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Is mechanical site-specific weed control a fallow management option?

with Andrew Guzzomi, Senior Lecturer, School of Engineering, The University of Western Australia

There’s nothing more old school than a chipping hoe when it comes to weed control. Chipping has so many practical benefits – simple, low tech, no survivors, cheap, environmentally friendly, non-chemical, works on all weed species and sizes and keeps the kids out of the house for hours.

The downside of course is that it is slow, hot, boring work and requires teams of people to make any difference to weed numbers. So, how good would it be to have multiple chipping hoes mounted on a bar travelling at 10 km/h and poised at the ready to chip out any weeds?

UWA Agricultural engineer, Dr Andrew Guzzomi led the engineering component of a project to develop a tactical tillage machine, that can help manage herbicide resistance risk in conservation cropping fallow management. (Photo: Ryan Early, Anvil Media)

To see if this was feasible, GRDC funded a project for an expert team* of agricultural engineers and weeds researchers, along with grower and industry advisors, to build and test prototypes in various cropping areas around Australia. Their efforts have culminated in the development of the ‘Weed Chipper’, a cultivator fitted with hydraulic response tynes and commercially-available optical sensors that is ready for commercial trials and validation.

Agricultural engineer and senior lecturer at the School of Engineering at The University of Western Australia, Dr Andrew Guzzomi led the engineering development of the rapid response tyne, which was based on the Shearer Trashworker hydraulic breakout system.

“Like the optical sprayers, the ‘Weed Chipper’ is fitted with commercially-available optical sensors that can detect weeds and trigger individual tynes to rapidly chip out the weeds,” says Dr Guzzomi. “Its best fit is in fallow situations where the weed density is low – around 1 weed per 10 square metres.”

“Being well-suited to the control of larger weeds, this machine provides a fantastic non-chemical option to clean up survivors in a double-knock operation within a conservation cropping system.”

This machine has the potential to revolutionise herbicide resistant weed management and help farmers keep weed numbers low. Field trials showed the implement is a highly effective way to manage key summer weed species, such as windmill grass, feathertop Rhodes grass and awnless barnyard grass, even when these weeds are up to 70 cm in diameter.

Any tactic that can consistently achieve 90 to 100 per cent weed control needs to be taken seriously. See Table 1 below.

How much damage does the chipping do to the soil in a no-till system?

Short answer: Very little at low weed densities.

Longer answer: Targeted tillage is suitable for use at low weed densities, i.e. 1 plant per 10 mor less. At low weed densities the ‘Weed Chipper’ disturbs only a very small portion of the paddock’s surface during weed control. The response tyne is designed so that the amount of soil disturbance can be altered as required to control the target weeds. The cultivation depth and duration can be increased to target large tap-rooted and fibrous weed species, or reduced for smaller, shallow-rooted species.

Photos: Michael Walsh

How fast does it operate?

Short answer: The ‘Weed Chipper’ has been developed to operate at a ground speed of around 10 km/h.

Longer answer: The response tyne system was designed to operate at a nominal 10 km/h where weed densities are 1 plant per 10 mor less. Although higher operational speeds are possible, this would increase system loads and the potential for misses of targeted weeds. With less environmental limitations impacting on safe operation, the ‘Weed Chipper’ can be operated 24/7 if necessary.

What is the best-fit for an optical chipper in an integrated weed control program?

Short answer: As a non-herbicide fallow weed treatment option in low weed density situations.

Longer answer: The best-fit for the ‘Weed Chipper’ is to target low density (1 plant per 10 m2) weed populations in fallow paddocks – the same situation where optical sprayers are currently being used. This will remove survivors and reduce weed seed-set, to potentially prevent or delay resistance evolution. The Weed Chipper also has the benefit of being able to be used across a range of environmental conditions that prevent the application of herbicide treatments. This facilitates more timely and effective weed control.



* The Weed Chipper team led by Dr Michael Walsh (University of Sydney) is comprised of agricultural engineers Dr Andrew Guzzomi (UWA) and Dr Carlo Peressini (formerly UWA) and weed researchers Dr Michael Widderick (QDAF), Dr Adam McKiernan (QDAF) and Dr Bhagirath Chauhan (UQ).

A new era of tactical tillage

Technology heralds new approach to weed control. Targeted tillage machine can chip out weeds at densities of one plant per 10 square metres, while travelling at 10km/hour.

