Giving summer legumes the competitive edge
Fifty years ago, many farmers and researchers expected herbicides to be the panacea for weed control. The reality has been that no weed species has been eradicated through the use of herbicides alone, and in fact, many weeds have increased in population in the presence of herbicide application in farming systems. This is what motivates researcher Dr Bhagirath Chauhan, Principal Research Fellow with the Queensland Alliance for Agriculture and Food Innovation (QAAFI) to study how crops can do their own weed control through the application of cultural practices. Dr Bhagirath Chauhan says narrower row spacing in summer legumes such as mungbean and soybean will take the pressure off in-crop herbicide applications, provided the crop is sown into clean paddocks and weeds are controlled for at least three weeks after planting. He says that his agronomic trials in summer legume (mungbean and soybean) crops are consistently showing that early canopy closure results in lower weed biomass and higher crop yield. “We have demonstrated that narrower row spacing in summer legumes such as mungbean and soybean will take the pressure off in-crop herbicide applications, provided the crop is sown into clean paddocks and weeds are controlled for at least three weeks after planting,” he says. “Increased yield is a significant benefit and will support growers’ decisions to set up their planting gear to suit farming on narrower row spacings.” In the 2015 and 2016 mungbean seasons, 30 kg/ha of Jade-AU was grown at 25, 50, and 75 cm row spacing and Rhodes grass was sown to provide a known weed density in all plots. There were four levels of weed infestation applied 1. Weeds sown at planting, 2. Weeds sown 3 weeks after planting, 3. Weeds sown 6 weeks after planting, 4. Weed-free throughout the growing season. Rhodes grass was spread at 300 seeds per m2 to create even competition across the site, which is generally not possible if researchers rely on the natural weed seed bank of the experimental site providing the competition. Agronomic trials in summer legume (mungbean and soybean) crops are consistently showing that early canopy closure results in lower weed biomass and higher crop yield. “In plots where the mungbean crop and the weed emerged together, it didn’t matter which row spacing was used – the weed biomass was high and the grain yield was low, less than 360 kg/ha,” says Dr Chauhan. “If the 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. We know from other studies that weed biomass correlates well with weed seed production, so reducing biomass can be expected to also reduce seed production in the weed.” In both seasons, 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. Even in a completely weed-free growing environment the two narrower spacings generated higher yield than the 75 cm rows. “This trial demonstrates the value of reducing row spacing to 50 cm in mungbeans and keeping crops weed-free for the first six weeks after planting,” says Dr Chauhan. “After this point the crop has the competitive edge and any later germinating weeds struggle to get established.” Dr Chauhan has also conducted trials to determine weed densities that will cause a 50 per cent reduction in mungbean yield. These weed densities can provide a guide to growers and agronomists about the level of infestation that will affect yield and allow them to determine an economic response, keeping in mind that any weeds that set seed are building up in the weed seed bank for following seasons. A 50 per cent yield reduction in mungbean can be expected in the face of 31 bladder ketmia plants/m2, 22 feathertop Rhodes grass plants/m2, 33 windmill grass plants/m2, 21 liverseed grass plants/m2 or 23 button grass plants/m2. In a similar row spacing trial in soybeans, Dr Chauhan saw similar results to those of the mungbean trial. He also confirmed that row spacing, not plant population, is the key driver to reducing weed growth. “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,” says Dr Chauhan. “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. Mungbean crop density has a direct effect on the morphology of weeds such as barnyard grass where the weed is quite prostrate when there is no crop competition, and has a more upright habit when there is more crop competition. This may have implications for weed control strategies such as harvest weed seed control.
SwarmFarm: Targeting small weeds all year
Just five years ago Central Queensland grain farmer Andrew Bate was in a tractor, spraying a wheat crop, and thinking about ways to farm better and more efficiently. His idea to create a ‘swarm’ of small, lightweight machines that could work autonomously and cooperatively, is now a commercial reality. SwarmFarm operations manager and leader of field development, Will McCarthy, says the robots are the ultimate weed scouts, tracking down escapes and eliminating them before they have a chance to set seed. Along with his wife Jocie, Andrew is founding director of SwarmFarm Robotics. The headquarters of their agricultural technology company is their farm ‘Bendee’ at Gindie, south of Emerald, where their team of seven software and mechatronics engineers and technicians is building and testing world-first robotic technology specifically for agricultural applications. “There are currently seven SwarmFarm robots working on grain farms, turf farms and in an environmentally-sensitive area on a mine site,” says Andrew. “Our commercial release of 50 robots setup for spraying weeds using the WeedIT optical sprayer technology is now underway.” Weed control provides an excellent opportunity for robotics to shine. A time-consuming but ‘simple’ task that robots can do very effectively at a slower pace, ensuring every weed in the paddock is accurately and effectively controlled while still at a small size. Weighing just 2 t fully loaded, each robot is only 10 per cent of the weight of a conventional sprayer and they fit in perfectly with zero till and controlled traffic farming systems. The cost benefit of robots applying herbicide lies in the frequency of treatment, accuracy and ability to safely operate any time of the day or night. While a grower may hesitate to go spraying, concerned that there might be another rain event and subsequent germination, the robots can ‘go now and go later’, always targeting small weeds at their most susceptible growth stage. Weighing just 2 t fully loaded, each robot is only 10 per cent of the weight of a conventional sprayer and they fit in perfectly with zero till and controlled traffic farming systems. The SwarmFarm robots optimise the use of existing optical sprayer technology to identify and target small weeds in a green-on-brown situation (i.e. in fallow) by enabling more frequent applications that are slower and more accurate. The ability to go over the same paddock every few weeks is the standout difference that robotics can bring to the management of herbicide resistance. SwarmFarm operations manager and leader of field development, Will McCarthy, says the prescription spraying used on ‘Bendee’ involves the robots passing over the fallow paddocks once every two weeks. “This way, no weed will get bigger than the 50 cent piece size that is optimal for effective control,” he says. “We can apply a wider range of herbicide modes of action, more robust rates for chemicals registered for this use pattern and potentially reintroduce products and brews that may have had reduced efficacy as broadacre sprays in the past.” “The robots are the ultimate weed scouts, tracking down escapes and eliminating them before they have a chance to set seed. Constantly targeting small weeds and preventing seed set is the only way to keep weed numbers low and avoid herbicide resistance.” The SwarmFarm robots enable the optical sprayer technology to really come into its own because the robots can operate slower, the cameras and sprayers can be closer together and the robots can go over the paddock repeatedly so there is no concern about getting the timing right. Every weed can be treated at an early growth stage for the herbicide to have maximum effect, tackling herbicide resistance at the source by applying constant downward pressure on the weed seed bank. The SwarmFarm concept is ideal for new technology developments, as it allows easy integration of third party products as they are being developed, such as green-on-green technology. Will says the robots would then be able to distinguish between a weed and a crop plant and even between weed species. This will