Adjuvants’ role in combatting herbicide resistance  

Andrew Somervaille, Jubilee Consulting has been evaluating the performance of herbicides for more than three decades and says the role of adjuvants is often either over-rated or under-estimated. This may seem a contradiction, but the fact is that sometimes adding an adjuvant is beneficial and sometimes it is detrimental; and there is an art to knowing how to best deploy these additives.
When weeds are susceptible to the applied herbicides, the effectiveness of adjuvants generally goes un-noticed. However, correctly applied adjuvants can reduce the impact of low level herbicide resistance by helping to maximise the amount of herbicide taken up by the plant.<!
Andrew Somervaille, research agronomist with Jubilee Consulting suggests growers and advisors should not overlook or over-rate the use of adjuvants.
“In the best case scenario, the correct use of an adjuvant can optimise performance of a single herbicide, or a herbicide mix,” he says. “This results in the most efficient control of the target weeds, minimises seed set and reduces weed numbers into the future. All research points to low weed numbers as the only sustainable way to manage herbicide resistance.”
“In the worst case scenario, the incorrect use of an adjuvant can reduce herbicide performance, may compromise the physical compatibility of mixtures and can alter the function of tank mix components,” says Andrew. “This may result in a sub-lethal dose of herbicide being applied, which is known to amplify herbicide resistance if there are low levels of resistance present in the weed population.”
“Once populations are highly resistant then the impact of adjuvants is reduced,” says Andrew. “This highlights the importance of being very deliberate and calculated when making recommendations or decisions about adjuvants.”
In one experiment Andrew conducted with two formulations of glyphosate, he measured the negative effect of the adjuvant when in the presence of 2,4-D to control awnless barnyard grass. “We know there is antagonism between 2,4-D and glyphosate in a tank mix in some situations that results in a reduction in the level of control expected from glyphosate alone,” he says. “What we observed in this experiment was that one glyphosate+surfactant formulation mixed with 2,4-D achieved just over 80% control while a second glyphosate+surfactant formulation mixed with 2,4-D achieved 94% control.”

Andrew says that although some herbicide products are manufactured with an adjuvant included as part of the formulation, there may still be a benefit gained from adding another type of adjuvant prior to application, depending on the other products in the mix, the water quality or the target weed.
In another experiment, Andrew investigated the effect of different adjuvants (LI 700 and Liase) on the efficacy of a RoundUp Power Max (glyphosate) and Amicide (2,4-D) mix. The results clearly showed that while Liase improved the performance of the mix, LI 700 reduced performance to less than 80% control of barnyard grass.

Grower experience, research trials and computer modelling all point toward high levels of herbicide performance, coupled with the removal of survivor plants, to reliably and sustainably extend the useful lifespan of herbicides by removing potential carriers of resistance traits.
“Even small incremental losses in control at the ‘top end’ can have a large effect on the total seed-bank load,” says Andrew. “While 95% control might still be considered a good result from a herbicide application that could have potentially achieved 98% control, this three per cent loss in efficiency could be the start of a substantial increase in weed numbers and allow herbicide resistance to gain a foothold.”
Keeping weed numbers low allows the targeted use of more expensive products (e.g. through an optical sprayer), makes manual control methods economical, and even allows the use of less efficient products to maintain or slightly reduce numbers while adding diversity to the program (provided there is no cross-resistance).
What adjuvants do
An adjuvant may modify the physical, chemical and biological activity of the herbicide on the target. For example, an adjuvant may be added to improve the physical properties of the spray such as its spray quality, or to allow products to dissolve or mix in water. Adjuvants may also alter the chemical properties of the formulation to counter poor water quality or activate certain components in the herbicide, and from a biological perspective an adjuvant may be used to influence uptake through the plant cuticle and even movement across cell membranes.
Very small amounts of surfactants are required to achieve adequate ‘wetting’ of the plant surfaces and adding more surfactant will not necessarily increase the performance of foliar applied herbicides. However, some adjuvants (including surfactants) are added specifically to activate the active ingredients and so are an essential component of the formulation or mixture.
Adjuvants can play an incremental role in improved herbicide performance and assist with keeping weed numbers low and reducing the risk of sub-lethal resistance traits. However, their use does not over-ride the need for correct chemical use and application under the right conditions.
The same adjuvant may even perform different functions when included in different mixes or added to different formulations. There are also specific responses in certain weeds to certain surfactants, some giving superior results and other inferior. It is not possible to give rule-of-thumb recommendations – each scenario needs to be examined carefully, taking into account the target weed species, the condition of the weeds, the water quality and the specific herbicide formulations.
They are not always beneficial and can result in sub-lethal doses being applied if they are not used correctly. Combinations of surfactants can modify the functions of the individual components and it cannot be assumed that the effects are additive to herbicide performance.
Also, be aware that the characteristics of the leaf cuticle are not the primary limiting factor associated with the uptake of foliar applied herbicides. Plant stress is usually the main limiting factor and this may only be partly overcome through the use of an adjuvant.
Clearly, it is not a simple matter of making recommendations or decisions to include an adjuvant. Growers and their advisors need to have an appreciation of the chemistry behind the adjuvant’s use and the way that it may impact on the uptake of the herbicide into the target weed.
Useful resources
Adjuvants – Oils, surfactants and other additives for farm chemicals


Weed control program has expanded crop choices

Darren and Leanne Eather have been growing cotton east of Narrabri for some 20 years. Speaking to their CottonInfo Regional Extension Officer, Darren Eather outlined how they are managing the threat of resistant weeds.
Darren encounters a range of weeds each season, particularly fleabane, windmill grass, liverseed, barnyard grass, turnip, milk thistle and volunteer cotton.
Darren Eather
Darren’s approach to weed management varies between his irrigated and dryland cotton, and is tailored field by field and crop by crop.
In his dryland crop, he strategically uses rotations to control weeds, along with different chemical modes of action. He applies a pre-plant residual herbicide, followed by an at-plant residual and glyphosate as a pre-emergent knock down.
“For grass control, Group D chemicals were our preferred option in order to rotate chemistries. However, we’ve found that windmill grass in particular is not consistently controlled and we get some escapees, which is why we’ve moved towards Group As as an alternative. We’re finding it is providing good control,” Darren said.
In his irrigated country, he combines cultivations with two or three applications of post-emergent glyphosate. He aims to meet with his consultant each year following picking to discuss his approach.
Darren tends not to apply a layby residual, due to the long-lasting effects and subsequent reduction of options. He believes weed control in non-crop areas of his farm is very important, and actively manages this.
“We purchased a farm around a decade ago where we had a very serious issue with black oat and black bindweed,” Darren said. “As a result, we were unable to grow chickpeas on that farm. Now, 10 years later, with a good strategy of weed control and careful crop selection we have controlled the weeds in our system to the stage where we can now grow chickpeas.”

Darren and Leanne Eather

Cropping area
Total hectares: 3,000 ha.
Cotton: 400 ha irrigated and 300 ha dryland

Soil type
Vary from deep black soils to river loams and some hard setting soils

Two-year rotation: Irrigated – cotton (summer); wheat (winter); long fallow (summer and winter); cotton (summer)
Dryland – Cotton (summer); chickpeas, canola, wheat (winter); long fallow (summer and winter); cotton (summer)

Source: CottonInfo Weed Control case studies
Further information: CottonInfo weed management page


Efficient herbicide use pays off

He is faced with a series of weeds each season, including barnyard grass, fleabane, milk thistle, red pig weed, peachvine, feathertop Rhodes grass, bathurst burr, black oats, phalaris and turnip weed. Resistance has been confirmed in barnyard grass (resistant to Group M herbicides – glyphosate) and black oats (resistant to Group A).

