if cool conditions are met.
“Where several light rainfall events occur after dry seeding the rate of pre-emergent herbicide breakdown is more rapid. If these conditions are forecast, applying a higher registered dose and mixing compatible products can extend the efficacy of the herbicide,” he says. “If the initial application decays before the weeds germinate it may be possible to apply early post-emergent residual product such as Boxer Gold, keeping in mind that the result may not be fully effective and the operation will be expensive.”
Pictured with Craig White, Bayer Crop Science Technical Advisor and Leader of Integrated Weed Management, Dr David Minkey is determining the decay curves for a range of pre-emergent herbicides under different season conditions.
The potential for herbicide resistance is high if dry sowing is not managed well, and many supporting strategies must be in place to eliminate survivors. “Pre-emergent herbicides must be supported by other weed control practices,” says Dr Minkey.
“Clean paddocks are a must,” he says. “Then consider planting crops with early vigour or crops with in-crop herbicide options, such as RR and TT canola, include harvest weed seed control and invest in summer weed control to preserve moisture and nitrogen to support early crop growth.”
The WeedSmart 10 point plan provides information on a diverse range of crop management practices that can be used to eliminate weed survivors throughout the season.
What is the safest way to manage pre-emergent herbicides at seeding?
Using tank mixes to extend herbicide ‘life’
Soil behaviour of pre-emergent herbicides in Australian farming systems: a reference manual for agronomic advisors
Making clean seed your business
In 1980 the Bach family of Toowoomba diversified their farming operation to include a commercial grain storage and handling facility. With a background in grain production the family knows what’s needed to provide an efficient and safe grain handling service for other farmers.
Not only do they know the importance of cleaning and grading grain to bring it ‘up to spec’, they also understand the value of removing weed contamination from seed that is being retained for planting, removing extraneous matter that can lead to problems in long-term storage and selecting the largest seed with the highest germination percentage and early vigour.
David Bach from Toowoomba Grain and Storage suggests harvesting much more grain than you need for seed, getting it cleaned and keeping the largest grain aside for seed.
David Bach manages the family’s grain handling facility near Toowoomba, on Queensland’s Darling Downs. He says that since retained seed must be stored for longer than most grain is held on farm, it must be stored in optimal conditions. “Grading grain at harvest will remove trash such as leaf and stem material that can attract insects and mould while the grain is in storage, either awaiting sale or being retained for seed,” he says. “Once cleaned the seed then needs to be kept cool and dry to maintain seed quality.”
“When planning to retain seed on farm, select the best part of the paddock and harvest it first,” says David. “This way you will have collected the seed with the greatest vigour, which will provide the most competition for weeds in the early growth phase.”
If there is not an area of the paddock that is clearly better than the rest, David suggests harvesting much more grain than you need for seed, getting it cleaned and keeping the largest grain aside for seed.
“Grading it hard means that you have the best chance to remove a large proportion of the weed seeds present and you will also have a more consistent line of seed with the highest germination percentage,” he says.
“It is very important that grain is cleaned at harvest, before it is stored. Clean seed that is stored and managed properly can remain viable for over 9 years.”
David’s brother, Peter Bach manages the family’s farming operation—1620 ha of barley, wheat, sorghum, corn, mungbean and some faba bean—50 km west of Toowoomba.
Retaining seed not only represents a cost saving for them, it also provides a back-up if some or all of a paddock needs to be re-seeded for any reason.
Having a good supply of seed on hand means that growers can take advantage of favourable seasonal conditions. “We try to store enough seed here to plant half of the farm’s cropping area as soon as the soil moisture conditions allow,” says David. “This way we can make last-minute decisions and be confident that the seed we plant is clean and good quality.”
“Especially when the price is up it can be difficult to source seed, so we clean five times as much seed as we expect to use and store it,” he says. “To get that seed we might clean 120 tonne of grain and just keep the best 10 tonne for seed knowing that it has been thoroughly cleaned and graded.”
David cites black oats as the main problem in their area for barley and wheat crops, and sees that the wild turnip is soon going to be a major concern for growers.
“It pays to clean mungbean seed very hard,” he says. “Just one tonne of seed is required to sow 40 ha so it makes sense for that tonne of seed to be the very best that you have available, and free of weed seed contamination.”
Johnstone grass is the most difficult weed to remove from sorghum and maize crops in summer and David sees the herbicide tolerant hybrids providing some useful options for grass control in these summer crops.
However, he has noticed an increasing problem with herbicide resistant crops growing as volunteers in other crops and contaminating that grain. “For example, imi-tolerant sorghum might grow as a volunteer in another, conventional crop, and will not be controlled by the herbicides applied in that crop,” he says. “Further cross contamination can occur if that seed is unintentionally kept for planting. It is easy to become complacent about the herbicide tolerant crop plants growing on roadsides and the potential flow of seed from roadsides into grain paddocks.”
