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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.
Practical tips

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

Detailed findings
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.
Research papers

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

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!
Register now
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.

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.

Practical tips:

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.

Detailed findings
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.

WeedSmart conclusion
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.
References
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

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.
Resources:

WeedSmart podcast with Tim Bartimote

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.
More resources:

AHRI Insight – World-first: glyphosate resistant capeweed
Management of herbicide resistant weeds on fencelines
Don’t jeopardise glyphosate for clean fencelines

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

Wild oats

Wild radish

Annual ryegrass

Continuous cropping
 
 
 

No IWM

268 (± 35)

-9 (± 27)

284 (± 34)

IWM

332 (± 38)

315 (± 37)

335 (± 38)

Crop + pasture rotation

No IWM

288 (± 29)

157 (± 25)

284 (± 28)

IWM

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.

You can listen to this article being read above!

WeedSmart extension agronomist, Peter Newman is urging growers to think twice before mixing glyphosate with 2,4-D when treating weeds with low to medium levels of resistance to glyphosate.
Antagonism between the two products has been widely known for some time and in most circumstances careful product choice and a slight rate adjustment is all that is required to compensate for the compromised performance of glyphosate.
New research from the Australian Herbicide Resistance Initiative (AHRI) has shown that high label rates of glyphosate can often control grass weeds with low level glyphosate resistance; but adding 2,4-D amine or ester to the glyphosate can result in these weeds surviving the spray.
Once low to medium glyphosate resistance has established in a population of awnless barnyard grass, using a tank mix of glyphosate and 2,4-D is not going to work.
With investment from the Grains Research and Development Corporation and others, AHRI researcher Jingbo Li and others studied two populations of awnless barnyard grass with relatively low-level glyphosate resistance and found this phenomenon is due to the 2,4-D dramatically affecting uptake and translocation of glyphosate from the tank mix.
“For awnless barnyard grass a susceptible population was 100 per cent controlled using 0.5 L/ha glyphosate 540 while 11 per cent survived when the same rate of glyphosate was mixed with 1 L/ha of 2,4-D amine 700,” says Peter. “For the low-level resistant population, a higher rate of 1 L/ha of glyphosate was required to achieve 100 per cent control but when this rate of glyphosate was mixed with the 1 L/ha of 2,4-D amine, 90 per cent of the weeds survived. A similar result was found using 2,4-D ester.”
Survival of awnless barnyard grass seedlings with low level resistance to glyphosate. Left: Zero survival from 1 L/ha Glyphosate 540 application. Right: 85 per cent survival to 1 L/ha Glyphosate 540 + 1.03 L/ha 2,4-D Ester 680 mixture.
In another, more resistant, population of awnless barnyard grass the same scenario played out, albeit with an even higher rate of 3 L/ha of glyphosate to achieve 100 per cent control. In this population the survival rate was 77 per cent for the tank mix.
“What this means for growers is that once glyphosate resistance has established in a grass weed population, using this particular tank mix is not going to work,” says Peter. “A grower with glyphosate resistant grass weeds would be better served by applying the higher rate of glyphosate on its own, or perhaps with a different mixing partner, to achieve maximum control. It is then necessary to look at building in additional tactics to keep weed numbers low into the future with less reliance on glyphosate.”
While not examined in this study, 2,4-D antagonism of glyphosate is reported on several other species including Johnson grass, wheat, barley and wild oats. 2,4-D is also reported as antagonistic of Group A herbicides on species such as wild oats and annual ryegrass. Although mixing these two herbicides can provide a valuable multi-shot control of both grass and broadleaf weeds, the pros and cons need to be carefully evaluated.
“The other thing to remember is that the maximum level of control when using glyphosate is achieved when the best formulation is applied to young weeds at higher label rates,” says Peter. “These factors are generally within the grower’s control, even if they cannot control the weather conditions or plant stress levels, which also impact on glyphosate efficacy.”
To keep glyphosate as a viable option into the future Peter also recommends applying a double knock tactic after each application of glyphosate. He says following glyphosate with paraquat has been an effective double knock for many years but there are other options to consider, including strategic tillage and alternative herbicides.
Other resources

AHRI Insight: 2,4-D antagonises glyphosate, especially in glyphosate resistant weeds