allow the removal of volunteer crop plants and even target broadleaf weeds like sowthistle in a broadleaf crop such as chickpea. Although the current focus is on herbicide application, there is great potential to use the same platform to implement non-herbicide tactics such as targeted tillage or robotic chipping, steam or any other non-herbicide tactic found to be effective. “Using the robot concept, microwave technology becomes a realistic option because the robots can stop at every weed and apply the necessary microwave blast to kill each weed, something that is simply not feasible for a tractor operator,” says Will. WeedIT cameras capture data from a 1 m wide band on the ground using NIR and IR light to detect green weeds in a ‘brown’ paddock. The SwarmFarm concept allows easy integration of third party products as they are being developed, such as green-on-green technology or non-herbicide tactics such as targeted tillage or robotic chipping, steam or any other tactic found to be effective. “Aside from weed management, the robots will enable direct management of a crop’s plant population to maximise yield potential for the available soil moisture,” he says. “There is no reason why the SwarmFarm platform can’t be utilised for planting and applying fertiliser precisely and economically, controlling insect pests and even harvesting the crop. The system is in place to support any application really and all that is needed is the planter, cultivator or harvester to be engineered and bolted on.” Andrew reckons that a 10 000 ha property like ‘Bendee’ would only need two SwarmFarm robots to take care of all their weed control operations. SwarmFarm robot features Working through the features of the SwarmFarm robots highlights their simple and robust construction and numerous safety features. Will says the team has worked hard to make diagnosis as simple as possible and the modular components have minimal opportunity for failure. “If there is a problem, the replacement parts can be easily fitted on-farm without specialist technicians and the maintenance is straightforward and well within the capability of any farmer,” he says. Multiple safety features built into the robots, which make them safer than a person operating a spray rig or tractor, include: obstacle detection sensors (can determine if the terrain ahead is suitable to traverse and also stopping if there is something in its path e.g. a vehicle or person), paddock definition (it maps the paddock to show boundaries, fencelines, trees, troughs, dams etc. then uses software to generate A–B lines. The robot then drives itself around these fixed obstacles), remote control using an iPad (allows the operator to stop and restart the robot when within the local farm network), a bumper sensor to turn off the machine as a back-up to the obstacle detection sensors (slow operating speed means any damage would be minimal if this was activated), a geo-fence that turns off the machine if it crosses the line. Each robot has an 8 m boom fitted with eight WeedIT optical cameras, 40 nozzles and a 600 L spray tank. Depending on weed density across the paddock this could last all day or an hour. The robot monitors the volume of spray in the tank and makes a decision whether it can reach the end of a run or not before running out of spray. It then returns to a docking station for refilling. At the moment, a person is required to refill the spray tank but plans are in place to fully automate the refilling operation within the next 8–12 months. The 60 L diesel fuel tank on board gives the machine an operating time of 18 hours between refueling. WeedIT cameras capture data from a 1 m wide band on the ground using NIR and IR light to detect green weeds in a ‘brown’ paddock. There are five individual sprayer solenoids per camera span, giving one spray nozzle every 20 cm. The cameras are set to turn on three nozzles over a weed to ensure good coverage, which is particularly important if there is a breeze blowing. The genius of the SwarmFarm system lies in the ‘smarts’ of the SwarmHive base station computer. Located in the grower’s office the SwarmHive takes care of the robots’ activity and decision making and coordinates the workflow of all the robots operating in the paddock. “If one machine is going slower due to higher weed numbers then the SwarmHive will reallocate the other robots to cover the extra area so that the whole paddock operation is completed at about the same time and all the robots come back to the docking station together,” says Will. “It is updating in real time and making decisions about the weather conditions, mapping weed density, ensuring the robots are operating efficiently and sending alerts if any problems arise, such as a pump malfunction or an obstacle detected.” An on-site automatic weather station located at the docking station monitors key parameters such as Delta-T, wind speed and direction and ensures the robots only operate within the label directions. The SwarmHive automatically turns off the robots and then restarts them when the conditions are within the acceptable range. Integration with an on-site, automatic weather station also provides a reliable record of spray activities and the real-time environmental conditions during the spray operation, such as wind speed and direction. The grower also has ‘on-the-go’ access via an iPad app to monitor and control the robots if necessary, when within the local farm network. Other resources Andrew Bate SwarmFarm Robotics podcast
Beating multi-resistant weeds in the Northern region
Dryland cotton and grains farmer Paul Slack is battling weeds that have high levels of resistance to several important herbicide modes of action. Managing 4850 ha of cropping land within 30 km radius of his home farm at Gurley, east of Moree, NSW Paul is using summer and winter cropping, strategic tillage and a long fallow spray program to get on top of resistant annual ryegrass and black oats populations. Tony Lockrey (AMPS Agribusiness agronomist), Paul Slack (Moree farmer) and Caleb Torrance (AMPS trainee agronomist) inspecting Paul’s corn crop, which is adding diversity to his cropping and herbicide program. A few years ago Paul was faced with a blow-out situation of 90–100 annual ryegrass plants per square metre in a Clearfield canola crop, along with some black oats and barley grass. The ryegrass present was tested for resistance, revealing an alarming profile of resistance to five herbicide mode of action groups. Groups A, B, M and two others, where the lowest level of resistance was to mesosulfuron-methyl (Atlantis, Group B) at 15 per cent. The annual ryegrass present was tested, revealing resistance to five herbicide mode of action groups. A dedicated program aimed at preventing seed set has seen a huge reduction in weed numbers with annual ryegrass now confined to a few isolated patches near waterways. With stubble cover required for the fallow ahead of a dryland cotton crop, Paul and his agronomist Tony Lockrey, AMPS Moree decided to run down the weed seed bank, starting with a double knock application to treat the first germination in autumn. Residual herbicides were incorporated using a Kelly chain through the canola stubble prior to sowing a short season wheat crop. “Sakura + Avadex Xtra provided upwards of 80 per cent control of ryegrass, but it must be used wisely in the rotation — we would try not to use this combination more than twice every four years,” said Tony. “In our experience, Group K herbicide Sakura is more robust in a variety of conditions, proving more stable than other residuals in dry conditions following planting, which often occurs in this region.” An in-crop application of the selective herbicide (Group B Atlantis) in wheat followed by a long fallow and a dryland cotton crop resulted in a huge reduction in weed numbers. Paul said annual ryegrass is now confined to a few isolated patches near waterways. “Using the Kelly chains in canola stubble represented some challenges but it was much easier than in other crop stubbles,” said Paul. “Now that the chain is worn-in I think it could be used in a chickpea crop to incorporate pre-emergent herbicides without damaging the crop. Mostly though we use it to chop chickpea stubble and to control weeds on the headlands.” Having had success with this regime to control multi-resistant annual ryegrass Paul decided to use a similar program to rein in Group A resistant black oats that had spread over about 100 ha. Black oats has been a problem for the last 15 years or so after gaining Group A resistance from