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Agronomists, researchers, growers answering important questions

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Improving pre-emergent herbicide efficacy in high stubble situations

with Catherine Borger, Researcher, Department of Agriculture and Food, Western Australia
Maintaining high stubble has many benefits but there is a downside when it comes to applying pre-emergent herbicides, which must be applied to the soil surface to be effective.
An application of 2.5 L/ha of trifluralin or the full label rate of Sakura® would usually be expected to achieve 70–90% ryegrass control in crop, however Department of Agriculture and Food (DAFWA) researcher, Dr Catherine Borger, has shown that the carrier volume has a large effect on the level of control achieved.
Across four trial sites Dr Borger’s research demonstrated that ryegrass control with trifluralin or Sakura® increased from 53% control when the carrier volume was 30 L/ha to 78% control when the carrier volume was increased to 150 L water/ha in high crop residue ground cover situations.
“The good news is that the effect was consistent, regardless of droplet size—from medium to extremely coarse,” says Dr Borger. “Surprisingly, Sakura® responded similarly to trifluralin even though these two herbicides have quite different properties.”
“Our four trial sites had stubble covering 50–90 per cent of the ground surface, a factor known to influence pre-emergent herbicide efficacy,” she says. “At all sites the average ryegrass control achieved increased as the carrier volume increased.”
Expert boom spray set-up may be able to improve efficacy of pre-emergent herbicides at lower water rates but since this is hard to achieve, simply adding more water or slowing application speeds could increase ryegrass control levels with pre-emergent herbicides applied to paddocks with high levels of stubble cover.

Is increasing the water volume the best way to improve ryegrass control with pre-em herbicides?
Short answer: Yes, in high stubble situations high water volumes are the best option.
Longer answer: To increase ryegrass control when spraying trifluralin in high crop residue situations the only options are to increase the herbicide rate, or increase the carrier volume (water rate). Since the top label rate of trifluralin is 2.9 L/ha pre-sowing of wheat, there is little scope to increase the herbicide rate.
Why did such different products show the same response to water volume?
Short answer: All pre-emergent herbicides work best when the product is applied to the soil.
Longer answer: Trifluralin and Sakura® herbicides have different solubility and adsorption properties. Trifluralin has low solubility and is highly adsorbed to organic matter and Sakura® is the opposite, with higher solubility and low adsorption to organic matter. In these trials ryegrass control was similar for both herbicides and increasing the water rate also gave a similar response with both products.
If droplet size didn’t make any difference to weed control rates, does that mean it isn’t important?
Short answer: No, droplet size is important for several reasons and label instructions must be followed.
Longer answer: Set the droplet size to suit factors other than stubble load e.g. drift risk, delta T (bigger droplets for higher Delta T), mixing partner (e.g. medium droplets for paraquat). To evaluate spray jobs, place some water sensitive paper on the ground (in and between old stubble rows) and some vertically on standing stubble. For best results keep ground speeds below 25 km/hr when applying pre-emergent herbicides.

Dr Catherine Borger from DAFWA has demonstrated the clear benefits of using high water volumes when applying pre-emergent herbicides in high stubble load situations.
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Herbicide resistant crops becoming weeds in the bush/along roadsides

with Roberto Busi, Research Fellow, Australian Herbicide Resistance Initiative, The University of Western Australia
Back in 2009, before the commercial release of transgenic canola in Western Australia, some windrowed Round-up Ready (RR) canola plants from a trial site were blown into an adjacent area of bushland during a storm.

For the next four years, Roberto Busi, a weeds researcher at The University of Western Australia monitored the site where the plants had shed their seed. “Plants established in a on-farm bushland area that wasn’t actively managed for weeds,” says Dr Busi. “We monitored the population each year, counting the number of plants, recording plant traits and measuring the amount of seed the plants produced.”
“By 2013 the RR canola population was extinct on this site,” he says. “Research done in U.S.A., Canada, Europe and Japan show that canola can establish and grow outside agricultural fields but generally doesn’t persist. Our data show that, in Australia, transgenic RR canola doesn’t persist for more than three years.”
UWA Research Fellow, Roberto Busi monitored sites where RR ready canola had established as a weed and has shown that RR canola does not persist in the environment for longer than three years, the same as conventional canola varieties.
What if RR canola in non-farming areas is actively managed with herbicide?
Short answer: It can persist for longer if other plants around it are removed.
Longer answer: At another site, we monitored RR canola plants growing in a median strip area near a grain depot. This median strip was sprayed repeatedly with glyphosate, which removed all other plants, leaving the RR canola to grow without any competition for resources. These plants grew well and produced large quantities of seed.
Is it possible to control RR canola in these situations?
Short answer: Yes, it just requires a change in management.
Longer answer: RR crops remain susceptible to a wide range of herbicide modes of action. So although they will thrive if the affected area is treated with glyphosate alone, the addition of a different herbicide will effectively remove these plants. We monitored this median strip for two years and in that time the roadside management team introduced the use of different herbicide mixes, mowing and hand weeding to remove the RR canola plants from this site.
Does RR canola pose any additional weed threat along roadsides or in non-crop areas around farms?
Short answer: No, provided land managers are aware that they must not rely on glyphosate alone in these areas.
Longer answer: All herbicide resistant crop types have the potential to become volunteer weeds in-crop and along roads and fences. Strategies must be put in place to rotate herbicide groups, as well as non-herbicide tactics like mowing, weeding and even haymaking, to control volunteer plants.
When these canola plants were treated with glyphosate, only the non-transgenic plants died, leaving the RR canola to grow without competition. As soon as a herbicide mix was applied, along with mowing and hand weeding, the site was weed-free again.
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Do canola biotypes have allelopathic effects on weeds?

with Jim Pratley, Research Professor in Agriculture, Charles Sturt University
Allelopathic traits were never important in plant breeding in the past because there was a suite of herbicides available to control weeds. Now, with high levels of herbicide resistance, around the world there is a renewed focus on allelopathic effects in several crops, including wheat, rice and canola.

