Here you’ll find expert columns answering questions about weed control concepts and news updates on the latest information on herbicide resistance and weed control.
Don’t sow wild oats
Ranked as the third most costly weed in Australian grain cropping, three weedy Avena spp. – wild oat, sterile oat and slender oat – are estimated to infest over two million hectares, causing crop yield losses of 114,596 t and a national revenue loss of $28.1 million.
In the southern and western regions, the main species found is wild oats (A. fatua), while in the northern region, sterile oat (A. sterilis ssp. ludoviciana) is the more problematic species. Both have evolved resistance to multiple herbicide groups in Australia.
QAAFI weed researchers Gulshan Mahajan and Bhagirath Chauhan have recently published a series of papers on their weed ecology studies of Avena spp., providing growers and agronomists with more information to use when formulating integrated management plans for these weeds in crops.
Both wild oat and sterile oat can survive in soil moisture conditions of 60 per cent water holding capacity (WHC). Sterile oat even produced seed at 40 per cent WHC.
Seedlings of these weeds can emerge from a depth of 10 cm, but greater emergence occurred from 2 and 5 cm depths. Emergence commenced at the start of winter (May) and continued until spring (October).
Early emergence plants produce the most seed, but later emergence plants can still produce enough seed to support reinfestation.
In a no-till system there is low persistence of seed on the soil surface. A 2-year assault on the weed seed bank can result in complete control of infestations.
Weed density of 15 wild oat and 16 sterile oat plants/m2 resulted in a 50 per cent reduction in wheat yield. Lower weed density (just 3 plants/m2) can still support reinfestation.
Sterile oat is a better candidate than wild oat for harvest weed seed control (HWSC).
Wild oat is best managed through early weed control (pre and post sowing) and strong crop competition.
An integrated approach to weed management can reduce Avena weed biomass by up to 90 per cent.
Experimental design features
We are summarising the finding from four related research papers:
Biological traits of six sterile oat biotypes in response to planting time. https://doi.org/10.1002/agj2.20507
Influence of soil moisture levels on the growth and reproductive behaviour of Avena fatua and Avena ludoviciana. https://doi.org/10.1371/journal.pone.0234648
Seed longevity and seedling emergence behaviour of wild oat (Avena fatua) and sterile oat (Avena sterilis ludoviciana) in response to burial depth in eastern Australia. https://doi.org/10.1017/wsc.2021.7
Interference of wild oats (Avena fatua) and sterile oats (Avena sterilis ludoviciana) in wheat. https://doi.org/10.1017/wsc.2021.25
Sterile oats growth and seed production for early and late emergence cohorts
Six biotypes of sterile oats were collected from sites in southern Qld and northern NSW and planted in field conditions at the Gatton research farm in the winter cropping seasons of 2018 and 2019. The weed seed was sown early, mid and late season and the growth and reproductive potential of the six biotypes was monitored.
Averaged across the biotypes, the early planted weeds produced 2660 seeds/plant. Weeds sow mid-season produced 21 per cent less seed and the late-season weeds produced 84 per cent less seed than the early-season plants.
Although seed production was more prolific from the early and mid season plants, the late season plants produced sufficient seed to support reinfestation the following season.
A clean seed bed and competitive crop environment is the best strategy to suppress sterile oat seed production.
Effect of moisture stress on biomass and seed production of wild oats and sterile oats
Seeds of wild oat and sterile oat used in this study were collected from Warialda, NSW, in October 2017 and multiplied at the University of Queensland, Gatton Research Farm in the winter season of 2018. The pot trial to investigate the effect of 20, 40, 60, 80 and 100 per cent water holding capacity (WHC) on these two Avena weed species was conducted in 2019.
Results revealed that wild oat did not survive, and failed to produce seeds, at 20 and 40 per cent WHC. However, sterile oat survived at 40 per cent WHC and produced 54 seeds/plant, suggesting that this species is likely to compete strongly with crops in water stressed situations.
In favourable moisture conditions, both species will produce copious quantities of seed, suggesting that high infestation rates for both species may be a risk in irrigated crops.
