Early weed control pays dividends
Weeds that are allowed to germinate with summer crops significantly reduce yields and can set a vast amount of seed. Conversely, crops that get away to a clean start for the first three to six weeks can often maintain most of their yield potential.
Competitive crops, grown on narrow row spacing, can reduce the number of weeds that germinate later in the crop and although these weeds may have minimal impact on yield, any survivors will add to the weed seed bank and must be controlled with other tactics.
In sorghum and maize crops, weeds like barnyard grass, liverseed grass and fleabane can be hard to kill once they emerge, and they have the potential to reduce yields by 30 per cent or more if left untreated.
In mungbean, University of Queensland researchers found a 50 per cent crop yield reduction can be expected as a result of weed populations as low as 20–35 plants/m2 of bladder ketmia, feathertop Rhodes grass, windmill grass, liverseed grass or button grass.
Many growers are looking for opportunities to safely include residual herbicides into their summer cropping weed control program to complement pre-plant knock-down herbicide applications.
Mark Congreve, ICAN senior consultant said pre-emergent herbicides can provide a six to eight week weed-free environment for crop establishment and their inclusion increases the diversity of herbicide modes of action used in the farming system – a useful delaying strategy for herbicide resistance.
Mark Congreve, ICAN senior consultant says pre-emergent herbicides can provide a six to eight week weed-free environment for summer crop establishment.
Starting the summer crop clean usually involves an effective knock-down herbicide applied prior to sowing to treat weeds germinating on the planting rain, followed by an effective pre-emergent.
“It is generally easier to target grass weeds in a broadleaf crop and vice versa. For example, if grass weeds are the main problem in a paddock then grain sorghum may not be a good crop choice,” he said. “There are no effective in-crop post-emergent herbicides for control of emerged grass weeds in sorghum, so any weeds that escape the pre-emergent herbicide treatment are likely to set seed and replenish the seedbank.”
Broadleaf crops such as cotton, mungbeans, soybeans and sunflowers have more pre-emergent and in-crop herbicide options to treat grass weeds, potentially making these crops a better choice where grasses are the main problem.
Crop competition and pre-emergent herbicides are complementary tactics, with their combined effect reducing weed seed numbers and supporting crop yield. Once the crop has been selected and the pre-emergent program chosen, implement all practices that promote vigorous crop growth and early canopy closure.
An over-reliance on pre-emergent herbicide use will select for herbicide resistance, just as it has for post-emergent herbicides. To minimise this, it is important to use a diverse range of weed management tactics, including non-herbicide tactics such as inter-row cultivation or chipping, to remove survivor weeds before they set seed. Where possible, rotate pre-emergent herbicide modes of action between years.
“Most growers find that there are one or two weed species that are really driving their decision making, so having a dedicated program to drive down numbers of these key weeds to ensure the summer crop is clean at harvest, will make future weed control easier and allow greater flexibility in crop choice.”
How can summer crops help drive down weed seed numbers?
Creating stiff competition against summer weeds
Why diversity is so important in the battle with herbicide resistant weeds
Creating stiff competition against summer weeds
With soil moisture at a premium, there is nothing spare to waste on summer growing weeds.
With four of the most difficult to control summer weeds, feathertop Rhodes grass (FTR), awnless barnyard grass, common sowthistle and flaxleaf fleabane, now with confirmed cases of glyphosate resistance, the pressure is on to find effective non-herbicide control tactics. These weeds can produce 40,000, 42,000, 25,000 and 110,000 seeds per plant respectively. Other studies have found these numbers could be even higher, so every effort to reduce seed production is worthwhile.
Dr Michael Widderick, is the lead DAF researcher in the mungbean and sorghum crop trials to better understand the agronomic factors that increase the crop’s competitive advantage over feathertop Rhodes grass and awnless barnyard grass.
Various trials in mungbean, soybean and sorghum crops have consistently found that planting these summer crops in competitive configurations can reduce weed biomass and seed production while maintaining or increasing crop yield.
The change in row configuration may involve set up costs to modify planting equipment but does not require an increase in seeding rate as the effect has been demonstrated using the same established populations for mungbean and soybean. For sorghum, the competitive edge comes through increased plant population rather than narrower rows.
Along with narrowing the row spacing in mungbean and soybean, there are also benefits in early weed control. Keeping crops weed-free for the first three to six weeks seems to be a valuable rule of thumb to give crops the head start required to drive down weed numbers. Weeds that germinate in-crop after the 3–6 week mark are fewer in number and individual plants also produce less seed.
As a non-herbicide control tactic, crop competition is very important in any integrated weed management program.
With investment from the GRDC, researchers from NSW Department of Primary Industries, University of Sydney and Queensland Department of Agriculture and Fisheries are conducting trials to identify ways to increase the competitiveness of sorghum and summer pulses.
In the 2017–18 summer sorghum was sown in trials using three row spacings (50, 75, 100 cm) and two crop densities (5 and 10 plants per m2). In a separate trial, mungbean plots were established using three row spacings (25, 50 and 75 cm) and two crop densities (20 and 35 plants per m2). Seed of feathertop Rhodes grass and awnless barnyard grass was spread at planting and the plots were irrigated to ensure optimal crop and weed emergence.
With only one year of data from this trial so far it is difficult to make recommendations. Average seed head production in non-crop treatments was 1745 heads/m2 for feathertop Rhodes and 1525 heads/m2 for awnless barnyard grass. Growing either crop more than halved the number of weed seed heads produced, even in the least competitive configuration for the two crops.
Row spacing in mungbean had a clear impact on weed seedhead production for both weed species. This effect has been demonstrated numerous times in a range of agronomic trials with mungbean. Narrowing row spacing to 25 cm reduced feathertop Rhodes grass seed heads to 32 plants/m2. For awnless barnyard grass, narrowing row spacing to 50 cm or less reduced seed heads to less than 10 per m2.
Note: this represents one year of data only.
In these plots, mungbean sown on row spacing 25 cm at low crop density (20 plants/m2) competed strongly with awnless barnyard grass (left) and feathertop Rhodes grass (right).
In these plots, mungbean was sown on row spacing 75 cm at low crop density (20 plants/m2). The plot on the left was kept weed free while the centre plot shows limited suppression of feathertop Rhodes grass. Being a low growing weed, the level of infestation of awnless barnyard grass (right) is difficult to see in this photo.
For sorghum it seems increased plant population had the best effect, significantly reducing weed seed head numbers for both weed species. Unlike mungbeans and soybeans, sorghum yields were reduced in plots with narrower row spacing configurations. At each row spacing, the higher plant density (10 plants/m2) treatment yielded more than the lower density plant population.
In a separate trial, University of Queensland researchers confirmed that row spacing, not plant population, is the key driver to reducing weed growth in soybean crops. In soybeans, weed biomass was reduced by 89 per cent under narrow rows (25 cm) and 75 per cent under wider rows (75 cm) when the crop was kept weed-free for the first three weeks after planting. If weeds were controlled for the first six weeks, then weed biomass was reduced by 98 per cent under narrow rows and 88 per cent under wider rows.
In weed-free plots there was a 20 per cent yield benefit in changing from 75 cm row spacing to 25 cm. This yield difference was 65 per cent in plots where weeds were introduced 6 weeks after planting and a huge 121 per cent higher when the weed infestation occurred three weeks after planting.