Canola plants have varying abilities to interfere with the germination and growth of other nearby plants, including weeds. This interference is usually a combination of allelopathy and competition. Determining which canola varieties carry the genetic capability to produce allelopathic substances that suppress important weeds is of great interest to Charles Sturt University (CSU) research professor, Jim Pratley and his team.
“Modern plant breeding programs assess the productivity and other traits of a new hybrid in a weed-free environment,” says Professor Pratley. “This means that genotypes that have allelopathic capability do not have an opportunity to demonstrate their value to a real-world farming system where weed pressure is an inevitable part of plant production.”
Research at CSU has focused on screening canola genotypes for their effectiveness in controlling ryegrass and found that some varieties in the current genotype collection provide good non-herbicide control over the target weed, annual ryegrass, and a number of other species such as shepherd’s purse, Paterson’s curse and brassica weeds like wild turnip.
“More non-herbicide tools are required to help manage herbicide resistant weeds,” he says. “Crops that can be sown into weedy paddocks and that can suppress weed seed germination using allelopathy or reduce the number of weeds setting seed through crop competition offer a useful tool for growers.”
Professor Jim Pratley (right) and PhD student Md Asaduzzaman are very keen to see the research work on allelopathy in canola continue as the importance of non-herbicide weed control measures become increasingly important in farming systems.
How do you identify the difference between allelopathy and competitiveness in a hybrid?
Short answer: It can’t be done in the field. This is for the lab only.
Longer answer: In the laboratory, the canola plants are grown in agar, in a weed-free environment for one week. Then the target weed seed is introduced and the development of the weed root system is monitored. The plants do not need to compete for light, nutrients or water so any suppression of root development can be ascribed to the herbicidal effect of the substances that the canola plants have exuded into the agar.
Can a variety be both competitive and allelopathic to weeds?
Short answer: Yes, but not always.
Longer answer: Once the allelopathic capability of a variety is established, researchers grow the variety in the field with no herbicides applied to control weeds. Any additional weed control that is achieved in the field can be ascribed to the ability of the hybrid to out-compete the weeds present for light, nutrients and water resources.
The team at CSU have screened half of the available canola genotypes to rank them according to their ability to contribute the genes required to produce the allelopathic substances to suppress annual ryegrass and other key weed species.
Is canola the only crop that has allelopathic potential?
Short answer: No. Researchers around the world are also finding wheat and rice cultivars vary in their allelopathic capability. In other countries, this information is being actively pursued in the respective plant breeding programs. Research in this field in Australia is sadly lagging behind.
Longer answer: Work in the area of allelopathy is not new but it has not featured as a high priority trait in plant breeding programs in modern agriculture, particularly since herbicides have been widely used in cropping systems. Many older varieties and cultivars have much stronger allelopathic ability than modern ones. This trait can be re-introduced into high performance varieties and hybrids using either traditional plant breeding methods or gene transfer. Keep in mind that weeds also have allelopathic capability and are not afraid to use it.
How to ask a WeedSmart question
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How can summer crops help drive down weed seed numbers?

with Paul McIntosh, Northern Region Extension Agronomist, AHRI
In the northern region, summer crops present great opportunities for growers to challenge problem weeds while still turning a profit. Sorghum, mungbean, sunflower, maize, cotton, millets, peanut and soybean all bring different benefits to the crop sequence or rotation. Along with the ability to use different selective herbicides there are also opportunities to include non-herbicide tactics like cultivation, grazing, hay or silage making and green manuring.

Paul McIntosh, Australian Herbicide Resistance Initiative northern region extension agronomist says growers can be creative and add more diversity to their cropping system—confronting herbicide resistance in weeds head-on.
“Taking a paddock-by-paddock approach there are many things to consider when deciding what to plant and when,” he says. “Along with tackling weeds there is an opportunity to break disease cycles, better utilise stored soil moisture and manage key nutrients such as nitrogen and phosphorus.”
“The weed spectrum, the size of the weed seed bank and the presence of herbicide resistant plants are important pieces of information,” says Paul. “If there is an impending weed blow-out in your main crop then it is even more important to introduce more non-herbicide tactics to your weed control plan.”
Paul McIntosh, Australian Herbicide Resistance Initiative northern region extension agronomist says there are great opportunities for growers to be creative and to add more diversity to their cropping system—confronting herbicide resistance in weeds head-on with summer crops.
What are the best options if I have grass weed problems, including volunteer cereals?
Short answer: Broadleaf crops like mungbeans, sunflowers and soybeans, or competitive grass crops like maize, forage and grain sorghum.
Longer answer: There are more in-crop herbicide options for maize than sorghum, including a maize hybrid that is imidazolinone tolerant. Forage sorghum or millet cut for hay or green manure could be a competitive option. Mungbeans, soybeans and sunflowers have both pre-emergent and in-crop herbicide options to treat grass weeds.
Sorghum, mungbean, sunflower, maize, cotton, millets, peanut and soybean all bring different benefits to the crop sequence or rotation for weed control, disease breaks and to manage nutrients and soil moisture.
How can I make the most of the weed control benefits in a summer crop?
Short answer: Use as many tactics as possible.
Longer answer: All crops are prone to weed competition early. Sowing into clean paddocks is the aim so it is best to implement as many pre-sowing treatments as practical, including knockdown and residual herbicides prior to or at sowing and possibly using some mechanical removal of old or large plants. If intending to double crop your winter crop ground then pre-harvest desiccation is very effective on in-crop weed escapes. Any planting operation should have an effective plant establishment population, coupled with uniform seedling emergence and plant spacing.
What’s the best way to deal with resistant grass weeds using crop rotation?
Short answer: Implement an effective plan with at least a 2-season sequence to target grass populations.
Longer answer: Determine which herbicide options are still effective on the problem weeds and choose a crop that is compatible with that herbicide. Also consider crops that allow non-herbicide tactics to be implemented to run down the weed seed bank as quickly as possible. Some weeds have very persistent or dormant seed, so a long-term strategy is essential.
How to ask a WeedSmart question
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Moisture and nitrogen is wasted on weeds in summer