Effect of seed burial on emergence, growth and persistence of wild oats and sterile oats
The seed longevity and emergence pattern of wild oat and sterile oat were monitored in field conditions at Gatton, Narrabri and St. George. Fresh weed seed was placed into nylon bags and buried at depths of 0, 2 and 10 cm in November 2017. Bags were exhumed at 6-month intervals over 30-months to evaluate seed germination, viability and decay.
For both species, 50 per cent of seeds at the surface and 10 cm depth had decayed within the first six months. Shallow burial (2 cm depth) of the seed increased persistence, with a significant percentage of seed being viable in the following winter cropping season.
The largest cohort of both species began to emerge at the start of the winter season (May). To ensure the seed bed is clean prior to planting, consider using tillage, herbicide application and cover crops to control this early cohort of Avena weeds. Tillage will bury seeds below their maximum depth of emergence and subsequent tillage should not be performed for 3–4 years to avoid bringing seeds back to the ‘emergence’ depth. Later emerging cohorts (through to October) will be suppressed using strong crop competition or a winter fallow if the infestation is severe.
The results of this research suggest that management strategies that can control all emerged seedlings over two years and restrict seed rain in the field could lead to complete control of weedy Avena spp. in the field.
Effect of wild oats and sterile oats infestation on wheat yield
The interference of wild oat and sterile oat in a wheat crop was examined through field studies in 2019 and 2020 at Gatton, Qld. Infestation levels of 0, 3, 6, 12, 24 and 48 plants m2 of both weed species were evaluated for their impact on wheat yield.
At an infestation level of 15 and 16 plants per m2 for wild oats and sterile oats respectively, wheat yield was halved as a result of reduced spike number per m2.
At the highest weed infestation level (48 plants per m2), wild oat and sterile oat produced a maximum of 4800 and 3970 seeds per m2, respectively. At wheat harvest, wild oat exhibited lower seed retention (17 to 39 per cent) than sterile oat (64 to 80 per cent), with most of the wild oat seeds having fallen from the seed heads before crop maturity.
The results of this study suggest that harvest weed seed control is likely to be a useful tactic in paddocks infested with sterile oat. An integrated weed management strategy that uses both chemical and nonchemical tactics is required to avoid severe crop yield loss, increased weed seed production and weed seedbank replenishment when these weed species are present.
This body of research highlights the benefits of an integrated weed management program that takes the ecology of the target weed into account.
This research was conducted by researchers from the University of Queensland, a WeedSmart scientific partner, with investment from the Grains Research and Development Corporation a WeedSmart sponsor.
Mahajan, G., & Chauhan, B. (2021). Biological traits of six sterile oat biotypes in response to planting time. Agronomy Journal,113: 42-51 https://doi.org/10.1002/agj2.20507
Sahil , Mahajan G, Loura D, Raymont K, Chauhan BS (2020). Influence of soil moisture levels on the growth and reproductive behaviour of Avena fatua and Avena ludoviciana. PLoS ONE 15 (7): e0234648. https://doi.org/10.1371/journal.pone.0234648
Mahajan, G., & Chauhan, B. (2021). Seed longevity and seedling emergence behavior of wild oat (Avena fatua) and sterile oat (Avena sterilis ludoviciana) in response to burial depth in eastern Australia. Weed Science, 1-10. https://doi.org/10.1017/wsc.2021.7
Mahajan, G., & Chauhan, B. (2021). Interference of Wild Oats (Avena fatua) and Sterile Oats [Avena sterilis ssp. ludoviciana (Durieu)] in Wheat. Weed Science, 1-20. https://doi.org/10.1017/wsc.2021.25
WeedSmart Week goes to Esperance, WA
Growers and agronomists in each region and on each farm can adapt the WeedSmart Big 6 principles to bring more diversity to their farming system and bamboozle weeds.
Each year growers and agronomists are invited to attend WeedSmart Week, somewhere in Australia. This year the 3-day event will be held in Esperance, WA, beginning with a 1-day forum at the Civic Centre on Tuesday 17 August. The following two days will be spent touring farms in the Esperance region to see how growers are implementing the WeedSmart Big 6 tactics to minimise the impact of herbicide resistance on their businesses. The WeedSmart Week theme, ‘Diversify and Disrupt – Use the BIG 6 to beat crop weeds’, says it all!
