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

WEEDSMART DRY SEASON SPECIAL
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.”

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Testing informs herbicide choice

Buying a new property brings with it a number of unknowns and it can take some time for the new owners to become familiar with the soils, the terrain and the weeds.
When the Birch family of ‘Catalina Farms’, Coorow, WA purchased a neighbouring property, historic ‘Koobabbie’, they had reason to expect that herbicide resistance would be less of a problem because there had been limited use of herbicides on ‘Koobabbie’ in its 114 history of farming.
Daniel Birch, who farms with his wife Jen, parents Rod and Shelley, and long-term team member, Justin Passamani, says the purchase conveniently coincided with the opportunity to be involved with independent herbicide resistance testing in 2020.
“The testing was arranged through the Liebe Group and conducted at the Australian Herbicide Resistance Initiative,” he says. “We were keen to test some theories we had about resistance status of weeds on Catalina and also to get some baseline information about Koobabbie to help us plan our herbicide program.”
Daniel Birch of ‘Catalina Farms’, Coorow, WA says the big take home message from recent herbicide resistance testing was the power of mode of action mixtures.
The testing revealed a number of things that surprised the Birches. To start with, they found there was Group 2 [B] resistant ryegrass in paddocks on Koobabbie that had no history of Group 2 herbicide use.
“This demonstrated just how easy it is for herbicide resistance to move in seed or hay, or on machinery,” says Daniel. “Obviously we can use this information to avoid using chemistry that we know has little to no efficacy on ryegrass.”
“The other major finding was that Group 12 [F] resistance in wild radish was much higher than we expected across the farm,” he says.
This has led the Birches to include more premium products in their program to target resistant ryegrass and wild radish, and drive down the weed seed bank as quickly as possible. On the flip side, they also discovered that they can save money by using trifluralin at lower rates in seasons with good growing conditions, where there is less need for a long residual effect.
“In those years where the crop gets off to a good start, the crop competition effect kicks in early to suppress weeds,” says Daniel.
By taking on board the resistance testing results from weed seed samples collected across the state, Daniel was reassured by the fact that the resistance issues they faced were essentially the same as other growers.
“The big take home message for us was the power of mode of action mixtures,” he says. “From the overall survey results across Australia, the resistance frequency to stand-alone pre-emergence herbicides ranged from 10 to 34 per cent, yet resistance to herbicide mixtures ranged from 0 to 6 per cent.”
Armed with knowledge about the resistance profile of their weeds, the Birches are combining some older chemistry with newer, premium herbicides for maximum effect.
The AHRI resistance testing program is led by Dr Roberto Busi. In 2019, annual ryegrass seed samples from 298 farms were submitted, representing 579 populations from four states in Australia, and these were tested for resistance to 21 herbicides applied at the recommended rate – 12 standalone and nine two-way mixtures.
In total, 15 876 individual resistance tests were conducted to screen two million seeds against registered herbicides and herbicide mixtures at the recommended label rate.
Dr Busi says the mixtures that growers can confidently incorporate in their annual ryegrass program are trifluralin + Sakura mix, Luximax + triallate, and clethodim + butroxydim.
“When applied at full rate for each component, these mixtures can achieve a better outcome than the same herbicides applied as stand-alone treatments against annual ryegrass with known resistance,” he says.
For wild radish, 200 samples were tested over a period of two years and resistance to Groups 2 [B], 4 [I] and 12 [F] were all over 50 per cent resistant. At 70 per cent resistant, Group 2 [B] herbicides should probably be dropped from most wild radish herbicide programs and Group 4 [I] is under threat.
“In 2021, we found that mixing Group 12 [F] with Group 6 [C] herbicide bromoxynil vastly improved control of wild radish,” he says. “From 51 per cent resistant to Group 12 [F] down to less than 15 per cent of samples resistant to the 12 [F] plus 6 [C] mixture.”
“It is important to emphasise that herbicide resistance testing is conducted on small, actively growing weed seedlings in a glasshouse environment,” says Roberto. “In the field, spray failures can easily occur, even in susceptible weed populations, if the herbicide is applied under the wrong conditions or to plants that are too large or stressed. This is particularly true for wild radish.”
AHRI Podcast: Interview with Daniel Birch about herbicide testing
AHRI Insight: Mixtures rock
Herbicide testing options

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Safe sorghum planting while controlling weeds