with Colin McMaster, Research and Development Agronomist, NSW DPI
In winter-dominant rainfall areas, less attention is given to the value of the rain that falls over summer and the contribution that the fallow period makes to nitrogen mineralisation and nitrogen removal via the weed. Likewise, little attention is given to controlling weeds growing over summer, provided they were ‘cleaned-up’ prior to planting crops in autumn.

Colin McMaster, NSW DPI and a group of other researchers have recently shared data from trials conducted in 2011–12 that shed some light on exactly how much soil moisture and nitrogen is wasted if weeds are allowed to grow over summer.
“We reckon that controlling summer weeds is like ‘buying a spring’,” says Colin. “Our trials in central NSW showed that summer rainfall and fallow nitrogen contribute to 50 per cent of the following winter crop’s yield potential, and reduce the probability of moisture stress during the spring.”
Colin McMaster, NSW DPI says that controlling summer weeds is like ‘buying a spring’. A trial in central NSW showed that summer rainfall and fallow nitrogen contribute to 50 per cent of the following winter crop’s yield potential, and reduce the probability of moisture stress or severity during the spring.
“In the two trial years an extra 86 mm (2011) and 50 mm (2012) of rainfall was stored in the profile when weeds were controlled compared to the control sites,” he says. “An additional 69 kgN/ha (2011) and 45 kgN/ha (2012) was also available for the following crop. In summary, for every millimeter of moisture that was lost through summer fallow weed growth, mineral nitrogen levels reduced by 0.56 kgN/ha. The economic benefit of every dollar/ha spent on herbicides to control those weeds was $8/ha.”
The results of this trial in central NSW support the findings of other trial work conducted in Victoria and South Australia.
How much extra grain was produced when weeds were controlled over summer?
Short answer: an extra 1 t/ha of canola, and between 0.5 and 1.7 t/ha for wheat.
Longer answer: As a result of excellent summer weed control the canola was planted into a profile with an additional 85 mm of plant available water (PAW) and 69 kgN/ha, giving the crop the best possible start. More soil moisture promotes more rapid mineralisation of nitrogen so the two go hand in hand. For every extra mm of stored soil moisture, an additional 0.6 kgN is mineralised. Water and nitrogen increase grain yield through grain number (more tillers and more grains per head) and grain size. Controlling weeds is even more benefical than maintaining stubble.
Crops grown after a weed-free fallow (background) benefit enormously from the additional stored soil moisture and nitrogen that would otherwise be wasted on growing summer weeds.
Does the presence of weeds have an impact on other nutrients such as P, K and S?
Short answer: not as much as on nitrogen.
Longer answer: Summer weeds do not appear to extract large amounts of P, K or S to a depth of 90 cm.
Is there a difference in the returns on applied nitrogen when weeds are controlled over summer?
Short answer: Yes, there is a considerable financial benefit.
Longer answer: When weeds are not controlled in summer, side-dressing winter crops returns $1 for each $1 spent on fertiliser application. Where weeds are controlled over summer, each $1 spent on side-dressing returned $3 in grain value. The benefits of summer weed control on profitability and resource efficiency have been confirmed.
How to ask a WeedSmart question
Ask your questions about the value of controlling summer weeds in the southern regions on  Facebook or Twitter @WeedSmartAU or leave a comment below.

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Is crop rotation an economic option for managing weeds?

Adding a broadleaf break crop to the cropping sequence helps keep wheat profitable in a sustainable cropping system.

Tony Swan, CSIRO says their 5-year GRDC funded project illustrated that adding at least one break crop, and preferably two, to the crop rotation was beneficial for weed control and nitrogen management, and can be as profitable or more profitable than continuous wheat. A series of experiments were established to challenge the idea that break crops are risky and not profitable.
“Many farmers in south-east Australia are sceptical about growing break crops such as pulses and canola,” says Tony. “The problem is, once high populations of herbicide resistant annual ryegrass become apparent, the profitability of continuous wheat significantly reduces.”
“Rotations that include a break crop in paddocks with high populations of resistant annual ryegrass were more profitable than continuous wheat and had significantly less ryegrass numbers after three years, provided all the available tactics were used to reduce germination and prevent seed set,” he says. “Our experiments demonstrated that it is cheaper and more effective to control ryegrass using one of the many break crop options than attempting to achieve control in wheat using expensive herbicides.”
What break crop options did you trial?
Short answer: RR canola, TT canola, lupins for grain, field peas for brown manure, fallow and wheat cut for hay.
Longer answer: The combination of a fallow or break crop in year 1 followed by a second break crop in year 2 resulted in the greatest reduction in annual ryegrass seed bank population and panicle number after 3 years. This sequence was significantly more profitable than continuous wheat, but not as profitable as a RR canola–wheat (high input) –wheat rotation.
What was the most effective option in a weedy situation?
Short answer: A 2-year break crop option.
Longer answer: The double break rotations of lupins grown for grain followed by RR canola, or RR canola followed by wheat cut for hay provided a very high level of weed control while also generating high average annual 3-year gross margins of $790/ha/yr and $834/ha/yr, respectively. This compared to the most profitable 3-year sequence of RR canola followed by wheat (high input) / wheat of $883/ha/year. However, this sequence did not achieve the same reduction in annual ryegrass and grass herbicides cost over $140/ha in the wheat crops. Sequences that included fallow or brown manures followed by RR canola were extremely effective at reducing the annual ryegrass seed bank but were not as profitable as continuous cropping.
Where herbicide resistant annual ryegrass is a major problem, an alternate three year sequence of wheat-hay (sprayed afterwards) in year 1, pulse-grain (spray topped) in year 2, and RR canola in year 3 can be profitable and also reduce the seed bank to extremely low levels.
A two-year break crop can break the weed cycle without breaking the bank.
What is the key recommendation from this trial work for annual ryegrass control?
Short answer: Two consecutive years of total annual ryegrass control using break crops and implementing all available weed seed control options.
Longer answer: Break crops work and can be profitable. Two or more years of effective ryegrass control using break crops and other management options including strongly competitive crops, rotating herbicide groups, pre and post emergent timing and prevention of seed set using crop topping, hay making and brown manuring along with fallow management and harvest weed seed control such as narrow windrow burning.
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How to protect herbicide tolerance technologies in crops

