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Does delayed sowing help manage weed populations?

with Dr Gurjeet Gill, Associate Professor – Weed and Crop Ecology, University of Adelaide

The answer to this question is a very simple ‘no’. Waiting for weed seeds with longer dormancy to germinate before sowing costs yield and weeds often set more seed in late sown crops.  

Dr Gurjeet Gill, Associate Professor of Weed and Crop Ecology at The University of Adelaide says sowing a competitive crop ‘on time’ has better outcomes for both crop yield and suppressing weed seed production.

Dr Gurjeet Gill, Associate Professor of Weed and Crop Ecology at The University of Adelaide is one of the four experts presenting the new WeedSmart Crop Competition 101 online course.

“The lack of effective in-crop herbicides leaves growers with few chemical options when it comes to controlling weeds like annual ryegrass and brome grass that are emerging later in the crop,” says Gurjeet. “Our field trials in South Australia demonstrated that sowing ‘on time’ is the best way to maximise crop yield and suppress weeds that germinate in-crop, both with and without the use of pre-emergent herbicides.”

The time of sowing x seeding rate x herbicide field trials were conducted at several sites in South Australia in 2018 and 2019 with GRDC investment.

“The other aspect of these trials was investigating the effect of seeding rate on weed density and seedhead production,” says Gurjeet, “Higher seeding rate increased the yield in wheat at Minnipa at both times of sowing, and did not increase screenings.”

Early sown crops consistently produce more crop and less weeds.

Dr Gill is one of the presenters in WeedSmart’s new Diversity Era ‘Crop Competition 101’ course, which can be completed online in less than 10 hours, giving you a solid grounding in ways to effectively implement tactics that give crops a competitive advantage over weeds.

This free online course can be found at www.diversityera.com/courses/crop-competition-101

Why are weeds in cropping systems becoming more dormant?

Short answer: Increased cropping intensity and routine use of pre-emergent herbicides selects for the longer dormancy trait in annual ryegrass and brome grass.

Longer answer: Weed populations have a mix of individuals with different levels of seed dormancy. In self-regenerating pastures, there is likely to be a penalty for high seed dormancy and germinating later than the neighbouring plants. Therefore, high dormancy late germinating plants remain a minority in the population.

However, the situation changes when growers switch to intensive cropping where knockdown herbicides routinely kill the very early germinating plants. In such systems, weeds that emerge with or soon after the crop have a greater survival because they escape the effects of the knockdown herbicides. After several years of cropping, weed populations change from being early germinating to later germinating. These later germinating weed populations are less responsive to delayed sowing and are now common in southern farming systems.

See the Short answer: Grow the most competitive crop possible – cultivar, seeding rate, row spacing, row orientation and time of sowing all have an impact.

Longer answer: In these trials, time of sowing was by far the major contributor to weed suppression and crop yield – even when no pre-emergent herbicide was applied.

Delaying sowing to wait for weeds to germinate after breaking rain is usually counter-productive, unless the delay results in better soil moisture conditions for pre-emergent herbicides. But even when there is a weed control benefit from the later sowing date there is likely to be a penalty on crop yield of at least 20 per cent.

This was demonstrated at the Minnipa site in 2018, where a delay in sowing of wheat reduced in-crop ryegrass density and its seed production, but there was a yield penalty of 25 to 43 per cent.

Also in 2018, the delayed sowing treatment at Marrabel, saw a large reduction in brome grass plant density in barley — however, weed seed production on these fewer plants was high. Delayed seeding also reduced barley grain yield by almost 30 per cent.

In barley, the additional in-crop use of Intervix completely prevented weed seed set at both the on-time and delayed time of sowing. As resistance to Intervix is still quite rare in brome grass, use of Clearfield® crops can be a highly effective part of the management program.

Field trials at various locations across South Australia clearly demonstrated that sowing a competitive crop ‘on time’ has better outcomes for both crop yield and suppressing seed production on annual ryegrass and brome grass. Photo: University of Adelaide.

What is the effect of seeding rate?

Short answer: Higher crop seeding rate can greatly reduce weed seed production.

Longer answer: In these trials, doubling the crop seeding rate from 100 to 200 plants per m2 usually reduced weed seed production by 30 to 40 per cent. In the barley trial, the performance of the late planted crop was improved when the higher seeding rate was used.