Pre-emergent herbicide, metolacholor and S-metolachlor, have been used in Australia primarily to control grass weeds for over 40 years. To date, no instances of resistance to this active ingredient have been documented in Australia and even internationally the few cases of resistance are in broadleaf weeds only. Resistance in northern summer grasses is probably low at present.
Syngenta’s field biology manager, Rob Battaglia, says some recent changes to the label use patterns and the launch of a new seed safener product will further expand the usefulness of S-metolachlor products, such as Dual Gold® and Primextra Gold®, in summer crops while protecting sorghum seedlings from herbicide damage.
Syngenta’s field biology manager, Rob Battaglia, says the new use patterns for Dual Gold® in sorghum, cotton and fallow centre on extending application flexibility and residual activity of the herbicide on target weeds.
“S-metolachlor is registered for use in a wide range of crops, including sorghum, maize, sweetcorn, soybean, sunflower and cotton, as well as in fallow situations,” he says. “The new use patterns in sorghum, cotton and fallow centre on extending application flexibility and residual activity of the herbicide on target weeds.”
In sorghum, a rate of 1.0 to 2.0 L/ha can be applied either as a single application before the crop or weeds emerge, or as a split application pre and post crop emergence (up to 6-leaf stage). Similarly, the full rate can be applied after harvest to establish the fallow or 1.0 to 1.5 L/ha can be applied pre-emergent to weeds at fallow establishment followed by 0.5 to 1.0 L/ha within 4 weeks. Be sure to observe replant intervals when planning the next crop.
In cotton, a single application of 1 L/ha either pre-emergent (before, at, or immediately after, planting), over-the-top or directed stand-alone spray or over-the-top mixed with Roundup Ready herbicide on Roundup Ready FLEX® cotton.
“There are several factors that affect the length of effective residual activity of S-metolachlor, some of which are outside the grower’s control,” says Rob. “The product needs to be incorporated and has interactions with rainfall, temperature, soil type, soil organic matter and stubble. Having more flexibility in the application timing gives growers the ability to compensate for some of these factors and control new waves of weed germinations for longer.”
Untreated strips are a good indicator of the seed bank and potential weed burden in a field. The pre-emergent herbicide treated area on the left has far fewer weeds than the untreated section of the field on the right, which suffered significant yield loss due to feathertop Rhodes grass infestation.
S-metolachlor is only active on weeds that have not yet emerged and has no effect on weeds that have already emerged from the soil or that do not come in contact with the herbicide as they emerge from the soil. It is essential to control already-emerged weeds first and then apply the pre-emergent to control subsequent flushes of weed germinations.
“There are some situations where the residual herbicides will appear to be less effective than they should, but this can often be attributed to compromised application timing, limited rainfall and soil conditions, rather than herbicide performance,” says Rob. “The difficultly in achieving perfect application timing and incorporation with rainfall is one of the main reasons why the new use patterns for Dual Gold® have been registered. Pre-emergent herbicides are best used within the WeedSmart Big 6 integrated weed management program and not as a stand-alone control measure.”
While other crops listed on the S-metolachlor label can metabolise the herbicide, sorghum requires a seed safener to protect the germinating plants from the herbicidal effects of S-metolachlor. The new Epivio C® seed safener replaces Concept II® and provides better overall protection for sorghum seed.
S-metolachlor damage is seen as distorted and twisted growth in sorghum, the rightmost plant is unaffected (protected by seed safener).
In Syngenta’s product development trials in Queensland and NSW, they measured an 11 per cent increase in plant stand and a 7 per cent increase in sorghum yield in crops where the seed was treated with Epivio C®compared to Concept II®. Epivio C® has also demonstrated improvements in seed safety and shelf-life for carry-over seed.
“Epivio C® is applied as seed treatment and when the seed is planted into moist soil the product is taken up into the plant and improves the metabolism of the herbicide in the seedling, resulting in no crop symptoms,” says Rob. “The result is better crop establishment and healthier plants that can better compete against weeds that germinate later in the season.”
Epivio C plot trial demonstrating the benefit of the seed safener in supporting robust germination and establishment. No safener (left) compared to Epivio C safener (right).
There is evidence that growers can maintain crop yield and reduce summer grass seed production by planting sorghum crops at a density of 10 plants/m2 and a row spacing of 50 cm.
The WeedSmart Big 6 includes diversity in crops and pastures, crop competition and mixing and rotating herbicide mode of action groups.
More information

Podcast: Seed safener explainer (starts around the 22 min mark) 
Article: Weaponise sorghum crops to take out feathertop Rhodes grass and awnless barnyard grass

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Ask an Expert

Can pulse cover crops tackle multi-resistant ryegrass in irrigated systems?