Similarly, if a mungbean crop is kept weed-free for the first 3 weeks after planting, then the narrower row spacings of 25 and 50 cm saw a reduction in weed biomass. In both the 2015 and 2016 summers, the combination of keeping the crop weed-free for at least the first 3 weeks and planting on the narrower rows (25 or 50 cm rather than 75 cm) generated a yield increase of 159–197 per cent in 2015 and 198–223 per cent in 2016.
Summer crops are an integral part of many farming systems and play an important role in an integrated weed management program. Many summer weeds only remain viable on the soil surface for a period of around 12 months so if a competitive summer crop is followed with a competitive winter crop and harvest weed seed control, there are more opportunities to drive down weed numbers in the soil seed bank.
Giving summer pulses the competitive edge
How can summer crops help drive down weed numbers?
Reducing the glyphosate resistant weed seeds in cotton fields
The Australian cotton industry has benefited enormously through widespread adoption of Roundup Ready technology, with close to 100 per cent adoption since 2010/11.
A downside of the uptake of this technology has been the shift in the weed spectrum found in cotton fields. Department of Agriculture and Fisheries (DAF) researcher Dr Jeff Werth says growers have achieved better control of Cyperus sp. (nutgrass and other sedges), but surface germinating and glyphosate tolerant species, along with reduced control of vines and legumes have found the new cropping system favourable.
Jeff Werth (DAF) collecting data at the patch eradication trial at Hermitage Research Facility, Warwick. The dense patches of awnless barnyard grass seen in this image are the glyphosate-only treatments.
“This shift has taken years to occur and now we are seeing weedy patches establishing in cotton crops,” he said. “We have used crop modelling to show that managing these patches is effective in awnless barnyard grass and now we have field experimental results demonstrating that the industry’s best management practice weed control strategy works.”
Dr Werth and the DAF weed research team designed an experiment to field-test the 2+2&0 (2 non-glyphosate tactics in crop plus 2 non-glyphosate tactics in fallow and no survivors) strategy in a dryland cotton system.
They chose a dryland cotton field with a dense population of glyphosate-susceptible awnless barnyard grass (ABG) and used a low rate of glyphosate (250 g/ha Roundup Ready®) to simulate glyphosate-resistance with approximately 70% control of the barnyard grass each time glyphosate was used.
The treatments applied were designed to compare ‘glyphosate only’ with the ‘2+2&0 strategy’ to see the effect on ABG emergence and seed bank. They also tested a few ‘eradication’ tactics, which were applied at three different times in the cropping season – 1. Early (Oct to mid-Dec), 2. Mid (mid-Dec to mid-Feb) and 3. Late (mid-Feb to March). The experiment ran for a four-year cotton-fallow-fallow-cotton rotation.
Table 1. Example of control tactics used on Awnless barnyard grass comparing glyphosate only, to BMP and additional eradication treatments.
1. Early season
3. Late season
Best Management Practices (BMP or 2+2)
Paraquat + Pendimethalin (PPPE)
Additional Eradication tactics (Erad)
(Treatments 3, 6, 7 and 9)
(Treatments 3, 6, 7 and 9)
(Treatments 5, 7, 8 and 9)
“When glyphosate was used without applying any other weed control tactics, weed emergences were almost always higher than where multiple tactics were used,” said Jeff. “The most consistent tactic for reducing barnyard grass emergences was metolachlor applied six weeks prior to planting in the ‘Eradication phase 1’ treatments.”
Pendimethalin applied at planting seemed to have little effect on the major flush of weed germination that usually occurs in spring, but did have some effect on later germinations.
“Including residual herbicides in the weed control program can really reduce the number of weeds that emerge, and as a result, there is less pressure on post-emergence herbicides applied later in the crop,” he said.
This simulation experiment also demonstrated that the 2+2&0 strategy was effective in driving down the weed seed bank, which is the ultimate goal of any integrated weed management program.
Jeff said that it took three years to really see the differences between the two main treatments, so persistence is required if you don’t see immediate changes in weed numbers. Eradication treatments applied early in the crop season were also the most effective in driving down the weed seed bank.
Table 2. Total Awnless barnyard grass emergence in each year of the experiment. Means with the same letters are not significantly different.
1. Glyphosate only
3. BMP + Erad (phase 1)
4. BMP + Erad (phase 2)
5. BMP + Erad (phase 3)
6. BMP + Erad (phase 1 and 2)
7. BMP + Erad (phase 1 and 3)
8. BMP + Erad (phase 2 and 3)
9. BMP + Erad (phase 1,2 and 3)
*Means back-transformed from Log(x+1)
The time taken to see a decline in weed populations is related to the longevity of the seed of different weed species. In a cotton farming system small seeded species such as feathertop Rhodes grass, windmill grass, fleabane and sowthistle generally last in the soil for 2–3 years; larger seeded species such as awnless barnyard grass and liverseed grass last about 4–6 years and harder seeded species like bladder ketmia and peachvine last 6–10 years.
Awnless barnyard grass produces a large quantity of seed if it escapes weed control tactics, resulting in patches of potentially resistant weeds in cotton fields.
“It is reasonable to expect that the longer the 2+2&0 strategy is applied, along with early season eradication tactics, the greater the difference there would be compared to a glyphosate-only approach,” he said. “In years with multiple rainfall events across the summer, there would be increased benefit from employing early eradication tactics to avoid a potential build-up of the weed population. It is critical to ensure that survivors are controlled to prevent potentially resistant seed re-entering the soil.”
The 2+2&0 strategy forms the basis of the Cotton Herbicide Resistance Management Strategy and is best applied in all cases to minimise the risks of resistance development. If a problem is detected early, the 2+2&0 can form the baseline response, and extra tactics can be incorporated to limit potential spread of the patch in the field, and drive down the seed bank.
How do you manage summer weeds without spraying at night?
Concerns are being raised about the practical implications of this for summer weed control programs.
Mary O’Brien, a private consultant with extensive experience in managing spray drift, is keen to see growers fully adopt spray application practices that maximise herbicide efficacy and minimise off-target drift.
Mary O’Brien says the ‘community drift’ that can occur when a number of applicators are each putting a small amount of product in the air at the same time can have very damaging effects on off-target sites.
“The bottom line is that allowing spray to drift is like burning money,” she says. “Any product that doesn’t hit the target is wasted and the efficacy of the spray job is reduced, mildly resistant biotypes may survive as a result of low dose application and there is potential damage to sensitive crops and the environment.”
“The difficulty is that many growers want to spray at night to cover more ground when conditions are cooler and potentially weeds are less stressed. Having a restriction on night spraying does restrict the time available to cover the areas required.”
Having heard these concerns from growers across the country Mary keeps coming back to the fact that if there was a limitation to capacity at planting or at harvest, growers would scale up to get the job done in a timely manner.
“Buying another spray rig or employing a contractor is an additional cost, especially after a couple of tough seasons, but I really think this is insignificant against the cost of losing key products and the resultant escalation in herbicide resistance to the remaining herbicides,” says Mary. “This problem is not confined to 2,4-D or even to herbicides. I recently spoke to a stone fruit grower who was forced to dump his whole crop after a positive MRL return for a fungicide he had never even heard of, let alone used.”
