Can multi-species planting provide effective weed control?
Crop competition is one of the most effective weed control tools available to growers, but some crops simply don’t have a competitive edge.
Dr Andrew Fletcher, a farming systems scientist with CSIRO, says companion planting and intercropping is an option that growers can consider to bolster the competitiveness of an otherwise uncompetitive but valuable crop in the rotation. International research suggests that it can!
Andrew Fletcher, CSIRO farming systems researcher sees potential for multi-species plantings to compete with weeds. Photo: GRDC
“When two or more species are grown together they can occupy ecological niches that might otherwise be taken up by weeds,” he says. “Multi-species plantings have several potential benefits including increased crop yield and improved soil health, but the right combination can also reduce weed biomass by over 50 per cent.”
Multi-species plantings can be quite challenging to integrate into a grain cropping rotation but are more easily used in mixed grain and livestock operations and in intensive pastures for dairy cattle. International research suggests there is a significant untapped opportunity to increase the use of these systems in Australian grain production systems. However, relevant Australian data is scarce and more research is required to understand this untapped potential in Australian systems.
A mixed-species cover crop can provide multiple soil health benefits, grazing and fodder for livestock and weed control through crop competition and stopping weed seed set.
“Crop competition is a non-herbicide pillar in the WeedSmart Big 6, with the potential to do some serious heavy lifting in terms of weed control,” says Andrew. “Intercropping and companion planting offers a means to bolster the competitiveness of some crops and to keep them in the rotation without risking a weed blow-out.”
What is intercropping, companion planting and mixed-species planting?
In brief: These systems all involve planting two or more crop species together. The combinations are almost limitless.
The details: Intercropping involves planting two or more species together and harvesting the grain of multiple crops. This generally relies on the grain species having different size seed and compatible harvest times.
Companion planting involves two or more species planted together with the intention to harvest grain from one species only after grazing or terminating the other species before seed set.
Sowing a low-growing species like clover between the rows of cereal can compete with weeds in the inter-row area, fix nitrogen and provide the basis of a pasture after the cereal grain is harvested. This is one example of companion planting.
Mixed-species planting is used to describe plantings of several species grown together primarily for the soil health benefits, and that may have potential for grazing and or forage conservation.
How do these systems suppress weed growth?
In brief: These multi-species systems are designed to take up the ecological space that might otherwise present and opportunity for weeds to fill.
The details: Intercropping and companion planting provide additional weed control in situations where one of the species is a relatively poor competitor as a sole crop. By maximising competition, weed growth is suppressed by up to 58 per cent compared to the least competitive species grown on its own. If a competitive crop such as barley is sown in the most competitive configuration possible, there is little additional benefit from adding a second species.
The downside of using this multi-species strategy for weed control is that in-crop herbicide options the choice of herbicides is limited. This is mainly due to the common combinations being a grass crop with a legume or brassica, meaning grass and broadleaf herbicide options can’t be used, except for when one species is terminated. This needs to be factored into decisions around intercropping and companion cropping.
What are the best-bet combinations for enhanced weed control?
In brief: It depends on the farming system and the other reasons for considering a multi-species planting.
The details: If the aim is to produce grain, the species selected should have easily separated seed. A well-known example is peaola (field pea plus canola). A recent review of historical trials showed that the median yield increase was 31 per cent compared to sole crops of peas and canola, but the weed control effects of peaola in Australia are unquantified.
An effective companion planting combination is wheat undersown with tillage radish and a legume. The broadleaf companions are sprayed out at stem elongation, leaving the cereal to mature through to harvest.
If there is livestock in the farming system, dual purpose combinations such as grazing canola plus vetch and oats can provide excellent weed suppression. This mix could be grazed and then terminated as hay or silage at stem elongation.
Multi-species plantings add a layer of complexity to the farming system, but many growers have taken on the challenge and are reaping the rewards in crop yield, soil health and weed suppression.
Investigate adverse experiences when using herbicides
A shuttle of glyphosate applied over the top of a Roundup Ready cotton crop was recently shown to also contain a damaging level of 2,4-D impurity, resulting in significant crop injury and yield loss.
The grower involved did not accept the suggestion that the crop damage was due to poor sprayer decontamination or spray drift from a fallow application of 2,4-D, and he was able to prove the problem was due to product impurity.
2,4-D herbicide injury in cotton after the crop was sprayed with glyphosate product contaminated with 2,4-D.
Other shuttles of the same batch may have been applied to fallow weeds where the residual 2,4-D in the glyphosate would have gone unnoticed. Full rate 2,4-D in glyphosate is known to compromise glyphosate efficacy, but studies of low-rate 2,4-D impurity in glyphosate could not be found.
Where can impurities come from?
While the agricultural chemical manufacture and supply chain in Australia is considered first-class and is highly regulated, there is an acceptance that the nil-impurity requirement for the manufacture of agricultural chemicals is unattainable in facilities that use multi-purpose equipment for synthesis, formulation and packaging of products.
Companies therefore apply their own quality assurance standards before releasing products for distribution and sale. If the level of risk posed by certain residual impurities in a product is underestimated, there is potential for instances of crop injury, pesticide residue in produce or poor performance of the product on the intended target weed, fungus or pest.
Mistakes can and do happen within the manufacturing process and chemical supply and distribution chain. To ensure that risks of contamination are minimised and that quality assurance protocols are followed carefully, it is important that any breaches or errors are identified quickly, reported and investigated.
Keep good records of each spray event, including batch numbers of applied product, to help identify the cause of adverse experiences with herbicides.