the short wheat / chickpea / wheat rotation commonly used at the time, where there was heavy use of Group A products such as Verdict and Topik. Black oats pressure is clearly seen in this area where the crop was not sown. The addition of dim chemistry (Group A) in the chickpeas has helped extend the life of Group A chemistry, even though there is 50 to 60 per cent resistance to Verdict (a Group A ‘fop’) in the black oats population. Paul has used a long fallow period to regain control of resistant black oats, bringing the problem back to a very small area. “We did a long fallow after wheat then back to cotton followed by chickpea and then a short fallow and back to wheat,” he said. “This meant we could apply glyphosate in the fallow and two fop + dim mixes in the following chickpea crop. The addition of dim chemistry (Group A) in the chickpeas has helped extend the life of Group A chemistry, even though there is 50 to 60 per cent resistance to Verdict (a Group A ‘fop’) in the black oats population.” “We might repeat this clean-up program to really run down the black oats seed bank before returning to our normal program, which now features both summer and winter crops and a broader range of herbicide tactics,” said Paul. Black oats blow-outs are most likely in wheat crops and Paul is considering cutting weedy crops to make round bale silage. However, without a strong nearby market for the fodder and the large nutrient loss involved in baling, this decision will be a hard one to make. Milk thistle is looming as another challenging glyphosate-resistant weed that will require a different non-herbicide solution, possibly a mechanical innovation. “We know that zero-tillage farming led to a 30 per cent yield improvement in this environment,” says Paul. “We can’t afford to be forced back to a full tillage system to control weeds.” “There is potential to cut the crops lower in problem areas to expose the summer weeds to knock-down control and to improve herbicide-soil contact and efficacy with residuals,” he says. “In doing so we then also need to find ways to better spread the increased volume of crop residue out the back of the header, right across the 12 m to maximise fallow moisture efficiency. Using a chaff deck system to gather weed seeds at harvest and place the chaff on the wheel tracks is also something we are considering.” The current rotation of corn and cotton in the summer and wheat and chickpea in winter has helped bring all resistant weeds back to a manageable level. Bringing Group B chemistry into the herbicide program has been an effective tool in reducing the number of multi-resistant grass weeds and adding a Clearfield wheat into the rotation is another option for the future.
Taking the competition to the weeds
Competitive crops are the ultimate weapon against herbicide resistant weeds as a non-herbicide tactic that suppresses weed germination and boosts crop yield. When stubble retention and harvest weed seed control are also enlisted in the tactical war on weeds then productivity and profitability become the clear winners. Greg Condon (right) is working with the Fox family at Marrar to build an integrated weed management system that also promotes productivity. Greg Condon, Grassroots Agronomy says growers can consider putting together several components that enable narrower row spacing, which is known to generate a one per cent increase in grain yield for every inch reduction in row spacing. “It is important to remember that this benefit can be gained without going to ultra-narrow rows,” he says. “I’d suggest growers go as narrow as they can within the constraints of their existing equipment and farming system to gain as much yield benefit and weed suppression as possible. Then, when it’s time to buy new equipment growers could look at what’s available that would enable more competitive configurations.” Green crops provide the best possible shading effect to suppress weed growth with a competitive crop having shading power equivalent to 10 t/ha retained stubble. This varies considerably with crop architecture where erect cereals provide less shading of weeds than varieties with a prostrate plant structure. Narrowing the row spacing increases the competition while also contributing to crop yield. One system that some of Greg’s clients are using combines stubble management and narrow rows using a disc seeder at planting and a stripper front at harvest. “The real advantages of stubble retention are seen in improved water holding capacity in the soil, which enables earlier sowing,” says Greg. “One year of retained stubble is generally not a problem for any of the modern seeders but we are seeing disc seeders do a better job handling stubble accumulated over a few years and in various stages of decay.” Front loss from a stripper front can be 1–2 per cent but in some cases this can be compensated by reduced harvest cost and header depreciation as stripper fronts harvest faster. Correct harvester and front setup is essential to minimise harvest losses. Including canola and pulses in the crop rotation also plays an important part in managing the quantity and decomposition of crop residue, particularly in a narrow row configuration. Pulses and canola crops assist in stubble management across the rotation. “Using a stripper front at harvest greatly reduces the amount of crop residue that is cut, chopped and spread out the back of the harvester,” says Greg. “The rearwards rotating rotor and stripper fingers strip the grain off the heads and leave the bulk of the crop residue standing in situ. About 85 per cent of the threshing occurs at the front end of the harvester, enabling double the harvest capacity of a draper, using 60–70 per cent less engine power and 60 per cent less fuel.” Grower experience suggests that stripper fronts work well in lodged crops and still allow the operator to have the front down low to collect any weed seeds, such as annual ryegrass and brome grass, present at harvest. This is particularly effective if the crop is sown on narrow rows, forcing the weed seed heads upwards in the crop canopy. Research into the efficacy of this system for harvest weed seed control is planned for the coming harvest. “The tall stubble left after harvest provides very effective soil shading and protection from drying winds over summer to retain more moisture and enable early sowing, rather than growers needing to wait for breaking rains,” says Greg. “The stubble dries and becomes brittle rather than rotting on the soil surface, making it easier for the disc seeder to operate.” The tall stubble left by the stripper front at harvest has widened their planting window all the way from January to May, giving them incredible flexibility in their cropping program decisions. The chaff line is subject to a high level of shading and higher moisture conditions that suppress germination very effectively. Harvest weed seed control is an additional component that growers like Daniel Fox at Marrar, NSW are adding to the ‘strip and disc’ system. Daniel has moved away from narrow windrow burning and now uses a chafflining chute to deposit the small amount of chaff, along with weed seeds, into a thin band behind the harvester. This contains the weed seeds to a very small portion of the paddock where they either rot away or have to face stiff competition from the following crop. “We are putting much more emphasis on cultural weed control methods now and reducing our reliance on herbicides,” says Daniel. “The tall stubble has really widened our planting window all the way from January to May, which gives us incredible flexibility in our cropping decisions. The system is easier to manage if you start with low weed numbers but we really believe that it can be used to drive down weed numbers after a blow-out too.” In a nut-shell, Daniel and his father David have implemented a controlled traffic farming system based on 16.5 cm row spacing, competitive cultivars, disc seeding, stripper front, chafflining, double-break cropping and sowing east-west where it is practical. On the herbicide front they have a strong pre-emergent program and utilise double-knocks and crop topping to manage in-crop survivors. The Fox family of Marrar, NSW have implemented a controlled traffic farming system based on 16.5 cm row spacing, competitive cultivars, disc seeding, stripper front, chafflining, double-break cropping, and sowing east-west where it is practical. On the herbicide front they have a strong pre-emergent program and utilise double-knocks and crop topping to manage in-crop survivors.