What about just slowing down and lowering the boom during night spraying?
Short answer: This, coupled with a good nozzle, will reduce drift but it will never eliminate it.
Longer answer: The correct ground speed and boom height will have a large effect on the amount of product that remains in the air. The problem is that it only takes 1 per cent of the product remaining in the air to cause off-target damage.
Once there are a few operators putting just 1 per cent of their product in the air at the same time, the amount of product quickly accumulates and can potentially be very damaging. Mary calls this ‘community drift’.
Isn’t it better to spray weeds at night when it’s cooler?
Short answer: Not really.
Longer answer: Research by Bill Gordon showed that even if you keep everything else the same, night spraying can put at least three times more product in the air than daytime application, even if weather conditions are similar and there is no temperature inversion in place. The main difference between day and night is how the wind is moving across the landscape, rather than the wind speed.
Under inversion conditions, the air moves parallel to the ground surface and this means that the product can move significant distances away from the target before coming to the ground.
To achieve the best results through daytime spraying, applicators should focus on treating small, actively growing weeds. When there is good soil moisture, weeds are unlikely to be stressed even when the temperature is quite high.
Temperature inversion conditions are more common at night and in the early morning. These conditions generate a laminar flow of air across the landscape allowing small droplets to travel many kilometres away from the target site before coming to ground.
Can I use other products at night and just avoid using 2,4-D?
Short answer: The current changes to 2,4-D labels has drawn a lot of attention but the problem is the same for all crop protection sprays – herbicides, fungicides and insecticides.
Longer answer: Different products have different properties and some may work better at night but the problem is the sensitivity of some crops to certain products, such as 2,4-D. All products are tested for their efficacy and the label provides detailed information about the required spray quality and spray application conditions. Many products have explicit label instructions regarding wind speed, temperature inversions (or laminar flow) and night spraying.
Given the high risk of drift at night, applicators need to be very confident that there is no inversion present, and weather conditions should be measured at least every 15 minutes to ensure wind speed remains above 11 kilometres per hour. An on-board weather station is the best way to monitor conditions.
A visual demonstration using smoke to simulate the the lateral movement of small spray droplets when a temperature inversion is in place.
What can I do to improve spray efficacy and avoid spray drift?
Short answer: If you do just one thing – change your nozzle.
Longer answer: All the factors that increase drift also reduce efficacy. To improve efficacy and reduce drift, use a better nozzle (larger spray quality) and appropriate water rates (matched to spray quality and stubble load), slow down and keep the boom low. Wind is required to push product downward and onto the target, and remember that the 3–15 km/h wind speed is for day time conditions only, this does not apply at night.
Using your harvester to destroy weed seeds
Collecting and destroying weed seeds as part of the harvest operation is recognised as the most efficient and effective way to implement harvest weed seed control (HWSC) within an integrated weed management system.
Acknowledged as the ‘holy grail’ of the WeedSmart Big 6 tactics to manage herbicide resistance, HWSC was implemented on over 40 per cent of Australian grain farms in 2014 and adoption is expected to increase to 80 per cent by 2020.
Chaff impact mill machines render the weed seed unviable, causing the destruction of over 95 per cent of the weed seed that enters the mills.
There are currently six HWSC methods being used in Australia, all of which have been invented, adapted and adopted by Australian farmers. When choosing between these methods growers must consider the set-up cost, nutrient removal costs and labour requirements.
Most agree that the ultimate HWSC tool would complete the weed seed control in one pass at harvest, retain all stubble and nutrients and not require any follow-up work such as marketing hay or burning chaff. There are currently two machines that meet these requirements – the iHSD (Integrated Harrington Seed Destructor) and the Seed Terminator.
These two chaff impact mill machines render the weed seed unviable, causing the destruction of over 95 per cent of the weed seed that enters the mills. In 2017 harvest, both the iHSD and Seed Terminator machines were in commercial operation on farms around Australia and both experienced the teething problems that can be expected for new technology moving into the real-world. They have since been in field trials overseas – the Seed Terminator in Canada and the iHSD in France – where both machines completed around 200 hrs work without a hitch.
Kondinin Group Manager of Research and Development, Ben White interviewed 20 growers using either the iHSD or Seed Terminator during the 2017 harvest and reported that their observations suggested both brands were achieving over 95 per cent reduction in seed viability.
“Both types of impact mills ran into the same real-world problems of handling high flow rates of chaff, choking on green crop or weed matter and significant damage to the mills from any sand or soil that is picked up by the harvester,” he said. “Both machines also caused a reduction in harvester capacity of between 12 and 20 per cent in wheat, even though the harvesters had been remapped. This is a significant cost that growers must allow for through machine depreciation and base hourly operating costs, as more hours are needed to harvest the same area of crop.”
Over 70 per cent of the growers that the Kondinin Group engineers visited had remapped their harvesters to improve the harvest capacity.
Operators should note that chaff impact mill technology pulverises the entire chaff fraction, which generates significant levels of dust during the harvest operation. This means that more frequent machine clean-down may be required to minimise fire risk, as well as increased frequency of filter cleaning or replacement.
“Although the problems outlined here are important, they are all likely to be resolved as this technology matures in the commercial world,” said Ben. “There is enormous interest in this method of harvest weed seed control and this will ensure that the problems are solved.”
Seed Terminator, WA.
Australian Herbicide Resistance Initiative’s cost comparison of HWSC methods suggests that the chaff impact mills cost $16-17/ha compared to $22/ha for narrow windrow burning and $6-7/ha for chaff tramlining and chaff lining, depending on crop yield and area. In addition to the weed control benefits achieved through the use of any HWSC method, the chaff impact mill option also reduces crop volunteers.
Research into the efficacy of impact mills is an ongoing process, particularly while the two types of machines are undergoing rapid developmental changes. Recent research by Michael Walsh (AHRI and Sydney University) with help from John Broster at Charles Sturt University (CSU), shows that despite the problems that have been experienced with the new machines, iHSD mills are passing the research tests with flying colours.
Their research concluded that:
The iHSD achieved 96–99% destruction of the seeds of 11 weed species when processed in wheat chaff.
Weed seed destruction varied by about 10% depending on crop chaff type – ryegrass seed kill was greatest in lupin (98%) > wheat (92%) > canola (90%) > barley (88%).
Weed seed kill dropped by about 4% when chaff moisture increased above 12%.
Weed seed kill increases with mill speed and 3000 rpm is the accepted optimal speed for the iHSD.
SAGIT funded research conducted by Trengove Consulting in 2017 found similar results for the Seed Terminator:
Ryegrass weed seed kill was 93% at 2250 rpm and increased to 98% or greater at normal operating speeds (2500–3000 rpm).
Greater than 99% control of several other species including wild radish, brome grass, wild oat, bifora, bedstraw and tares.
Increasing chaff flow rate (harvest rate) did not reduce control of these species.
Samples analysed in 2018 by the Weed Science Research Group, at The University of Adelaide showed that the Seed Terminator could consistently kill 96% of weed seeds when operated at 2750 rpm.
These kill rates refer only to the weed seed that enters the impact mills. To achieve high level of weed control it is essential that all efforts are made to ensure the weed seed enters the front of the header and is then separated and directed into the mills.