with Ian Taylor, General Manager, Cotton Research and Development Corporation
The cotton industry has led the way in Australian agriculture in the development and adoption of varieties with herbicide tolerance traits. When Roundup Ready® Flex cotton was released in Australia in 2000 it was embraced as a way to reduce costs and reduce the use of residual herbicides. Within two years 40 per cent of the Australian crop was Roundup Ready® and these days it is grown on almost 100 per cent of cotton fields.

Dr Ian Taylor, CRDC says the industry’s adoption of the herbicide tolerant technology 15 years ago revolutionised weed management and growers recognised the need to implement the Roundup Ready and Roundup Ready Flex stewardship plan to minimise the risk of glyphosate resistance in weeds.
“Roundup Ready technology has served the cotton industry well,” says Ian. “Primarily it allowed growers to reduce their use of residual chemistries that can cause crop damage and potentially remain in the environment for many months after application.”
“Across the whole cropping rotation there is extensive use of glyphosate and there is no doubt that glyphosate resistant weeds could find their way into cotton fields,” he says. “Along with the naturally tolerant species such as feathertop Rhodes grass and sowthistle, glyphosate resistant barnyard grass in the northern cropping region is of increasing concern.”
Dr Ian Taylor, CRDC general manager sees glyphosate resistant awnless barnyard grass as the first major weed to threaten the effectiveness of herbicide tolerant cotton varieties.
What weeds are the most challenging in cotton systems?
Short answer: Feathertop Rhodes grass, sowthistle, fleabane (conyza), barnyard grass and windmill grass.
Longer answer: Feathertop Rhodes grass, sowthistle and fleabane are naturally tolerant of glyphosate and are well-suited to the no-till farming system in the northern region. Their control relies on strategies other than glyphosate and this needs to be factored in to the weed management program. There are almost 100 confirmed cases of glyphosate resistant barnyard grass in the northern region and this weed is particularly difficult to control due to multiple germinations potentially occurring during the summer cropping season.
Are there glyphosate weeds present in cotton growing regions?
Short answer: Yes. Awnless barnyard grass is of particular concern for cotton growers.
Longer answer: If uncontrolled, resistant barnyard grass will reduce the weed control effect of the Roundup Ready technology in cotton crops. The adoption of other treatments to supplement the Roundup Ready program, such as increased use of residual herbicides and strategic cultivation, is essential. This is an example where one weed has the potential to greatly reduce the value of the herbicide tolerance traits in a crop.
Roundup Ready Flex cotton has provided growers with a very useful weed control tool for the last 15 years but even robust technologies require the support of a diverse weed control program.
What are some strategies that have proven successful in treating glyphosate survivors?
Short answer: Double-knock always and sometimes a spring tickle.
Longer answer: Double-knock treatments involve using two herbicides or a herbicide and non-herbicide tactic on the same generation of weeds. The traditional glyphosate followed by paraquat is one option but others include using a knock-down + residual herbicide or knock-down + cultivation. A spring tickle will initiate weed germination prior to planting without sacrificing soil moisture.
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What are weeds costing the grains industry?

with Rick Llewellyn, Farming Systems Researcher, CSIRO
In the last 15 years there have been some incredible changes in Australian farming systems and practices. These changes have also had an impact on the weed spectrum on many farms and the control options that farmers are implementing.