Tristram Herstlet, Reardon Farms and Michael Brosnan B&W Rural
Tristram expects to have additional problems with resistance in the future, particularly with feathertop Rhodes grass and glyphosate.
To manage the existing and potential future resistance threats, Tristram has implemented an integrated approach to weed management, in collaboration with consultant Michael Brosnan of B&W Rural at Mungindi.
“We regularly discuss our weeds management program with our agronomist. By the time we plant cotton, we have a program in place and know what we want to do,” Tristram said.
“Our strategy is threefold: to minimise future resistance, prolong the use life of each herbicide, and avoid the problem of weed shifts.
“We do this by having different tactics, such as rotating our chemical groups, cultivation, crop rotation and farm hygiene. We are well aware of the dire consequences of having multiple grass and broadleaf weeds resistant to glyphosate – and we are currently managing two cases of resistance, in barnyard grass and black oats,” Tristram said.
Tristram takes a holistic approach to integrated weed management.
“We use control methods at a whole farm level, at a management unit level, down to an individual field basis. And we apply as many options as possible to try and control the weeds that are resistant, and prevent further resistance developing,” Tristram said.
“We use a Group L treatment instead of glyphosate (Group M) to give the field a break. More recently, we are using a Group K as a post-plant, pre-emergent in cotton. We intend to follow this with cultivation. We haven’t felt that we’ve needed a layby (a residual herbicide used to control weeds in-crop) as the other control methods are currently working.
“We also have a range of other tactics. All of our contractors are required to have clean gear when they come on farm. We practice ‘Come Clean Go Clean’ primarily for disease protection, but an added benefit is weed management.
“We also use chippers, predominately in non-crop areas, but also in-crop if necessary, such as if fleabane has become an issue. We try to keep everything spotless, as the fewer the weeds, the fewer the seeds for next year.”
Tristram believes the future of weed management lies in robotics and microwave technology.
“On our dryland areas we use a camera spray system, which we purchased in 2016. The driver behind the purchase was the ability to spray the dryland fallow area when cotton is in, and the ability to put on higher rates of chemical to kill fleabane without using a hormone,” Tristram said.
“The system was a serious investment – a total of $640,000, including a tractor, sprayer and boom – but we’re spraying at the moment and it costs only $5 per hectare with this new system, versus $11.60 per hectare with a full boom.
“Based on this, we’re saving around $2,000 per day in chemical with the new system. Most importantly, we’re really happy with the results: the cameras work on the chlorophyll in the plants, and the results are fantastic: we’re getting a good weed kill,” he said.

Source: CottonInfo Weed Control case studies
Further information: CottonInfo weed management page


Getting weeds early is key

Tom and Charm Arnott have been growing cotton on ‘Fairfield’ at Boggabilla for 24 years. Speaking to their CottonInfo Regional Extension Officer, Tom Arnott outlined how they are managing the threat of resistant weeds.
They encounter a wide range of weeds in their farming operations, including peachvine, barnyard grass, chinese lantern, feathertop Rhodes grass, fleabane, sesbania, climbing bellvine, pigweed, milkthistle, turnip and native vetch. Tom finds climbing bellvine a particular challenge in cotton, because once it is in the crop it is difficult to control, wrapping around the cotton and causing issues at harvest.
Tony Taylor, Taylor Ag Consultancy and Tom Arnott “Fairfield” Boggabilla
Tom is working with consultant Tony Taylor to implement an integrated weeds management strategy.
“Resistance in weeds is already impacting on our business. We have seen how quick it can get away and it can be hard to get back. There’s also a cost associated with trying to get back,” Tom said.
“As a result, Tony and I are mapping out a strategy to combat resistance. It’s our first year, so it’s very much a work in progress. To date, I have relied on an over the top glyphosate application and a light cultivation for weed control. We’ve learnt that spraying when the plants are too big doesn’t work – you have to get in when they’re small.
“We are now implementing a residual program, which will involve using selective pre-emergents at planting to target hard-to-kill weeds on a field-by-field basis.
“We will then use a mix of glyphosate in a double- knock approach, and conduct a more aggressive cultivation with discs and knives to cultivate closer to the plant line.
“We’ll also apply a broadleaf ‘layby’ (a residual herbicide used to control weeds in-crop) to all fields to control late weeds coming through.
“We also chip weeds: particularly in the school holidays, with the whole family doing their bit. I always have a hoe in the ute!”
Tom found the recent CRDC, CottonInfo and ICAN weed management workshops useful in learning strategies to control glyphosate-resistant weeds.
“There was plenty of information, and a lot of experience among the growers in the group. It was good to kick around ideas and talk to the agronomists,” Tom said.

Tom & Charm Arnott

Cropping area
Total hectares: 1400 ha. Cotton: 1400 ha irrigated

Soil type
Black clay Vertosol

Two-year rotation: cotton (summer); double crop winter cereal or chickpeas (winter); long fallow (summer and winter); cotton (summer).

Source: CottonInfo Weed Control case studies
Further information: CottonInfo weed management page


Diverse approach to feathertop Rhodes grass control

As an early adopter of zero tillage and controlled traffic farming in Central Queensland, Brendan Swaffer is fully convinced of the benefits, and is well aware of the potential impact of weeds like feathertop Rhodes grass.
Since taking over the family farm near Clermont in 2007, Brendan and his wife Jody have been building a robust cropping program with wheat and chickpea in winter and, if soil moisture permits, dryland cotton and sorghum in summer across their 4000 ha of cultivation.
Brendan and Jody Swaffer, Clermont have reintroduced tillage to their zero till controlled traffic farming system to manage weedy patches, primarily feathertop Rhodes grass. Dryland cotton has also been a useful addition to the rotation and provides another opportunity to manage FTR.
“In the summer fallow our main weeds are summer grass, Johnson grass and fleabane but we are most concerned with the small patches of feathertop Rhodes grass that are appearing,” says Brendan. “We are using a mechanical and chemical double knock to manage these patches of FTR and it has been very effective for us in preventing its spread.”
Early in summer Brendon targets any patches of persistent weeds – mostly feathertop Rhodes that has survived under the winter crop. Starting with cultivation of the affected areas, Brendan then follows up a few days later with an application of metolachlor (Group K) to provide short-term residual control of any new germinations that are triggered by the cultivation.
“We might spend two days ploughing but only cultivate 150 ha in total,” he says. “For the rest of the year we carry a hoe in every vehicle and stop to chip out small areas of weeds when we see them. We have been enjoying the enormous benefits of zero till and controlled traffic since the 1990s – there is no going back to full cultivation, but it is a useful management tool to target weedy patches before they get out of hand.”
Preventing seed set for a couple of consecutive seasons is known to rapidly rundown FTR seedbank as the seed on the surface or even slightly buried only persists for 12 to 18 months.
Adding dryland cotton to their rotation has also helped minimise the spread of FTR. The Swaffers produced cotton in four of the five summers from 2010–11 to 2014–15, which enabled the application of Roundup Ready Plantshield to keep pressure on FTR and reduce seed set.
Brendan has built in several non-glyphosate weed control measures including cultivation, along with other knock-down and residual herbicides, to take the pressure off glyphosate in their farming system.
“Feathertop Rhodes grass is not a problem in conventional systems but the more area farmed the more difficult it is to keep clean,” he says. “It also seems to prefer scrub soils that are a bit lighter textured than the open downs country and alluvial soils we have on this property, giving us a slight advantage.”
The fallow starts with spraying out sorghum in June with glyphosate to kill the crop, make harvest easier and kill the weeds. During summer, Brendan applies glyphosate, 2,4D-amine and small amount of metsulfuron (Group B) as a tank mix to target weeds when they are small and actively growing after a rainfall event. The metsulfuron is targeting parthenium and can also provide an additive effect on glyphosate when applied in a tank mix.
After each spray application Brendan looks for, and manages any survivors or areas where the sprayer has missed, to minimise the number of weeds that escape and set seed later in the season. In recent years he has moved to more robust rates to ensure efficient weed control and to avoid the need to go over a paddock a second time.
With the variable rainfall experienced in Central Queensland, chickpeas are now the Swaffer’s most reliable crop, using moisture seeking planting techniques. “We can plant in April or early May on rain received in February by planting the chickpea seed up to 18 cm deep,” says Brendan. “Chickpea is the only crop that has a long coleoptile that allows emergence from such depth.”
Planting chickpea on 50 cm rows using moisture seeking techniques has established chickpea as the Swaffer’s most reliable crop. Brendan has found that a post-plant pre-emergence application of Terbyne (Group C) controls weeds up to canopy closure and no other in-crop herbicide is needed until the crop is desiccated prior to harvest.
“Emergence can take three weeks, but we can establish a crop on stored moisture and have it up and away before in-crop rain initiates a fresh flush of weed germination, giving the crop a distinct competitive advantage.”
The timing of a moisture seeking planting needs to factor in the frost risk in the district to avoid having the chickpea crop flowering when there is a high chance of frost.
In some years there is moisture higher in the profile, allowing both chickpea and wheat to be planted about 10 cm deep. Wheat is also planted a little later in the Clermont district than in other areas of CQ, to avoid frost. Most growers prefer to accept the small yield penalty for planting later rather than risking a crop failure.
“Strzelecki wheat is a slow maturing spring wheat of semi dwarf habit that is popular in CQ due mainly to its longer coleoptile length that allows us to plant to a depth of 10 cm,” he says. “But this variety is about to be re-classified to Hard 2 instead of Prime Hard and so many growers will be looking for alternative wheat for the future,” he says.
Strzelecki wheat has been a mainstay variety in the Clermont district because of its ability to emerge from a depth of 10 cm. A change in classification of this wheat will most likely drive growers to look for a replacement variety suited to the conditions.
Higher levels of crop competition can be achieved in the winter crops compared to the summer crops, with chickpea and wheat both sown on 50 cm rows. Brendan plants chickpea in his cleanest paddocks and uses a post-plant pre-emergence application of Terbyne (Group C) to control weeds up to canopy closure. No other in-crop herbicide is applied except for desiccation for harvest management. Brendon avoids using Balance due to the long plant-back period and the need for a lot of rain to breakdown the residual.
Sorghum crops are sown on single skip metre rows, with cotton planted in double skip configuration of 2 in and 2 out to optimise yield and quality. Brendan previously planted sorghum in solid 1 m single rows but has changed to planting a single skip – 2 in and 1 out – and increased intra-row plant density. The soil on the Swaffer’s property requires about 200 mm of steady soaking rain to fill the profile and initiate a summer crop planting. Last season there was no summer crop planted due to a lack of soil moisture however the outlook year is looking more promising for sorghum but they have not planted cotton this season.
“We are concentrating on achieving even intra-row spacing using a double disc precision planter to increase weed competition within the row,” he says. “This also promotes even maturity and reduces tillering. The combined effect encourages a shorter flowering period and makes grub and midge control easier, along with reducing the risk of ergot infection.”
Sorghum is planted in January and early February following an application of glyphosate, Dual Gold and 2,4D, provided there is no cotton planted nearby. Brendan also applies atrazine and fluroxypyr to provide in-crop weed control. Metolachlor applied in the fallow ahead of cotton provides some residual weed control but the main in-crop weed control strategy is RR Plantshield. Brendan puts far greater emphasis on timeliness of weed control than on specific rates and products.
At harvest, Brendan uses perforated screens in the header to remove as much Mexican poppy, and turnip weed seed and soil as possible out of the chickpea grain sample. He also keeps about 100 t of both chickpea and wheat seed that has been graded hard to ensure the cleanest possible seed goes back in the ground the following season.
Brendon does all his own spraying with a John Deere 4030R self-propelled sprayer and likes to keep their spray technology up to date. He considers the sprayer to be their main tractor now and changes the sprayer unit every 5 years or so to always have new gear that works well and minimal downtime.
“Our groundwater is quite hard so we use ammonium sulfate, especially when spraying out sorghum with glyphosate,” says Brendan. “Although we now have more access to rainwater, storing water is very costly so we have been assessing the difference between rain water and groundwater this year in terms of cost and efficacy on weeds. We expect to invest more in rainwater storage in the future.”
Being in full control of the spray program means Brendan can ensure his neighbours are always informed regarding cotton plantings and he only sprays when conditions are suitable. “When sensitive crops are nearby it is all about working in the right conditions and being careful about product selection,” he says.
Other resources
Mark Congreve explains the key features of FTR, considerations when developing a control strategy, fallow herbicide options and fallow application recommendations in a new series of Grains Research and Development Corporation (GRDC) Know More videos.