Timing, cutting height, operating speed, weed and crop type and harvester set-up all play a part in achieving maximum harvest weed seed control. As is recommended for all HWSC methods that treat the chaff fraction, separation of the chaff and straw through the harvester often requires the addition and fine-tuning of a baffle plate to achieve greater efficiency.
Setting up and operating harvesters to achieve the best weed control outcomes often involves some modification and compromise. By taking the time to get things right, growers usually find that they end up with more grain in the bin and a better sample, making the extra effort worthwhile.
WeedSmart has secured the rights to distribute an electronic version of Kondinin Group’s Research Report: Residue Management at Harvest, which is available in the Resources section of www.weedsmart.org.au. WeedSmart encourages growers and advisors to support Kondinin Group’s independent research through subscription to Farming Ahead.
Confirmed resistance to the double-knock tactic in tall fleabane
For weeds that have a natural tolerance to glyphosate, the double-knock has provided growers with an excellent tool to take two swipes at weeds like fleabane, sowthistle and feathertop Rhodes grass and achieve a greater level of control.
NSW Department of Primary Industries weeds researcher, Dr Md Asaduzzaman (Asad) has uncovered disturbing evidence of double-knock resistance in tall fleabane (Conyza sumatrensis) samples collected during weed surveys funded by the Cotton Research and Development Corporation (CRDC).
NSW Department of Primary Industries weeds researcher, Dr Md Asaduzzaman (Asad) has uncovered disturbing evidence of double-knock resistance in tall fleabane (Conyza sumatrensis) samples collected during weed surveys funded by the Cotton Research and Development Corporation (CRDC).
“Our surveys in 2016 and 2017 showed that cotton fields were generally weed-free but herbicide resistance is building in weeds along farm roadsides, drains and channels and around infrastructure,” he said. “We identified two tall fleabane biotypes that have resistance to glyphosate, paraquat and the double-knock tactic of an initial glyphosate application followed with an application of paraquat 7 days later.”
The rate response analysis showed that one of these biotypes is 4.9 times more resistant than the susceptible biotype, requiring 2.5 L/ha Paraquat-250 to kill 50 per cent of the plants from the resistant population compared to just 0.5 L/ha to achieve the same result in the susceptible population (see Table 1). While this level of resistance is generally considered ‘moderate’ it is clear that resistance is building and must be taken very seriously given the importance of the double-knock tactic in most cotton and grain production systems in Australia.
Table 1 Resistance levels of tall fleabane screened against paraquat, glyphosate and glyphosate + paraquat (R-resistant > 50% survival; DR-developing resistance < 50% and > 20% survival and S-susceptible < 20% survival)
Tall fleabane biotype
% Plant survived under
Paraquat-250 @ 2 L/ha
Glyphosate-540 @ 1.2 L/ha
Glyphosate followed by Paraquat (Double-knock)
These two populations, collected near Nandi, Queensland and Coleambally, NSW, are the first paraquat-resistant tall fleabane to be identified in Australia. Resistance to paraquat in this species has previously been recorded in Japan, Sri Lanka and Taiwan.
Location of confirmed cases of double knock resistance in tall fleabane.
“Although the tall fleabane plants from these two populations showed signs of herbicide damage, such as narrowing of leaves and slow growth, when the double-knock was applied, they were able to survive and produce seed,” said Dr Asad. “This species produces a large quantity of seed, germinates quickly and the seed can travel over 10 km in the wind so dispersal of paraquat / glyphosate resistance traits will be impossible to contain.”
Tall fleabane seedlings 28 days after the double knock (glyphosate + paraquat) tactic was applied.
This discovery makes tall fleabane the second species in Australia to have confirmed resistance to both glyphosate (Group M) and paraquat (Group L), the first being a population of annual ryegrass identified in Western Australia in 2013. Having demonstrated resistance to the dual application of these herbicides in the otherwise effective double-knock tactic is cause for great concern.
Weed populations take longer to evolve resistance to paraquat and glyphosate compared to some other modes of action, but it will happen after years of regular applications without survivor control.
Like other fleabane species, tall fleabane is susceptible to crop competition but flourishes in poorly competitive, wide-row crops such as dryland cotton. Combatting herbicide resistance and keeping weed numbers low will require the implementation of a wider range of weed control tactics rather than relying heavily on the double knock tactic.
“Growing more competitive crops and using a wider range of pre- and post-emergence herbicides and strategic tillage will help manage this weed,” said Dr Asad. “Above all is the need to monitor and remove any survivor weeds in line with the cotton industry’s weed control strategy of ‘2 + 2 and 0’ that recommends two non-glyphosate tactics in-crop plus two non-glyphosate tactics in the fallow and zero survivors.”
In other research Dr Asad is testing the opportunities for cotton growers to use cover crops to create additional opportunities for herbicide rotations, run down the seed bank and delay the adaption of weed populations by reducing the frequency of single modes of action herbicide use.
Paraquat resistance has previously been confirmed in 10 species in Australia, including flaxleaf fleabane (Conyza bonariensis).
Resistance risk to knock-down herbicides on irrigated cotton farms
Protecting knock-down herbicide options
What are the herbicide options for the summer fallow
Is weed seed control an option for managing northern region weeds?
with Dr Michael Walsh, The University of Sydney
Harvest weed seed control (HWSC), is a key component in the WeedSmart Big 6 tactics for managing herbicide resistance on grain farms. In the northern region there has been some scepticism surrounding how effective HWSC will be in capturing seed from common weeds in the region.
Dr Michael Walsh, Director of Weed Research at The University of Sydney is a pioneer of harvest weed seed control research and is convinced that the tactic can be effectively deployed against northern region weeds.
“The concern people usually have is that several key species in the north start shedding seed before the winter harvest,” he says. “Pre-harvest shedding does reduce the impact that HWSC can have on the seed bank of some species, but there is also solid evidence that the harvest operation can collect a significant amount of seed even if the seed shedding process has begun.”
“Even in weeds such as sowthistle where the seed is light and wind borne, not all the seed in each seed head matures at the same time. This means that while some might be lost, the immature seeds can be collected and destroyed using any of the harvest weed seed control tools.”
Dr Walsh says chickpea crops are a great place to start with HWSC because the crop requires a low harvest height, which increases the amount of weed seed that is collected, and many crops are desiccated prior to harvest, which may assist with seed retention in some weed species.
“The concept of harvest weed seed control was invented in Australia in response to herbicide resistance, and has been adopted by about 50 per cent of grain producers nation-wide,” says Dr Walsh. “The idea is now being adopted internationally and is proving beneficial in driving down the seed bank of many weed species.”
What northern weed species are the best candidates for HWSC?
Short answer: African turnip weed, turnip weed, wild oats, annual ryegrass, feathertop Rhodes grass, sowthistle and bladder ketmia.
Longer answer: These species are rated as very high to intermediate for seed retention at harvest. This means that at least 20 per cent, and up to 100 per cent, of the seed is present on the plant at harvest and able to be collected through the front of the header. Once in the header, destruction of the seed is almost guaranteed through impact, burning or breakdown, depending on the HWSC method used.
What about flaxleaf fleabane and awnless barnyard grass?
Short answer: Seed retention of these two species at harvest is highly variable.
Longer answer: Both flaxleaf fleabane and awnless barnyard grass have a wide range of seed retention at harvest. Ranging from less than 20 per cent to over 90 per cent, the impact of HWSC on the seed bank is not a sure thing, but in some years could be very effective.