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The common practice of planting sorghum on wide rows has made this crop notorious as a weak link crop that can allow key summer grass species to set bucket loads of seed.
The take home message from four years of research at Narrabri, NSW and Hermitage, Qld, is that halving sorghum row spacing can halve weed seed production in both feathertop Rhodes grass and awnless barnyard grass.
With investment from the GRDC, researchers from the University of Sydney and Queensland Department of Agriculture and Fisheries (DAF) have conducted a range of field trials to identify ways to increase the competitiveness of sorghum and summer pulses.
Dr Michael Widderick, DAF principal research scientist says the findings from these field trials have shown that a change to narrower row spacing for sorghum greatly suppressed weed growth and seed production, without reducing crop yield.
“This is very significant for sorghum growers who have struggled with controlling these grass weeds in wide-row configurations,” he said. “These weeds are difficult to control with herbicides and there are few chemical options available to growers to control grasses in a grass crop. Any non-chemical strategies that reduce seedbank replenishment are very valuable to growers.”
Dr Widderick said sorghum is often grown on one metre row spacing with an expectation that the crop will have access to more soil moisture. A considerable downside to planting on the wider row configuration is that canopy closure does not occur, allowing weeds to proliferate in the inter-row.
With soil moisture at a premium, there is nothing spare to waste on growing summer weeds. For a sorghum crop to be competitive against weeds it requires adequate stored soil moisture (or access to irrigation) to establish the crop and achieve canopy closure as quickly as possible. This is most reliably done at a row spacing of 50 cm and this trial demonstrated that row spacing did not significantly impact crop yield within a season.
 
Left: weeds growing uninhibited in the inter-row space of sorghum sown at 1 m row spacing. Right: Fewer weeds can establish when the canopy closes in sorghum sown at 50 cm row spacing.
Two of the most difficult to control summer weeds, feathertop Rhodes grass (FTR) and awnless barnyard grass (ABG) can produce 40,000 and 42,000, seeds per plant respectively. Other studies have found these numbers could be even higher, so every effort to reduce seed production is worthwhile.
Both these species have populations confirmed as resistant to glyphosate, and recently a population of FTR was confirmed to be resistant to haloxyfop (Group A). Including a poorly competitive sorghum crop in the crop rotation provides a weak link in any strategy to reduce the weed seedbank for these weeds, and potentially allows a blow-out in herbicide resistant biotypes, making future control in other crops or summer fallows very difficult.
Dr Widderick said sorghum competitiveness across all seasons and both sites was increased with narrow row spacing (50 cm) and a plant density of 10 to 15 plants/m2.
In the 2017/18 season at Hermitage, the researchers demonstrated that planting sorghum at a density of 10 to 15 plants/m2 reduced seed production of both weed species reduced by over 50 per cent compared to the seed production at the low crop density of 5 plants/m2. In the same season, cultivar choice, sorghum density (5, 10, 15 plants/m2) and row spacing (50, 75 and 100 cm) had no statistically significant effect on crop yield.
Armed with this information, the 2018/19 sorghum trial at Hermitage was sown at a crop density of 10 plants/m2, and the effect of row spacing (50 cm and 100 cm) on weed production was measured. Biomass and seed production of ABG was reduced by 55 per cent and 65 per cent, respectively when the sorghum was sown at the narrower spacing. Similarly for FTR, the 50 cm row spacing reduced biomass and seed production by 48 per cent and 56 per cent, respectively.

Graphs: Awnless barnyard grass (ABG) (left) and feathertop Rhodes grass (right) biomass and seed production as affected by sorghum row spacing at Hermitage, Qld 2018/19. Within each graph, different letters indicate significant (P<0.05) difference after pairwise comparison.
Crop competition is a ‘free kick’ non-herbicide tactic in the WeedSmart Big 6 strategy to manage herbicide resistance in weeds. There is now solid evidence that growers can maintain crop yield and reduce summer grass seed production by planting sorghum crops at a density of 10 plants/m2 and a row spacing of 50 cm.
There are some residual herbicide options for the control of these summer grass weeds in sorghum. However, their efficacy can differ greatly depending on the season and will rarely provide full control of FTR and BYG. A combination of residual herbicides and a competitive crop is likely to have an additive effect and reduce seed production on surviving weeds.
This research project also demonstrated that mungbeans are more competitive on 50 cm row spacing, making any changes to seeding equipment worthwhile as it would suit the whole summer crop program in the northern region.
Central Queensland sorghum grower experience
Organic grain producers Paul and Cherry Murphy have always relied on crop competition as an integral component of their weed management program in all crops, including sorghum, at ‘Kevricia’, near Capella in Central Queensland.
With many years of experience growing sorghum on 50 cm row spacing Paul says the suppressive ability of the closed canopy certainly reduces weed growth and seed set in-crop.
Paul Murphy, Central Queensland, has been sowing sorghum on 50 cm row spacing for many years to suppress weeds in their organic farming system.
“We have been working off a plant density of around six plants per metre square as a rule of thumb that seems to work in most seasons on our farm,” he says. “In seasons where soil moisture might be limiting we have seen higher density crops fall over, and so have leant towards the lower planting rate. But the 10 plants per metre square would certainly increase the competitiveness of the crop in seasons where there is sufficient moisture.”
Paul is pleased to see researchers doing more work on row spacing and plant density, which is difficult to really tease out in commercial settings where there are too many potential variables. “In sorghum there is a complexity associated with plant density, tillering and row spacing that needs scientific trials like this to really determine the optimal combination for maximum yield and weed control in a variety of seasonal scenarios,” he says.
This season Paul will be breaking with tradition and planting sorghum on wider row spacing as he now has a Garford camera-guided inter-row cultivator. He hopes the wider spacing will only be required for this season while he makes the adjustments required to have the machine suit their controlled traffic configuration.
The Murphy’s inter-row cultivator is capable of working in crops planted on 50 cm row spacing once it has been adjusted to suit their CTF configuration.
“Once we are ready to plant the winter crop I hope to be able to plant on 50 cm spacing again and still use the inter-row cultivator,” he says. “The cameras on the cultivator guide the alignment of the tynes to follow the plant row with a 1 cm accuracy, and can be used when the crop is 10 to 40 cm high.”
As organic growers the Murphys don’t use any herbicides and so early weed control can be difficult, but this inter-row cultivator will help remove any weeds that emerge with the crop and then crop competition can suppress any later germinations.
Other resources