In many other trials, very high seeding rates (such as 400 plants per m2) have been shown to vastly reduce annual ryegrass numbers. Using variable rate seeding, growers can consider sowing known weedy patches at very high seed rates (e.g. 250 to 300 plants per m2) simply to outcompete weeds.

Using a sowing rate at the upper end of the recommended range for the chosen cultivar is good practice to help support the efficacy of pre-emergent herbicides early in the season and provide strong competition for weeds that emerge later in the season.

Seed dormancy explainer

Seed dormancy is usually associated with a ‘dark’ requirement, where seeds can remain dormant on the surface, where they are exposed to light, but when they are ‘planted’ the dark requirement is filled and germination follows. These weeds are the target of pre-emergent herbicides in no-till farming systems.

What growers have observed, and researchers have tested, is that some weeds remain dormant even after the dark requirement has been fulfilled, suggesting that some other trigger may be at play. In brome grass, for example, it has been demonstrated that some seeds do not germinate until a ‘cold requirement’ has been fulfilled. These weed seeds remain dormant in the soil until the night temperature reaches 4 degrees C, often well after any pre-emergent herbicide applied at seeding has degraded.

Once the weeds with the cold dormancy trait have established and set seed they can become the dominant in-crop weed pressure to impact crop yield, and future applications of pre-emergent herbicides will have a limited effect on the population.

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What can I do to control large FTR grass in fallow?

Feathertop Rhodes grass (FTR) is a major weed in chemical fallows in Australia, and is notoriously hard to kill with glyphosate.
Bhagirath Chauhan, professor at the University of Queensland’s Centre for Crop Science, says some other herbicide control measures have potential to manage large FTR plants (40 to 50 leaf stage) that have escaped earlier treatment.
Professor Bhagirath Chauhan says there are some tank mixes and herbicide sequences that growers could deploy to help manage FTR and stop seed set.
“Feathertop Rhodes grass is an aggressive weed that can establish in bare fallow situations and produce a large quantitiy of seed if left uncontrolled,” he says. “Several biotypes of this species are resistant to glyphosate and can also survive a double knock of glyphosate followed by paraquat, particularly once the weed is larger than 4 to 5 leaf stage.”
To give growers more options, a study was conducted to assess the potential of other herbicides and use patterns that can control large feathertop Rhodes plants or stop seed set.   
Alternative herbicide options are available to help manage large FTR and reduce seed set in fallow.
“An integrated approach is essential to controlling feathertop Rhodes grass,” says Bhagirath. “In applying the WeedSmart Big 6 to FTR in a bare fallow situation we have identified some tank mix and herbicide sequences that growers could deploy to help manage this difficult weed and stop seed set.”
Can anything be done to improve the efficacy of glyphosate or the double knock against large FTR plants?
In brief: Adjuvants did not improve glyphosate efficacy on mature (40 to 50 leaf) FTR plants. In glyphosate resistant populations, the second knock product is doing the heavy lifting when applied to large (8 to 10 leaf) FTR plants.
The details: None of the commercially available adjuvants improved the efficacy of glyphosate (740 g a.e. per ha) as a single product application on FTR at the 40 to 50 leaf stage. All the plants survived and produced seed after being treated with glyphosate, indicating that the population used in the study was resistant to glyphosate at this rate and weed growth stage.