Which HWSC method is best?
Short answer: All are equally effective at reducing the weed seed bank.
Longer answer: The HWSC methods currently available to growers are narrow windrow burning, impact mills, chaff lining, chaff tram-lining, chaff carts and bale direct. All except the impact mills have some level of nutrient removal or concentration. Narrow windrow burning and chaff carts usually involve a burning operation the following autumn and the bale direct system relies on a market or on-farm use for the hay.
The impact mills, bale direct and chaff cart systems require sizeable investment and harvester modification while the other systems are very cheap and relatively easy to set up.
All HWSC methods are equally effective at reducing the weed seed bank. The chaff tramlining system shown here delivers the chaff component of the crop residue onto the tramlines where it is left to decompose over time, killing much of the weed seed present.
How often does HWSC need to be done to be effective?
Short answer: The more the better but it doesn’t have to be every year in every paddock.
Longer answer: Long term focus paddocks in WA have shown that HWSC every second year, in addition to in-crop herbicide applications, will consistently drive down the weed seed bank to very low levels.
Within a crop rotation many growers choose to implement HWSC in certain crops. For many, the break crops are the preferred crops due to lower levels of stubble, and for pulses, lower harvest heights.
Getting the edge on brome grass on non-wetting soils
On water-repellent soils, such as the Mallee sands of South Australia and Victoria, stronger crop competition has been shown to have an immense impact on brome seed production. In a difficult environment, where crop establishment is often patchy, researchers have demonstrated that water and nutrient harvesting within the crop furrow can result in better establishment when crops are sown on or near last year’s row.
Results from four years of trials on non-wetting sands at the Mallee Farming Systems (MSF) trial site at Karoonda, SA consistently show that the increased surface soil moisture, crop establishment, crop biomass and crop / brome competition effects achieved through edge-row sowing can reduce brome grass seed set.
Blue dye shows the higher moisture that can be found beneath last year’s row (left) and crop sown using edge row placement into last year’s crop stubble. Photos: Bill Davoren CSIRO
Dr Rick Llewellyn, research group leader (agricultural systems) with CSIRO says that brome grass is the most costly weed for Mallee farmers to manage, even though herbicide resistance in brome grass is currently low in the region.
“A series of trials have investigated the effect of different herbicide use, particularly pre-emergence herbicides, and row placement on crop yield and brome grass seed production,” he says. “To successfully manage brome grass it is necessary to keep weed seed numbers low and avoid blow-out situations. Our aim was to find the best options for reducing seed set.”
When this year’s crop can make use of the resources in last year’s furrow there is more reliable crop germination and it can allow earlier sowing in years with marginal rainfall. On non-wetting sands, the practice of edge-row sowing has led to reductions in brome seed set of at least 55 per cent, and as high as 75 per cent, over the past four seasons.
“Using edge-row seeding to achieve stronger establishment and crop competition on these difficult but widespread sands has resulted in a large and relatively reliable impact on brome grass seed production,” says Dr Llewellyn. “Growers will need to weigh this benefit against any potential disease risk and any stubble management issues in parts of a paddock that are not non-wetting sands, before changing their seeding set-up for edge-row sowing.”
One important observation was the extent of brome grass germination in the crop row at the time of applying the pre-seeding knockdown. With seven times more brome grass seedlings emerging on the row, a successful knockdown is very important and a great opportunity to give the pre-emergence herbicide the best chance of success.
Pre-emergent herbicides have the potential to achieve up to 75 per cent control of brome grass under optimal conditions but these products are notoriously variable from season to season. Trifluralin (1.5 L/ha) was commonly the least effective pre-emergent herbicide option while Sakura mixed with Avadex consistently achieved the greatest reduction in brome grass seed set.
“In the 2015 trials at Karoonda, Sakura alone resulted in 55 per cent less brome panicles than trifluralin alone, and Sakura + Avadex resulted in 72 per cent less brome panicles than trifluralin alone,” says Dr Llewellyn. “Reducing seed production is the most important component of effective weed management. In some seasons, while the trifluralin and metribuzin treatments halved early brome grass density, they did not result in significant seed set reduction.”
“Unfortunately, the more reliable Sakura plus Avadex treatment, with its extended period of activity is too costly for common use in low input regions. This leaves growers to get the most from lower cost but less reliable pre-emergence herbicide options and increases the need to include non-herbicide tools, such as improved crop competition through tactics like edge-row sowing.”
Brome grass is a weed that demands an integrated approach to keeping the seedbank low, especially where growers are wanting to reduce their reliance on Group B tolerant crops.
Crop row placement (MSFP)
Crop Row Placement – 10 things to consider
Take a look at edge-row sowing submitted by one of our MSF members. Travelling at 9km/hr and sowing with a John Deere Conserva Pak, and StarFire guidance system giving 2cm accuracy. The dry start meant there was very little moisture mid row but moisture was found in the previous year's crop row. Sowing as close to the moisture band helps to get a better germination in dry conditions. However, the practicalities of edge-row sowing are still being worked through. It can be hard to manage stubble and keep it standing when sowing so close to last year's row. In this situation it has worked with accurate guidance systems that allow you to sow on the same row year after year and always in the same direction. Thanks to the GRDC Stubble project you can learn more about crop row placement in stubble retained systems specific to the Mallee at the link belowhttp://www.msfp.org.au/farmtalk-crop-row-placement
Posted by Mallee Sustainable Farming on Friday, 22 June 2018
Get along to WeedSmart Week 2018
As a whole, the northern grains region has had a very tough start to the 2018 winter cropping season. With a few showers around there are hopes of some reprieve from a long, dry spell.
Unfortunately, weeds will most likely be the first to respond to rainfall and growers will need to make some difficult decisions surrounding their cropping program, pre-emergent herbicide applications and maximising the competitiveness of any crops that get established.
Brad Jackson, Gurley, will speak at the WeedSmart Week 2018 forum and host a bus tour of his family’s property, outlining his experience with pre-emergent herbicides, optical weed detection, narrow windrow burning and using a diverse cropping program to stay ahead of herbicide resistance.
By August the dice will have rolled on the season and growers and agronomists are invited to attend the 2018 WeedSmart Week event in the Narrabri and Moree districts. The 3-day program consists of a 1-day forum at Lochabar Station near Narrabri on 20 August. The following two days will be filled with bus tours to farms around Narrabri and Moree where growers have but in place integrated weed management programs to minimise the impact of herbicide resistance on their businesses.
The theme for the event is ‘Diversify and Conquer – manage weeds using the BIG 6’. At the forum and on the bus trips growers, agronomists and researchers will have all the options and ideas on the table for discussion.
Three growers from the region who attended the 2017 WeedSmart Week in Wagga Wagga will be making presentations over the three days, outlining ways they have tackled herbicide resistance head on.
Farming north-west of Moree, Jason Rogers drops a GPS marker on any suspect weed patches he finds so he can treat them with spot spraying, chipping or strategic tillage. In recent years he has moved into applying residual herbicides soon after harvest and is also mixing and rotating fallow sprays to extend the efficacy of all available modes of action. He says barley and Clearfield canola provide early canopy closure and the strongest crop competition for winter weeds.