Program leader, Lisa Mayer says the first WeedSmart Week event was held in Perth in 2016 and it’s now a highly anticipated annual event hosted by the WeedSmart program. Having now been held in Queensland, New South Wales, Victoria and South Australia over the last five years, this year sees the flagship event returning to Western Australia. WeedSmart Week is supported by the GRDC as the major sponsor and a wide range of herbicide and machinery companies – all with skin in the weed control game. This will be the seventh WeedSmart Week event.
“The herbicide and non-herbicide tactics that form the WeedSmart Big 6 have been researched and demonstrated in the field – we know they work,” said Ms Mayer. “Low weed seed banks underpin all profitable farming enterprises. Keeping weed numbers low and quickly regaining control of blow-outs is the sole purpose of the WeedSmart program.”
WeedSmart is committed to exploring and promoting farming systems and technologies that produce ‘more yield, fewer weeds’ every year.
WeedSmart Week brings together a wealth of knowledge and experience from local and inter-state growers, researchers, advisors and technology experts – putting the spotlight on herbicide resistance and weed management. Growers can see first hand what is and isn’t working and consider how key principles can be applied directly to their own farming operation.
At the forum and on the bus trip growers, agronomists and researchers put all the options and ideas on the table for discussion. Greg Warren from Farm and General in Esperance is one of the local agronomists assisting with the planning for 2021 WeedSmart Week. As one of the forum speakers Greg will be sharing his thoughts on the control of weeds like summer-germinating ryegrass, marshmallow, fleabane and portulaca.
He says the growers around Esperance are tackling glyphosate resistance in annual ryegrass, along with brome and barley grass and other emerging weeds using a range of integrated control tactics.
“We know we can’t take the foot off the pedal when it comes to weed control,” he says. “Growers are always assessing their options and making decisions based on good science and demonstrated benefits – and that’s what events like WeedSmart Week bring to a district.”
Greg is encouraging local growers to register their interest early and is keen to welcome growers from other regions and inter-state to look, learn and discuss tactics that work.
There will be a focus on both herbicide and non-herbicide tools and plenty of chances to see how mechanical tactics like harvest weed seed control can fit into a variety of farming systems to drive down weed numbers.
The growers, agronomists and researchers speaking and participating in expert panels at the Day 1 forum will spark important discussions about herbicide resistance and how the Big 6 tactics can be used to target the weed species and farming systems of the high rainfall zones of southern and western Australia. There’s one thing for sure – doing nothing is not an option.
Day 2 and 3 will be bus tours to farms in the Scadden and Howick areas surrounding Esperance. The bus trips will highlight how growers in the region are implementing the Big 6 weed management tactics in a variety of farming systems and environments.
This year, Ben White, Kondinin Group’s research manager will host the very popular technology and machinery field demo, where attendees will have the opportunity to see and discuss cutting-edge innovations such as the latest sprayer and weed detection technology and a range of harvest weed seed control implements, including impact mills and chaff decks.
Register for this important 3-day event for the ‘early bird’ single ticket price of $190 (GST incl), guaranteeing a seat on both the bus tour days as well as the forum, all fully catered. Early bird price is available until 31 July, 2021.
WeedSmart is committed to the health, safety and well-being of everyone working in, and in support of, the Australian grains industry. WeedSmart Week may be postponed in response to any coronavirus outbreak, and will be held in accordance with Australian Government advice in relation to social distancing.
Remove the fetters from crops and they’ll trample the weeds
Weeds can exploit situations where crops fail to germinate or grow less vigorously. This does not usually mean that the weeds prefer soils that have constraints such as acidity, compaction or low nutrition status.
While crop responses to changes in soil pH are extensively researched, there is far less research available that quantifies the impact of the amelioration of soil acidity on weed growth.