There are two important things to note: firstly, the current regulations specify that crop protection products must contain nil impurities (other than manufacturing impurities listed in the APVMA standard); and secondly, companies are required to recall product batches when contamination issues are identified. The Australian Pesticide and Veterinary Medicines Authority (APVMA) oversees a highly regulated system of registration, compliance and enforcement on crop protection products.
Assess potential application issues
When misapplication (wrong product applied, incorrect mixing, contaminated product etc) occurs, symptoms of affected plants are usually uniform throughout the treated area. It is often suggested that poor application technique or poor sprayer decontamination is the reason for crop injury or poor weed control results – suggesting a grower ‘own-goal’. Such potential errors must be considered, but if best practice spray techniques and spray rig decontamination procedures have been followed, product impurity should also be considered and investigated.
The chemistry of the product will determine the risk of residues being held within the tank and spray lines of the application rig. This is why there are differences in the sprayer hygiene requirements after using particular products.
Most modern spray rigs have impervious rubber and plastic, or stainless steel components, drastically reducing the risk of chemical absorption and subsequent extraction. Residues on the rubber surfaces are the main concern, and all registered cleaners will physically remove residues when used as directed, but cracked rubber components can present a contamination risk. All filters/strainers must be cleaned and all actuators and taps musts be cycled as the cleaner is run through the spray boom and tank loading system, agitators and tank.
Crop injury or poor weed control that is associated with just one sprayer tank load would suggest sprayer contamination. Effects from contaminated tanks are usually worse at the beginning of the spray run, with damage diminishing with spraying and tank reloading. The field pattern can provide clues to the sprayer filling routine in the field where the crop damage occurred.
The other major reason commonly cited for crop injury in spray drift. Although there is always some small amount of drift when agricultural chemicals are sprayed from a ground rig, the amount is down to ‘virtually safe’ levels within a few tens of metres. If the conditions are very windy, or the boom is too high, or the droplet size too small, spray could drift a few hundred metres from the application ground rig.
Spray droplets may travel a few feet to several kilometres from the targeted area, depending on weather conditions and spray application; but the potential for drift damage decreases with distance because droplets are deposited or become diluted in the atmosphere. The pattern of injury is normally seen most prominently on the section of the field closest to the sprayer that generated the spray drift, and decreases across the field.
During inversion conditions, a similar amount of product is subject to drift, but the drifting product will not dilute as much in the air, so concentrations at specific locations can be higher than expected in non-inversion conditions.
What to do if your crop is damaged or weeds don’t die as expected?
Along with several other possible causes, unintended application of contaminated product should be considered as a potential explanation for crop injury or poor weed control.
Keep in mind that if product impurity is the problem, it is most likely due to a low-dose effect that may be difficult to diagnose or may take longer to express in the target weeds or susceptible crops.
Finding the cause of an ‘adverse experience’ with herbicide is one of the most important reasons to keep accurate and detailed spray records.
If a problem occurs:
Take detailed, time-stamped photographs of the crop or weeds and record everything you know about the crop or fallow management, weather conditions in the weeks prior to the damage being seen, spray history of the field etc. If possible, geotag the photos so they can be easily associated with the correct field.
Record the relevant batch numbers of the chemicals used, which can be checked against the retention samples at the factory if necessary. Collect samples from drums of product used prior to the injury being observed (up to 14 days prior to symptoms being obvious). When you take samples, make sure there are witnesses who can vouch for the voracity of the evidence you have collected. Testing for one impurity (e.g. 2,4-D in glyphosate) costs less than $500 per sample.
Document the injury over time. For example, injury in cotton from low rates of 2,4-D will grow out in two weeks, but injury from higher rates, could last three to four weeks and are the most likely to result in yield loss. Similarly with weeds although the impact may be more difficult to document.
Mark out the affected area in the field to help assess crop yield loss at the end of the season. Note the pattern and intensity of the problem across the field.
Eliminate as many possible causes as you can. Re-assess the application technique and equipment, consider the pattern of damage in the field, look at the weather conditions for the relevant period of time and so on.
Test for herbicide resistance in weeds.
Report the crop damage or poor weed control. The APVMA administers the Adverse Experience Reporting Program, which allows anyone to report a problem with an agricultural chemical, including crop and plant damage, for example, plant death, severe stunting or significant yield loss. This is also the way to report poor weed control outcomes.
The APVMA acknowledges there is likely under-reporting of adverse experiences. The magnitude of under-reporting is unknown and provides limitations in quantifying product risk.
Investigations of spray drift are conducted by the relevant state government body, for example: NSW EPA (call Environment Line: 131-555), Biosecurity Queensland (call 132-523) and Chemical Standards Officer (Victoria) (call 03 5430 4463). Industry organisations will also support growers impacted by chemical damage to crops.
If the damage is due to factors other than spray drift, the affected party will need to take legal action and seek compensation themselves.
Is poor weed control due to herbicide resistance?
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.
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.
Feathertop Rhodes grass and sorghum – does this work?
Avoid getting stuck in the imi-cycle
Competitive sorghum to reduce FTR and BYG pressure
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.
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.
Review: evolutionary drivers of agricultural adaptation in Lolium spp., Maor Matzrafi, Christopher Preston and Caio Augusto Brunharo, 2021, Pest Management Science
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
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)
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.
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
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.
Agricultural pesticides formulations (SmartTrain course notes)
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):
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.
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.
Herbicide residues in soil – the scale and significance (GRDC Update paper)
Herbicide residues in soil (GRDC Podcast)