Surveys highlight the cross-resistance to herbicides
John Broster, senior technical officer (herbicide resistance) at Charles Sturt University leads the weed survey project in NSW and has just completed crunching the numbers and finalising the results of the 2016 survey. John Broster, CSU “We gathered samples from the northern NSW and plains regions in 2016, which means we now have survey results from across the major grain growing regions of NSW,” he said. “The target weeds that we collect and test for herbicide resistance are, in order of importance, annual ryegrass, wild oats, sowthistle, barley grass, brome grass, wild radish and Indian hedge mustard.” The weeds that are collected in the surveys are tested for their susceptibility to the major herbicide groups used for their control. “Across NSW the extent of ryegrass resistance to Group A ‘fops’ and Group B ‘SU’ herbicides is by far the highest with about 60 per cent of samples being resistant, and 46 per cent of samples being resistant to Group B ‘Imi’ herbicides. There was less extensive resistance to the Group A ‘dims’ and Group D herbicides at 10 and 11 per cent of samples respectively,” said John. “When we looked at cross-resistance in the ryegrass samples collected in NSW we found that while 24 per cent of samples were susceptible to all five of the major herbicide groups used for ryegrass control, a huge 43 per cent were resistant to three or more of these five herbicide categories.” Speaking at the recent WeedSmart Week forum, John Broster, senior technical officer (herbicide resistance) at Charles Sturt University presented results from the latest weed survey conducted in NSW where 43 per cent of surveyed annual ryegrass plants were resistant to three or more of the five important herbicide categories used to control this weed. These surveys clearly illustrate the level of cross-resistance that exists in cropping paddocks across NSW, and in other states around Australia. At the recent WeedSmart Forum in Wagga Wagga where John presented the latest survey results, 170 growers, agronomists and researchers compared notes and experiences about a whole raft of weed management tools, some largely untested but showing promise in the field. Harvest weed seed control options were of great interest, as were the demonstrated benefits of double-break cropping, utilising livestock in mixed farming enterprises and ways to extend the effective use of herbicides. The ‘Big 6’ WeedSmart tactics known to drive down weed numbers and drastically reduce the impact of herbicide resistance on Australian farms are: Rotate crops and pastures, Mix and rotate herbicides, Increase crop competition, Use the double-knock, Stop weed seed set and Implement harvest weed seed control. Herbicide resistance can’t be beaten with herbicides alone — that’s the take-home message from these surveys. However, even in paddocks where herbicide resistant weeds are almost as common as susceptible ones, this doesn’t mean ‘game over’, as many Western Australian farmers have shown. It does mean that farming systems need more diversity and weed management programs need to include a range of tactics aimed at reducing weed numbers at every opportunity during the year.
Setting up harvesters to capture weed seed in the chaff
Getting weed seeds into the chaff fraction Separating the chaff (including the weed seeds) from the straw is a great way to retain more crop residue at harvest. There are several harvest weed seed control (HWSC) systems that manage just the chaff, including chaff decks, chaff carts and chaff lining. All these systems rely on the weed seed entering the front of the header and then being captured in the chaff stream. Chaff lining – new, cheap, simple [but not fully tested] Chaff lining is a new harvest weed seed control method that has sparked incredible interest from growers throughout Australia. Similar in concept to the chaff deck system (called chaff tramlining), chaff lining places the chaff fraction directly behind the harvester rather than on the CTF tram tracks. It is a grassroots grower solution to HWSC that is cheap to try and seems to be a very effective tool. Essentially weed seeds are collected at harvest and dropped in a narrow line behind the harvester. There is no burning required and all the straw is spread behind the harvester so there is very little loss of ground cover. Getting started involves the construction of a simple chute that is then fitted to the harvester. The chaff lining chute can be used in all crops and does not affect harvester operation. Like all harvest weed seed control methods, the harvester must be set up and operated, correctly to ensure the maximum number of weed seeds enter the front of the header and are contained within the chaff fraction. Below are a number of videos and farmer case studies showing how to implement chaff lining. This method of HWSC is well suited to controlled traffic farming but it can also work in non-CTF systems provided the harvester runs in the same lines for a few years in a row. Grower case studies Mic and Marnie Fels have developed a farming system where herbicides are used to back-up their cultural practices, rather than the other way around. Mic Fels has used a modified version of chaff tramlining as their harvest weed seed control strategy. The idea is that the chaff component is funneled into a narrow strip in the middle of the CTF runs behind the header. In a controlled traffic system this means that the weed seeds collected through the header are concentrated into the same zone every year and any seeds that germinate through the mulch are subject to the full force of crop competition. Similar to the experience of growers using a chaff deck to channel the chaff into CTF permanent wheeltracks, Mic finds that the chaff and the weed seeds simply rot away and there is no need to burn the chaff to gain the benefits of this weed control measure. Daniel (left) and David Fox are pleased with the chaff lining chute they have introduced as a harvest weed seed control method on their Marrar farm near Wagga Wagga. Marrar farmer Daniel Fox is chasing higher yields across his 2100 ha cropping program while also driving down weed seed numbers. For a few years Daniel has been adding components to his system to conserve moisture and keep herbicide resistant weeds at bay. Having used narrow windrow burning for a few years and seeing the benefit of capturing seed from late germinated weeds at harvest, the Foxes have now built a chaff lining chute for the header and are delivering the chaff component, including weed seeds, into a 250 mm chaff line in the middle of the 12 m CTF lap. This maintains most of the crop residue evenly across the paddock and avoids the need for burning. “Having the weed seed concentrated in a narrow band reduces the amount of seed that germinates and also reduces the chance of weed seed being buried and ‘stored’ underground at planting now that we are using a disc seeder,” says Daniel. Setting up harvesters to capture the weed seed in the chaff https://www.weedsmart.org.au/resources/hwsc/
Spray out low yield potential crops now or crop top later?