Gauz Azam and Catherine Borger
To help fill this knowledge gap, research scientists Catherine Borger, Gaus Azam, Chris Gazey, Andrew van Burgel and Craig Scanlan from the Department of Primary Industries and Regional Development, Western Australia (DPIRD), have recently published the results from long-term studies measuring the impact of ameliorating soil acidity on the growth of annual (rigid) ryegrass (Lolium rigidum) in wheat.
In acidic soils, the application of lime increases soil pH and improves the crop’s competitive ability against annual ryegrass.
Lime applications increase initial growth of both wheat and ryegrass.
The application of lime in previous years reduced ryegrass density, biomass, and seed production in wheat crops in 2018.
Lime increased wheat tiller number and, at one location, increased yield.
Crop and weed establishment may be poor in the season following soil amelioration. The crop often ‘catches up’ later in the season.
Reacidification is common. An ongoing liming program is likely to be required to maintain the competitive edge of crops over weeds such as annual ryegrass.
Most crops and pastures grow best in soils with a pH between 5.5 and 8, but some crops, such as barley, are more sensitive to soil pH than others. Similarly, some weeds are able to grow in hostile environments but will often grow better when the pH is in the optimal range for crop growth. For example, annual ryegrass competes very strongly with wheat in low pH soils, but actually grows best in the same pH range as crops. On the other hand, there is some evidence that wild radish prefers acidic soils.
Identifying soil constraints can involve detailed investigations and there are commonly multiple constraints at play. With approximately half of the agricultural soils in Australia having a surface pH of 5.5 or less, this constraint alone can be responsible for significant yield loss. Conversely, South Australian farmers are more likely to have to contend with high pH soil constraints, with 60 per cent of agricultural soils in that state being highly alkaline.
Experimental design features
These experiments were conducted at field sites in the Merredin and Wongan Hills shires in Western Australia. The scope of this research included two experiments:
A field experiment was conducted from 2016 to 2018 at DPIRD’s Merredin Research Facility on naturally acidic soil to investigate the effect of crop rotation (continuous wheat and wheat–chemical fallow), lime incorporation (nil and to 15 cm) and lime rate (0, 2, 4 and 6 t/ha). Wheat and annual ryegrass production was measured in the 2018 season.
A field experiment at DPIRD’s Wongan Hills Research Facility was established in 1994 on soil with low pH as a result of agricultural practices. The trial investigated the long-term effect of lime rate (0, 0.5, 1, 2 and 4 t/ha applied in 1994) and top-up applications of 0 or 1.5 t/ha in 1998 and 0 or 3 t/ha in 2014. In 2018 soil was cultivated to a depth of 0, 15 or 25 cm prior to seeding. Wheat and annual ryegrass production was measured in the 2018 season.
Crop rotation and lime at Merredin
Within the continuous wheat rotation at Merredin, increasing rates of lime increased surface soil pH (0–5 cm) from 4.9 to 6.0 and pH at depth (10–15 cm) from 4.3 to 4.7 with no incorporation. Increasing rates of lime reduced density, biomass, and seed production of ryegrass and increased wheat tiller number and yield.
Incorporation of lime had no significant effect on wheat yield or ryegrass biomass, even though incorporation increased pH at depth (10–15 cm) from 4.2 to 5.1.
A wheat-fallow rotation reduced ryegrass density, biomass and seed production and increased yield compared to the continuous wheat system. Lime rate and incorporation within the wheat-fallow system increased soil pH (0–5 cm) from 4.9 to 5.8, but had no effect on ryegrass due to uniformly low weed pressure. Fallowing is a very effective weed control measure, but is unlikely to be a profitable option unless weed pressure is very high.
Long-term effects of lime application at Wongan Hills
Cumulative lime application at the Wongan Hills site increased soil pH from 5.6 to 6.4 (0–10 cm), 4.6 to 5.4 (10–20 cm), and 4.1 to 4.9 (20–30 cm).
Lime applications in 1994 and 2014 had long-lasting impact on weed growth, resulting in reduced ryegrass density, biomass and seed production in the 2018 crop. The lower rates applied in 1998 had no significant impact on ryegrass density and seed production.