Posted by Grains Research and Development Corporation on Tuesday, 14 May 2019



More resources

https://grdc.com.au/resources-and-publications/grdc-update-papers/tab-content/grdc-update-papers/2019/03/weed-chipper-site-specific-tillage-for-fallow-weed-control

https://grdc.com.au/news-and-media/audio/podcast/mechanical-weeder-marks-a-new-era-in-tactical-tillage



Table 1. Response tyne efficacy on three summer weed species at three growth stages, Gatton and Hermitage, Qld 2017

Trial site location, summer 2016/2017

Weed size

Barnyard grass

Feathertop Rhodes grass

Windmill
grass

Control (%)

Gatton

Medium
(20-40cm)

100 94 100
Large
(40-70cm)
96 91 100
X-Large
(>70cm)
99 87 96
Hermitage Medium
(40cm)
98 78 100
Large
(40-70cm)
92 33 100
X-Large
(>70cm)
83 8

The dash (–) indicates that established weeds were either missed by the tyne (alignment issue) or the tyne did not activate.

This table is reproduced from the Weed chipper site-specific tillage for fallow weed control GRDC Update Paper.

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What’s the benefit of a double paraquat knockdown?

The ‘double knock’ strategy has long been used and promoted as a valuable tool in the battle against glyphosate resistance in weeds, with paraquat typically applied to control weeds that survived the ‘first knock’ of glyphosate.  
James Jess, research and technical services manager, Western AG in Ballarat, says growers in his client group and beyond have used a double paraquat application to great effect this year, and avoided a very serious blow-out of glyphosate resistant annual ryegrass.
James Jess, research and technical services manager, Western AG in Ballarat. (Source: Syngenta)
“The 2020 season did not provide an opportunity for an effective pre-seeding knockdown and growers across the higher rainfall zones of Victoria found many large, well-tillered ryegrass plants flourishing in their crops,” he says. “We sent samples to Peter Boutsalis at Plant Science Consulting and the results of the Quick Test showed high levels of glyphosate resistance.”
The live plant samples sent to Plant Science Consulting were tested for their response to rates from 2 to 5 L/ha of 600 g active ingredient glyphosate, and many survived rates of 4 L/ha and above. Given the high level of resistance, Western AG put down a trial to compare a range of double knock options so they could give their clients more confidence going into the 2021 season.    
“We knew that the surviving plants growing in fields across the district would be setting a huge amount of seed and that growers would face a devastating situation unless we took decisive and effective action in 2021,” says James. “In the badly infested patches growers also took measures to reduce the amount of seed entering the seed bank at the end of the 2020 season using hay cutting or harvest weed seed control. In paddocks with mainly glyphosate susceptible ryegrass, desiccating feed barley is also a good way to drive down weed numbers.”
‘Double-knock to protect glyphosate’ is one of the WeedSmart Big 6 tactics, which will be the centre of discussion at WeedSmart Week in Esperance, WA in August this year. This flagship event always attracts growers from interstate keen to see how other farmers are keeping weed numbers low in different systems. Early bird registration is now open.
What double-knock options did you trial?
In brief: 1. Glyphosate followed by paraquat and 2. two sequential paraquat applications.
The details: A series of timings were also tested for both the double knock options. Each of the ‘second knocks’ were applied 3, 7 and 12 days after the first knock application.
The two paraquat applications either 3 or 7 days apart were clearly effective in the trial, with the first paraquat application providing 90 per cent control of the glyphosate resistant ryegrass. This means that in a year where it is not possible to implement a double knock, we know that a single application of paraquat at robust rates will still do a good job of reducing weed numbers.

Get more details from the Western AG trial site report.
What advice did you give your clients going in to the 2021 season?
In brief: Delay seeding and implement a double paraquat knockdown.
The details: It was essential to get on top of the glyphosate resistance in ryegrass and avoid a blow-out. Last season the resistance level was high but the plants were still mainly found in manageable patches. Given the amount of seed that was likely added to the seed bank and potentially spread during harvest, it was essential that extra emphasis was put on having a clean seedbed going in to the 2021 season.
Once an effective knockdown has been applied we then recommend growers use a pre-emergent herbicide to reduce weed emergence when the crop is young. Later germinations are then suppressed by the competitive crop. In our trial we used Sakura incorporated by sowing (IBS), which provided excellent early weed control.
Has the recommendation been adopted successfully?
In brief: Yes, rapid and widespread adoption.
The details: There was immediate adoption of this tactic in response to the situation that emerged last season in the high rainfall zone of Victoria. Over 70 per cent of the Western AG client base in the higher rainfall western districts of Victoria implemented a double-paraquat application pre-seeding to target glyphosate resistant ryegrass before seeding the 2021 winter crop. This high level of adoption was a result of the strategy being actively promoted to clients, with the trial results giving growers the confidence to implement the recommendation.
The Western AG double knock trial last year provided growers with the confidence they needed to take decisive action on glyphosate resistant ryegrass before seeding the 2021 crop.
The blow-out was a real eye-opener for growers about how important it is to keep weed numbers low and the resistance mechanisms in play for all agricultural chemicals – not just herbicides.
The double paraquat tactic is also a good knockdown prior to sowing Roundup Ready canola to meet the stewardship requirements for using the RR technology.
Although resistance to paraquat is currently quite rare in annual ryegrass, it has been found in situations where paraquat has been applied at sub-lethal rates over a long period of time. There have also been new cases of paraquat resistance confirmed, and identified as developing, in ryegrass populations in WA, SA, Victoria and NSW this year.
With this in mind, a simple switch to double paraquat as a pre-seeding knockdown is not recommended as a standard practice but rather as a strategic tactic to contain glyphosate resistance in ryegrass. Once that has been achieved, a set of diverse strategies, including herbicide mixes, must be implemented and any survivors must be removed before they set seed.
Resources