WEEDSMART DRY SEASON SPECIAL As dry conditions continue to impact on crops in many regions of Australia, affected growers will be faced with difficult decisions that will have long lasting effects on the weed seed bank. The lack of crop competition in drought-affected crops leaves the door wide open for more weeds to germinate and set seed, adding to weed control costs in future years. North Tenindewa, WA grower Glenn Thomas is considering the best option for this low yielding canola crop to manage future wild radish weed pressure. Faced with a similar situation in 2001, and again in 2006, North Tenindewa, WA grower Glenn Thomas made the hard decision to sacrifice his low yield potential lupin crops to escape the consequences of an inevitable blowout in annual ryegrass and brome grass. “In 2001 we had a paddock with a very poor lupin crop and about 140 ryegrass plants per square metre,” he said. “In that August we decided to spray the crop out using a glyphosate mix, leaving the best part of the paddock for comparison, and in the hope of late rains that might save the crop. Although we lost the value of a 250 kg/ha lupin crop, we measured a 700 kg/ha yield increase in the 2002 wheat crop as a result of fewer weeds, more nitrogen and more soil moisture. This was particularly pleasing given that 2002 was also a drought year in this district.” The un-sprayed section was still managed with selective herbicides to control weeds in-crop, however this section of the paddock stayed weedy for six or seven years, while the sprayed out area had no weeds, clearly illustrating how much Glenn had saved in chemical and operating costs in the area he had sprayed out. When the same scenario played out in the 2006 season Glenn again decided to spray out the low yielding lupin crop in the same paddock to avoid a build-up of weeds. This time he sprayed out the whole paddock, being more confident in the long-term benefit of this decision. This photo from 2016 shows how clean Glenn’s sprayed out paddock has remained, 10 years after the last drought-affected crop was sprayed out. In both 2001 and 2006 the drought conditions did not improve as the year progressed so Glenn had no reason to regret his decisions to forego the crop and spend the $25–30 per ha to spray out the failed crops and reset the weed seedbank. In the end, Glenn’s decision has created a legacy of very clean paddocks and he has taken a diverse approach to his weed management program including narrow windrow burning, chemical fallow and brown manuring in weedy paddocks. With annual ryegrass now well under control Glenn is more concerned about managing wild radish. Glenn’s experience was also demonstrated in a 4-year trial at Mingenew conducted by AHRI communication lead, Peter Newman while working for DAFWA. “In this trial we sprayed out a weedy lupin crop and compared this to harvesting the lupin crop and returning ryegrass seeds to the seed bank,” he said. “The following year 163 ryegrass/m2 germinated in the area where lupins were sprayed out compared to 1433 ryegrass/m2 germinating in the area where lupins were harvested.” This trial ran for 4 years and the difference in ryegrass seed bank was evident for the duration of the trial. “Although a full spray out now might provide the best outcome for weed control, it is still an expense. While there is no cash flow into the business, there is also no further expense with harvesting a low yielding crop where grain losses can also be high, particularly in short pulse crops,” he said. “Another option is to crop top in October, which will still reduce seed set while preserving some cash flow from this year’s crop. Implementing a harvest weed seed control tactic such as chaff lining or using a chaff cart will further reduce the risk of a future weed blowout from a low rainfall year.” Other advantages of crop topping over an earlier spray out is that paraquat is a lower cost herbicide application and there will be more ground cover left to protect the soil surface from the wind over summer. “Although crop topping these paddocks in October will not necessarily achieve as good a result as spraying the paddock out in August, it is a lot better than simply harvesting the crop and returning weed seeds to the seed bank,” said Peter. As the current season unfolds Glenn is once again looking at weed numbers in low yielding crops. He reckons there will be a percentage of his lupin crop that will be sprayed out along with a small area of canola where wild radish numbers are of concern. “This year we probably won’t actually spray crops out until later in the season when we have grown as much biomass as possible,” he says. “In 2001 and 2006 the season started well but then deteriorated so the weeds grew with the crop and needed to be sprayed in August when it was clear the crops could not compete with the weeds. This year, the dry start has kept weed numbers relatively low and we are only just seeing a response to the couple of double-digit rainfall events from the last week or two.” Glenn is now watching the weeds respond to the rain and will monitor them carefully so he can prevent seed set while still maximising biomass and nitrogen production in the lupins. He is also watching his wheat crops that have already come to head and the weeds are growing but it’s too late for in-crop herbicide applications. “This is a real dilemma for us and others in the district,” he says. “There might be a case for a pre-harvest herbicide application in these wheat crops to target late germinating weeds but there is a lot to consider before making this decision.”