Wheat density was not affected by lime, but tiller number increased with increasing rates of lime applied in 1994 and 2014. The slight increase to wheat yield following application of lime was not significant and incorporation of lime in 2018 did not affect ryegrass or wheat production.
Deep tillage increased pH at depth (20–30 cm) from 4.2 to 5.2. The interaction between lime application in 2014 and incorporation of lime in 2018 was significant for ryegrass, with weed density, biomass and seed production decreasing with increasing depth of tillage in those plots where lime was not applied in 2014 (0 t/ha treatment). Deep tillage did not significantly affect ryegrass in plots where 3 t/ha of lime was applied in 2014, as ryegrass density was already very low across all tillage treatments. By 2018, the lime applied in 2014 had already done the heavy lifting in terms of reducing weed pressure in the 3 t/ha plots.
Applying and incorporating lime is the best way to increase the pH of acidic soils, but it usually takes several years before a surface lime application has a measurable effect on soil pH at depth. Incorporation is the best way to speed up the process and also releases other soil nutrients to boost crop growth.
By default, the incorporation of lime by tillage or inversion also buries weed seed, placing at least a portion of the seed bank deeper in the soil profile and prohibiting germination. Annual ryegrass seed has optimal emergence from a depth of 1 or 2 cm. Emergence reduces with increasing depth and ryegrass does not emerge from depths of 10 cm or more. Even when buried, some seed can remain viable and emerge if the next sowing operation brings the seed back near the soil surface.
This trial work confirms the importance of crop competition in a diverse weed control program. Addressing soil constraints, such as low pH (and the associated aluminium toxicity), enables the crop to compete strongly with weeds such as annual ryegrass – reducing weed growth and seed production.
This research was conducted by researchers from the Department of Primary Industries and Regional Development, Western Australia and was supported by the Grains Research and Development Corporation, a WeedSmart financial partner, through the Soil Constraints Initiative—Innovative Approaches to Managing Subsoil Acidity (DAW00252) project.
Borger CPD, Azam G, Gazey C, van Burgel A, Scanlan CA (2020) Ameliorating soil acidity–reduced growth of rigid ryegrass (Lolium rigidum) in wheat. Weed Sci. 68: 426–433. doi: 10.1017/wsc.2020.38
Central NSW growers investigate IWM options
Having completed a two-year demonstration of chaff decks with investment from the GRDC, Tim, along with cropping officers from adjacent LLS regions, are capitalising on the interest in integrated weed management tactics to counter the insidious rise of herbicide resistance in weeds.
“Annual ryegrass is one of the main weeds causing growers concern in-crop,” he said. “There is known resistance to Group 1 [A] and 2 [B] herbicides, and there are strong indications that glyphosate resistance is evolving on some farms.”
Tim Bartimote, Local Land Services (LLS) in Dubbo, says many grain growers in the Central West region of NSW are keen to see the benefits of integrated weed management tactics demonstrated in their area.
Harvest weed seed control has been commonly practiced in the region for many years, primarily as narrow windrow burning or simply broadacre stubble burning. Tim says there is a definite shift in interest toward technologies such as impact mills, although the price of these machines is a barrier to immediate and wide-spread adoption.
“Through discussions with grower groups we found that a few growers had moved into using chaff decks and chaff-lining, but these options were not well-known to others in the area,” says Tim. “We decided to demonstrate chaff decks, which are less expensive than impact mills and are well-suited to the controlled traffic systems used on a few properties in the region.”
The two growers who demonstrated the use of chaff deck systems both identified resistant ryegrass as their main weed target for harvest weed seed control.
“At the demonstration site at Parkes, the ryegrass population was evenly spread across the paddock at a density of 26 plants per metre square,” says Tim. “For the purposes of monitoring the effect of the chaff deck operation, we chose four sites within the paddock and found 4, 19 and 9 plants per m2 away from the wheeltracks and 68 plants per m2 on the wheeltracks.”
“This clearly demonstrated the shift of ryegrass seed from being spread across the paddock to being concentrated on the wheeltracks where seedlings can be controlled with other tactics as required.”
