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Maximising the weed control value of my crop rotation

with Kevin Morthorpe, Trait & Seed Technology Stewardship Manager, Pioneer Seeds

A diverse crop rotation is the twine that holds a good farming system together and underpins an effective weed management program.  

Kevin Morthorpe, Pioneer Seeds’ Trait & Seed Technology Stewardship Manager says herbicide tolerance traits in crop hybrids can be used to maximise competition against weeds and increase the herbicide options available to growers while optimising yield and profitability of the crop sequence in rotations.

Kevin Morthorpe (left) – Pioneer Seeds’ Trait & Seed Technology Stewardship Manager, with Dr Ray Cowley – Canola Research Scientist, Corteva Agriscience and Pioneer Seeds’ Rob Wilson – Strategic Customer & Market Development Manager and Clint Rogers – Western Regional Sales Manager & Canola Product Lead at a canola research trial near Jindera in southern NSW.

Plant breeders continue to introduce herbicide tolerance traits in a number of crops in Australia, including corn, canola, pulses, cereals, grain sorghum, summer forages and cotton.

“For example, in canola there are several herbicide tolerance traits and they are primarily available in hybrids,” he says. “This means growers get both improved crop performance due to hybrid vigour and more flexibility in herbicide use patterns.”

The increased vigour of canola hybrids also generates greater biomass production and early canopy closure that suppresses growth and seed set of weeds that germinate in-crop, complementing the use of pre-emergent herbicides.

“Hybrids super-charge crop competition through a strong root system and vigorous growth,” Kevin says. “From an economic angle, hybrids optimise yield in both high input and tough environments. In fact, we see more growers selecting hybrids when producing canola in tough conditions.”

Since the release of the first herbicide tolerant canola in 1991, the popularity of herbicide tolerance has seen a 98 per cent adoption of canola varieties with tolerance to imidazolinone (Clearfield), triazine (TT) or glyphosate (RR). In the last 15 years, the area sown to hybrid canola has risen to an impressive 47 per cent in Australia. With glyphosate tolerant canola hybrids entering South Australia in 2021 and new hybrid releases, the hybrid percentage will increase further over coming years.

With glyphosate tolerant canola hybrids entering South Australia in 2021 and new hybrid releases, the hybrid percentage will increase further over coming years.

Kevin says that Pioneer Seeds have seen increasing demand for Clearfield canola in recent years following a dip in popularity. Through strategic application of herbicide tolerant traits in diverse crop rotations it seems that farmers are overcoming the resistance problems that were prevalent with the Clearfield technology and can now re-introduce these varieties and take advantage of the weed control benefits and high yields they offer, and manage herbicide residues in the soil.

“A diverse rotation of crops and pastures is one of the WeedSmart Big 6 tactics, which Pioneer Seeds endorses wholeheartedly to protect the longevity and effectiveness of herbicide tolerance traits,” he says. “Through an effective crop rotation you can tick off all the herbicide and non-herbicide tactics needed to drive down weed numbers.”

How do I make the most of a hybrid crop?

In brief: Employ best practice agronomy.

The details: Grain hybrids are vigorous plants that produce increased biomass and grain yield. To do this, they must be supported with adequate crop nutrition. When properly fed, hybrids will provide increased crop competition and achieve greater water use efficiency compared to their conventional counterparts.

Growing a hybrid crop with herbicide tolerance traits does not equate to a full weed control program. These crops must be used within the WeedSmart Big 6 framework, within a diverse crop rotation and using herbicide tactics such as double knocking alongside cultural practices such as harvest weed seed control and crop competition to reduce seed set. They also combine well with pre-emergent herbicides to achieve excellent early weed control and suppress seed set in any late germinating weeds.

Can I use hybrid crops with herbicide tolerance to fix a weed blow-out?

In brief: No. This technology is not suitable for salvage operations.

The details: When Roundup Ready canola varieties were first released there was an expectation that these traits could be used to reverse a weed infestation. This proved not to be the case.

Hybrid crops are best used in low weed density situations where they can effectively drive down the weed seedbank. They should be grown in rotations that include an effective double-break, brown manure crop or a pasture phase.

Having hybrid crop options for both summer and winter growing seasons increases the opportunities to tackle weeds throughout the year or to use different fallow herbicides while maintaining the ability to safely grow crops in the following season.

New glyphosate tolerance traits (Truflex® and Optimum GLY®) and the stacking of herbicide tolerance traits of triazine tolerant and Clearfield® (TT+CL) have expanded the safe window for herbicide application in canola.

Are residues in grain a concern when using stacked trait herbicide tolerant hybrids?

In brief: Not if the stewardship program is followed.