Planned approach to rotations helps manage weeds

Ross, Ingrid, David and Margot Uebergang have been growing cotton for 27 years at Miles on the Darling Downs. Speaking to their CottonInfo Regional Extension Officer, Ross Uebergang outlined how they are managing the threat of resistant weeds.
They combat a series of weeds each year, in both their cotton and winter crops, including fleabane, feathertop Rhodes grass, barnyard grass, liverseed grass, bladder ketmia, black pigweed, caustic creeper, caltrop, volunteer cotton, milk thistle, fireweed, black oats and phalaris. Bellvine is an emerging problem on the farm, one Ross Uebergang suspects may become a larger problem down the track.
Ross Uebergang, Jess Mickelborough and Tim Richards
The Uebergangs are yet to do any resistance testing – something Ross hopes to implement this season – but suspect they may have resistant grass weeds.
As a result, they have implemented a whole-of-farm approach to integrated weed management, involving multiple weed-control tactics.
“If you keep relying on one tactic no matter what it is, a problem is going to arise,” said Ross. “We are trying to manage resistance and also the buildup of problem weed seeds. If we don’t, resistant or hard-to-kill weeds will bring the whole farming system unstuck.”
“For us, grasses are the main problem. We have barnyard grass and feathertop Rhodes grass and we’re unsure if they’re resistant or just hard to kill,” he said.
Ross doesn’t rely on glyphosate: his approach includes pre-plant residuals, pre-emergent knock downs, and ‘laybys’ (residual herbicides used to control weeds in- crop), plus non-herbicide tactics including cultivations and spot chipping.
“Our current strategy is to apply a residual six-weeks prior to cotton planting and then to pre-irrigate to allow the volunteers and other hard-to-kill weeds to emerge.
“We follow this with pre- and post-planting knockdowns, which include gramoxone (Group L). In crop, we apply two Group M (glyphosate) sprays and a Group A spray to target feathertop Rhodes grass and will also apply in-crop residual chemicals with shield spray in problem fields.
“After picking, we mulch and rootcut and then do heavy tillage passes to remove ratoon cotton and compaction and then the system starts again. For fields going into fallow, a layby is applied immediately post winter harvest, keeping fields clean for first flush of spring grasses,” said Ross.
“We review our practices every year. Pre-season and post-season we have a meeting with our consultant, Tim Richards of MCA, to review our strategy.
“This is where we work out our rotations and fields, highlight problem areas and develop our residual program. We have a whole farm approach, but treat fields separately due to different weed spectrum and soil types.
“This is our third season of implementing this weed control program and it has really streamlined the whole operation of growing the crop, resulting in greater timeliness of operations which equates to better yields,” he said.
Tim Richards of MCA says a successful integrated weed management system means taking a long-term approach.
“If a grower is looking further ahead than just this season and is willing to commit to a rotation, then it is easy to implement an integrated weed management system like this one. The spin-off benefit of such a system is superior operational timeliness, as we have – and are adhering to – a plan,” Tim said.

Ross and Ingrid Uebergang, David and Margot Uebergang, Uebergang Agriculture

Cropping area
Total hectares: 1100 ha. Cotton: 400 ha irrigated

Soil type
Clay loam, brigalow belah, deep cracking self-mulching clays

Three-year rotation: cotton (summer); covercrop back into cotton (winter, then summer); barley or wheat (winter); fallow (summer and winter); and cotton (summer)

Source: CottonInfo Weed Control case studies
Further information: CottonInfo weed management page


Are you going spraying or killing weeds?