Western AG Double knock field demo results – 2020
Double paraquat and RR canola – Podcast with Mark Lawrence 

Article
Ask an Expert

How do you manage summer weeds without spraying at night?

Concerns are being raised about the practical implications of this for summer weed control programs.
Mary O’Brien, a private consultant with extensive experience in managing spray drift, is keen to see growers fully adopt spray application practices that maximise herbicide efficacy and minimise off-target drift.
 
Mary O’Brien says the ‘community drift’ that can occur when a number of applicators are each putting a small amount of product in the air at the same time can have very damaging effects on off-target sites.
“The bottom line is that allowing spray to drift is like burning money,” she says. “Any product that doesn’t hit the target is wasted and the efficacy of the spray job is reduced, mildly resistant biotypes may survive as a result of low dose application and there is potential damage to sensitive crops and the environment.”
“The difficulty is that many growers want to spray at night to cover more ground when conditions are cooler and potentially weeds are less stressed. Having a restriction on night spraying does restrict the time available to cover the areas required.”
Having heard these concerns from growers across the country Mary keeps coming back to the fact that if there was a limitation to capacity at planting or at harvest, growers would scale up to get the job done in a timely manner.
“Buying another spray rig or employing a contractor is an additional cost, especially after a couple of tough seasons, but I really think this is insignificant against the cost of losing key products and the resultant escalation in herbicide resistance to the remaining herbicides,” says Mary. “This problem is not confined to 2,4-D or even to herbicides. I recently spoke to a stone fruit grower who was forced to dump his whole crop after a positive MRL return for a fungicide he had never even heard of, let alone used.”
What about just slowing down and lowering the boom during night spraying?
Short answer: This, coupled with a good nozzle, will reduce drift but it will never eliminate it.
Longer answer: The correct ground speed and boom height will have a large effect on the amount of product that remains in the air. The problem is that it only takes 1 per cent of the product remaining in the air to cause off-target damage.
Once there are a few operators putting just 1 per cent of their product in the air at the same time, the amount of product quickly accumulates and can potentially be very damaging. Mary calls this ‘community drift’.
Isn’t it better to spray weeds at night when it’s cooler?
Short answer: Not really.
Longer answer: Research by Bill Gordon showed that even if you keep everything else the same, night spraying can put at least three times more product in the air than daytime application, even if weather conditions are similar and there is no temperature inversion in place. The main difference between day and night is how the wind is moving across the landscape, rather than the wind speed.
Under inversion conditions, the air moves parallel to the ground surface and this means that the product can move significant distances away from the target before coming to the ground.
To achieve the best results through daytime spraying, applicators should focus on treating small, actively growing weeds. When there is good soil moisture, weeds are unlikely to be stressed even when the temperature is quite high.
Temperature inversion conditions are more common at night and in the early morning. These conditions generate a laminar flow of air across the landscape allowing small droplets to travel many kilometres away from the target site before coming to ground.
Can I use other products at night and just avoid using 2,4-D?
Short answer: The current changes to 2,4-D labels has drawn a lot of attention but the problem is the same for all crop protection sprays – herbicides, fungicides and insecticides.
Longer answer: Different products have different properties and some may work better at night but the problem is the sensitivity of some crops to certain products, such as 2,4-D. All products are tested for their efficacy and the label provides detailed information about the required spray quality and spray application conditions. Many products have explicit label instructions regarding wind speed, temperature inversions (or laminar flow) and night spraying.
Given the high risk of drift at night, applicators need to be very confident that there is no inversion present, and weather conditions should be measured at least every 15 minutes to ensure wind speed remains above 11 kilometres per hour. An on-board weather station is the best way to monitor conditions.
A visual demonstration using smoke to simulate the the lateral movement of small spray droplets when a temperature inversion is in place.
What can I do to improve spray efficacy and avoid spray drift?
Short answer: If you do just one thing – change your nozzle.
Longer answer: All the factors that increase drift also reduce efficacy. To improve efficacy and reduce drift, use a better nozzle (larger spray quality) and appropriate water rates (matched to spray quality and stubble load), slow down and keep the boom low. Wind is required to push product downward and onto the target, and remember that the 3–15 km/h wind speed is for day time conditions only, this does not apply at night.