GRDC Update paper: Growing competitive sorghum and mungbean crops to suppress summer weeds 
Creating stiff competition against summer weeds 
Managing barnyard grass in summer crops and fallow 

With investment from GRDC, researchers led by Professor Bhagirath Chauhan at the University of Queensland, have shown that both windmill grass and feathertop Rhodes grass can greatly reduce yield in mungbean, yet both weed species retain a large portion of their seed when the mungbean crop is ready for harvest.
This gives growers the opportunity to use several tactics to reduce the seedbank of these two species while growing mungbean.
Professor Chauhan says that even at the most competitive row spacing of 50 cm, mungbean yield was halved when there were around 40 windmill grass plants/m2 or just 11 feathertop Rhodes grass plants/m2 growing in the crop.
Feathertop Rhodes grass competes strongly and produces masses of seed if it gains a foothold in a mungbean crop.
“The good news is that both species have a high level of seed retention at harvest because mungbean is such a quick growing crop,” he said. “This gives growers the chance to vastly reduce the amount of new seed entering the seedbank.”
“Even though these weeds have high seed retention at harvest they also produce a huge quantity of seed,” he says. “At peak weed density in our field trials feathertop Rhodes grass produced over a quarter of a million seeds per metre square and windmill grass produced around 100,000 seeds per metre square. So, even if a small portion of this seed enters the seedbank it can still equate to a large number of seeds to potentially germinate the following spring.”
Feathertop Rhodes grass is known to begin germinating in late winter and early spring, well before a mungbean crop is planted so every effort should be made to eliminate all flushes of this weed prior to planting mungbean. Haloxyfop is currently registered for fallow control of feathertop Rhodes grass ahead of mungbean production and can be used to reduce the weed burden prior to planting mungbeans in the most competitive configuration of 50 cm row spacing. To reduce the risk of Group A resistance, use a double knock in this pre-plant situation to control any Group A herbicide survivors of these difficult grass weeds. Paraquat is the usual chemical double-knock partner in these situations and should be applied to small, unstressed weeds within 7 to 10 days after the application of haloxyfop.
Both these weed species can germinate close to the same time as the mungbean crop, so early weed control is essential to maximise yield and minimise early weed competition. Although these two grass species are susceptible to several pre-emergent herbicides, only flumioxazin (Valor) is registered for use in mungbean. This Group G herbicide can be applied at least two months pre-sowing to provide enhanced knockdown and residual control of feathertop Rhodes grass in mungbeans, taking care to follow the ‘critical comments’ to avoid crop injury.
Extra emphasis should be put on ensuring the paddock is as clean as possible prior to planting mungbeans. Inter-row cultivation may be an option provided the young plants are not injured, as wounds can allow entry of diseases such as tan spot or halo blight.
Clethodim applied before the mungbeans begin to flower will provide effective in-crop control of small, late germinating grass weeds.
Mungbean crops are commonly desiccated prior to harvest using either Reglone or glyphosate. Both of these Chloris weed species are generally unaffected by these herbicides as mature plants, so the desiccation of the crop is unlikely to stop weed seed set. Mechanical options such as swathing are currently under investigation and may provide a more reliable way to stop seed set on these weeds prior to harvest.
Professor Bhagirath Chauhan, University of Queensland, says windmill grass and feathertop Rhodes grass both retain a large portion of their seed at the time of mungbean harvest, making harvest weed seed control an practical option to help reduce the weed seedbank.
“Mungbean is a good candidate for harvest weed seed control, using chaff lining, impact mills and the like, because the crop is harvested at ground level so any weed seed held on the plants should enter the harvester front,” says Professor Chauhan.
The WeedSmart Big 6 approach to help manage resistant and hard to control weeds combines the power of multiple tactics throughout the year and across a full crop sequence to reduce weed seed set.
Although feathertop Rhodes grass and windmill grass both produce vast quantities of seed, the seed is very short-lived. If left on the soil surface the seed remains viable for only one to two years. All efforts to prevent seed set will be rewarded with a rapid decline in the weed seedbank for these two difficult grasses.
GRDC has recently updated the ‘Integrated weed management of feathertop Rhodes grass’ manual, which provides detailed information on the ecology of this important weed, along with the tactics and strategies that can be used throughout a cropping sequence to manage the seedbank.
Other resources