The best weed control comes from tactics that also bring other benefits to a farming system.
Greg Sefton, principal agronomist with Sefton Agronomics in the Riverina, says multi-resistant annual ryegrass is becoming a major problem in irrigated systems.
Greg Sefton, principal agronomist with Sefton Agronomics in the Riverina, says legume cover cropping is providing effective control of multi-resistant annual ryegrass in irrigated systems.
“Herbicide resistance can move easily through irrigation areas, particularly when the control methods used on the supply channels are limited to just a few herbicides,” he says. “The ryegrass here is generally accepted to have resistance to glyphosate (Group 9 [M]), Group 1 [A] such as clethodim, Group 2 [B] and Group 3 [D], such as trifluralin. Growers are now relying heavily on Group 15 [K] products such as Sakura, and doing their best to rotate out of the problem.”
To regain control, Greg is working with growers to incorporate a multi-purpose fallow crop such as field pea into the system as a winter fallow clean with the added benefit of contributing biological nitrogen into the soil ahead of planting rice or wheat. 
Earlier maturing varieties of field pea provide better weed control options than Kaspa field pea, chickpea and lupin, all of which generally mature later, sometimes after the target weeds have set seed.
“A competitive pulse crop terminated at maximum biomass is an excellent way to reduce weed seed set,” says Greg. “It is a cultural control that also enables the use of some herbicides that are rarely used in our system. Combining the herbicide and cultural methods in the WeedSmart Big 6 is an effective way to keep our cropping options open and to maximise the value of applied water.”

What is the best fit for the legume crop as a winter clean?
In brief: In the Riverina, the optimal place in the rotation is ahead of rice.
The details: Fields selected for rice production are usually bare fallowed for the preceding winter. The aim of the fallow is to control weeds and conserve soil moisture.
Some growers are having success with field pea sown in May as a winter cover crop then terminated for silage or as a brown manure in early September. This fits well with preventing seed set in annual ryegrass, including late germinating plants.
Field pea is a competitive legume and can suppress weed germination and growth when planted in the most competitive configuration possible with minimal soil disturbance and no gaps.
A knockdown treatment of glyphosate (Group 9 [M]), clopyralid (Group 4 [I]) and carfentrazone (Group G [14]) is applied at planting then a mix of pendimethalin (Group 3 [D]), clomazone (Group 13 [Q]) and paraquat (Group 22 [L]) is applied after an irrigation flush to initiate rice germination and prior to rice germination to knockdown both newly emerged barnyard grass (BYG) and persisting ryegrass. This provides a double knock on ryegrass whilst applying a pre-emergent herbicide for barnyard grass in the rice phase.
When implemented once every 4 or 5 years, with a diverse rotation of winter and summer crops in-between, growers can keep a lid on herbicide resistant annual ryegrass populations. 
Field pea is a competitive legume crop that can reduce annual ryegrass germination in the paddock and halt encroachment from the crop borders.
How do you manage weeds on the non-crop areas?
In brief: The same herbicide mix is applied to the whole paddock, including the weeds growing in the check banks.
The details: Weed seed, often carrying herbicide resistance genes, travels easily through irrigation systems and can colonise non-crop areas. Seed from these plants readily infests the cropping areas if not controlled effectively. The control measures used on non-crop zones are often limited to herbicide tactics, so it is important to make sure the herbicide is applied to maximum effect to prevent seed set.
Farm hygiene and physical removal of isolated weeds will also have a positive impact on weed seed production. 
What farming system benefits come with growing a legume cover crop?
In brief: A legume crop grown for biomass rather than grain can improved soil tilth and reduce crusting on some sodic soils. This practice also allows better soil nutrition management and keeps the grower’s options open if the water allocation situation changes.
The details: The field pea crop will fix atmospheric nitrogen and this allows the grower to use 100 to 150 kg/ha less urea to grow the following rice crop without any yield penalty. If there is insufficient irrigation water available for a rice crop, then the fixed nitrogen is still available for a winter crop of canola or wheat.
The phosphorus fertiliser required for rice can be applied when the field pea crop is planted, giving the phosphorus time to become more available in the soil and ready for uptake when the rice is planted.
Field pea is quite drought tolerant, so if irrigation water is not available for rice, the field pea can be grown through to harvest the grain and will usually yield 0.7 t/ha, which can be more profitable than, say, a 1 t/ha drought-affected wheat crop.
Building an integrated farming system based on methods that have multiple benefits is fundamental to staying ahead of weed pressure.
Practical tips for growing field peas as a brown manure crop
Pulses to attack weeds on many fronts

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Optical spray has a good fit in CQ

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Farming moisture : Beating weeds

Across the 4500 ha cropping area at ‘Narratigah’, the weed numbers are low as a result of the Single’s ‘farming moisture’ philosophy, which involves planting whenever there is sufficient subsoil moisture to establish a competitive crop on their heavy clay soils. Read the case study here.

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