Glyphosate and the double knock tactic can often provide good control of resistant FTR plants if the herbicide is applied when the plants are small and actively growing.
The traditional double knock of glyphosate (Group 9 [M]) or glyphosate + 2,4-D, followed by paraquat (Group 22 [L]) or glufosinate (Group 10 [N]), applied to older FTR plants (8-10 leaf) achieved increased phytotoxicity through improved mortality, reduced biomass or fewer seed panicles.
However, the double knock was no better than using paraquat or glufosinate alone when applied to 8 to 10 leaf FTR plants. FTR is not listed on glufosinate labels in Australia but is used to control other weeds in fallow situations at the rate (750 g a.i. per ha) tested in this study. For best results, glufosinate needs to be applied in warm, humid conditions, which is not a common scenario for summer fallow situations.
Rate response (0, 187.5, 375 and 750 g a.i. per ha) to glufosinate applied to large FTR plants.
Are clethodim or haloxyfop suitable alternative herbicides to treat large, glyphosate resistant FTR plants?
In brief: Possibly. Excellent results were achieved in pot trials conducted in an open environment, but will be more difficult to achieve in the field.
The details: Clethodim and haloxyfop were tested on FTR plants at the 24 to 28 leaf stage. Clethodim is registered for use against FTR in a number of summer crops, but without any crop competition many FTR plants survived the registered rate (90 g a.i. per ha), although weed biomass and seed production was severely curtailed.
Haloxyfop efficacy against FTR at this growth stage was 100 per cent at the registered rate of 80 g a.i. per ha.
A combination of these two treatments also resulted in 100 per cent control. The effective use of these two herbicides (both Group 1 [A]) relies on excellent coverage and application when the plants are actively growing. This is difficult to achieve in field conditions, which is why the label recommendations are typically for younger weeds.
A combination of clethodim and haloxyfop can provide good control of large feathertop Rhodes plant and curtail seed production.
These herbicides are known to readily select for resistant biotypes so when applied in a chemical fallow situation (with no competition), it is necessary to target small weeds with robust application rates and to apply a second knock with a contact herbicide, such as paraquat. 
Did you find any new and exciting prospects for controlling mature FTR plants?
In brief: Yes, it seems that there is a truly synergistic effect when isoxaflutole (Group 27 [H]; e.g. Balance) is mixed with paraquat.
The details: Neither of these herbicides provided useful control of FTR at the 40 to 50 leaf stage when applied individually. When mixed together, these herbicides achieved a higher level of weed mortality and prevented panicle production. For example, a tank mixture of isoxaflutole 75 g a.i. per ha, with paraquat 600 g a.i. per ha, resulted in 92 per cent FTR mortality and no panicle production.
Even at a paraquat rate of 300 g a.i. per ha mixed with isoxaflutole 75 g a.i. per ha, only 17 per cent of the large FTR plants survived when the mixture was applied to both the plant and the nearby soil – allowing uptake through both the leaves and the roots.
The benefit of this mixture may be reduced if the weed patch is dense, potentially reducing the amount of the isoxaflutole that reaches the soil. Even the prevention of seed set in large FTR plants is of significant value in managing the seed bank of this invasive weed, as FTR seed remains viable for less than 12 months.
Such a use pattern is not currently specified on product labels, although both products are registered for weed control in fallow situations.  
Web resources
Read the research paper.