Spray drift is of great concern for sensitive crops and environments, along with the fact that if the spray doesn’t hit the intended target, you do your dough and your weeds live.
Bill Gordon, spray application consultant with Nufarm Australia says the focus of spraying herbicide needs to be on doing the job right so the weeds receive the correct dose and die, and this includes reducing the air borne fraction to a bare minimum.
Bill Gordon, Nufarm spray application consultant says most growers are implementing best spray practice when it comes to boom height and nozzle selection but the temptation to spray at higher speeds and into the night can over-ride common sense at times.
“In many cases this means not spraying at night if the wind speed is too low,” he says. “Many product labels prohibit night spraying due mostly to the risk of spray drift. Our studies have shown that with a coarse spray quality drift may travel up to 300 to 400 metres during the day after any inversion has broken, but spraying at night can leave up to five times as much chemical in the air using the same products, nozzles and ground speed. This can result in spray drifting 10 to 20 kilometres or more at night, and this is unacceptable for other farmers, the community and the environment.”
Mr Gordon says most growers are implementing best spray practice when it comes to boom height and nozzle selection but the temptation to spray at higher speeds and into the night can over-ride common sense at times.
“The flip side of this is that it is becoming more evident that using Delta-T as the main determinant of safe day-time spraying conditions may not be the best approach,” he says. “What really matters most is the wind speed and whether the weeds are stressed or not.”
“If the weeds are not stressed on a hot day, due to ample soil moisture, then spraying at a higher Delta-T may still be effective and safe, with course droplets surviving quite well and not becoming air-borne.”
This can essentially increase the number of daytime hours available to growers to get the job done without spraying at night or early in the morning when the risk of spray drift is the highest.
“Getting onto paddocks as soon as possible after rain and using up the daytime hours to target priority paddocks will maximise weed control and minimise spray drift risk,” says Mr Gordon. “Spraying can continue into the evening in summer if the soil remains warm and the wind speed stays above 12 km/hr. If the wind drops off, then spraying should cease, usually by about 10 pm through to a few hours after sunrise.”
The bottom line is that it is very difficult to determine a safe night-time spraying conditions.

When environmental conditions are borderline in terms of wind turbulence, the safe spray window can be extended slightly through the use of coarser sprayer quality where the proportion of droplets less than 150 microns is 10 per cent or less, keeping drift to a minimum. The trade-off, however, is reduced efficacy when using very coarse droplet size, particularly when the target is small, vertical or hard to wet.
“When buying new nozzles, check them against the new standard, which shows spray quality with adjuvants rather than water only,” says Mr Gordon. “The GRDC has recently updated and published the 2017 Nozzle Selection Chart for growers to use as a reference. Using the correct nozzle and adjuvant combination can have a positive impact in reducing spray drift and maintaining efficacy.”
[A new factsheet provides advice to help growers meet the new 2,4-D application requirements]
“Reducing ground speed by just 5 km/hr can also make a big difference to spray coverage and efficacy of weed control, particularly if there is a high stubble load present,” he says. “Water sensitive paper, in combination with apps such as ‘SnapCard’, is a good way to test the coverage, penetration and spray pattern achieved under different conditions, such as different ground speeds.”
Mr Gordon’s rule of thumb for effective weed control when applying fully translocated products (e.g. glyphosate and Group I) is a minimum of 6–8 per cent coverage, while coverage of 10–12 per cent or more is required for contact herbicides. “Pre-emergent herbicides are the most difficult to judge due to the number of variables involved in their effective application, but as a rule of thumb I generally look for coverage of at least 15–20 per cent,” he says.
Getting onto paddocks as soon as possible after rain and using up the daytime hours to target priority paddocks will maximise weed control and minimise spray drift risk. Spraying can continue into the evening in summer if the soil remains warm and the wind speed stays above 12 km/hr. If the wind drops off, then spraying should cease, usually by about 10 pm through to a few hours after sunrise.
If you are going out killing weeds, you need to get everything right.
Bill Gordon’s 10 Tips for Reducing Spray Drift

Choose all products in the tank mix carefully.
Understand the product mode of action and coverage requirements.
Select (and check) the coarsest spray quality that will provide effective control.
Expect that surface temperature inversions will form as sunset approaches and will likely persist overnight and even beyond sunrise on many occasions. DO NOT SPRAY.
Use weather forecasts to inform your spray decisions.
Only start spraying when the sun is about 20 degrees above the horizon and when the wind speed has been above 4–5 km/hr for more than 20–30 minutes, and clearly blowing away from any adjacent sensitive crops or areas.
Set the boom height to achieve a double overlap of the spray patterns.
Avoid higher spraying speeds.
Leave buffers unsprayed if necessary and come back.
Continue to monitor conditions, particularly wind speed, at the site during the spray operation.
Watch what happens in this demonstration of fine particle movement under surface temperature inversion conditions.

Other resources:

‘Spray wisely and well’ Webinar – Bill Gordon
Ask an expert May 2017 – Bill Gordon
Maintaining efficacy with larger droplets – New 2,4-D application requirements
SnapCard App
GRDC Nozzle Selection Guide 2017
Cotton Field Awareness Map (CottonMap)
Nufarm Spray Wise Decisions
Spray Application GrowNotes


Giving summer legumes the competitive edge

Fifty years ago, many farmers and researchers expected herbicides to be the panacea for weed control. The reality has been that no weed species has been eradicated through the use of herbicides alone, and in fact, many weeds have increased in population in the presence of herbicide application in farming systems.
This is what motivates researcher Dr Bhagirath Chauhan, Principal Research Fellow with the Queensland Alliance for Agriculture and Food Innovation (QAAFI) to study how crops can do their own weed control through the application of cultural practices.
Dr Bhagirath Chauhan says narrower row spacing in summer legumes such as mungbean and soybean will take the pressure off in-crop herbicide applications, provided the crop is sown into clean paddocks and weeds are controlled for at least three weeks after planting.
He says that his agronomic trials in summer legume (mungbean and soybean) crops are consistently showing that early canopy closure results in lower weed biomass and higher crop yield.
“We have demonstrated that narrower row spacing in summer legumes such as mungbean and soybean will take the pressure off in-crop herbicide applications, provided the crop is sown into clean paddocks and weeds are controlled for at least three weeks after planting,” he says. “Increased yield is a significant benefit and will support growers’ decisions to set up their planting gear to suit farming on narrower row spacings.”
In the 2015 and 2016 mungbean seasons, 30 kg/ha of Jade-AU was grown at 25, 50, and 75 cm row spacing and Rhodes grass was sown to provide a known weed density in all plots. There were four levels of weed infestation applied 1. Weeds sown at planting, 2. Weeds sown 3 weeks after planting, 3. Weeds sown 6 weeks after planting, 4. Weed-free throughout the growing season.
Rhodes grass was spread at 300 seeds per m2 to create even competition across the site, which is generally not possible if researchers rely on the natural weed seed bank of the experimental site providing the competition.
Agronomic trials in summer legume (mungbean and soybean) crops are consistently showing that early canopy closure results in lower weed biomass and higher crop yield.
“In plots where the mungbean crop and the weed emerged together, it didn’t matter which row spacing was used – the weed biomass was high and the grain yield was low, less than 360 kg/ha,” says Dr Chauhan. “If the crop is kept weed-free for the first 3 weeks after planting then the narrower row spacings of 25 and 50 cm saw a reduction in weed biomass. We know from other studies that weed biomass correlates well with weed seed production, so reducing biomass can be expected to also reduce seed production in the weed.”
In both seasons, the combination of keeping the crop weed-free for at least the first 3 weeks and planting on the narrower rows (25 or 50 cm rather than 75 cm) generated a yield increase of 159–197 per cent in 2015 and 198–223 per cent in 2016. Even in a completely weed-free growing environment the two narrower spacings generated higher yield than the 75 cm rows.
“This trial demonstrates the value of reducing row spacing to 50 cm in mungbeans and keeping crops weed-free for the first six weeks after planting,” says Dr Chauhan. “After this point the crop has the competitive edge and any later germinating weeds struggle to get established.”
Dr Chauhan has also conducted trials to determine weed densities that will cause a 50 per cent reduction in mungbean yield. These weed densities can provide a guide to growers and agronomists about the level of infestation that will affect yield and allow them to determine an economic response, keeping in mind that any weeds that set seed are building up in the weed seed bank for following seasons.
A 50 per cent yield reduction in mungbean can be expected in the face of 31 bladder ketmia plants/m2, 22 feathertop Rhodes grass plants/m2, 33 windmill grass plants/m2, 21 liverseed grass plants/m2 or 23 button grass plants/m2.
In a similar row spacing trial in soybeans, Dr Chauhan saw similar results to those of the mungbean trial. He also confirmed that row spacing, not plant population, is the key driver to reducing weed growth.
“In soybeans, weed biomass was reduced by 89 per cent under narrow rows (25 cm) and 75 per cent under wider rows (75 cm) when the crop was kept weed-free for the first three weeks after planting,” says Dr Chauhan. “If weeds were controlled for the first six weeks then weed biomass was reduced by 98 per cent under narrow rows and 88 per cent under wider rows.”
In weed-free plots there was a 20 per cent yield benefit in changing from 75 cm row spacing to 25 cm. This yield difference was 65 per cent in plots where weeds were introduced 6 weeks after planting and a huge 121 per cent higher when the weed infestation occurred three weeks after planting.
Mungbean crop density has a direct effect on the morphology of weeds such as barnyard grass where the weed is quite prostrate when there is no crop competition, and has a more upright habit when there is more crop competition. This may have implications for weed control strategies such as harvest weed seed control.