Zero row spacing could take crop competition to a new level
Grain grower Leigh Bryan doesn’t shy away from an unconventional idea. His crop-guided shielded sprayer is working a treat, he is confident with chaff lining and now he is testing the possible advantages of zero row spacing. His aim is weed-free paddocks with crops growing at their full potential across the 2400 ha of cultivation within 30 km of Swan Hill in Victoria’s Mallee region. “Being crop-guided, the shielded sprayer provides precise control over weeds and crop volunteers,” he says. “It makes it possible to suppress medic and other hard-to-kill weeds in lentil crops and take self-sown wheat plants out of barley paddocks. Keeping crops as clean as possible.” Shielded sprayer used to apply paraquat for control of medic in lentils. Using the shields strategically over the past seven years, Leigh has drastically reduced weed numbers and can concentrate on preventing seed set in just the odd plant here or there. The shields allow Leigh to use more cost-effective pre and post emergence herbicide options to prevent seed set of grass and broadleaf weeds in cereal crops. “The shielded sprayer also makes it possible for me to crop top at the optimum time to have maximum impact on weed seed set without damaging the crop, regardless of the crop maturity stage,” he says. The shielded sprayer has played a significant role in managing herbicide resistant brome grass and annual ryegrass. Leigh’s crop-guided shielded sprayer in action, controlling brome grass in a barley crop. As a harvest weed seed control tactic, narrow windrow burning was a good ‘get out of gaol’ tactic but Leigh has been chaff lining now for three years. He says depositing the chaff-only fraction in a narrow line between the harvester wheeltracks is a very effective way to keep weed numbers low. “Dropping the chaff line between the wheeltracks, rather than into the wheeltracks, minimises disturbance of the seeds, reduces weed seed contact with soil and promotes more rotting down over summer,” says Leigh. “The chaff lining chute is also cheaper to make and install than a chaff deck.” Last year Leigh used the chaff lining chute in all crops except canola and found there was no problem pulling the tines through at planting this year. “We first tested the idea in clean paddocks hoping to keep them clean,” he says. “We are now completely confident in the system and will be using the chaff lining chute in all crops across the farm this season. It has proven particularly valuable in lentil crops.” If weeds become a problem in the chaff line Leigh is prepared to band spray the chaff line with pre-emergent herbicide, but this has not been necessary to date. For the last three years Leigh has also been trialling zero row spacing in a bid to capture optimal yield from his crops. The planter has been fitted with splash plates that spread 35 per cent of the seed at random while the remaining 65 per cent of the seed is sown conventionally, down the tube. Leigh uses splash plates fitted to his planter to spread 35% of the seed randomly across the soil surface to achieve ‘zero row’ spacing. Close-up on the splash plate. “This will only work in relatively clean paddocks,” he says. “With no trifluralin applied the in-crop weed control is reliant on strong crop competition and very, very low weed seed bank. Next year I will be looking at different pre-emergent herbicide options and how they might fit in this system.” The seeding operation is faster than normal and this helps throw sufficient soil to enable successful germination and establishment for the seeds spread on the surface. In Leigh’s on-farm side-by-side trials, the zero row spacing strips are showing that the idea has promise. “There are others playing around with the idea of zero row spacing too and I think it has potential,” he says. “In 2016, a strip-trial in barley resulted in the zero row spacing strip yielding 4.994 t/ha compared to 4.889 t/ha in the conventionally sown crop at 375 mm spacing.” This result gave Leigh the confidence to sow one-third of the farm using zero row spacing this season. To achieve the best results Leigh sows considerably more seed but says this is cheaper than pre-emergent herbicide and results in potentially higher yielding crops. The results of this large scale trial will be very interesting in terms of both weed management and crop yield. Leigh takes a weed-point view in zero row spacing (left) and 375 mm spacing (right). “The random placement of stubble is easier to sow through the next year. It still provides trellising for pulse crops and shades the soil to conserve moisture and reduce soil surface temperatures,” he says. “There can be a 10-day delay in germination between seeds sown on the surface compared to those sown into the soil because the surface seed needs rain to germinate.” When it comes to growing a competitive crop Leigh emphasises the need to get the crop planted on time, keep it healthy and well-fed. Leigh’s continuous, no-till cropping system includes wheat, barley, canola, lentil, field pea and lupin brown manure. When planning the crop rotation for paddocks he ranks them according to weed burden and uses a 4-year break crop system to clean up paddocks with high levels of grass weeds. “In a weedy paddock we start with a pea/lupin brown manure, then plant canola, in year 3 we plant a cereal and use the shielded sprayer in-crop then finish with a lentil crop,” he says. “This gives us a 4-year run at grass weed control and then we can return to ‘normal’ rotation with a low weed seed bank.” Once weed numbers are low, Leigh says it is relatively easy to keep them low through the consistent use of both herbicide and cultural practices. “When weed numbers are low it is possible to look at ways to improve yields and reduce production costs,” he says. “That’s what I hope to achieve with zero row space planting, supported by crop rotation and chaff lining to manage survivors. If there is evidence that weed numbers are increasing, then we have other tools at the ready to stop a weed blow-out.”
