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What’s next in controlling herbicide resistant broadleaf weeds?

With Alicia Merriam, University of Adelaide.

A new IMI-tolerant lentil variety, PBA Kelpie XT, is about to be released.

PBA Kelpie XT is the fifth lentil variety released with tolerance to Group B herbicides, imazamox and imazapyr, along with one IMI-tolerant faba bean.

University of Adelaide weeds researcher and PhD candidate, Alicia Merriam, says resistance to the IMI herbicides and other Group B chemistry, particularly the sulfonylureas (SU), is making control of some broadleaf weeds very difficult.

University of Adelaide weeds researcher and PhD candidate, Alicia Merriam says screening has shown resistance to IMI herbicides in over 75 per cent of populations of both weeds collected in random weed surveys in South Australia.

“Imi-tolerant lentils have been very popular with growers and have increased the weed control options in this important crop, but resistance in sowthistle and prickly lettuce is very widespread in the southern region,” she says. “Screening has shown resistance to IMI herbicides in over 75 per cent of populations of both weeds.”

With investment from GRDC and an Australian Government Research Training Program Scholarship, Alicia conducted a trial at two sites in South Australia to investigate options to implement the best practice recommendation for lentils – to control weeds in the preceding wheat crop and again at sowing or crop emergence in the lentils.

Both sowthistle and prickly lettuce are renowned for their prolific seed production when growing in non-competitive situations and wind dispersal of seed enables recruitment of resistance from crop borders, and far beyond. Consequently, eradication is not a realistic proposition.

“Sequencing the gene that controls resistance to Group B herbicides has uncovered a large variety of different mutations in these species across the Mid North and Yorke Peninsula in SA,” says Alicia. “Most mutations of this type cause SU resistance, but some cause IMI resistance and the effect can vary between weed species. Crucially, we found all these mutations within a single grower paddock, which shows that they are widely distributed.”

“Crop rotation and increased crop competition are essential components of the WeedSmart Big 6 to help run down the seed bank and suppress seed production by all means available,” she says. “Herbicide tolerance in pulse crops is a useful tool when coupled with strong competition and other herbicide options in as many crops as possible in the rotation.”

What is the current resistance status of sowthistle and prickly lettuce in the southern region?

In brief: Widespread resistance to Group B SU and IMI chemistry exists in both these broadleaf weeds.

The details: In surveys conducted in the Mid North and Yorke Peninsula regions the percentage of SU-resistant populations of prickly lettuce increased from 66 per cent in 1999 to 82 per cent in 2004 and 100 per cent in 2019. The populations screened in the 2019 survey were all resistant to Group B IMI chemistry.

Sowthistle surveys in the Mid North and Yorke Peninsula have found SU resistance in 89 per cent of populations and IMI resistance in 76 per cent of populations. Surveys also show that Group B resistance in sowthistle is very common across the rest of the southern cropping region.

Sample populations screened with SU and IMI herbicides where Population 1 is susceptible to SU and IMI herbicides, Population 2 is moderately resistant to SU but susceptible to IMI herbicides, and Population 3 is resistant to both these Group B herbicides.

Did crop competition or herbicide treatments affect weed seed production in the wheat phase or weed numbers in the following crop?

In brief: The herbicide treatments used in the 2018 wheat crop had an impact on the sowthistle population in the next crop, but had little effect on prickly lettuce. Crop competition treatments did not reduce weed density in the following growing season.

The details: The weed populations at both sites were confirmed resistant to Group B herbicides but susceptible to glyphosate. The three in-crop treatments were 1. no in-crop herbicide, 2. ‘conventional’ herbicide application of metsulfuron-methyl (Ally) + MCPA and 3. ‘proactive’ herbicide application of bromoxynil + picolinafen + MCPA (Flight EC). Two levels of crop competition (seeding rate 60 and 90 kg/ha) were also applied.

In the 2018 wheat crop the proactive treatment gave the best control of sowthistle in that crop and this resulted in a reduction in numbers in the 2019 crop. Although the conventional treatment provided some weed control benefit in the 2018 crop, the benefit did not flow on to the next crop, probably because the sowthistle population was resistant to the residual action of the metsulfuron-methyl component of the conventional treatment.

The herbicide treatments both reduced prickly lettuce density better than the untreated option but there was no additional benefit from the more expensive proactive treatment in either the year of application or the following crop.

Crop competition is a well-established practice for reducing weed seed production, so it was surprising to find that increased crop competition did not reduce weed numbers in the following year. This could be due to conditions in the year of the trial and the mobility of seed of these species.

In less competitive situations (right) sowthistle and prickly lettuce produce vast quantities of seed whereas in competitive situations (left) seed production is considerably reduced.

What’s the take-home message for using herbicide tolerant lentils in the rotation?

In brief: Herbicide tolerant crops are an important tool but must complement a diverse arsenal of weed control tactics. Short rotations are a very risky option and will lead to yield-reducing numbers of these prolific seeding weeds.

The details: Sowthistle and prickly lettuce can be expected to become increasingly difficult to control in the lentil phase. Neither crop competition nor proactive herbicide regimes alone are likely to provide sufficient downward pressure on these weeds in a short rotation. Building in a longer break away from lentils is likely to be a more effective strategy.

The number of different resistance mutations found in the cropping regions of the Mid North and Yorke Peninsula show that Group B resistance is widespread, and here to stay. This highlights the importance of diversity in crop and herbicide groups rotations, including the strategic use of herbicide tolerant crops.

The new Group G herbicide Reflex, with planned registration for IBS (knife point press wheel) application in lentils, will also be a welcome addition to improve weed control options in this crop.

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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

Ask an Expert

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.

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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