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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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Can multi-species planting provide effective weed control?

Crop competition is one of the most effective weed control tools available to growers, but some crops simply don’t have a competitive edge.
Dr Andrew Fletcher, a farming systems scientist with CSIRO, says companion planting and intercropping is an option that growers can consider to bolster the competitiveness of an otherwise uncompetitive but valuable crop in the rotation. International research suggests that it can!
Andrew Fletcher, CSIRO farming systems researcher sees potential for multi-species plantings to compete with weeds. Photo: GRDC
“When two or more species are grown together they can occupy ecological niches that might otherwise be taken up by weeds,” he says. “Multi-species plantings have several potential benefits including increased crop yield and improved soil health, but the right combination can also reduce weed biomass by over 50 per cent.”
Multi-species plantings can be quite challenging to integrate into a grain cropping rotation but are more easily used in mixed grain and livestock operations and in intensive pastures for dairy cattle. International research suggests there is a significant untapped opportunity to increase the use of these systems in Australian grain production systems. However, relevant Australian data is scarce and more research is required to understand this untapped potential in Australian systems.    
A mixed-species cover crop can provide multiple soil health benefits, grazing and fodder for livestock and weed control through crop competition and stopping weed seed set.
“Crop competition is a non-herbicide pillar in the WeedSmart Big 6, with the potential to do some serious heavy lifting in terms of weed control,” says Andrew. “Intercropping and companion planting offers a means to bolster the competitiveness of some crops and to keep them in the rotation without risking a weed blow-out.”
What is intercropping, companion planting and mixed-species planting?
In brief: These systems all involve planting two or more crop species together. The combinations are almost limitless.
The details: Intercropping involves planting two or more species together and harvesting the grain of multiple crops. This generally relies on the grain species having different size seed and compatible harvest times.
Companion planting involves two or more species planted together with the intention to harvest grain from one species only after grazing or terminating the other species before seed set.
Sowing a low-growing species like clover between the rows of cereal can compete with weeds in the inter-row area, fix nitrogen and provide the basis of a pasture after the cereal grain is harvested. This is one example of companion planting.
Mixed-species planting is used to describe plantings of several species grown together primarily for the soil health benefits, and that may have potential for grazing and or forage conservation.
How do these systems suppress weed growth?
In brief: These multi-species systems are designed to take up the ecological space that might otherwise present and opportunity for weeds to fill.
The details: Intercropping and companion planting provide additional weed control in situations where one of the species is a relatively poor competitor as a sole crop. By maximising competition, weed growth is suppressed by up to 58 per cent compared to the least competitive species grown on its own. If a competitive crop such as barley is sown in the most competitive configuration possible, there is little additional benefit from adding a second species.
The downside of using this multi-species strategy for weed control is that in-crop herbicide options the choice of herbicides is limited. This is mainly due to the common combinations being a grass crop with a legume or brassica, meaning grass and broadleaf herbicide options can’t be used, except for when one species is terminated. This needs to be factored into decisions around intercropping and companion cropping.  
What are the best-bet combinations for enhanced weed control?
In brief: It depends on the farming system and the other reasons for considering a multi-species planting.
The details: If the aim is to produce grain, the species selected should have easily separated seed. A well-known example is peaola (field pea plus canola). A recent review of historical trials showed that the median yield increase was 31 per cent compared to sole crops of peas and canola, but the weed control effects of peaola in Australia are unquantified.
An effective companion planting combination is wheat undersown with tillage radish and a legume. The broadleaf companions are sprayed out at stem elongation, leaving the cereal to mature through to harvest.
If there is livestock in the farming system, dual purpose combinations such as grazing canola plus vetch and oats can provide excellent weed suppression. This mix could be grazed and then terminated as hay or silage at stem elongation.
Multi-species plantings add a layer of complexity to the farming system, but many growers have taken on the challenge and are reaping the rewards in crop yield, soil health and weed suppression.

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Rod Birch on how the Big 6 is keeping weeds at bay at Catalina Farms

Catalina Farms is approximately 13,000 ha of 100% cropping, with 65% wheat, with the other 35% comprising of 20% canola and 15% lupins. The longterm rainfall is about 330ml per year and around 250-280ml of growing season rainfall.
Taking on a new property usually has its challenges, but there can be upsides too. Some of the challenges at Koobabbie were related to the fact it had been a livestock property and cropping machinery had never having been run through it. However, Rod explained that they have had some great news about the herbicides they are able to use.
Through the University of Western Australia’s resistance testing service, run by Dr Roberto Busi at AHRI, they found out some older chemistries were still effective, which was a pleasant surprise. Daniel Birch explains below in the Regional Update Podcast what they found out and how they used this information in their planning.

Video breakdown

1:10 Crop rotations – Rod’s favourite rotation is Lupins – wheat – canola – wheat. It provides a lot of diversity for Modes of Action, as well as allows for nitrogen to go back into the soil through the lupin phase.
2:10 Crop Competition – the Birches are big fans of crop competition and it’s an essential part of their approach to controlling weeds.
3:00 Double knock – conditions haven’t been suitable for a double knock since 2016, but when the opportunity arises, it’s an important tool.
4:21 Mix and rotate herbicides – the crop rotations used at Catalina allow for really diverse chemical groups to be used, which is a great tactic to keep resistance at bay.
5:52 Stopping weed seed set – the Birches are trying to eliminate as many weeds in the crop as possible. Crop topping is a tool that they employ, as well as late spraying where necessary.
6:49 Harvest weed seed control – seed destruction is on the horizon at Catalina, but logistically has been a bit tricky.
7:21 Acquiring Koobabbie – it has been exciting for the Birches to be able to introduce more diverse rotations. They’ve been able to use Modes of Action which have never been used before.
9:48 Soil amelioration  – liming has been a really beneficial tool for Catalina Farms. They also put out pot ash and gypsum. Deep ripping has also been a great tool to remove the compaction layer.
11:29 Big 6 benefits  – controlling weeds is such an important strategy at Catalina Farms. Rod Birch said “We’ll never have a ceasefire on the war on weeds!”.