Giving summer legumes the competitive edge
FTR grass demands attention to stop seed set 
Creating stiff competition for summer weeds
GRDC manual: Integrated weed control for feathertop Rhodes grass 2020 update

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Whether they are resistant to herbicide or not, weeds generally do not compete well with vigourous crops, but in the fallow they can rapidly take advantage of the lack of competition for resources.
Department of Agriculture and Fisheries researcher, Dr Annie Ruttledge has been running experiments at Kingaroy to investigate the benefits of bringing crop competition into the fallow phase of cropping systems in southern Queensland.
Dr Annie Ruttledge, Department of Agriculture and Fisheries, Queensland weeds researcher is conducting trials to identify cover crop species suited to southern Queensland that have weed-suppressive traits.
With investment from GRDC the cover crop project, led by Charles Sturt University, is investigating the weed-suppressive power of various cover crop species suited to either summer or winter fallows at three locations in the northern grain growing region – Kingaroy, Narrabri and Wagga Wagga.
At the Kingaroy site, both winter and summer-growing cover crops were shown to suppress weeds by over 85 per cent and up to 95 per cent, compared to an untreated fallow where the sown weeds were not inhibited by a cover crop. While this level of control is worthwhile on its own, it is also backed up with either a chemical or non-chemical tool to terminate the cover crop and kill any survivor weeds.
“In winter in Kingaroy, the best cover crops for weed control were grazing oats and tillage radish,” says Annie. “These species provided early season ground cover and suppressed our mimic annual grass weed, Italian ryegrass, by up to 94 per cent relative to the weeds-only fallow. None of the cover crop species we tried were able to suppress the quick-growing mimic broadleaf weed, Oriental mustard.”
Winter-growing cover crop monocultures and mixtures.
In summer, Annie says the best cover crop options for Kingaroy were white French millet, Japanese millet, forage sorghum and buckwheat. Again, early-season biomass and ground cover was the key to suppression of both grass and broadleaf weed mimics by up to 95 per cent when compared to the weeds-only fallow.
Summer-growing cover crop monocultures and mixtures.
So far in this trial, there has been no measurable weed suppression benefit in sowing mixed species cover crops rather than monocultures. However, a mixed species cover crop may be preferred if a grower is wanting to achieve multiple outcomes. For example, grazing oats may be selected as a fast growing and highly competitive species and teamed with a less competitive legume to boost soil nitrogen stores.
“Obviously, the species selected will depend on the growing region and soil type,” she says. “Cover crops also provide many other services to the farming system and so the grower could select a cover crop species, or mix of species, that would also provide a break from disease or insect pressure, increase moisture infiltration, build up organic matter or break down compaction.”

Source: Charles Sturt University
Cover crops are an extension of the WeedSmart Big 6 tactic of providing crop competition to suppress weed growth and reduce the weed seed bank in an integrated weed management program.
Annie says that light interception is a critical driving force in the effectiveness of cover cropping for weed control. In selecting cover crops for weed suppression, choose species that grow well in your locality and that restrict light penetration to the soil through strong early growth and the development of a dense canopy. For greatest benefit, terminate cover crops at maximum biomass, which should coincide with the beginning of flowering; however, earlier termination may be required if soil moisture is limiting.
There is a large body of research work now underway to investigate other aspects of incorporating cover cropping into farming systems in various regions. While this work focuses on weed suppression, other researchers are looking into soil water and nutrient use efficiency under different conditions and in various cropping systems.

Other resources

Summer cover crops video DAF
Day family case study
Cotton cover crops
Cover crops research update video presentation
GRDC Update paper – Cover crops to provide groundcover in dry seasons