SwarmFarm: Targeting small weeds all year

Just five years ago Central Queensland grain farmer Andrew Bate was in a tractor, spraying a wheat crop, and thinking about ways to farm better and more efficiently. His idea to create a ‘swarm’ of small, lightweight machines that could work autonomously and cooperatively, is now a commercial reality.
SwarmFarm operations manager and leader of field development, Will McCarthy, says the robots are the ultimate weed scouts, tracking down escapes and eliminating them before they have a chance to set seed.
Along with his wife Jocie, Andrew is founding director of SwarmFarm Robotics. The headquarters of their agricultural technology company is their farm ‘Bendee’ at Gindie, south of Emerald, where their team of seven software and mechatronics engineers and technicians is building and testing world-first robotic technology specifically for agricultural applications.
“There are currently seven SwarmFarm robots working on grain farms, turf farms and in an environmentally-sensitive area on a mine site,” says Andrew. “Our commercial release of 50 robots setup for spraying weeds using the WeedIT optical sprayer technology is now underway.”
Weed control provides an excellent opportunity for robotics to shine. A time-consuming but ‘simple’ task that robots can do very effectively at a slower pace, ensuring every weed in the paddock is accurately and effectively controlled while still at a small size.
Weighing just 2 t fully loaded, each robot is only 10 per cent of the weight of a conventional sprayer and they fit in perfectly with zero till and controlled traffic farming systems.
The cost benefit of robots applying herbicide lies in the frequency of treatment, accuracy and ability to safely operate any time of the day or night. While a grower may hesitate to go spraying, concerned that there might be another rain event and subsequent germination, the robots can ‘go now and go later’, always targeting small weeds at their most susceptible growth stage.
Weighing just 2 t fully loaded, each robot is only 10 per cent of the weight of a conventional sprayer and they fit in perfectly with zero till and controlled traffic farming systems.
The SwarmFarm robots optimise the use of existing optical sprayer technology to identify and target small weeds in a green-on-brown situation (i.e. in fallow) by enabling more frequent applications that are slower and more accurate. The ability to go over the same paddock every few weeks is the standout difference that robotics can bring to the management of herbicide resistance.
SwarmFarm operations manager and leader of field development, Will McCarthy, says the prescription spraying used on ‘Bendee’ involves the robots passing over the fallow paddocks once every two weeks. “This way, no weed will get bigger than the 50 cent piece size that is optimal for effective control,” he says. “We can apply a wider range of herbicide modes of action, more robust rates for chemicals registered for this use pattern and potentially reintroduce products and brews that may have had reduced efficacy as broadacre sprays in the past.”
“The robots are the ultimate weed scouts, tracking down escapes and eliminating them before they have a chance to set seed. Constantly targeting small weeds and preventing seed set is the only way to keep weed numbers low and avoid herbicide resistance.”
The SwarmFarm robots enable the optical sprayer technology to really come into its own because the robots can operate slower, the cameras and sprayers can be closer together and the robots can go over the paddock repeatedly so there is no concern about getting the timing right. Every weed can be treated at an early growth stage for the herbicide to have maximum effect, tackling herbicide resistance at the source by applying constant downward pressure on the weed seed bank.
The SwarmFarm concept is ideal for new technology developments, as it allows easy integration of third party products as they are being developed, such as green-on-green technology. Will says the robots would then be able to distinguish between a weed and a crop plant and even between weed species. This will allow the removal of volunteer crop plants and even target broadleaf weeds like sowthistle in a broadleaf crop such as chickpea.
Although the current focus is on herbicide application, there is great potential to use the same platform to implement non-herbicide tactics such as targeted tillage or robotic chipping, steam or any other non-herbicide tactic found to be effective.
“Using the robot concept, microwave technology becomes a realistic option because the robots can stop at every weed and apply the necessary microwave blast to kill each weed, something that is simply not feasible for a tractor operator,” says Will.
WeedIT cameras capture data from a 1 m wide band on the ground using NIR and IR light to detect green weeds in a ‘brown’ paddock. The SwarmFarm concept allows easy integration of third party products as they are being developed, such as green-on-green technology or non-herbicide tactics such as targeted tillage or robotic chipping, steam or any other tactic found to be effective.
“Aside from weed management, the robots will enable direct management of a crop’s plant population to maximise yield potential for the available soil moisture,” he says. “There is no reason why the SwarmFarm platform can’t be utilised for planting and applying fertiliser precisely and economically, controlling insect pests and even harvesting the crop. The system is in place to support any application really and all that is needed is the planter, cultivator or harvester to be engineered and bolted on.”
Andrew reckons that a 10 000 ha property like ‘Bendee’ would only need two SwarmFarm robots to take care of all their weed control operations.
SwarmFarm robot features
Working through the features of the SwarmFarm robots highlights their simple and robust construction and numerous safety features. Will says the team has worked hard to make diagnosis as simple as possible and the modular components have minimal opportunity for failure. “If there is a problem, the replacement parts can be easily fitted on-farm without specialist technicians and the maintenance is straightforward and well within the capability of any farmer,” he says.
Multiple safety features built into the robots, which make them safer than a person operating a spray rig or tractor, include:

obstacle detection sensors (can determine if the terrain ahead is suitable to traverse and also stopping if there is something in its path e.g. a vehicle or person),
paddock definition (it maps the paddock to show boundaries, fencelines, trees, troughs, dams etc. then uses software to generate A–B lines. The robot then drives itself around these fixed obstacles),
remote control using an iPad (allows the operator to stop and restart the robot when within the local farm network),
a bumper sensor to turn off the machine as a back-up to the obstacle detection sensors (slow operating speed means any damage would be minimal if this was activated),
a geo-fence that turns off the machine if it crosses the line.

Each robot has an 8 m boom fitted with eight WeedIT optical cameras, 40 nozzles and a 600 L spray tank. Depending on weed density across the paddock this could last all day or an hour. The robot monitors the volume of spray in the tank and makes a decision whether it can reach the end of a run or not before running out of spray. It then returns to a docking station for refilling. At the moment, a person is required to refill the spray tank but plans are in place to fully automate the refilling operation within the next 8–12 months. The 60 L diesel fuel tank on board gives the machine an operating time of 18 hours between refueling.
WeedIT cameras capture data from a 1 m wide band on the ground using NIR and IR light to detect green weeds in a ‘brown’ paddock. There are five individual sprayer solenoids per camera span, giving one spray nozzle every 20 cm. The cameras are set to turn on three nozzles over a weed to ensure good coverage, which is particularly important if there is a breeze blowing.
The genius of the SwarmFarm system lies in the ‘smarts’ of the SwarmHive base station computer. Located in the grower’s office the SwarmHive takes care of the robots’ activity and decision making and coordinates the workflow of all the robots operating in the paddock.
“If one machine is going slower due to higher weed numbers then the SwarmHive will reallocate the other robots to cover the extra area so that the whole paddock operation is completed at about the same time and all the robots come back to the docking station together,” says Will. “It is updating in real time and making decisions about the weather conditions, mapping weed density, ensuring the robots are operating efficiently and sending alerts if any problems arise, such as a pump malfunction or an obstacle detected.”
An on-site automatic weather station located at the docking station monitors key parameters such as Delta-T, wind speed and direction and ensures the robots only operate within the label directions. The SwarmHive automatically turns off the robots and then restarts them when the conditions are within the acceptable range. Integration with an on-site, automatic weather station also provides a reliable record of spray activities and the real-time environmental conditions during the spray operation, such as wind speed and direction. The grower also has ‘on-the-go’ access via an iPad app to monitor and control the robots if necessary, when within the local farm network.
Other resources