The details: New glyphosate tolerance traits (Truflex® and Optimum GLY®) and the stacking of herbicide tolerance traits of triazine tolerant and Clearfield® (TT+CL) have expanded the safe window for herbicide application in canola. This gives more options, more flexibility and more crop safety through the rotation.

The stewardship program for the herbicide tolerant trait hybrids describe herbicide use patterns that growers must follow to confidently avoid the accumulation of herbicide residue in the grain and ensure that Australian maximum residue limits (MRLs) will not be exceeded. [Note that MRLs in other countries may be different to the Australian MRL. Find out more at Grain Trade Australia

To avoid problems with crop safety within the rotation it is important to maintain accurate paddock records to avoid applying herbicide to the wrong crop variety and ensure susceptible crops are not sown into paddocks with herbicide residues in the soil.

On the flip-side, herbicide tolerance in crops increases the options for crop selection within the rotation.
Also, keep in mind the importance of controlling any volunteers from a herbicide tolerant crop in the summer fallow or following crop.

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Ask an Expert

What can I do at harvest to reduce my future weed burden?

As crops mature and harvesters begin reaping, consider the potential fate of seeds ripening on weeds that escaped in-crop control measures.
Peter Newman, WeedSmart’s western extension agronomist, says harvest time is an important opportunity to assess weed burden across the farm and be proactive about driving down the weed seed bank.
“Harvest can either be a super-spreader or a weed suppressing event,” he says. “Small patches of weeds can quickly expand when seed is blown out the back of the harvester. On the other hand, the harvester can be a powerful weed management tool if any one of the harvest weed seed control options are implemented.”
WeedSmart’s western extension agronomist, Peter Newman says efforts made to reduce the spread of weed seed at harvest will soon pay off for growers.
Australian growers have led the world in inventing and adopting harvest weed seed control tools such as impact mills, chaff carts, chaff decks and chaff lining, all of which can reliably destroy over 90 per cent of the weed seed that enters the front of the harvester.    
“In addition to harvest weed seed control there are several other actions in the WeedSmart Big 6 that growers can implement just prior to, during and immediately after harvest that will make a measurable difference to the weed burden in future growing seasons,” says Peter. “The WeedSmart Big 6 tactics are scientifically-proven to reduce the risk of herbicide resistance through diverse herbicide use and cultural control to prevent weed seed set.”
What can I do before harvest to manage late emerged weeds?
In brief: Scout for and map weedy patches. Consider sacrificing small areas of high density weeds. Swathing can be a very effective way to stop seed set of late emerged or resistant weeds. Collect weed seeds for herbicide susceptibility testing.
The details: Growers across Australia use a variety of methods to map weeds – from the simple to the sublime. ‘Dropping a pin’ using the tractor’s GPS mapping system as you travel through a weedy section when spraying or harvesting is easy and provides useful information about the distribution of weeds in the paddock. Many growers have their own drones and use them the collect images or video footage of the crop that can be viewed or analysed to identify high density weed patches.
Collect seed for herbicide susceptibility testing – knowing what still works is vital information for planning next season’s herbicide program. There are three herbicide testing facilities in Australia that are equipped to test weed seed samples – Plant Science Consulting, CSU Herbicide Resistance Testing and UWA Herbicide Resistance Testing.
Collecting weed seed before or at harvest is the most common method used. The collected seed must be mature, from green to when the seed changes colour. Before harvest, collect 30 to 40 ryegrass seedheads or several handfuls of wild oats seed. After harvest, it is common to find seedheads still in the paddock or samples of contaminated grain can be sent for analysis.
Keep samples from different locations separate and details noted on the bag. Only use paper bags (double layer) to collect and send seed samples. Ensure bags are sealed so that the samples don’t mix during transit.
Which harvest weed seed control tool is best for my situation?
In brief: There are six harvest weed seed control tools used in Australia – impact mills, chaff decks, chaff lining, chaff carts, bale direct and narrow windrow burning. Choose the one that best suits your system and budget.
The details: Impact mills are best suited to continuous cropping situations. Residues are retained and evenly spread. Chaff decks have lower capital cost and are well-suited to controlled traffic situations. Chaff carts are popular with grain producers who also run livestock. Bale direct is also expensive but has a good fit in locations where there is access to straw markets. Chaff lining is currently the best ‘entry level’ system and can be used in CTF or non-CTF systems, with best results where the harvester runs on the same track each year. Chaff lining has essentially superseded narrow windrow burning, overcoming the time required and risks involved in burning and reducing the loss of nutrients from the system.
If you haven’t used harvest weed seed control tools before, it doesn’t take long to build and fit a chaff lining chute ready for use this harvest season.
What should I be ready to do straight after harvest?
In brief: Spraying weeds immediately after harvest is fairly common practice. Weeds present may be close to maturity or fresh germinations of summer-active weed species.
The details: Some growers get in early with knockdown herbicide applied under the cutter bar when swathing barley or canola. Consider using the double knock strategy, heavy grazing pressure and possibly a soil residual herbicide that is compatible with your planned crop rotation. Pay particular attention to any weedy patches identified before or during harvest. Stopping seed set at every opportunity is the crux of an effective weed management program.
Give some thought to what might be the underlying cause of weedy patches – fixing problems such as pH and soil nutrition imbalances, waterlogging and spray practices that routinely deliver low doses of herbicide.