Byron Birch is implementing a 5-year rotation of cereals, pulses and dryland cotton on a 4500 ha lease block on ‘Morven’, north-west of Narrabri. He is contending with annual ryegrass that was delivered in the 2016 flood and appears to have some tolerance to both Group A and B chemistry, and there are some small patches of glyphosate-resistant barnyard grass emerging. Byron has used dryland cotton to provide strong competition for moisture over summer to suppress the expansion of weedy patches on the farm.
Brad Jackson farms with his father Peter and brothers Phil and Matt at Gurley where they usually have a robust winter cropping program featuring wheat, barley, chickpea, canola and linseed. Following 2017 WeedSmart Week they have reintroduced the use of pre-emergent herbicides to their herbicide program and are taking every opportunity to mix and rotate herbicide modes of action. The Jackson’s WeedIT optical sprayer has revolutionised their herbicide program, allowing them to spray low weed density paddocks more frequently and target small, fresh seedlings. Brad and Phil are also developing valuable weed management tactics, such as camera-guided inter-row cultivator and green manuring, that can be employed in an organic farming system at Westmar.
WeedSmart Week attendees will have several opportunities to see and discuss cutting-edge technologies such as optical sprayers, autonomous tractors and emerging ‘green-on-green’ spray sensors. Beefwood Farms’ manager, Glenn Coughran has used optical sprayers as the foundation of a rejuvenated weed control program over the last 12 years and seen the benefits of combining this technology with autonomous tractors over the last three years. Glenn is keen to see ‘green-on-green’ optical weed detection become a reality and is working closely with AgriFac to have this technology integrated into their spraying equipment.
In a region where mixed farming is far less common now than it was a few decades ago, Tom Lampe uses livestock to deal with weeds that are hard-to-kill with herbicide. Using three 250 ha paddocks, each with a 5 ha ‘cooler’, Tom sprays the paddocks soon after harvest and then allows the sheep to graze the wheat, barley, chickpea and sorghum stubble, providing additional feed for the stock through the year.
These are some of the growers ready to host bus tours, even though the weather may prevent them doing what they normally would to manage their cropping program. These experienced and thoughtful farmers have implemented weed management programs that ‘Diversify and Conquer – manage weeds using the BIG 6’ in all seasons.
The growers, agronomists and researchers that will speak and participate in expert panels at the Day 1 forum will spark important discussions about herbicide resistance and how the Big 6 tactics can be used to target the weed species and farming systems in the northern cropping region. There’s one thing for sure – doing nothing is not an option.
Register for this important 3-day event for the single ticket price of $95, guaranteeing a seat on both the bus tour days as well as the forum, all fully catered, at https://www.weedsmart.org.au/northern-weedsmart-week-august-2018/
Going for the Big 6 on Yorke Peninsula farms
Chris Davey, partner and director of YP AG at Kadina has worked with growers on the Yorke Peninsula of SA for over 20 years, assisting them to devise weed control programs that reduce the impact of herbicide resistance.
His group of 20 clients farm between Port Broughton and Arthurton with annual rainfall ranging from 300 to 500 mm and very diverse soil types. Chris initiated the Northern Sustainable Soils farmer group in 2007 to provide growers with the opportunity to research farming system tactics and discuss their ‘fit’ for the highly variable soils found on the Peninsula.
YP-AG partner and director, Chris Davey works with his grower clients including Gary Bruce (left), to plan integrated weed management programs that include as many of the WeedSmart Big 6 tactics as possible.
“The soils here range from shallow sheetrock and limestone to grey calcareous loams, dune and swale systems to heavy red fertile clay,” he says. “This variability drives many management decisions and has a direct relationship with many of our weed problems.”
Chris has used resistance testing services to keep track of herbicide resistance in the main weeds, with growers managing resistance in annual ryegrass, brome grass and wild radish for some time and more recently finding milk thistle, Indian hedge mustard and prickly lettuce increasingly problematic. Annual ryegrass and brome grass on the Peninsula are known to have resistance to herbicides in Groups A (fop and dim), B, D and M and wild radish is resistant to Groups B, F and I.
“This area was the home of ryegrass resistance and growers are trying hard to avoid the same blow-out situation with brome grass,” says Chris. “Growers are well aware of the potential consequences if resistant brome grass gets out of hand so most are using some form of patch management and even chemical fallow in blocks where the brome grass has out-competed the crop.”
In response to increasing herbicide resistance, 85 per cent of Chris’ clients have adopted some form of harvest weed seed control within their weed management program. “Sixty per cent of my clients are using narrow windrow burning and 25 per cent are using either a chaff cart or chaff lining chute,” he says. “In the 2017 harvest there was also one iHSD and one Seed Terminator operating here.”
Chris is a strong supporter of all the tactics in the WeedSmart Big 6 and actively promotes the inclusion of all tactics in his clients’ integrated weed control programs.
1. Crop and pasture rotation
The close lentil – wheat rotation that has dominated farming systems on the northern Yorke Peninsula in recent years is acknowledged as a weak link in terms of weed control. This rotation has led to an increase in broadleaf weeds such as milk thistle and prickly lettuce, with bifora, tares and medic also exploding in the lentil phase in some years, leaving a high weed seed bank for the following year as well as increasing the risk of herbicide resistance evolution.
Chris says the economic drivers for the rotation can make weed control decisions difficult and there is a need for other profitable rotation options that can assist in reducing weed pressure.
The main problem with the short rotation is that weeds are exposed to the same herbicide modes of action every two years. Although imi-tolerant (Clearfield) varieties have been very useful, particularly PBA Hurricane lentils, allowing the use of Group B herbicides in the crop or in previous seasons, the alkaline soils on the Peninsula have expedited the more rapid evolution of Group B resistance in wild radish, mustard, milk thistle and ryegrass.
Lentils have provided several economic and weed control benefit to farming systems on the Yorke Peninsula but the short lentil / wheat rotation is a definite weakness when it comes to managing herbicide resistance.
2. Double knock to protect glyphosate
Glyphosate resistant ryegrass is widespread on the Yorke Peninsula, primarily along fencelines but as fences are removed to form larger paddocks, there is a significant risk that the resistance gene will be spread by headers. In 2013, the Peninsula had the dubious honour of having the first confirmed case of glyphosate resistant brome grass on a farm near Maitland, and this season, glyphosate resistance in barley grass was observed for the first time on northern Yorke Peninsula.
An annual double knock application before seeding is considered very important to help protect the efficacy of glyphosate and is widely practiced on the Peninsula. Sowing earlier to achieve a yield advantage and dry sowing can impact on the use of double knock. Chris advises his clients to avoid early or dry sowing in weedy paddocks and to hold off sowing until the double knock has been implemented, even though there could be a yield penalty. Under dry, dusty conditions most growers will choose two contact herbicides such as paraquat or paraquat / diquat rather than glyphosate / paraquat for the double knock.
3. Mix and rotate herbicide groups
There is a heavy reliance on pre-emergent herbicides on the Yorke Peninsula and in weedy paddocks growers need to use additional shots to drive down weed numbers to preserve yield. In cereals, the Boxer Gold and Sakura applications are often spiked with triallate to strengthen the pre-emergent efficacy because there are no in-crop herbicide options in wheat and barley crops.