Andrew Bate SwarmFarm Robotics podcast


Beating multi-resistant weeds in the Northern region

Dryland cotton and grains farmer Paul Slack is battling weeds that have high levels of resistance to several important herbicide modes of action.
Managing 4850 ha of cropping land within 30 km radius of his home farm at Gurley, east of Moree, NSW Paul is using summer and winter cropping, strategic tillage and a long fallow spray program to get on top of resistant annual ryegrass and black oats populations.
Tony Lockrey (AMPS Agribusiness agronomist), Paul Slack (Moree farmer) and Caleb Torrance (AMPS trainee agronomist) inspecting Paul’s corn crop, which is adding diversity to his cropping and herbicide program.
A few years ago Paul was faced with a blow-out situation of 90–100 annual ryegrass plants per square metre in a Clearfield canola crop, along with some black oats and barley grass.
The ryegrass present was tested for resistance, revealing an alarming profile of resistance to five herbicide mode of action groups. Groups A, B, M and two others, where the lowest level of resistance was to mesosulfuron-methyl (Atlantis, Group B) at 15 per cent.
The annual ryegrass present was tested, revealing resistance to five herbicide mode of action groups. A dedicated program aimed at preventing seed set has seen a huge reduction in weed numbers with annual ryegrass now confined to a few isolated patches near waterways.
With stubble cover required for the fallow ahead of a dryland cotton crop, Paul and his agronomist Tony Lockrey, AMPS Moree decided to run down the weed seed bank, starting with a double knock application to treat the first germination in autumn.
Residual herbicides were incorporated using a Kelly chain through the canola stubble prior to sowing a short season wheat crop.
“Sakura + Avadex Xtra provided upwards of 80 per cent control of ryegrass, but it must be used wisely in the rotation — we would try not to use this combination more than twice every four years,” said Tony. “In our experience, Group K herbicide Sakura is more robust in a variety of conditions, proving more stable than other residuals in dry conditions following planting, which often occurs in this region.”
An in-crop application of the selective herbicide (Group B Atlantis) in wheat followed by a long fallow and a dryland cotton crop resulted in a huge reduction in weed numbers. Paul said annual ryegrass is now confined to a few isolated patches near waterways.
“Using the Kelly chains in canola stubble represented some challenges but it was much easier than in other crop stubbles,” said Paul. “Now that the chain is worn-in I think it could be used in a chickpea crop to incorporate pre-emergent herbicides without damaging the crop. Mostly though we use it to chop chickpea stubble and to control weeds on the headlands.”
Having had success with this regime to control multi-resistant annual ryegrass Paul decided to use a similar program to rein in Group A resistant black oats that had spread over about 100 ha.
Black oats has been a problem for the last 15 years or so after gaining Group A resistance from the short wheat / chickpea / wheat rotation commonly used at the time, where there was heavy use of Group A products such as Verdict and Topik.
Black oats pressure is clearly seen in this area where the crop was not sown. The addition of dim chemistry (Group A) in the chickpeas has helped extend the life of Group A chemistry, even though there is 50 to 60 per cent resistance to Verdict (a Group A ‘fop’) in the black oats population.
Paul has used a long fallow period to regain control of resistant black oats, bringing the problem back to a very small area. “We did a long fallow after wheat then back to cotton followed by chickpea and then a short fallow and back to wheat,” he said. “This meant we could apply glyphosate in the fallow and two fop + dim mixes in the following chickpea crop. The addition of dim chemistry (Group A) in the chickpeas has helped extend the life of Group A chemistry, even though there is 50 to 60 per cent resistance to Verdict (a Group A ‘fop’) in the black oats population.”
“We might repeat this clean-up program to really run down the black oats seed bank before returning to our normal program, which now features both summer and winter crops and a broader range of herbicide tactics,” said Paul.
Black oats blow-outs are most likely in wheat crops and Paul is considering cutting weedy crops to make round bale silage. However, without a strong nearby market for the fodder and the large nutrient loss involved in baling, this decision will be a hard one to make.
Milk thistle is looming as another challenging glyphosate-resistant weed that will require a different non-herbicide solution, possibly a mechanical innovation.
“We know that zero-tillage farming led to a 30 per cent yield improvement in this environment,” says Paul. “We can’t afford to be forced back to a full tillage system to control weeds.”
“There is potential to cut the crops lower in problem areas to expose the summer weeds to knock-down control and to improve herbicide-soil contact and efficacy with residuals,” he says. “In doing so we then also need to find ways to better spread the increased volume of crop residue out the back of the header, right across the 12 m to maximise fallow moisture efficiency. Using a chaff deck system to gather weed seeds at harvest and place the chaff on the wheel tracks is also something we are considering.”
The current rotation of corn and cotton in the summer and wheat and chickpea in winter has helped bring all resistant weeds back to a manageable level. Bringing Group B chemistry into the herbicide program has been an effective tool in reducing the number of multi-resistant grass weeds and adding a Clearfield wheat into the rotation is another option for the future.


Taking the competition to the weeds

Competitive crops are the ultimate weapon against herbicide resistant weeds as a non-herbicide tactic that suppresses weed germination and boosts crop yield.
When stubble retention and harvest weed seed control are also enlisted in the tactical war on weeds then productivity and profitability become the clear winners.
Greg Condon (right) is working with the Fox family at Marrar to build an integrated weed management system that also promotes productivity.
Greg Condon, Grassroots Agronomy says growers can consider putting together several components that enable narrower row spacing, which is known to generate a one per cent increase in grain yield for every inch reduction in row spacing.
“It is important to remember that this benefit can be gained without going to ultra-narrow rows,” he says. “I’d suggest growers go as narrow as they can within the constraints of their existing equipment and farming system to gain as much yield benefit and weed suppression as possible. Then, when it’s time to buy new equipment growers could look at what’s available that would enable more competitive configurations.”
Green crops provide the best possible shading effect to suppress weed growth with a competitive crop having shading power equivalent to 10 t/ha retained stubble. This varies considerably with crop architecture where erect cereals provide less shading of weeds than varieties with a prostrate plant structure. Narrowing the row spacing increases the competition while also contributing to crop yield.
One system that some of Greg’s clients are using combines stubble management and narrow rows using a disc seeder at planting and a stripper front at harvest.
“The real advantages of stubble retention are seen in improved water holding capacity in the soil, which enables earlier sowing,” says Greg. “One year of retained stubble is generally not a problem for any of the modern seeders but we are seeing disc seeders do a better job handling stubble accumulated over a few years and in various stages of decay.”
Front loss from a stripper front can be 1–2 per cent but in some cases this can be compensated by reduced harvest cost and header depreciation as stripper fronts harvest faster. Correct harvester and front setup is essential to minimise harvest losses.
Including canola and pulses in the crop rotation also plays an important part in managing the quantity and decomposition of crop residue, particularly in a narrow row configuration.
Pulses and canola crops assist in stubble management across the rotation.
“Using a stripper front at harvest greatly reduces the amount of crop residue that is cut, chopped and spread out the back of the harvester,” says Greg. “The rearwards rotating rotor and stripper fingers strip the grain off the heads and leave the bulk of the crop residue standing in situ. About 85 per cent of the threshing occurs at the front end of the harvester, enabling double the harvest capacity of a draper, using 60–70 per cent less engine power and 60 per cent less fuel.”
Grower experience suggests that stripper fronts work well in lodged crops and still allow the operator to have the front down low to collect any weed seeds, such as annual ryegrass and brome grass, present at harvest. This is particularly effective if the crop is sown on narrow rows, forcing the weed seed heads upwards in the crop canopy.
Research into the efficacy of this system for harvest weed seed control is planned for the coming harvest.
“The tall stubble left after harvest provides very effective soil shading and protection from drying winds over summer to retain more moisture and enable early sowing, rather than growers needing to wait for breaking rains,” says Greg. “The stubble dries and becomes brittle rather than rotting on the soil surface, making it easier for the disc seeder to operate.”
The tall stubble left by the stripper front at harvest has widened their planting window all the way from January to May, giving them incredible flexibility in their cropping program decisions. The chaff line is subject to a high level of shading and higher moisture conditions that suppress germination very effectively.
Harvest weed seed control is an additional component that growers like Daniel Fox at Marrar, NSW are adding to the ‘strip and disc’ system. Daniel has moved away from narrow windrow burning and now uses a chafflining chute to deposit the small amount of chaff, along with weed seeds, into a thin band behind the harvester. This contains the weed seeds to a very small portion of the paddock where they either rot away or have to face stiff competition from the following crop.
“We are putting much more emphasis on cultural weed control methods now and reducing our reliance on herbicides,” says Daniel. “The tall stubble has really widened our planting window all the way from January to May, which gives us incredible flexibility in our cropping decisions. The system is easier to manage if you start with low weed numbers but we really believe that it can be used to drive down weed numbers after a blow-out too.”
In a nut-shell, Daniel and his father David have implemented a controlled traffic farming system based on 16.5 cm row spacing, competitive cultivars, disc seeding, stripper front, chafflining, double-break cropping and sowing east-west where it is practical. On the herbicide front they have a strong pre-emergent program and utilise double-knocks and crop topping to manage in-crop survivors.
The Fox family of Marrar, NSW have implemented a controlled traffic farming system based on 16.5 cm row spacing, competitive cultivars, disc seeding, stripper front, chafflining, double-break cropping, and sowing east-west where it is practical. On the herbicide front they have a strong pre-emergent program and utilise double-knocks and crop topping to manage in-crop survivors.