Chaff decks help concentrate the weed seed onto the wheel tracks during harvest.
At the second site, near Gilgandra, the weed population was found concentrated in patches. Tim and the grower, Daniel Volkofsky, GPS-marked sites within the paddock following the 2020 harvest and will monitor the shift in weed density over the next few years.
The growers used both commercial and home-made chaff deck systems in the demonstrations and found both options were effective. In addition to the traditional use of HWSC in winter crops, Daniel also tried using his chaff decks in a sorghum crop but ran into trouble with blockages on the leading edge of the baffle plate. Some growers have added cameras to help monitor stubble flow over the baffle plate and pre-empt blockages.
Tim says the LLS team wants to achieve a ‘weed management legacy’ from the investment of GRDC funds in the region.
“One of the outcomes of the GRDC-funded project was to build a network of growers with experience using different tactics in their integrated weed management programs,” says Tim. “We are now able to direct interested growers to speak to and visit growers in their region who can talk to them about what they have tried and what has worked well for them.”
“Some of the growers who have manufactured various harvest weed seed control devices on farm are willing to share their low-cost designs with others who are not ready to invest in the commercial models. There is also a pool of experience when it comes to the modifications to baffles and chutes required for different header makes and models.”
HWSC is one of the WeedSmart Big 6 tactics that under pin integrated weed management programs across Australia. Within each of the tactics growers are implementing a range of different methods that suit their own systems to keep weed numbers low.
Tim says they are capitalising on the interest generated through the project to now test and compare the efficacy of a range of pre-emergent herbicides on the market.
WeedSmart podcast with Tim Bartimote
Mix up your approach to fenceline weeds
Glyphosate has been the go-to product for keeping weeds in these areas under control for a long time but unfortunately it is often the only product used and the weeds are commonly quite large when they are sprayed. The result is that glyphosate resistance can, and does, quietly build up in these zones in a wide variety of weed species.
Fencelines will always be a potential source of weed seed but there are ways to ensure that the seed from these areas is not already resistant to the herbicides when it blows into the production areas.
Farmanco agronomist, Brent Pritchard, collected the suspect capeweed samples on a farm near Borden in Western Australia. The capeweed had evolved resistance to glyphosate in an un-cropped drainage area, where it had routinely been sprayed with glyphosate, and had then invaded the adjacent field. The cropped area had been managed with a diverse rotation of wheat, TT canola, pasture and fallow over a 17-year period.
The capeweed samples also showed signs of resistance to metosulam (Eclipse®) and diflufenican (Brodal®), but were susceptible to a range of other herbicides including clopyralid, MCPA, bromoxynil, diuron, metribuzin, simazine, Spray.Seed® and Velocity®.
Dr Yaseen Khalil, a researcher in the agronomy team at the Australian Herbicide Resistance Initiative (AHRI), conducted the resistance screening and confirmed the resistance status of the capeweed population.
AHRI’s Dr Yaseen Khalil confirmed the resistance status of the capeweed samples and is urging growers to take a more diverse approach to weed management in non-cropped areas around the farm
“There is no doubt that an integrated approach to weed management needs to be applied to non-production areas such as fencelines, around buildings, along tracks and roads and around irrigation infrastructure,” says Dr Khalil.
“Probably the first step is to stop using glyphosate alone in these areas unless you are able to reliably apply a double knock to every application. Evolving resistance to this useful herbicide in non-productive zones is counter productive at the least.”
Wherever possible, apply glyphosate in a mix with other herbicides effective on the target weeds, then follow with a second knock.
The main problem on fencelines is the lack of competition to weeds. If pastures are part of the crop rotation it may be possible to establish the pasture species along the fenceline and leave them in place when the paddock returns to the cropping phase. Similarly, the crop can often be sown right up to the fence and the first round or two mown or baled for hay prior to harvest. If there are livestock in the production system they can be used to graze the perimeter in the fallow or in suitable crops.
Mowing or baling the perimeter of the crop can halt the incursion of weeds into the crop area.
Establishing cover using desirable perennial species and eliminating fenceline spraying could be a long-term solution to stop fencelines being a source of herbicide resistant weeds.