Controlling glyphosate-resistant grass in irrigated cotton
When glyphosate-resistant summer grass starts to cause yield losses in cotton, growers obviously need to add some non-glyphosate options to their system to protect yields and prevent further weed blowouts. The questions then become where to add non-glyphosate tactics to get most benefit, and how many are needed? Developer David Thornby used the Barnyard Grass Understanding and Management tool, BYGUM to investigate three key questions relevant to irrigated cotton systems. Scenario #1 – Ask BYGUM: the value of glyphosate What’s the remaining value of glyphosate in rotations with glyphosate-resistant awnless barnyard grass? BYGUM developer David Thornby has developed a series of scenarios to demonstrate this new decision support tool. Since the first confirmation of glyphosate resistance in awnless barnyard grass, many other resistant populations have been found, and these populations don’t all display the same level of resistance. While glyphosate is no longer effective as a stand-alone control measure against any of them, some populations are less strongly resistant than others. In the case of the first-confirmed population, field rates still had around a 40 percent efficacy on small seedlings. For other more recently confirmed populations, efficacy even on small seedlings is much lower. Given that glyphosate is going to be applied to these populations anyway, it’s important not to overstate the usefulness of glyphosate by hoping to be able to rely on it for some level of control. BYGUM can test the difference for us, between populations with strong resistance and those with moderately strong resistance. David said he used a simple irrigated rotation, with modest use of non-glyphosate options in an otherwise glyphosate dominated system. He varied the effectiveness of glyphosate from around 40 percent (‘moderate’ resistance) to around five percent (‘strong’ resistance). Key outcomes The results show two key things. First, both systems are still making money after five years. High levels of crop competition keep seed production per escaping weed low, and the addition of a few effective tactics reduces the number of surviving plants to moderate/know levels of between six and 14 plants per square metre at the end of the fifth season. Secondly, however, both systems are heading towards failure. Weed and seedbank numbers are increasing, however slowly. And while gross margins are the same at the end of season one, there is a predicted difference of around $500/ha between the gross returns in season five. David says there are three lessons here. First, strong crop competitive effects might mask the seriousness of resistance issues in irrigated cotton, should they be present. Second, there are good reasons to determine just how strongly resistant your resistant awnless barnyard grass population is, if you’re going to be sticking with a system that is predominately about the use of glyphosate. And third, allowing a slow decline with somewhat-inadequate weed management looks likely to have a substantial cost as the years pass by. We’ve made many assumptions in this example – in particular, that irrigated cotton is planted and provided with resources to allow for strong competitiveness against the weed. We’ve also made assumptions about crop and herbicide costs, average yields and prices. “You could run BYGUM with a different set of assumptions that fit your experiences, and see if the results change. Scenario #2 – Testing the value of a cover crop Can summer cover crops be used to get on top of weed populations? In a one-in-one-out rotation of dryland cotton, summer fallows offer a chance to get on top of weed populations through vigorous use of non-crop herbicides. However, with no crop competition present, they can also offer weed escapes an opportunity to set a lot of seed, especially when the key herbicide in both crop and fallow, glyphosate, is no longer effective. “Cover crops allow growers to maintain some competition even in non-crop seasons,” David Thornby says. “A good stand of millet (as simulated in BYGUM), sprayed out before seed set, allows for a combination of late season herbicide use to clean up survivors and mid-season competition with glyphosate- resistant barnyard grass, reducing seed set per plant.” The first scenario is a basic one-in- one-out rotation. This contains the assumption that the barnyard grass population is resistant to glyphosate, and that an early season residual and mid-season inter-row cultivation are used to provide some control in crop: summer fallows use two cultivations and a double knock. In scenario 2 David replaced the second summer fallow with a cover crop. The cover crop includes cultivation, a double knock, spray out (assuming this is with a non-glyphosate product effective on glyphosate, such as paraquat), and a late application of paraquat over the now-dead millet. “The cover crop is more expensive than the summer fallow, and actual plant numbers per square metre are not reduced all that much (1.1 to 0.8 per square metre),” David says. “But there is a substantial difference in seeds returned to the seed bank.” The comparison scenario shows a substantial increase in the yield from the final cotton crop, due to the strong reduction in seed bank numbers at the end of the cover crop season. “The benefits of the cover crop come due, as expected, in the following crop, where the seed bank has been driven down and emerging weed numbers are low,” David said. “Over the course of the whole rotation, incorporating one cover crop every second summer fallow is predicted to be worth almost $200/ha in increased yield. “There is more than one way to protect future yields in a dryland rotation, but using cover crop competition certainly seems to bear looking at. “We’ve made many assumptions in this example – in particular, that planting time and summer rainfall are conducive to good cover crop growth, resulting in high competition, that the barnyard grass population is strongly resistant to glyphosate, and that the cover crop doesn’t reduce moisture availability to the following cotton crop. “We’ve also made assumptions about crop and herbicide costs, average yields and prices. “Once again, users could run BYGUM with a different set of assumptions that fit their experiences, and see if the results change.” Scenario #3 – Residual answers to resistance Can you control a glyphosate-resistant grass by adding a residual in irrigated cotton? In situations where a glyphosate-resistant summer grass starts to cause yield losses in cotton, growers obviously need to add some non-glyphosate options to their system to protect yields and prevent further weed blowouts. The questions then become where to add non-glyphosate tactics, to get most benefit, and how many are needed? David Thornby used BYGUM to investigate this question. First he compared a system with glyphosate-resistant barnyard grass where only glyphosate is used with the same system with an early-season (pre- or at-planting) residual added. Residuals tripled gross margins The yield results of adding a single residual are striking. The glyphosate-only system is still producing some yields (Figure 2 – primarily due to the competitiveness of irrigated cotton), but end-of-season weed numbers are very high and the potential of the system is being seriously under-utilised. “Adding a single residual can reduce early-season weed numbers dramatically, and because this is when most of the competition effects occur, this has a huge benefit for the bottom line,” David said. Figure 1: Irrigated cotton rotation with an early-season residual. While we’ve used ‘a residual’ in the notation here, a rotation of suitable products from different modes of action should be used in the field. The glyphosate-alone system is the same as this one, without the early season residual. Figure 2: A comparison of gross margin and barnyard grass numbers at end-of-season between glyphosate alone (top) and glyphosate plus a single, early-season residual. “Gross margins are almost tripled compared to glyphosate alone when the weed population is strongly glyphosate resistant. “However, end-of-season weed numbers (and seedbank density) are still unacceptably high, so a single residual doesn’t appear to be enough of an addition, despite the dramatic effect. “A single year of poor control from the residual (rather than the average of around 85 percent efficacy) would certainly result in a blowout.” Adding a mid- or late-season tactic provides some insurance against weed blowouts and seed production. Because late weed germinants in vigorous cotton stands don’t produce a lot of seed per plant, the effects on yield aren’t so dramatic. However reducing surviving plants and especially reducing the seed bank size are critically important insurance against future blowouts and selection of resistance for other modes of action. David tried two different tactics in BYGUM, adding either a layby residual to each crop or a mid-season knockdown. Adding a layby reduces the seed bank somewhat, and cuts surviving plants at end-season down to around 25 per square metre. This still appears to be too many survivors for comfort, but it does represent a substantial improvement over an early- season residual alone, and offers insurance against future blowouts. BYGUM predicts that it can be sustained at least for the five-year rotation. This comes at a cost, however: the reductions in late-season weed numbers are offset by the price of the extra residual and BYGUM includes a penalty due to phytotoxicity. In comparison with the layby system, a system with a mid-season knockdown, rotating between options including Group A herbicides, shielded paraquat and inter-row tillage improves the gross margin (due to a combination of taking out some weed competition and having some options with lower phytotoxicity-related yield penalties), but leaves more end-of-season survivors. “So this is not an ideal system either – but is certainly an improvement in all ways over a single-residual system,” David said. “These analyses show that while a single early-season residual can do a lot of heavy lifting in terms of reducing weed competitiveness, it’s not enough on its own for long-term sustainability. Late or mid-season tactics provide some insurance.” “BYGUM predicts that while good returns can be sustained at least for five years with this ‘plus two’ strategy, more non-glyphosate tactics would be needed to drive the seed bank to very low levels. We’ve made many assumptions in this example—in particular, that irrigated cotton is planted and provided with resources to allow for strong competitiveness against the weed, that resistance to glyphosate is quite strong, and that good efficacy is generally the case for residual applications. Pre-simulation weed numbers are assumed to be moderate and we’ve also made assumptions about crop and herbicide costs, average yields and prices.” Case study reproduced courtesy of CRDC, following publication in CRDC’s Spotlight magazine, Winter 2016. To access BYGUM, visit: www.cottoninfo.com.au/barnyard-grass-understanding-and-management-bygum.