Surveys highlight the cross-resistance to herbicides

John Broster, senior technical officer (herbicide resistance) at Charles Sturt University leads the weed survey project in NSW and has just completed crunching the numbers and finalising the results of the 2016 survey.
John Broster, CSU
“We gathered samples from the northern NSW and plains regions in 2016, which means we now have survey results from across the major grain growing regions of NSW,” he said. “The target weeds that we collect and test for herbicide resistance are, in order of importance, annual ryegrass, wild oats, sowthistle, barley grass, brome grass, wild radish and Indian hedge mustard.”
The weeds that are collected in the surveys are tested for their susceptibility to the major herbicide groups used for their control. “Across NSW the extent of ryegrass resistance to Group A ‘fops’ and Group B ‘SU’ herbicides is by far the highest with about 60 per cent of samples being resistant, and 46 per cent of samples being resistant to Group B ‘Imi’ herbicides. There was less extensive resistance to the Group A ‘dims’ and Group D herbicides at 10 and 11 per cent of samples respectively,” said John. “When we looked at cross-resistance in the ryegrass samples collected in NSW we found that while 24 per cent of samples were susceptible to all five of the major herbicide groups used for ryegrass control, a huge 43 per cent were resistant to three or more of these five herbicide categories.”
Speaking at the recent WeedSmart Week forum, John Broster, senior technical officer (herbicide resistance) at Charles Sturt University presented results from the latest weed survey conducted in NSW where 43 per cent of surveyed annual ryegrass plants were resistant to three or more of the five important herbicide categories used to control this weed.
These surveys clearly illustrate the level of cross-resistance that exists in cropping paddocks across NSW, and in other states around Australia. At the recent WeedSmart Forum in Wagga Wagga where John presented the latest survey results, 170 growers, agronomists and researchers compared notes and experiences about a whole raft of weed management tools, some largely untested but showing promise in the field.
Harvest weed seed control options were of great interest, as were the demonstrated benefits of double-break cropping, utilising livestock in mixed farming enterprises and ways to extend the effective use of herbicides.
The ‘Big 6’ WeedSmart tactics known to drive down weed numbers and drastically reduce the impact of herbicide resistance on Australian farms are: Rotate crops and pastures, Mix and rotate herbicides, Increase crop competition, Use the double-knock, Stop weed seed set and Implement harvest weed seed control.
Herbicide resistance can’t be beaten with herbicides alone — that’s the take-home message from these surveys. However, even in paddocks where herbicide resistant weeds are almost as common as susceptible ones, this doesn’t mean ‘game over’, as many Western Australian farmers have shown. It does mean that farming systems need more diversity and weed management programs need to include a range of tactics aimed at reducing weed numbers at every opportunity during the year.


Setting up harvesters to capture weed seed in the chaff

Getting weed seeds into the chaff fraction
Separating the chaff (including the weed seeds) from the straw is a great way to retain more crop residue at harvest. There are several harvest weed seed control (HWSC) systems that manage just the chaff, including chaff decks, chaff carts and chaff lining. All these systems rely on the weed seed entering the front of the header and then being captured in the chaff stream.

Chaff lining – new, cheap, simple [but not fully tested]
Chaff lining is a new harvest weed seed control method that has sparked incredible interest from growers throughout Australia. Similar in concept to the chaff deck system (called chaff tramlining), chaff lining places the chaff fraction directly behind the harvester rather than on the CTF tram tracks.
It is a grassroots grower solution to HWSC that is cheap to try and seems to be a very effective tool. Essentially weed seeds are collected at harvest and dropped in a narrow line behind the harvester. There is no burning required and all the straw is spread behind the harvester so there is very little loss of ground cover.

Getting started involves the construction of a simple chute that is then fitted to the harvester. The chaff lining chute can be used in all crops and does not affect harvester operation. Like all harvest weed seed control methods, the harvester must be set up and operated, correctly to ensure the maximum number of weed seeds enter the front of the header and are contained within the chaff fraction.
Below are a number of videos and farmer case studies showing how to implement chaff lining. This method of HWSC is well suited to controlled traffic farming but it can also work in non-CTF systems provided the harvester runs in the same lines for a few years in a row.
Grower case studies
Mic and Marnie Fels have developed a farming system where herbicides are used to back-up their cultural practices, rather than the other way around.
Mic Fels has used a modified version of chaff tramlining as their harvest weed seed control strategy. The idea is that the chaff component is funneled into a narrow strip in the middle of the CTF runs behind the header.
In a controlled traffic system this means that the weed seeds collected through the header are concentrated into the same zone every year and any seeds that germinate through the mulch are subject to the full force of crop competition. Similar to the experience of growers using a chaff deck to channel the chaff into CTF permanent wheeltracks, Mic finds that the chaff and the weed seeds simply rot away and there is no need to burn the chaff to gain the benefits of this weed control measure.
Daniel (left) and David Fox are pleased with the chaff lining chute they have introduced as a harvest weed seed control method on their Marrar farm near Wagga Wagga.
Marrar farmer Daniel Fox is chasing higher yields across his 2100 ha cropping program while also driving down weed seed numbers. For a few years Daniel has been adding components to his system to conserve moisture and keep herbicide resistant weeds at bay.
Having used narrow windrow burning for a few years and seeing the benefit of capturing seed from late germinated weeds at harvest, the Foxes have now built a chaff lining chute for the header and are delivering the chaff component, including weed seeds, into a 250 mm chaff line in the middle of the 12 m CTF lap. This maintains most of the crop residue evenly across the paddock and avoids the need for burning. “Having the weed seed concentrated in a narrow band reduces the amount of seed that germinates and also reduces the chance of weed seed being buried and ‘stored’ underground at planting now that we are using a disc seeder,” says Daniel.
Setting up harvesters to capture the weed seed in the chaff


Spray out low yield potential crops now or crop top later?

As dry conditions continue to impact on crops in many regions of Australia, affected growers will be faced with difficult decisions that will have long lasting effects on the weed seed bank.
The lack of crop competition in drought-affected crops leaves the door wide open for more weeds to germinate and set seed, adding to weed control costs in future years.

North Tenindewa, WA grower Glenn Thomas is considering the best option for this low yielding canola crop to manage future wild radish weed pressure.
Faced with a similar situation in 2001, and again in 2006, North Tenindewa, WA grower Glenn Thomas made the hard decision to sacrifice his low yield potential lupin crops to escape the consequences of an inevitable blowout in annual ryegrass and brome grass.
“In 2001 we had a paddock with a very poor lupin crop and about 140 ryegrass plants per square metre,” he said. “In that August we decided to spray the crop out using a glyphosate mix, leaving the best part of the paddock for comparison, and in the hope of late rains that might save the crop. Although we lost the value of a 250 kg/ha lupin crop, we measured a 700 kg/ha yield increase in the 2002 wheat crop as a result of fewer weeds, more nitrogen and more soil moisture. This was particularly pleasing given that 2002 was also a drought year in this district.”
The un-sprayed section was still managed with selective herbicides to control weeds in-crop, however this section of the paddock stayed weedy for six or seven years, while the sprayed out area had no weeds, clearly illustrating how much Glenn had saved in chemical and operating costs in the area he had sprayed out.
When the same scenario played out in the 2006 season Glenn again decided to spray out the low yielding lupin crop in the same paddock to avoid a build-up of weeds. This time he sprayed out the whole paddock, being more confident in the long-term benefit of this decision.