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The three things that the spray operator can and must control are nozzle choice, boom height and ground speed of the spray rig.
Recently, there has been plenty of attention on some of the new technologies involving weed detection and artificial intelligence (AI), and companies like Goldacres are keen to deliver these to growers as they come to market, but their sales and marketing operations manager, Stephen Richards says the technologies behind effective and reliable droplet delivery to the target remain central to their spray rig designs.
Goldacres sales and marketing operations manager, Stephen Richards says the technologies behind effective and reliable droplet delivery to the target are central to their spray rig designs.
“At the end of the day, if the droplets of product don’t hit the target at the required rate you might as well have left the spray rig in the shed,” he says. “The best way to ensure the correct dose is applied and avoid spray drift is to pay close attention to setting the rig up correctly and operating it well.”
In the last 20 years there has been a quiet revolution in nozzle design and much of this has been driven by the need to eliminate the risk of spray drifting downwind or being caught up in temperature inversion layers.
“Years ago the standard nozzle was the XRT-jet flat fan nozzle that operated at a pressure of 1 to 4 bar, which gave good coverage in ideal spray conditions, but also produced more fine droplets that easily drift,” says Stephen. “Modern nozzles have been designed with the emphasis on producing medium to coarse droplets and using higher water rates to achieve adequate coverage.”
The modern nozzles also have a wider pressure range of 1 to 6, or 1 to 8 bar, making the one nozzle type suitable for a variety of applications.
When considering ground speed, Stephen says the technologies behind even rate delivery through the boom have made it possible for machinery manufacturers to build sprayers that can operate at higher speeds and cover more area in a day.
“The Goldacres self-propelled sprayers have had a 3-tier nozzle system for about 20 years, where the first set of small nozzles come on when the machine is operating at 5 to 10 km/hr then the second and third sets activate when the machine is operating at higher speeds,” he says.
This ensures that the correct product rate is applied at the headlands and wherever the operator needs to slow down. Another option is the ‘pulse width modulation’ system to adjust the volume through the nozzles in response to changes in ground speed.
“Pulsing is particularly good for turn compensation with a large boom, where nozzles near the outside tip are typically moving twice as fast as nozzles near the machine,” says Stephen. “This means product would be under-applied at the tip and over-applied near the centre. Consistent under-dosing of herbicide is a particular risk in the evolution of herbicide resistance.”
As boom length increases so does the need for high tech suspension and rate compensation for variable speed and turning.
The boom height is also critical in reducing drift risk associated with the air turbulence behind the spray rig. A 20 cm change in height from the recommended 50 cm above ground to 70 cm can quadruple the quantity of air-borne droplets.
“With booms now as wide as 48 m the suspension system is more important than ever,” says Stephen. “Goldacres machines use a system that minimises yaw, roll and pitch of the boom to give a stable spray platform and optimise spray coverage in undulating or uneven paddocks.”
Before heading out to spray Stephen recommends operators check for blocked nozzles and at the start of each season, do a jug test to check for nozzle wear. The large investment in spray technology can be undone if nozzle choice and maintenance is neglected.
“The jug test needs to show that each nozzle is delivering within 10 per cent of the nominated volume per minute for the specific nozzle type and size,” he says. “The cost of a new set of nozzles pales in significance against the cost of product wastage, a spray failure or the evolution of herbicide resistance on your farm.”
Before heading out to spray Stephen recommends operators check for blocked nozzles and at the start of each season, do a jug test to check for nozzle wear.
The WeedSmart Big 6 tactics that form an integrated weed management program to reduce the risk of herbicide resistance in weeds are supported by companies like Goldacres, who understand the importance of effective and safe herbicide application.
Goldacres is working with Bilberry to perfect the artificial intelligence systems required to bring green-on-green weed detection to Australian farmers. These systems, along with the optical spraying technology that has been used for spot-spraying in fallows for over 20 years, are expected to deliver more targeted herbicide use into the future.

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This has been the key message of weed management experts in Australia ever since 1996, when Australia’s worst weed, annual ryegrass, was found to be resistant to our most useful herbicide, glyphosate.  
A few years later, the Australian Glyphosate Sustainability Working Group (AGSWG) was set up under the CRC for Australian Weed Management (Weeds CRC) to bring together commercial and research expertise from around the country with a determination to ‘keep glyphosate resistance rare’.
With investment from the Grains Research and Development Corporation, AGSWG established a database of confirmed cases of glyphosate resistance in Australia and developed information products for all users of this important weed control tool.
After guiding growers and agronomists through a critical 15 years of managing glyphosate resistance in Australia the AGSWG has been disbanded, however the work of advising farmers and other weed managers will continue.
Keep weed numbers low and do everything you can to prevent resistant weeds from setting seed.
Australian Herbicide Resistance Initiative (AHRI) director, Professor Hugh Beckie, says glyphosate means so much more than weed control to Australian farmers, particularly for dryland cropping.
“This herbicide has been the means of achieving incredible productivity increases in dryland crop production, initially providing an alternative to tillage for fallow weed control and thus conserving soil moisture over summer,” he said. “It is also now used as a broad spectrum knockdown pre-seeding and post-harvest in many crops and in RoundUp Ready cotton and canola.”
“As predicted, the incidence of glyphosate resistance is ramping up, having been heavily relied on for weed control since its introduction to Australia in 1976,” said Prof Beckie. “It is important to understand that glyphosate is not only used extensively on farms but also along roadways, fence lines, railway lines, in public parks and in home gardens. This means that resistance can, and does, evolve in many different settings and can move across the landscape in weed seeds and pollen.”
According to the International Herbicide-Resistant Weed Database there are currently 20 species and thousands of populations known to have evolved resistance to glyphosate in Australia.   
While this is a serious situation, and glyphosate resistance can no longer be considered ‘rare’, it is still possible to regain control of weed populations that have evolved resistance.
One of the useful products that AGSWG published was a series of factsheets outlining the practices that should be followed and those that should be avoided. These factsheets have recently been updated and published on the WeedSmart website.
There is a factsheet for each of the main glyphosate user groups – grain producers, cotton growers, horticulturalists, orchardists and vinegrowers, irrigators and managers of public lands and utilities. While the principles remain the same for all industries, there are some practical variations in implementation.
Using a diverse weed control program and taking care to apply glyphosate in the optimal way can tip the scales in the grower’s favour and keep this valuable product as an option well into the future.
Download glyphosate factsheets