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Ask an Expert

Does ambient temperature affect herbicide performance?

with Chris Preston, Associate Professor, Weed Management
 at 
The University of Adelaide
Temperature affects the absorption, translocation and metabolic degradation of herbicides applied to plants. Herbicides applied under the wrong conditions can appear to fail, however the reason may not be herbicide resistance.
Dr Chris Preston, Associate Professor, Weed Management
 at The University of Adelaide says most herbicides have a temperature range at which they are most effective in controlling target weeds.
“Applying herbicides outside the optimal temperature range is likely to contribute to a spray failure, even in susceptible populations,” he says. “Alternatively, applying herbicides within the correct temperature range can improve the control in populations known to have a level of resistance to that herbicide.”
Dr Chris Preston suggests testing whole plants rather than seed for responses to a range of post-emergent herbicides. The Quick-Test is conducted in the same growing season as herbicide will be applied so the testing will occur under similar conditions to field conditions.
Dr Preston says the effect of frost on the efficacy of clethodim is a striking example. Spraying clethodim in non-frosty conditions achieves vastly better results than spraying after three days of frost, even on populations that are resistant to this chemical mode of action.
“Combining the optimal temperature with optimal weed size will give the best results possible,” he says. “The current common practice of applying clethodim to tillered ryegrass in the coldest months is not making the best use of this herbicide.”
As a general rule of thumb, Group A (fops), paraquat (Group L) and glyphosate (Group M) are more effective at lower temperatures while Group A (dims), atrazine (Group C) and glufosinate (Group N) are more effective at higher temperatures. However, weeds that are resistant to paraquat become less resistant in warmer temperatures.
“The other implication of this research is the effect of ambient temperature on herbicide test results,” says Dr Preston. “Seed collected in winter and grown out in the glasshouse in summer will be tested for resistance in conditions that are not representative of field conditions when growers are next treating that weed species. The Quick-Test using whole plants overcomes this problem and improves the reliability of herbicide susceptibility testing.”
How can I get the best performance out of clethodim?
Short answer: Avoid applying clethodim during frosty periods.
Longer answer: Twice as much clethodim is required to kill susceptible annual ryegrass if the product is applied after three days of frost. Even higher rates are required if the plants have resistance to clethodim.
Planning to apply clethodim for grass control outside the coldest months of June and July, and avoiding night spraying in winter, will see better results in both resistant and susceptible populations, particularly in tillered plants. Clethodim is most active when temperatures are over 20 degrees C.
Weed seed that is tested during summer may return false negative results, which could translate into spray failure in the field the next season.
Twice as much clethodim is required to kill susceptible annual ryegrass if the product is applied after three days of frost. Even higher rates are required if the plants have resistance to clethodim.
When it is it too hot for glyphosate?
Short answer: Efficacy is much better at 20 degrees C than at 30 degrees C.
Longer answer: Spraying glyphosate resistant barnyard grass at lower temperatures is more effective than under hotter conditions. If barnyard grass is tested for herbicide resistance during the cooler parts of the year it may appear susceptible to the field rate of glyphosate but then when this rate is applied to the population in summer there may be many survivors.
When glyphosate is taken up rapidly it tends to limit its own translocation, which can mean that although symptoms may appear more rapidly in warmer temperatures, plant kill is less reliable.
Which herbicide resistance test should I use?
Short answer: The weed resistance Quick-Test for post-emergent herbicides.
Longer answer: The Quick-Test involves testing whole plants rather than seed for responses to a range of herbicides and rates. The Quick-Test is conducted in the same growing season as herbicide will be applied so the testing will occur under similar conditions to field conditions. The results of the Quick-Test are available within the same season, potentially giving growers an opportunity to apply an effective weed control tactic before the end of the season. The Quick-Test is not available for many pre-emergent herbicides.
The Quick-Test is available through Plant Science Consulting and results are normally available after four weeks.
Relevant links

Maximising clethodim performance and the impact of frost fact sheet
Keeping clethodim working in broafleaf crops
Plant Science Consulting herbicide resistance testing – Quick-Test
GRDC Update Paper – New developments and understanding in resistance mechanisms and management

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