Pre-emergent herbicides are also very important in lentils as the main break crop to reduce ryegrass numbers so there is less pressure on the clethodim / Factor mix in crop.
Trifluralin susceptibility in ryegrass has been very low since the late 1990s and so is not a tank mix option, unless targeting broadleaf weeds like wireweed or three corner jack.
4. Stop weed seed set
Chris says Yorke Peninsula growers generally use their late fungicide application in August or September to scout for weed escapes in crop. Taking a nil tolerance approach, growers might hand pull small areas, or spot spray.
Using paraquat or paraquat / diquat, growers can avoid using glyphosate on potentially resistant individuals when chemically fallowing areas of their crop. The permit for Weedmaster DST use to crop-top in barley provides a useful control tactic for radish and ryegrass at the end of the season, but is often too late for brome grass, which has usually already set seed by this stage of the crop. In blow-out situations Chris often advocates for the ‘short-term pain for long-term gain’ of a chemical fallow using paraquat or Spray.Seed.
Chris suggests that the chemical fallow is the best tool to use if brome numbers are building up in a paddock. In his experience, the performance of the following crop usually makes up for the one-year sacrifice due to increased nutrients and moisture availability. He says some growers plan for the inclusion of a small portion of the rotation to be sown as a chemical fallow, while a larger number would use chemical fallow only in a failed crop or for a weedy portion of a paddock as a patch management option.
5. Crop competition
Where the soil type allows, Yorke Peninsula growers have readily adopted east west sowing having seen the benefits of this row orientation promoted through AHRI and WeedSmart. Some soil types, such as the sand swales around Port Broughton, dictate sowing direction but it is an option in other areas.
Barley is the most competitive crop grown across the Peninsula and growers usually consider choosing the most competitive cultivars available. This is coupled with high sowing rates and narrower row spacing of 22–30 cm (9–12″) spacing, although there is local research that suggests there could be benefits of even narrower row spacing.
6. Harvest Weed Seed Control (HWSC)
Chris’ trial work with HWSC shows the importance of getting the weed seed into the header. He says brome grass can be difficult as its flexible stem doesn’t always get cut and can flick back up once the harvester has passed. Wild radish generally stands up well with 70–80% of seed entering the header. Even with a 50 to 60% capture of brome grass, depending on the season and how early harvest occurs, HSWC is still an important part of any weed management program. Capture of ryegrass seed is seasonal with the ryegrass lodging in some years and not picked up by the header, while it will stand up well and achieve 80% capture in other years.
Once weed seed is in the header, Chris’ research has shown that it doesn’t matter what HWSC method is used – all are effective. Chaff carts and narrow windrow burning have been widely adopted for many years but Chris expects that chaff lining and chaff decks is likely to increase while NWB will decrease in use. Chaff lining was widely adopted in 2017 harvest as an economical and easy way to manage weed-laden chaff. He also expects some growers with larger areas and some contractors to purchase iHSD and Seed Terminator modules, with one each of these operating in 2017 harvest on the Peninsula.
Chris Davey’s trial work with various harvest weed seed control methods has shown that the amount of weed seed entering the header can vary, but once in the header all methods are equally effective at reducing the weed seed bank for the next season.
Winter pulses can compete, even without herbicide
Chickpeas and faba beans now have a permanent place in most northern region farming rotations and, while they offer some diversity in herbicide options, they have been considered comparatively less competitive against weeds than other crops.
Back when growers had a number of effective herbicide options available, the lower competitiveness of these pulse crops was less problematic because the herbicides were keeping weed numbers low. In the light of increasing herbicide resistance in many weeds, strong crop competition is required to do more of the ‘heavy lifting’ within an integrated weed management program.
Dr Michael Widderick, Queensland Department of Agriculture and Fisheries principal research scientist, is leading the work on crop competition within the multi-facetted GRDC-funded ‘Innovative crop weed solutions for northern region cropping systems’ (US00084) project led by Dr Michael Walsh, University of Sydney. The first round of southern Queensland data was collected for faba bean (PBA Warda) and chickpea (PBA HatTrick) trials harvested in late October 2017, at the Hermitage site near Warwick.
Sowthistle is a major winter weed in pulse crops and growers are finding it is increasingly able to escape herbicide control, robbing the crop of valuable resources while setting huge numbers of seeds that increase weed pressure in the following season.
There are known populations of sowthistle with resistance to glyphosate and chlorsulfuron in the northern grains region (and 2,4-D resistance confirmed in SA), making non-herbicide control tactics all the more important in driving down numbers of this moisture-sucking weed.
Sowthistle is a major winter weed in pulse crops and growers are finding it is increasingly able to escape herbicide control, robbing the crop of valuable resources while setting huge numbers of seeds that increase weed pressure in the following season.
“We are looking at optimal row spacing and crop density to combat common sowthistle, and also determining if more competitive crops are also higher yielding,” says Dr Widderick. “The trials are being replicated at sites near Warwick, Narrabri and Wagga Wagga and will run for five years.”
The faba bean and chickpea crops were trialled at narrow and wider row spacing, and low, medium and high crop densities in both weedy (sown sowthistle) and weed-free plots. The sowthistle seed sown in the trials all came from the same, non-resistant population and no herbicide was applied to any of the plots.
Dr Widderick says sowthistle is definitely susceptible to crop competition, with even the least competitive faba bean and chickpea crops cutting weed biomass and seed production by at least 50 per cent compared to plots where the weed was allowed to grow free of any crop competition.
Sowthistle seed production was greatest for both crops at wide row spacing (50 cm) and low (20 plants/m2) crop density. Seed production was progressively reduced as crop competition increased.
“In chickpea we found that reducing row spacing from 50 cm to 25 cm further reduced weed biomass and seed production by about 50 per cent at a crop density of 40 plants/m2,” he says. “In this trial, row spacing had no effect on crop yield but increasing plant density did generate a significant and progressive increase in yield for both chickpea and faba bean in weedy plots.”
When sowthistle is present, increasing crop density from 20 plants/m2to 70 plants/m2in faba beans, or to 80 plants/m2in chickpea, generated around 0.5 t/ha yield increase. Averaged across all treatments for both crops, controlling sowthistle with crop competition alone (no herbicide applied) was worth approximately 0.25 t/ha in crop yield.
This research has measured the effect of increasing the competitiveness of chickpeas and faba bean without the use of any herbicide. On farms, growers usually have some herbicides at their disposal and other research shows that combining effective herbicides with strong crop competition is the best way to control herbicide resistant weeds.
In chickpea, reducing row spacing from 50 cm to 25 cm cuts sowthistle biomass and seed production by about 50 per cent at a crop density of 40 plants/m2.
Weed control is a package deal in SA Mid North
Agronomist Craig Davis has been assisting growers in the Mid North and Yorke Peninsula of South Australia to navigate their way around resistance to key herbicides such as trifluralin, Group A and Group B herbicides for 10 years or more.
“Some of this chemistry was cheap and dealing with the loss of these modes of action has been difficult,” says Craig. “The key to farming without these herbicides has centred around using crop rotations to control the weed seed bank.”
Mid North SA agronomist Craig Davis says harvest weed seed control has been widely accepted as a necessary tool to manage the seed bank in the Mid North and Yorke Peninsula. Narrow windrow burning (NWB) is commonly practiced in the region, and there has been considerable interest in chaff management systems with a few chaff decks operating and some growers now trialling chaff lining.