Seed cleaning equipment
There are several types of grain cleaning equipment available that vary in their efficiency when it comes to weed seed removal.
The Bachs use a rotary screen machine that has two main sections—1. an aspirator, where a fan sucks air through the grain, removing fine particles such as dust, and light material such as husks and some weed seeds and 2. the screens, where the grain rolls around inside a drum with different sized screens that allow the grain to be separated according to size.
“Usually the grain is sorted into two sizes plus the gradings or screenings, where the vast majority of weed seed is collected,” says David.
“Improving the grade of the sample is usually fairly simple, but cleaning for seed is much more time consuming and therefore costs more.”
“Sometimes growers think that their grain is cleaner than it really is,” says David. “On farms where the spraying is contracted out the farmer may not be as aware of the weed populations around their property.”
David says the value of having a commercial grain handling contractor do the seed cleaning lies in the contractor’s knowledge about how to set the machine up to achieve the best result. “The screens are expensive but it pays to use the right combination of screens to suit the grain and the weed spectrum,” he says. “It is probably not economic for a grower to invest in the large number of screens required to do the best job in all situations.”
“Grading table gear does an excellent job to remove weed seeds too,” says David. “These machines are most commonly found at commercial grain packing and processing facilities and could be a viable option for growers to use in some situations.”
Research conducted in Western Australia confirms David’s comments about the value of having seed cleaned by a specialist rather than using equipment, such as sieves or in-field rotary screens, that some growers use to clean their seed on farm.
Economics of seed cleaning
Growing seed for future planting needs to be a planned operation—start with clean seed, sow into a clean paddock, grow a competitive crop that suppresses weeds, keep the crop weed-free by taking action if individual plants survive treatment, harvest the best, cleanest part of the paddock, clean the seed hard and store it under optimal conditions. “Seed is very valuable and is worth investing in,” says David. “If you plant clean seed into clean paddocks the cost savings in time and herbicide will soon pay for the cleaning of the seed.”
To determine how many weed seeds are present in a potential seed lot, collect a 1 kg sample and separate the crop seed from all other material. 100 weed seeds per kilo of cereal or pulse seed sampled equals around one weed per square metre when the crop is sown.
A survey in Western Australia by the GRDC-funded Australian Herbicide Resistance Initiative found that un-cleaned seed samples can contain over 1500 weed seeds per 10 kg planting seed, which would add extraordinary pressure on the next crop. The AHRI survey found that the gravity table method of seed cleaning consistently produces the cleanest seed sample, reducing contamination to about 25 weed seeds per 10 kg. Sieves alone can bring the number down to about 150 weed seeds per 10 kg.
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Clean weed-free seed – don’t plant weeds
When is clean clean enough?
Fleabane control in the south and west
There are many hard to kill weeds, including flaxleaf fleabane, however one approach delivers consistent success – spray small weeds and use the double knock technique.
At the end of grain harvest in the southern and western regions flaxleaf fleabane will be establishing and growing strong root systems ready to take up any soil moisture available over summer. Flaxleaf fleabane plants feature hairy leaf surfaces, thick cuticle and few stomata, a combination that affords the weed a natural tolerance to herbicide.
Fleabane seedlings germinate throughout the spring and summer, making the timing of control very difficult.
In South Australia fleabane seeds start germinating in late winter to late spring, but initial seedling growth rate is very slow. With suitable spray conditions being few and far between over summer, fleabane is able to establish and take advantage of any small falls of rain to produce up to 120 thousand viable light fluffy seeds per plant that disperse on the wind and in runoff water over summer and autumn.
Weeds researcher Ben Fleet from the University of Adelaide says the timing and staggered germination of fleabane, coupled with the need to treat plants when they are small, is a combination that makes the weed very difficult to control with herbicides.
“All flaxleaf plants have a natural tolerance to herbicides but they are much more susceptible to herbicide control when the plants are young, less than a month old,” he said. “While rosette stage plants can be easily killed with lower rates of glyphosate, once stem elongation begins a far greater dose is likely to be required to achieve similar results.”
“In NSW and Queensland, glyphosate resistance has been identified in flaxleaf fleabane populations, indicating that while glyphosate has proven an effective tool on fleabane, increasing resistance will mean this herbicide will be less effective in the future.”
In summer fallow, herbicide control trials at Bute and Pinnaroo in South Australia, robust rates of glyphosate provided the greatest level of control. Use of paraquat in a double knock herbicide strategy helped to achieve high levels of control, but only when the first herbicide application was capable of providing at least 60 per cent control in its own right.
“Controlling fleabane in summer conserved 45 mm and 71 mm of soil moisture at the Bute and Pinnaroo sites respectively, as measured in April,” said Mr Fleet.