Northern grains and cotton factsheet
Winter grains and irrigation factsheet
Orchards and vineyards factsheet
Roadside and railways factsheet
Vegetable production factsheet

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GRDC Grower Relations Manager – North, Richard Holzknecht, says while it is important that growers control fallow weeds early to maximise efficacy, it’s equally important spraying is only undertaken when weather conditions are right. Spray equipment also needs to be set up and operated appropriately.
“While spraying at night and in the morning is not restricted, product labels state that chemicals should not be applied when hazardous inversions are present,” Mr Holzknecht said.
He warned off-target damage could occur from physical drift and inversion drift, which posed a significant risk during summer spraying as day/night fluctuations in temperature often result in inversions forming overnight and or early in the morning.
“Wind speed, in particular, should be monitored at least every 15 to 20 minutes and if the wind drops, spraying should stop,” he said.
The main factors influencing drift potential were weather conditions at the time of spraying and how spray machinery was operated in terms of spray quality, speed and boom height. Photo GRDC
“So, planning and being proactive is extra important. Growers need to talk with their neighbours to determine the location of any sensitive crops, such as cotton, and ensure they understand label recommendations and permit regulations, particularly those governing the use of 2,4-D.”
Mr Holzknecht said the main factors influencing drift potential were weather conditions at the time of spraying and how spray machinery was operated in terms of spray quality, speed and boom height.
In an inversion, chemical droplets can remain suspended in concentrated form and be carried significant distances.
“It is important growers understand the weather conditions that indicate an inversion is present and avoid spraying during these times.
“Surface temperature inversions are often associated with calm, low wind conditions, dust remaining suspended, fog or mist forming in low areas and sounds travelling long distances. All these signs indicate the risk of inversion drift is significantly high.”
Mr Holzknecht advised growers and spray contractors to closely monitor weather conditions.
The Grains Research and Development Corporation (GRDC) is actively investing in spray application research and training to assist industry in implementing best-practice spray systems, and it recently released a new video explaining the key factors affecting spray drift.

Source article: Spray safely to reduce drift risk this summer
Spray drift in-depth resources

GRDC Spray drift hub
Stop the drift webinar
Are you going spraying, or killing weeds?
How do you manage summer weeds without spraying at night?
Spray wisely and well webinar
Effect of formulation and environment on dicamba volatility webinar
Spray well – correct nozzles, adjuvants and water rates

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Livestock containment areas allow the Eagles to rest their pastures and fodder crops, efficiently use a variety of feeds and restrict the spread of weed seeds. On top of this they have also seen benefits in growth rates and lambing percentages.
Sam says the six or seven hectares they have available for containment was not expensive to build and has made it much easier to manage their livestock and cropping enterprises.