If this is not practical, or if the non-crop area must be kept bare for other reasons, such as managing insect pests, close attention must be paid to using alternative herbicides, double knocking, mixing and rotating herbicides and eliminating survivors.
Applying the WeedSmart Big 6 tactics to non-crop areas is a pre-emptive strike on ‘home-grown’ herbicide resistance.
AHRI Insight – World-first: glyphosate resistant capeweed
Management of herbicide resistant weeds on fencelines
Don’t jeopardise glyphosate for clean fencelines
Make seedbank management your priority this year
You can listen to the article being read above!
We all know that old saying – ‘one year seeding, seven years weeding’ or some variant of it, and know it is true. But it is easy to overlook just how important weed seedbank management is, until herbicide resistance begins to reduce the efficacy of previously reliable tools.
For a few decades herbicides really took the focus away from seedbank management because the chemical options were so effective at killing weeds that they appeared to be a complete solution to weed management.
But all along, growers, agronomists and researchers have known it was too good to last. The WeedSmart Big 6 strategy has struck a chord because it is a useful check list that can be used to prompt growers to consider using a selection of the many available weed control tools.
No one tool will do the job – just as herbicides alone have failed, so too will harvest weed seed control or crop competition if they are not part of a planned and multi-pronged assault on the weed seedbank. This is the underlying principle for integrated weed management.
In economic simulations conducted using the RIM and WeedRisk models in 2006, agricultural economists Randall Jones and Marta Monjardino showed that although many things impact on the economic assessment of weed management practices, there is strong evidence that when seasonal risk is taken into account, and the economic assessment is for a period of 20 years, integrated weed management consistently out-performs herbicide-only systems, regardless of the weed in question.
Herbicides provide high level control and are considered an essential component of broadacre cropping systems, however, other tactics that specifically target weeds that have escaped herbicide control are what make IWM systems more profitable in the long-run (see Table 1).
For weeds like wild radish, which produce large quantities of seed that can remain viable in the soil for many years, taking a non-integrated approach of using post-emergent herbicide only has the potential to ‘crash the system’, from an economic point of view.
It will always be a numbers game and IWM consistently wins, usually by a considerable margin, primarily due to lower weed seedbank numbers and conservation of the highly effective herbicide resource for tactical use over time in integrated weed management systems.
TABLE 1 The economic impact ($/ha) of different crop and IWM systems on meana annualised discounted returns for wild oats, wild radish and annual ryegrass in a southern New South Wales cropping system (4-year crop phase followed by 3-year perennial pasture phase).
Economic return ($/ha)a
268 (± 35)
-9 (± 27)
284 (± 34)
332 (± 38)
315 (± 37)
335 (± 38)
Crop + pasture rotation
288 (± 29)
157 (± 25)
284 (± 28)
319 (± 32)
300 (± 30)
320 (± 31)
a The shown in brackets following ± are the standard deviation.
Source: Jones R, Monjardino M and Asaduzzaman Md (contributors) (2019). Section 1: Economic Benefits of Integrated Weed Management, in: A.L. Preston (Ed) 2019. Integrated weed management in Australian cropping systems. Grains Research and Development Corporation.
Use the WeedSmart Big 6 to prepare an IWM plan for your farm
To develop an integrated weed management plan (IWM), it is useful to collate some historical information about past weed control activities, test weeds for herbicide resistance and use the WeedSmart Big 6 to match opportunities and weeds with suitable and effective control tactics, remembering that there are many weed control tools at your disposal.
With your agronomist’s assistance, aim to create a plan that maps out when each tactic will be applied. Ideally, try to include three or more of the Big 6 tactics in each crop, fallow or pasture phase.
Diversity is key. Some people prefer to have a set cropping sequence while others choose the crops in response to seasonal or market conditions, but either way it is important to look for ways to add as much diversity to your farming system as possible and to keep downward pressure on weed numbers at every opportunity.
While preventing weed seed production completely is unrealistic in the real world, a focus on the weed seedbank will pay dividends in the long run.
Ask an ExpertView all
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 is 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 is 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.