Annual ryegrass control has been and remains a significant cost to growers in these regions. Brome grass and wild oats are becoming increasingly important weeds that were previously suppressed by trifluralin, and stubble retention on many farms has also favoured some weeds, like brome grass.
Craig says these weeds are demonstrating increasing resistance to Group A (fop and dim) and Group B (SU and imi) herbicides, and some of the alternative pre-emergence herbicides are relatively ineffective, or variable in their control.
For some growers, oaten hay production for export has been a useful enterprise to reign in resistant annual ryegrass numbers. Although a very effective weed control option, it is not for everyone.
Brome grass and wild oats are not as well controlled through oaten hay production because a significant amount of seed is shed before the crop is cut.
For many hard-to-kill weeds, a breakthrough came with the introduction of new imi-tolerant lentil varieties in the early 2000s, which are now built into the rotation on most farms in the region. “Imi-tolerant lentils have been very useful in managing weeds, and the high grain prices and suitability to the rotation have made them the legume of choice for growers,” says Craig. “In practice though, field peas are the most effective legume option for grass weed control.”
Field peas can be sown later without suffering a yield penalty, are a more competitive crop and are a good option for effective crop-topping due to their early maturity. The downside of field peas is the likely build up of snail numbers.
Although lentils have been the pulse of choice for several years, field peas are the most effective legume option for grass weed control in the region because they can be sown later without suffering a yield penalty, are a more competitive than lentils and are a good option for effective crop-topping due to their early maturity.
Break crops currently make up around 50 per cent of the cropped area, due mainly to the recent high price for lentils. Craig expects this area of break crop to drop in response to the lower lentil price, however, growers generally recognise the benefits of maintaining diversity in their cropping program.
“Canola is more competitive than lentils and enables the use of triazine as an alternative chemistry for annual ryegrass control,” he says. “Hybrid canola is particularly competitive against weeds and can suppress seed set even when the herbicide package is not strong.”
A large portion of the canola crop is windrowed and there is a long history of spraying under the cutter bar to reduce seed set in lodged and late germinating ryegrass. Many growers have added narrow windrow burning (NWB) to their weed management program to kill any viable weed seed left at the end of the canola crop.
A cheaper option is to spray over the top to desiccate the crop, then direct harvest with a narrow windrow burning chute and burn the narrow windrows the following autumn, however Craig has witnessed significant wind damage to standing crops and recommends growers continue to swath and spray under the cutterbar, unless they can guarantee timely harvest. The higher harvest height of direct headed canola also means a significant amount of weed seeds are not captured in the header front.
Canola grown on cereal stubble has reached a yield plateau, and along with the increasing incidence of clethodim resistance, Craig expects growers to increase their use of double-breaks in their rotations. The traditional double break in the area was pasture followed by canola, which provides annual grass weed control, a cereal disease break and uses the residual soil nutrients.
“There is now an increase in the use of a double break of oaten hay or grain legume followed by canola,” he says. “This allows the canola to thrive in paddocks with lower weed burdens, lowers stubble residue levels and increases soil nutrient levels, particularly nitrogen.”
“Adding feed barley to the rotation offers growers the opportunity to implement a triple-break to control brome grass, wild oats and ryegrass, making use of the permit that allows spray-topping of feed barley to reduce weed seed set prior to harvest,” he says.
Craig works with his clients to optimise crop competition in tandem with a good pre-emergent herbicide package. He says that effective and relatively cheap pre-emergent herbicides have previously masked the true value of crop competition.
“Stubble retention makes the adoption of narrow rows more difficult but many growers have moved from 30 cm to 22 cm row spacing using tined planters,” says Craig. “More commonly, growers are using higher planting rates to achieve stronger crop competition. The other critical aspect is to seed all crops at the optimal time of sowing and to not plant everything early. Early sowing should only be considered for paddocks with low weed numbers.”
Mixing trifluralin with another pre-emergent herbicide is an effective tool provided both herbicides in the mix have some efficacy, say over 80 per cent efficacy as a stand-alone herbicide. With high levels of trifluralin resistance now widespread, the use of other effective MOAs in combination is increasing. With heavy stubble loads on the soil surface making it difficult to achieve high levels of control using pre-emergent herbicides, Craig says it is essential that growers diversify and implement other weed control tactics to remove any survivors and stop seed set.
Harvest weed seed control has been widely accepted as a necessary tool to manage the seed bank. Narrow windrow burning (NWB) is commonly practiced in the region, mostly in canola and lentil crops because the windrows tend to stay in place and support a hot fire that achieves a high level of weed seed kill.
“There has been considerable interest in chaff management systems in the district and several clients have used chaff carts in the past,” he says. “Now there are a few chaff decks operating and some growers are trialling chaff lining. There has been success with chaff lining pulses one year and placing the narrow windrows of the following canola crop on top of the pulse chaff line.”
“Growers using chaff decks are seeing improvements in summer spraying efficacy as a result of less dust coming off the tramlines,” says Craig. “The downsides are the increased difficulty in establishing crops in the tramlines and, although the weed seed mortality is high, there are still high numbers of weeds germinating on the wheeltracks.”
Getting started with chaff lining
Farming 4000 ha of light sandy to heavy clay soil in the medium rainfall district of Halbury and Salter Springs SA, Kevin Simon trialled chaff lining for the first time in the 2017 harvest.
Kevin planted early maturing PBA Wharton field peas to help bring annual ryegrass numbers back under control. The field peas yielded around 3–4 t/ha and, being early maturing, offered an opportunity to harvest early and catch the ryegrass before it lodged or set seed.
Farming in the medium rainfall district of Halbury and Salter Springs SA, Kevin Simon trialled chaff lining for the first time in the 2017 harvest. He planted early maturing PBA Wharton field peas to help bring annual ryegrass numbers back under control. Kevin plans to plant TT canola into this paddock in 2018 using a disc seeder to minimise disturbance of the chaff line.
“Harvesting low and early are important to stop ryegrass seed set but it also comes with difficulties because the ryegrass is still green and can bind up the rotors in the header,” he says.
Kevin plans to plant TT canola into this paddock in 2018 using a disc seeder to minimise disturbance of the chaff line. With limited in-crop herbicide options available, Kevin relies on late season cultural control.
“We spray over the top of the canola with a self-propelled sprayer then direct harvest to control ryegrass using the chaff lining chute,” he says. “Chaff lining is also a good way to collect volunteer crop seed from the previous season. The plan is to place the canola narrow windrows on top of the previous year’s pea chaff line, and burn the narrow windrows to control weed seeds collected during the harvest process.”
Last summer was very dry and so there was very limited germination of volunteers and weed seeds from the field pea chaff lines. In wetter years, Kevin expects that volunteers would be the most dominant plant type within the chaff line, with ryegrass being the next most prevalent species present. If necessary, Kevin is prepared to apply a range of chemical and cultural control measures to target the weeds growing in the chaff lines. Lime applied on other paddocks has also helped reduce the ryegrass population.
Pulses to attack weeds on many fronts
Setting up harvesters to capture weed seeds in the chaff stream
Mic Fels case study – Stacked rotation and Chaff lining
Warwick and Di Holding case study – From narrow windrow burning to chaff decks
How chaff lining works