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Ask an Expert

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 

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News

What to expect at WeedSmart Week 2021

Big 6 at WeedSmart Week 2021 – Double knock to protect glyphosate
The WeedSmart Forum is set for Tuesday 17 August, 2021 at the Civic Centre in Esperance WA. The program features growers, agronomists and researchers discussing ways to use the BIG 6 to beat crop weeds. You can register for the 3-day WeedSmart Week event here.
Greg Warren from Farm and General in Esperance will be sharing his thoughts on the control of weeds like summer-germinating ryegrass, marshmallow, fleabane and portulaca.
He says the growers around Esperance are tackling glyphosate resistance in annual ryegrass, along with brome and barley grass and other emerging weeds using a range of integrated control tactics. The double knock plays a key role in preserving glyphosate (and soil moisture) and providing a clean seed bed for planting crops.

 
Big 6 at WeedSmart Week 2021 – Increase crop competition
WeedSmart Week 2021 is set for Tuesday 17 to Thursday 19 August, 2021 in and around Esperance WA. The last two days feature local growers hosting visits to their farms and discussing how they use the BIG 6 to beat crop weeds. You can register for the 3-day WeedSmart Week event here.
One of the farms hosting a visit during WeedSmart Week is Warrakirri’s 12,800 cropping operation at Condingup. Farm manager Con Murphy has implemented a variety of tactics to combat their main weeds – annual ryegrass and wild radish. Since 2015 the farm has undergone an intensive soil amelioration program to improve the drainage and ameliorate the sandy soils across the farm.
Con says the benefits have been seen in better germination and establishment that sets their cereal, pulse and canola crops up to compete strongly with weeds. There is also a benefit at the end of the season where rain in August or September enters the soil profile without causing waterlogging, and providing a better finish for their crops.
Since 2016-17 about 80% of the farm has been ripped and a portion has been ripped 2 or 3 times because the sandy soils tend to slump after substantial rainfall events, recreating the hardpan.
Con will be showing the WeedSmart tour group how their ripping, drainage, liming and spading program has helped grow more crop and less weeds!
Listen to the podcast with Warrakirri’s Con Murphy talking about the impact of improved drainage on crop competition

Big 6 at WeedSmart Week 2021 – Implement harvest weed seed control
The WeedSmart Week machinery display is set for Wednesday 18 August, 2021 at Dave Campbell’s shed near Esperance WA. The 3-day WeedSmart Week program features growers, agronomists and researchers discussing ways to use the BIG 6 to beat crop weeds. You can register for the 3-day WeedSmart Week event here.
We’ve saved the harvest weed seed control discussion for the machinery session on Wednesday 18 August. Ben White from Kondinin Group will host the machinery session with spray and harvesting gear on display including impact mills from Seed Terminator, Redekop and iHSD (both hydraulic and belt-driven), Emar chaff deck, and spray technologies including Goldacres’ G6 Crop Cruiser series 2, and weed detection technologies using drones and weed identifying cameras (green on green).
Ben White, Kondinin Group (Photo: Melissa Powell, courtesy of GRDC)
Growers doing the WeedSmart Big 6
WeedSmart Week 2021 is set for Tuesday 17 to Thursday 19 August, 2021 in and around Esperance WA. The last two days feature local growers hosting visits to their farms and discussing how they use the BIG 6 to beat crop weeds. You can register for the 3-day WeedSmart Week event here.
One of the growers who will open up their farm for a visit is Adrian Perks who farms at Condingup, 70 km north-east of Esperance. Adrian runs a continuous cropping program on his 4300 ha property, growing canola, wheat, barley, faba beans and lupins. This diverse rotation allows him to mix and rotate both chemical and non-chemical weed control tactics. Over half of Adrian’s farm is sandplain, on which he has implemented a soil amelioration program to address non-wetting to increase the competitiveness of his crops. He currently uses chaff decks for harvest weed seed control and is introducing an impact mill this season. Adrian monitors the tramtracks for weed growth and if he feels the weed pressure is too high, he uses a shielded sprayer to reduce seed set. The bus tour will include four farm visits and a machinery display.
Listen to Adrian on the Regional Update podcast.
Adrian Perkins, Condingup WA
 

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