Dr Rick Llewellyn, CSIRO is a farming systems researcher interested in reducing the economic impact of weeds on Australian farms. He and a team of researchers and analysts have just completed a comprehensive study that shows how weed management has changed since the last major industry-wide study conducted 15 years ago by the Weeds CRC.
“Over this time there has been a major shift to no-till, large increases in herbicide resistance and some important new weed problems have emerged. Our study measured the economic impact weeds have on Australian grain farms,” says Rick. “Despite our relatively conservative assumptions, weeds are costing the grains industry even more than we had expected.”
“As paddock surveys also show, Australian farmers are doing well to keep weed densities low, but to achieve this they are needing to spend more and implement a larger number of weed management tactics,” he says. “The overall cost of weeds to Australian grain growers is estimated to be $3.3 billion, or over $140/ha in expenditure and losses.”
Dr Rick Llewellyn, CSIRO is a farming systems researcher says the overall cost of weeds to Australian grain growers is estimated to be $3.3b, or over $140/ha in expenditure and losses.
This GRDC-supported research includes the cost of yield loss due to in-crop and fallow weeds, weed control costs including herbicide and non-herbicide practices and grain contamination costs. The analysis showed that weed control costs, rather than yield losses, currently account for about three-quarters of the total economic impact of weeds on Australian grain growing businesses.
The study covers the 13 major agro-ecological zones across the Western, Southern and Northern grain growing regions and the major crop types of wheat, barley, oats, canola, pulses and grain sorghum.
Annual ryegrass is widespread across Australia and prone to herbicide resistance, so it accounts for the highest overall cost to Australian grain production but several other weeds are becoming increasingly costly to growers.
What is the best time to invest in weed control?
Short answer: In the fallow.
Longer answer: The study estimates $487 million is spent on fallow weed control however weeds in fallows are still estimated to be costing over $430 million through reduced yields in the following crop. This suggests that improved fallow weed management could generate the best ‘bang for your buck’.
What weeds are costing Australian farmers the most?
Short answer: No surprise—annual ryegrass.
Longer answer: Annual ryegrass is widespread across Australia and prone to herbicide resistance, so it accounts for the highest overall cost to Australian grain production. Despite serious increases in resistance in other weeds, the cost of herbicide resistance in ryegrass remains greater than the cost of resistance in all other weeds combined. Wild radish, wild oats, brome, wild turnip, barnyard grass, feathertop Rhodes grass, fleabane, sweet summer grass, melons and caltrop all feature among the most costly in different regions.
Are Australian farmers adopting integrated weed management (IWM)?
Short answer: Yes, there is widespread adoption of IWM practices. Recently there has been rapid adoption of narrow windrow burning and double-knock.
Longer answer: Adoption of IWM practices has been greatest in WA, where herbicide resistance has been most extensive, but is rapidly increasing in the southern and northern regions. Glyphosate resistance was found to be a major driver of IWM adoption in the northern region. Farmers were very conscious of the value of avoiding large weed seedbanks and are supportive of the development of more cost-effective and sustainable weed management options.
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What makes harvest weed seed control more efficient?

with Michael Walsh, UWA Senior Research Fellow, Australian Herbicide Resistance Initiative

Keeping paddocks weed-free is far easier and cheaper than doing battle with high density weed populations. Harvest weed seed control (HWSC) measures have proven their worth in reducing the amount of seed that returns to the seed bank each year and can quickly reverse a weed-density trend.
AHRI researcher Dr Michael Walsh specialises in studying the effectiveness of HWSC tactics such as chaff carts, narrow windrow burning, Harrington Seed Destructor and the like. He says all these tactics can achieve very similar results and the choice of tactic used will depend on factors such as crop type, location and markets.
Dr Michael Walsh, UWA Senior Research Fellow, Australian Herbicide Resistance Initiative says growing the most competitive crop possible will increase the efficiency of your chosen harvest weed seed control method.
“Something that we have looked at recently is the effect of crop competition on practical aspects of implementing HWSC,” says Michael. “In a nutshell, increased crop competition via crop density forces weeds to grow taller as they compete for light.”
“The result is that shade intolerant annual weeds produce less seed and the seed is located higher in the crop canopy where they are more easily captured using HWSC tactics.”
Michael says this GRDC supported research adds more weight to the benefits of implementing crop competition as a weed management strategy to achieve higher yields, support herbicide applications and ensure HWSC measures are as effective as possible.
This may mean that growers who effectively use crop competition to support their chosen HWSC tactic can potentially lift the harvest height a little and still be confident that they are capturing more than 90 per cent of the weed seed present at harvest.
In highly competitive crops annual weeds are forced to grow taller and tend to produce less seed.
What effect does crop competition have on weed plant structure?
Short answer: Weeds tend to grow taller.
Longer answer: Where crop biomass is low and there is a sparse, open crop canopy annual ryegrass populations tend to adopt a prostrate growth habit where seed is retained at low heights and more difficult to capture at harvest. In high biomass crops the combination of reduced seed production and elevated seed retention greatly reduces the amount of annual ryegrass, wild radish, brome grass and wild oats seed that potentially avoids HWSC.
How do seasonal conditions affect HWSC efficiency?
Short answer: In low rainfall years, it is important to harvest as low as possible.
Longer answer: In years that do not support robust crop growth the majority of annual ryegrass seedheads are likely to be located below a 20 cm harvest height. To capture more of this seed at harvest will require a lower harvest height. In more favourable years, over 75 per cent of the ryegrass seedheads are likely to be above 30 cm.
Can crop density influence weed seed production?
Short answer: Yes, weeds growing in high density crops produce less seed.
Longer answer: In a pot trial, wheat sown at the highest density (400 plants/m2) reduced seed production in annual ryegrass, wild radish, brome grass and wild oats seed production by 80, 93, 97and 96% respectively. Even at a crop density of 60 plants/m2 weed seed production was reduced by more than 50 per cent.
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How can I use a mechanical tactic as a second knock?

with Michael Widderick, Principal Research Scientist (Weeds), QLD Department of Ag and Fisheries.