“I’d definitely recommend building containment areas for sheep,” he says. “It is such a simple concept that has so many benefits. They really help to manage ground cover on your pastures and cropping paddocks, and in dry times they make feeding out much less stressful. In the last drought we had up to 6500 sheep in containment, including lambs, and I could feed them all in less than three hours, and didn’t have to feed every day.”
Sam and Emily use the containment areas for several purposes throughout the year. Although they generally keep their pasture and cropping paddocks separate, the sheep play an important role in weed management across the whole farm.
Horsham producer Sam Eagle uses every opportunity to maximise the synergies within a mixed farming operation.
“The containment areas allow us to bring in feed from outside if necessary and feed out screenings from our own grain, being confident that any weed seeds that come with that feed won’t be spread around the farm,” says Sam. “It is easy to manage any weeds that germinate in such a defined and small area of the farm.
“When we buy in sheep we shear them as soon as they arrive to remove any risk of them introducing weeds like Bathurst burr,” he says. “We use the containment areas to avoid overgrazing pastures so the sheep eat the weeds like barley grass as well as the more palatable species. They also provide an effective double knock effect for weeds that have herbicide resistance.”
Livestock containment paddocks boost productivity while stopping the spread of herbicide resistant weed seeds.
The Eagles cut weedy paddocks for hay or silage and feed it out in the containment areas where they can control any weeds that germinate. Sam says above-ground pit silage has been very cost effective at around $10 a cubic meter to cut the silage and store it under a tarp before feeding out in the containment paddocks.
“Silage is a very good weed control tactic,” he says. “You cut it early, so you are stopping weed seed set, and after three days of good weather you can spray out the paddock for a spray fallow.”
The Eagles prefer to either graze a crop fully or grow it for grain, having found that the ‘grain and graze’ tactic for dual purpose crops had an unacceptable yield penalty and opened up the canopy to allow weeds to grow through and compete in the grain phase.
Grazing cover crops and failed grain crops generates cash flow and helps manage weeds. Sam keeps an ungrazed reference area in dual purpose crops so he can remove grazing pressure at the right time if he wants to let the crop go through to grain.
They have found Moby barley plus clover to be the best cover crop to graze and then spray out. Oats and pasture are both cut either for hay or silage to conserve fodder and remove weed seeds.
“In the cropping paddocks sheep will eat most of the weeds that evolve herbicide resistance, like wild radish, annual ryegrass, fleabane and whip thistle. They also generate cash flow from cover crops and from grain crops that don’t go through to harvest due to drought, flood, weeds or frost,” says Sam. “Over summer the sheep reduce our herbicide costs and reduce the stubble load, which makes sowing easier. Once the feed supply runs out, we put the sheep into containment until they start to lamb. This allows the pastures and crops to get ahead and gives us good feed to put the ewes into for lambing.”
“The sheep can make inter-row sowing more difficult in our CTF system so we have to be careful to cut the stubble 300 mm or less above ground level so the stalks don’t lodge across the inter-row as the sheep graze the stubbles,” he says. 
Having used narrow windrow burning as their harvest weed seed control tactic for six years, Sam and Emily used a contract harvester with an impact mill for their harvester for the 2018 season. They were pleased with the job the mill did and are looking to purchase one of their own once the technology matures a little more. They use crop-topping in pulses and windrowing in canola to stop weed seed set and also spray herbicide under the cutter bar in canola.
“We test weeds for herbicide resistance so we know what still works and plan out a diverse herbicide program with multiple chemical groups used in a broad crop rotation,” says Sam.
Other than the grazing and weed management benefits, Sam and Emily have also found numerous productivity benefits for their 2500-strong merino flock. Using the containment yards for joining has seen increased conception rates and after preg-testing their ewes, Sam and Emily make separate mobs for the twins and singles so they can better manage the ewe’s nutrition while in containment. Once the lambs are weaned and are brought into containment their growth and feed utilisation rates are higher than when paddock grazed, meaning the returns on feed inputs are higher and the Eagles are able to either turn off hoggets earlier or at a higher weight.
Building and using containment areas
Size and design – they can be any size, provided an allowance is made for 2 to 5 m2 per sheep (2000 to 5000 sheep per ha). At the right stocking density the containment yards compact well and do not generate dust or strong odour. Place the food and water sources as far away from each other as possible in each containment yard – this helps keep the water troughs clean.
Water – sheep require 6 litres of water each per day and more in very hot weather. Flow is more important than pressure, so use thicker pipe (e.g. 30 to 50 mm) to supply the troughs.
Feeders – feed can be placed in self-feeders, feed troughs or on the ground.
Place the water and feed sources at opposite ends of each containment yard to keep the water clean for longer, and provide as much shade as possible.
Shade – think about shade when designing the containment areas and look for ways to provide as much shade as possible. Protect any established trees.
Feedstuffs – utilise a variety of feeds such as screenings, canola, hay, purchased grain and silage. Match the nutrient value of the feed with the class of animal you are feeding and supply any necessary mineral supplements. Get advice if you don’t have a good knowledge of animal nutrition.
Stock health – give sheep 6-in-1 vaccines and drench before putting a mob into containment.
Key benefits

Less feed wastage means feed costs are reduced and productivity is higher with more lambs produced (higher conception rate) and faster weight gain compared to paddock grazing.
The containment paddocks can have a variety of uses including being a fire break, lamb feedlot, shearing holding yard and joining paddock. Move sheep out once lambing commences.
Holding sheep in the containment paddocks allows the pastures and fodder crops to create a green wedge of feed before being grazed. They also provide a suitable place to hold sheep once the pastures and fodder crops have run out in summer, maintaining groundcover levels across the farm.
Good for your mental health in drought conditions as you don’t have to drive around dry paddocks every day, feeding doesn’t take as long each day, ground cover is preserved across the farm and the sheep can be kept in good condition.

More information

Eagle family case study
Sheep confinement area fact sheet

Unfortunately, glyphosate resistance seems to confer no such disadvantage on awnless barnyard grass.
In a recent random survey of summer-growing weeds in the northern grains region, 36 per cent of awnless barnyard grass (Echinochloa colona) populations proved resistant to glyphosate.