This photo from 2016 shows how clean Glenn’s sprayed out paddock has remained, 10 years after the last drought-affected crop was sprayed out.
In both 2001 and 2006 the drought conditions did not improve as the year progressed so Glenn had no reason to regret his decisions to forego the crop and spend the $25–30 per ha to spray out the failed crops and reset the weed seedbank.
In the end, Glenn’s decision has created a legacy of very clean paddocks and he has taken a diverse approach to his weed management program including narrow windrow burning, chemical fallow and brown manuring in weedy paddocks. With annual ryegrass now well under control Glenn is more concerned about managing wild radish.

Glenn’s experience was also demonstrated in a 4-year trial at Mingenew conducted by AHRI communication lead, Peter Newman while working for DAFWA. “In this trial we sprayed out a weedy lupin crop and compared this to harvesting the lupin crop and returning ryegrass seeds to the seed bank,” he said. “The following year 163 ryegrass/m2 germinated in the area where lupins were sprayed out compared to 1433 ryegrass/m2 germinating in the area where lupins were harvested.”
This trial ran for 4 years and the difference in ryegrass seed bank was evident for the duration of the trial.
“Although a full spray out now might provide the best outcome for weed control, it is still an expense. While there is no cash flow into the business, there is also no further expense with harvesting a low yielding crop where grain losses can also be high, particularly in short pulse crops,” he said. “Another option is to crop top in October, which will still reduce seed set while preserving some cash flow from this year’s crop. Implementing a harvest weed seed control tactic such as chaff lining or using a chaff cart will further reduce the risk of a future weed blowout from a low rainfall year.”
Other advantages of crop topping over an earlier spray out is that paraquat is a lower cost herbicide application and there will be more ground cover left to protect the soil surface from the wind over summer.
“Although crop topping these paddocks in October will not necessarily achieve as good a result as spraying the paddock out in August, it is a lot better than simply harvesting the crop and returning weed seeds to the seed bank,” said Peter.
As the current season unfolds Glenn is once again looking at weed numbers in low yielding crops. He reckons there will be a percentage of his lupin crop that will be sprayed out along with a small area of canola where wild radish numbers are of concern.
“This year we probably won’t actually spray crops out until later in the season when we have grown as much biomass as possible,” he says. “In 2001 and 2006 the season started well but then deteriorated so the weeds grew with the crop and needed to be sprayed in August when it was clear the crops could not compete with the weeds. This year, the dry start has kept weed numbers relatively low and we are only just seeing a response to the couple of double-digit rainfall events from the last week or two.”
Glenn is now watching the weeds respond to the rain and will monitor them carefully so he can prevent seed set while still maximising biomass and nitrogen production in the lupins. He is also watching his wheat crops that have already come to head and the weeds are growing but it’s too late for in-crop herbicide applications.
“This is a real dilemma for us and others in the district,” he says. “There might be a case for a pre-harvest herbicide application in these wheat crops to target late germinating weeds but there is a lot to consider before making this decision.”


Zero row spacing could take crop competition to a new level

Grain grower Leigh Bryan doesn’t shy away from an unconventional idea. His crop-guided shielded sprayer is working a treat, he is confident with chaff lining and now he is testing the possible advantages of zero row spacing.
His aim is weed-free paddocks with crops growing at their full potential across the 2400 ha of cultivation within 30 km of Swan Hill in Victoria’s Mallee region.
“Being crop-guided, the shielded sprayer provides precise control over weeds and crop volunteers,” he says. “It makes it possible to suppress medic and other hard-to-kill weeds in lentil crops and take self-sown wheat plants out of barley paddocks. Keeping crops as clean as possible.”

Shielded sprayer used to apply paraquat for control of medic in lentils.
Using the shields strategically over the past seven years, Leigh has drastically reduced weed numbers and can concentrate on preventing seed set in just the odd plant here or there. The shields allow Leigh to use more cost-effective pre and post emergence herbicide options to prevent seed set of grass and broadleaf weeds in cereal crops.
“The shielded sprayer also makes it possible for me to crop top at the optimum time to have maximum impact on weed seed set without damaging the crop, regardless of the crop maturity stage,” he says.
The shielded sprayer has played a significant role in managing herbicide resistant brome grass and annual ryegrass.

Leigh’s crop-guided shielded sprayer in action, controlling brome grass in a barley crop.
As a harvest weed seed control tactic, narrow windrow burning was a good ‘get out of gaol’ tactic but Leigh has been chaff lining now for three years. He says depositing the chaff-only fraction in a narrow line between the harvester wheeltracks is a very effective way to keep weed numbers low.
“Dropping the chaff line between the wheeltracks, rather than into the wheeltracks, minimises disturbance of the seeds, reduces weed seed contact with soil and promotes more rotting down over summer,” says Leigh. “The chaff lining chute is also cheaper to make and install than a chaff deck.”
Last year Leigh used the chaff lining chute in all crops except canola and found there was no problem pulling the tines through at planting this year.
“We first tested the idea in clean paddocks hoping to keep them clean,” he says. “We are now completely confident in the system and will be using the chaff lining chute in all crops across the farm this season. It has proven particularly valuable in lentil crops.”
If weeds become a problem in the chaff line Leigh is prepared to band spray the chaff line with pre-emergent herbicide, but this has not been necessary to date.
For the last three years Leigh has also been trialling zero row spacing in a bid to capture optimal yield from his crops. The planter has been fitted with splash plates that spread 35 per cent of the seed at random while the remaining 65 per cent of the seed is sown conventionally, down the tube.

Leigh uses splash plates fitted to his planter to spread 35% of the seed randomly across the soil surface to achieve ‘zero row’ spacing.

Close-up on the splash plate.
“This will only work in relatively clean paddocks,” he says. “With no trifluralin applied the in-crop weed control is reliant on strong crop competition and very, very low weed seed bank. Next year I will be looking at different pre-emergent herbicide options and how they might fit in this system.”
The seeding operation is faster than normal and this helps throw sufficient soil to enable successful germination and establishment for the seeds spread on the surface. In Leigh’s on-farm side-by-side trials, the zero row spacing strips are showing that the idea has promise.
“There are others playing around with the idea of zero row spacing too and I think it has potential,” he says. “In 2016, a strip-trial in barley resulted in the zero row spacing strip yielding 4.994 t/ha compared to 4.889 t/ha in the conventionally sown crop at 375 mm spacing.”
This result gave Leigh the confidence to sow one-third of the farm using zero row spacing this season. To achieve the best results Leigh sows considerably more seed but says this is cheaper than pre-emergent herbicide and results in potentially higher yielding crops. The results of this large scale trial will be very interesting in terms of both weed management and crop yield.

Leigh takes a weed-point view in zero row spacing (left) and 375 mm spacing (right).
“The random placement of stubble is easier to sow through the next year. It still provides trellising for pulse crops and shades the soil to conserve moisture and reduce soil surface temperatures,” he says. “There can be a 10-day delay in germination between seeds sown on the surface compared to those sown into the soil because the surface seed needs rain to germinate.”
When it comes to growing a competitive crop Leigh emphasises the need to get the crop planted on time, keep it healthy and well-fed.
Leigh’s continuous, no-till cropping system includes wheat, barley, canola, lentil, field pea and lupin brown manure. When planning the crop rotation for paddocks he ranks them according to weed burden and uses a 4-year break crop system to clean up paddocks with high levels of grass weeds.
“In a weedy paddock we start with a pea/lupin brown manure, then plant canola, in year 3 we plant a cereal and use the shielded sprayer in-crop then finish with a lentil crop,” he says. “This gives us a 4-year run at grass weed control and then we can return to ‘normal’ rotation with a low weed seed bank.”
Once weed numbers are low, Leigh says it is relatively easy to keep them low through the consistent use of both herbicide and cultural practices. “When weed numbers are low it is possible to look at ways to improve yields and reduce production costs,” he says. “That’s what I hope to achieve with zero row space planting, supported by crop rotation and chaff lining to manage survivors. If there is evidence that weed numbers are increasing, then we have other tools at the ready to stop a weed blow-out.”