The idea of a double knock for weed control is to use one tactic, usually a herbicide, to kill the majority of weeds and follow-up with another tactic, usually a herbicide from a different mode of action group, to kill any survivors. Our question this month considers the possibility of using a non-herbicide tactic as the second knock in a low weed density situation.
Dr Michael Widderick says using a non-herbicide tactic to remove potentially herbicide-resistant survivors is a valuable addition to a weed management plan. “We know that reliance on herbicides alone is not a wise approach,” he says. “Recent history in Australia proves that herbicide resistance will occur when herbicides are used repeatedly. The more diversity in weed control methods, the longer we will have access to effective herbicides.”
Non-herbicide tactics that can be used to eliminate survivors of a herbicide treatment in crop are harvest weed seed control (such as narrow windrow burning, chaff cart), grazing or slashing and hay making. In a fallow the best option might be targeted cultivation. Remember it is all about controlling low density populations of survivors.
“A herbicide double knock is all about timing and relies on using the second knock while weeds are still small,” says Dr Widderick. “With a non-herbicide second knock the timing is less critical, provided surviving weeds are not permitted to set seed.”
Dr Michael Widderick, Principal Research Scientist (Weeds), Queensland Department of Agriculture and Fisheries says new technology could make it easier to incorporate a non-herbicide option into the highly effective double knock weed control tactic.
How effective is cultivation in a zero-tillage system?
Short answer: Very effective.
Longer answer: In a double knock, the majority of weeds are killed with a first knock of herbicide, leaving a low density of weeds. These weeds can then be chipped or pulled and small patches can be cultivated without disturbing the rest of the paddock.
Are there any new technologies that avoid manual chipping?
Short answer: Yes, two exciting new projects are in progress.
Longer answer: Low density can still mean a fairly large number of weeds, especially if you plan to chip them out. Two new projects are aiming to mechanise the identification and removal of weeds using non-herbicide methods. The first is a Department of Agriculture and Fisheries funded project at the Queensland University of Technology that is investigating ways to use robotics to find weeds and either microwave or chip them out. The second is a Grains Research and Development Corporation funded project that plans to build and test a set of quick-release individual tyne mechanisms for use on weed detector technology booms.
Manual removal of low density weed populations may be a case of a ‘stich in time’.
Is harvest weed seed control a valid second knock?
Short answer: Yes, because it prevents surviving weeds from setting seed.
Longer answer: Harvest weed seed control methods include narrow windrow burning, chaff carts, direct baling and the Harrington Seed Destructor. All these methods provide a high level of weed control by collecting and destroying weed seeds present at harvest. Some weeds shed their seeds before harvest and may be better controlled with hay making, grazing or slashing and green manuring. These methods may be required if herbicide resistance has become a problem and there are too many survivors in crop.
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How can I get the best bang for my buck with a double knock?


As breaking rains begin to fall across southern Australia’s cropping regions many growers would be considering a pre-sowing ‘double knock’ herbicide treatment for annual ryegrass.
Tony Craddock, director and agronomic consultant with Rural Directions in South Australia says a well-timed and executed double knock is a very useful first step to reducing weed pressure and keeping a lid on glyphosate resistant annual ryegrass. “The no-till cropping system relies heavily on glyphosate as the primary knock-down product,” he says. “This has led to the inevitable evolution of glyphosate resistance in annual ryegrass, one of the most adaptive grass weeds in the world. Advising clients on tactics and systems to deal with herbicide resistant weeds is a key focus of my consultancy work, so the problem is real and widespread.”
The use of glyphosate as the first knock followed within one to seven days with the second knock application of paraquat or paraquat + diquat is increasing in southern Australia. Mr Craddock believes building the double knock treatment into a whole-of-season weed management plan provides opportunities to get more ‘bang for your buck’.
Tony Craddock, Rural Directions director and agronomist says this year has presented growers in many regions across southern Australia with the perfect conditions to use a double knock tactic to treat annual ryegrass and take some pressure off glyphosate.
What can be done to maximise the effectiveness of the double knock?
Short answer: Implement the tactic correctly.
Longer answer: Apply glyphosate as the first knock, followed by a second knock with paraquat or paraquat + diquat to take out any resistant plants that have survived the glyphosate. If the main weed problem is annual ryegrass then using paraquat on its own as the second knock is an appropriate choice. If there are also broadleaf weeds present then the paraquat + diquat combination (e.g. Spray.Seed®) will be more effective overall. Mixing the glyphosate and paraquat together is both ineffective and not registered. Applying the two sprays between one and seven days apart is optimum timing.
Since I am applying two lots of herbicide, can I cut the rates?
Short answer: No, always use full label rates.
Longer answer: The first knock is to kill all plants still susceptible to glyphosate—applying a lower rate risks higher survival rates, increasing the pressure on the second knock products. The second knock of Spray.Seed® or paraquat is to kill plants that survived the glyphosate. Reducing the rate of the second knock risks survival of potentially glyphosate resistant individuals and damages the integrity of the double knock tactic. Remember that paraquat and Spray.Seed® are contact herbicides and require robust water rates to ensure adequate coverage and allow for losses on stubble.
Glyphosate resistant annual ryegrass is a real challenge for southern region farmers and requires a multi-pronged approach that often will include a pre-sowing double knock.
How does a double knock treatment fit into a season-long weed management plan?
Short answer: By increasing the diversity of tactics.
Longer answer: Don’t rely on a pre-sowing double knock alone. Use pre-emergent herbicides, and focus on increasing the level of crop competition with narrow row spacing and varieties with vigorous early growth. Sow cereals at the optimal time to maximise competitiveness.
In weedy paddocks, consider the value of break crops such as pulses, canola or hay as a way of incorporating other in-crop and non-chemical options to manage annual ryegrass, such as grass-selective post-emergent herbicides, crop-topping, desiccation, spraying under the swath or narrow windrow burning where appropriate.
Can I enhance or ‘spike’ the double knock treatment?
Short answer: Yes.
Longer answer: If there is a mix of weeds present it can be useful to include a compatible herbicide ‘spike’ such as 2-4D low volatile ester, carfentrazone or oxyflouren to enhance control of broadleaf weeds. Be very mindful of plant-back requirements of some herbicide ‘spikes’ before planting sensitive crops such as pulses and canola.
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Will longer crop rotations really help manage weeds?