Flaxleaf fleabane, melons, sow thistle, windmill grass and feathertop Rhodes grass have all been associated more with the northern region but in fact they are all weeds that perform well in zero and minimum tillage systems. As these practices are becoming more common in the south and west, so the associated weeds are becoming more of a problem, particularly in years with mild, wet spring and autumn conditions.
“The mix of weeds present in a summer fallow varies dramatically between seasons in the southern region as these weeds respond to the prevailing seasonal conditions,” he said. “A few years ago, the combination of wet springs and good summer rainfall led to serious infestations of fleabane on many farms in South Australia. Then the subsequent run of dry spring and summers led to a decline in fleabane populations on farms to the point that researchers had difficulty in finding suitable trial sites.”
Mr Fleet emphasised that the efficacy of glyphosate on fleabane varied considerably in different seasons. “For example, glyphosate alone applied at 2 L/ha provided a modest 55% weed kill in 2012 but gave 97% control in 2014. In all seasons a double-knock of paraquat after glyphosate treatment ensured a higher weed kill,” he said.
Aside from herbicide controls, don’t underestimate the value of strong crop competition. Fleabane thrives along crop borders and in gaps that may appear within the crop.
Mr Fleet said that while herbicide control can be effective, particularly when plants are treated at the seedling stage (rosette), it is also important not to underestimate the value of crop competition in the winter cropping phase.
“Fleabane seedlings are highly sensitive to crop competition and any bare patches in a paddock provide an ideal environment for fleabane establishment,” he said. “Under moisture stress conditions in spring there tends to be extremely high seedling mortality.”
It’s time for a Glyphosate Intervention
If you knew some folks were outside your house and planning a home invasion would you encourage them or do what you could to intervene?
Of course you wouldn’t invite them in or give them the weapons they needed to wreck your home and steal your stuff.
The problem is that weeds along roadways and fences are getting ready to invade crops all over the country, having been afforded the opportunity to train with the best herbicide available and now more and more of them have the weapon they need—glyphosate resistance.
Weeds along roadways, fences and farm infrastructure are getting ready to invade crops all over the country, having been afforded the opportunity to train with the best herbicide available and now more and more of them have the weapon they need—glyphosate resistance.
At a meeting of concerned growers, agronomists and researchers at the Crop Updates in Perth earlier this year, discussion centred on the fact that the weeds growing on non-crop areas in and around farms are without doubt the main source of glyphosate resistant weed seed. There is also no doubt that these weeds are ready and able to move into crop areas, as soon as the opportunity arises.
The audience heard from Esperance grower, Chris Reichstein, who has brought glyphosate resistant ryegrass growing on a newly purchased property under control. Chris’ message was encouraging in that glyphosate resistance is manageable but it has placed a burden on his business that would have been avoided if better practices had been employed in the non-crop areas of the farm over the previous 15 or 20 years.
Chris has used every tactic possible to reduce weed seed numbers on the farm, including rotation planning to make the most of crop and herbicide combinations, including swathing and croptopping, narrow windrow burning, chaff carts, autumn tickle and a triple knock strategy of two chemical applications followed with a competitive crop. Containing the problem is now a permanent part of his farming system.
It is unrealistic to ‘ban’ the use of glyphosate along fences, roadways and around infrastructure, but it is clear that growers need to find other ways to manage these weeds without reaching for glyphosate as the first and only option.
There are other options, albeit more difficult, time consuming or expensive but imagine how difficult farming the cropping area would be without glyphosate. Depending on the situation, some fenceline tactics to prevent seed set could be to use different herbicides and change the timing of sprays to target weeds when they are small, planting the crop right to the fenceline and then cutting the first round for hay, using a ‘slash and spray’ approach, cultivation and knife rolling—anything but glyphosate!
Just last year a flood of evidence emerged, all pointing toward the imminent and widespread threat of glyphosate resistance:
Sally Peltzer, DAFWA, released the results of a GRDC funded survey that sampled ryegrass from 175 paddocks, finding glyphosate resistance in 40% of them.
John Moore, DAFWA, reported finding similar levels of glyphosate resistance when sampling trucks at CBH, Albany.
Dr Michael Ashworth, AHRI, reported the first case of glyphosate resistance in wild radish.
Adam Jalaludin, AHRI student, found a population of crowsfoot grass in Malaysia with resistance to glyphosate, glufosinate and paraquat.
In NSW, glyphosate resistant sowthistle was confirmed and Queensland announced glyphosate resistance had been confirmed in sweet summer grass.
Concerned agronomists raised the alarm as knockdown failures became evident in the field.
Now is the time for action and the implementation of a new system for managing weeds in the non-crop areas of Australian farms.