Awnless barnyard grass response to no crop competition – glyphosate resistant (GR) ABG with 0 mungbean plants/pot (left) and glyphosate susceptible (GS) ABG with 0 mungbean plants/pot (right).
Through a focused effort to better understand this problematic weed, GRDC invested in a series of studies on various aspects of its ecology. This work was done by QAAFI weed researchers, led by Dr Bhagirath Chauhan at the University of Queensland, Gatton.< “Awnless barnyard grass is one of the top three most problematic weeds of summer crops and fallows in Australia,” says Dr Chauhan.
“Our studies looked at environmental and cultural effects on germination, the impact of crop competition and early weed control, seed retention at harvest and the effect of low rates of glyphosate.”
These studies confirmed that awnless barnyard grass can emerge in spring, summer and autumn in Queensland, with temperature being the main driver of seed germination.“Germination is rapid for seed exposed to the light on the soil surface, as in no-till summer fallows,” he says. “As the temperature increases, seed buried up to 8 cm below the surface can also germinate. Covering the soil with crop residue suppressed germination by about 20 per cent, from 70 per cent without cover down to 47 per cent with sorghum trash.”
To run down the seedbank of awnless barnyard grass, whether glyphosate resistant or not, requires two to three years of no recruitment through ‘seed rain’. Strategic tillage is only useful if the seed bank is buried to a depth of more than 8 cm and then not disturbed again for many years as the seed will persist for longer than two years once buried.
“The best way to reduce seed production in this weed is to grow competitive crops in summer and to focus on controlling weeds for the first two weeks after crop emergence,” says Dr Chauhan.
“Both mungbean and sorghum crops can significantly suppress awnless barnyard grass growth and reduce the quantity of seed set over the warmer months.”
“Compared with weed plants grown alone, mungbean interference of four and eight plants per pot reduced weed seed production by 85 to 95 per cent. These reductions were similar for both glyphosate resistant and susceptible biotypes.”
Mungbean crop competition suppresses awnless barnyard grass (crop plants removed to show the effect on weed growth and habit. From left to right: GR ABG with 4 mungbean plants/pot; GS ABG with 4 mungbean plants/pot; GR ABG with 8 mungbean plants/pot; GS ABG with 8 mungbean plants/pot.
Likewise, even a sorghum crop at one metre row spacing, suppressed weed growth and seed production. Awnless barnyard grass produces 4000 seeds per plant when emergence is with the crop, 1000 seeds per plant when emergence is two weeks later and less than 100 seeds per plant when emergence is four and six weeks after crop emergence.
“This shows the importance of early weed control – even in widely-spaced sorghum,” says Dr Chauhan. “Plants that do emerge with the sorghum crop or within the first two weeks retain about 45 per cent of their seed at harvest.
Although larger plants produce more seed than smaller ones, plant size did not predict the level of seed retention at sorghum harvest.”
Awnless barnyard grass response to early weed control in wide-row (1 m) sorghum – BYG emerging with the crop (left) vs emerging after the crop (right).
While harvest weed seed control might be less practical in sorghum than other summer crops, removing almost half of the seed produced in-crop would be a valuable contribution to reducing the seed bank.
The random weed survey indicated that all populations, whether resistant to glyphosate or not, were susceptible to propaquizafop, clethodim and imazapic, providing some herbicide options for growers to achieve early weed control.n terms of pure plant ecology, there were few surprises – some awnless barnyard grass biotypes are more invasive than others, but growth and seed production of this weed at all moisture levels and environmental conditions ensures survival of the species and contributes to its weedy nature.
“In a study of ten awnless barnyard grass populations we saw large variations in many traits, but growth behavior and seed production potential in these populations did not help predict the likelihood of glyphosate resistance evolving,” he says. “Soil moisture is the main driver of weed growth and seed production.
However, when this weed is well-watered even the glyphosate-resistant populations were three times more susceptible to the herbicide than when the weed is water-stressed.”

In both resistant and susceptible biotypes, very low rates of glyphosate were shown to stimulate growth. This is known as the ‘hormesis phenomenon’, where a stress can stimulate a positive response.
Plants treated with glyphosate at active ingredient rates of 2.5 to 40 g/ha grew taller and produced more leaves, tillers, inflorescences and seeds than the control treatment. These rates are far lower than label rates for awnless barnyard grass and demonstrate the importance of accurate mixing and application of herbicides to ensure lethal rates are applied.

These weed ecology studies have demonstrated that glyphosate resistance in awnless barnyard grass does not confer any advantage or disadvantage over susceptible biotypes. The recommendation then is to treat all populations as resistant to glyphosate and to stack as many of the WeedSmart Big 6 tactics against it as possible, even if each tactic only provides a relatively small control benefit.

More resources

Webinar – Weed biology insight to improve the management of feathertop Rhodes grass and barnyard grass (the barnyard grass segment starts at the 30 min mark)  

The extent of herbicide resistance in summer grasses revealed 
Getting on top of barnyard grass in summer crops and fallow
Creating stiff competition against summer weeds