Peter Newman, Communication Leader with the Australian Herbicide Resistance Initiative says the real culprit is always a lack of diversity. He recalls a grain grower friend taking him to a paddock that had been in a lupin / wheat rotation for 30 years. “He said his wheat was always full of brome grass and his lupins were full of wild radish and blue lupins,” he says. “Then one year he decided to break the cycle and planted barley after wheat, instead of lupins, and the barley came up full of wild radish and blue lupins.”
This grower concluded that his weeds were responding to what had become a predictable farming system.
“Managing resistant weeds requires farming systems that are diverse—using different crops, different chemical modes of action and including non-herbicide control tactics,” Mr Newman says.
The days of simply changing from one chemical to another when the first one becomes ineffective are gone. All chemical options need the support of other tactics and this is where longer crop rotations can provide much more diversity to confuse the enemy than short rotations.”
“Diverse rotations can include longer rotations with different crop species, using varieties with different herbicide tolerance traits or sowing dates, and using an array of tactics to reduce weed numbers and prevent seed set.”
“We are seeing growers win in the battle against herbicide resistance and that is great news,” he says. “It’s time to get serious and to start looking for ways to make every farming operation as diverse as possible.”

Peter Newman, Communication Leader with the Australian Herbicide Resistance Initiative says the real culprit causing herbicide resistance is always a lack of diversity.
How does lengthening a crop rotation help manage herbicide resistance?
Short answer: By adding diversity.
Longer answer: A longer crop rotation means having more tools in the toolbox—better herbicide rotation, a range of seed set control options, varied planting times, competitive crop species or varieties and the ability to implement a variety of harvest weed seed control options.
What can I do if I need to keep a fairly tight rotation?
Short answer: Still look to add diversity.
Longer answer: If you can’t see your way clear to lengthen your crop rotation, look for ways to increase diversity within the crops you grow. Changing varieties may allow a different sowing time and in herbicide tolerant crops such as canola, you can rotate between the RR and TT hybrids. In a tight rotation harvest weed seed control and maximum crop competition are even more important.

Peter Newman, Tom Murphy and Rob Hughes discuss the benefits of narrow row spacing for yield and weed suppression.
What other benefits can I expect from longer crop rotations?
Short answer: A more sustainable farming system.
Longer answer: Increasing diversity in the crop rotation is good for pest and disease management as well as weed management. Adding a legume will also have a lasting impact on soil nutrition. In the longer term a diverse rotation reduces overall production risk.
Are there limiting factors?
Short answer: Yes.
Longer answer: Crop sequencing can have impacts beyond weed management. A rotation should be planned and the use of residual herbicides must take the following crop into account.

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Can planting a tight crop improve weed control?

with Dr Deirdre Lemerle, Professor of Agricultural Innovation at Charles Sturt University.
Given the significant up-front cost of buying good quality seed it is tempting to cut back on seeding rates to minimise costs.
Professor Lemerle advocates the use of highly competitive crops sown at robust rates to combat herbicide resistant weeds.
Lead scientist and director of the Graham Centre for Agricultural Innovation at Charles Sturt University, Professor Deirdre Lemerle takes the opposite view and is calling on growers to consider using high seeding rates and competitive cultivars to suppress weeds and achieve savings through reduced herbicide costs and more reliable herbicide performance.
“Weed control is one of the largest costs in most cropping systems and in the face of increasing herbicide resistance we recommend that growers use the crop itself as an important weed control tactic,” she said. “Cultivar selection, narrow row spacing, stubble management, seeding rate, sowing date and early vigour all contribute to the ability of a crop to suppress weed germination, growth and seed production.”
Professor Lemerle emphasises the need to develop regional agronomic management practices for weed control that suit the soil and weather conditions on your farm. She said that while the general principles are well tested, the application of the principles must be fine-tuned for each farm and farming system.
A combination of narrow rows, high seeding rates and competitive cultivars is a recipe for success when it comes to running down the herbicide resistant weed seed bank.
What makes a crop cultivar competitive?
Short answer: Strong emergence and rapid early growth.
Longer answer: Rye, triticale, barley and oats are more competitive than wheat, although some wheat cultivars are more competitive than others. Grain legumes like lupins and lentils are often poorly competitive, while canola is intermediate. The best weed suppression occurs in crops that quickly achieve canopy closure. Along with crop selection it is essential that whatever crop is grown is set up for success with everything in place to support strong emergence and rapid early growth.
What is the optimal row width?
Short answer: The narrower the better, depending on stubble levels.
Longer answer: Decisions surrounding row width are often strongly associated with stubble management. If there is sufficient stubble remaining to cover the soil and suppress weeds then wider rows may still provide adequate weed control. In low stubble situations broadleaf crops will maintain yield and cereals will increase yield at narrower row width while achieving much greater weed suppression than in wide rows.
How is planting at high seeding rates a cost effective weed control tactic?
Short answer: It is the single most effective way to increase wheat competitiveness.
Longer answer: Wheat crops sown at higher density will generally compete more strongly with weeds for resources such as moisture, nutrients and light. Sowing at the highest recommended rate will help compensate for wider rows, a less competitive cultivar and poor herbicide performance. Most recommended seeding rates do not take into account the significant impact herbicide resistant weeds can have on yield.
Can crop row direction suppress weeds?
Short answer: There is some evidence to suggest that sowing east–west is beneficial in wheat and barley.
Longer answer: Field trials in WA showed significant yield increases coupled with an up to 50 per cent reduction in weed biomass when wheat and barley crops were sown in an east–west orientation. If this can be easily implemented at minimal cost it could be very worthwhile trying it on your farm.
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How can studying resistance genetics in weeds help farmers?

with Dr Qin Yu, Research Associate Professor, Australian Herbicide Resistance Initiative.
The old saying ‘know thy enemy’ certainly applies to managing herbicide resistance. Dr Qin Yu’s research into the biochemical and molecular processes plants use to evade destruction is essential for the development of effective strategies to manage resistance.

Working with 33 populations of annual ryegrass that had survived treatment with Hoegrass® (diclofop), Dr Yu and her team identified the resistance mechanisms being used. Their findings were both alarming and illuminating—91% of the resistant populations studied possessed target site mutations, 80% possessed metabolic mutations, meaning 70% were using both types of mechanisms to survive herbicide treatment.
“Target site mechanisms are relatively easy to identify and study,” says Dr Yu. “We now have a good understanding of these mechanisms, which can lead to high levels of resistance. The resistant plants can usually be removed using alternative herbicides of the same mode of action.”
“Non-target site mechanisms are more difficult to study and harder to manage,” she says. “This type of resistance is related to the metabolism of the herbicide within the plant, making it more complex and unpredictable. This is harder to manage in the field.”

How can target site and non-target site mechanisms occur in the same individual plant?
Short answer: Through gene sharing.
Longer answer: Some weed species have a very high capacity to respond and adapt to environmental conditions, including herbicide applications. Cross-pollinator weeds such as annual ryegrass readily exchange genetic material, with each new generation having the capacity to accumulate resistance genes from different individuals.
What is the value of theoretic science such as resistance genetic studies?
Short answer: Better information for better decisions.
Longer answer: Advances in diagnostic technology to determine what mechanisms are present in a resistant weed population can identify effective management strategies. Finding ways to capitalise on the ‘fitness penalty’ that may come with herbicide resistance, such as increasing crop competition or using other non-chemical control tools, is an important outcome of this research.
What can resistance genetic studies tell us about using selective herbicides?
Short answer: Resistance can occur within just a few years if low rates are used or survivors are ignored.
Longer answer: Selective herbicides work by taking advantage of a crop’s greater ability to metabolise a herbicide compared to the target weed’s lesser ability to metabolise the herbicide. Studies of annual ryegrass and wild oats have proven that both weeds possess the genetic capacity to breakdown selective herbicides if these herbicides are applied at sub-lethal rates. Resistant populations can be resistant to other selective herbicides, even ones that are not yet on the market. Applying full label rates and removing any survivors is essential to preserving these chemicals for use in-crop.
What are some ways to improve herbicide efficacy in resistant populations?
Short answer: Ambient temperature makes a difference to the efficacy of some herbicides and the resistance level.
Longer answer: Some resistance mechanisms are temperature-dependent. Understanding these mechanisms can help researchers optimise the way growers use herbicides that the weeds are resistant to. For example, glyphosate should be applied during relatively cool (but not warm) temperatures for better control of resistant winter-grown ryegrass, and applied at warm (but not hot) temperatures to improve control of resistant summer-grown barnyard grass.
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Fallow weed management with Tony Cook

With no crop competition in the fallow, weeds are free to grow bigger and produce about ten times more seed per plant than they can if they grow in-crop.
DPI NSW weeds technical specialist, Tony Cook, suggests using pre- and post-emergence herbicides early in the fallow phase to prevent herbicide resistant weeds setting large numbers of seed.
As a weed technical specialist with NSW DPI, Tony Cook has seen the effect of a serious weed blow-out in fallow. “With grass weeds particularly, the control measures used in the fallow make a huge difference to the amount of seed in the weed seed bank going into the next crop,” he says. “Grasses can quickly form dense patches if just a few plants are allowed to grow and produce seed.”
Not only do these patches become a problem in crop and at harvest, they can also represent a biosecurity risk for the rest of the farm.
“In a demonstration a few years ago we monitored the effect of treating 11 glyphosate resistant ryegrass plants with glyphosate in a fallow situation compared to treating another 11 resistant plants with different chemistry,” he says. “The glyphosate-treated plants survived and produced large quantities of seed. Within two years the 11 plants had become a dense patch of ryegrass, like a bowling green. The 11 plants treated with different chemistry were killed and no new seed was added to the weed seed bank.”
Just a few awnless barnyard grass plants in a fallow can consume stored fallow moisture and return large numbers of weed seed to the soil.
Glyphosate is relatively cheap and still effective, why change?
Short answer: To preserve the efficacy of this useful chemistry.
Longer answer: We know that repeated use of the same chemistry results in resistance to that mode of action. If we want to keep using glyphosate we must include other herbicide and non-herbicide treatments in the fallow.
What are the advantages of a fallow weed management program?
Short answer: Weeds are easier to see and to treat in the fallow.
Longer answer: In the fallow it is easier to monitor weed presence and full sweep treatments will reduce the chance of weed escapes. The key to success is to target small weeds when using post-emergent herbicides and to check two weeks later to see how effective the treatment has been. Follow-up treatments are essential to remove any weeds that escape as no single tactic is likely to be 100 per cent effective.
What other control options are available for fallow weed control?
Short answer: Pre-emergent chemistry, optical sensor technology and strategic cultivation.
Longer answer: Pre-emergent herbicide applied soon after harvest is an effective fallow management tactic that can also reduce time pressures if you are doing double-knock treatments in some paddocks. Many factors, such as stubble cover, organic matter and rainfall, influence the level of control achieved with pre-emergent products and 100 per cent control is unlikely. Check for weed growth a few weeks after rain to assess the level of escapes. A follow up with spot spraying using optical spray technology, or strategic cultivation when weeds are small will further reduce seed set in the fallow.
What’s new in pre-emergent chemistry?
Short answer: More registrations are now in place.
Longer answer: The GRDC and chemical companies have been investing in research and the registration of pre-emergent herbicides for fallow use. Check for label updates and also be aware of plant back periods and of the effect of environmental factors on herbicide breakdown in the soil.
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