Paired rows take off on Twitter
Recently we started a hashtag on Twitter to share information about paired row sowing. We used the hashtag #Pairedrow. You can still use it now to engage in the discussion and tag WeedSmart. Thank you to everyone who has already contributed to the discussion about paired row sowing on Twitter. Here is a summary of the pros and cons. You’ve got to love the names of some of the brands out there! Below are the names of some of the paired row sowing options that growers had tried. Rootboot, Stiletto boot, Agmaster twin seeker, Bourgault, Ausplow, Conservapac, Morris C2 boot, Seed master, Burando Hill.
Testing for weed susceptibility to herbicide pays off
Sam and Emily Eagle run 2500 merino ewes on their 3000 ha mixed farm near Horsham, Victoria. They say the livestock and cropping activities complement each other, keeping their pastures and crops performing at their best. Herbicide resistant annual ryegrass is their main weed challenge with one test revealing resistance to glyphosate (65 per cent) and clethodim (80 per cent), and full susceptibility to chlorsulfuron (Group B, Glean). Sam and Emily Eagle run a mixed farming enterprise near Horsham, Victoria where grazing and cropping are mutually beneficial for weed management. “We test annual ryegrass from two or three paddocks each year to monitor any changes in susceptibility to the herbicides we use,” said Sam. “Knowing which herbicides are effective makes it easier to plan our herbicide use without relying solely on the products that still work. Every year we have at least one tactic in place specifically to reduce the weed seed bank.” Knowing that the tested weeds were susceptible to Glean gave Sam an opportunity to regain control of a potential blow-out situation, using a herbicide that is much cheaper than alternatives that he might have chosen if he had to make the decision without the herbicide resistance test results. The Eagle’s agronomist usually collects the seed for testing and the results are considered to be representative of the whole paddock, each one being around 35 to 70 ha. “We can fairly safely assume that all our weeds have some level of resistance so we concentrate on managing survivors, mostly treating with a double-knock whenever possible,” he said. “Annual ryegrass is our main problem weed however we are keeping a close eye on brome grass that is present on one of our lease blocks.” Triazine resistance on one block precludes the use of TT canola so the Eagles grow conventional canola on this block, aiming for the most competitive, highest yielding crop possible. Along fence lines Sam uses a 2-year program where he slashes in spring in one year and then sprays a knockdown + residual herbicide mix the next year. “When we slash, we know that the weeds will still set seed. We keep the slasher low to the ground to ensure any seed heads present at harvest are below header height so they won’t get spread,” said Sam. The pasture paddocks are de-stocked over summer with the sheep grazing on the stubble. If the stubble runs out early the sheep are returned to the containment area where they are fed screenings, hay and grain until the pastures are ready. The ewes return to the pastures to lamb in autumn. Sam and Emily use narrow windrow burning in the canola as their harvest weed seed control tool. They have had trouble using this tactic in cereals, where the fires often don’t burn right to the ground, leaving weed seed concentrated in bands. On the other hand, the canola burns well, destroying the weed seed, and Sam is able to safely burn several paddocks on the one day. Grazing the canola narrow windrows has not caused any problems with burning or with weed seed being spread. Narrow windrow burning in canola has worked very well for Sam, driving down herbicide resistant ryegrass numbers. “Canola actually gives us a few opportunities to control late germinating weeds,” said Sam. “Firstly with an over the top spray to desiccate the crop, secondly windrowing the crop early and third, using the narrow windrow chute at harvest in preparation for narrow windrow burning in autumn. We also spray top wheat and barley, with the sheep providing the second knock for any survivor weeds.” Growing faba beans, canola, wheat and malt barley enables them to use a different pre-emergent herbicide each year of the rotation. At the end of this 4-year program any paddocks that are carrying a weed burden are thoroughly cleaned using a pasture and a 3-year hay program. Moby forage barley sown with clover gives a nutrient boost to the perennial ryegrass pasture phase, which may last up to ten years. “We supply hay for export and generally grow two oat crops and one vetch,” said Sam. “Any failed crops or additional production is stored as silage in underground pits to drought-proof our breeding flock. Silage is a particularly good way to clean up weeds because we spray out when the crop is actively growing and not under any moisture or heat stress, then cut in early September.” At the end of a 4-year cropping program any paddocks that are carrying a weed burden are thoroughly cleaned using a pasture and a 3-year hay program. The Eagles supply hay for export and generally grow two oat crops and one vetch. The 2017 seeding represents the beginning of the Eagles’ fully aligned controlled traffic farming (CTF) system. The transition to 12 m wide CTF has taken several years but Sam and Emily are convinced that the efficiencies gained will be well worth the investment. They sow all crops on 300 mm row spacing and aim to achieve the most competitive crops possible. Although Sam knows 380 mm row spacing would make some management operations easier, they pick up extra yield and suppress weeds with the narrower spacing. In the seven years that Sam and Emily have been managing the farms they have seen the benefits of the rotation in keeping weed numbers low. “All of our worst paddocks have now had the ‘rotation treatment’ and we have avoided weed blow-outs,” said Sam. “Two wet years in a row could potentially challenge our weed management but having the sheep in the system gives us more options while still earning income from each paddock.” Related links 10 Point Plan – Test for resistance to establish a clear picture of paddock-by-paddock farm status Plant Science Consulting herbicide resistance testing CSU Herbicide resistance testing
Ask an Expert
Does ambient temperature affect herbicide performance?
with Chris Preston, Associate Professor, Weed Management at The University of Adelaide Temperature affects the absorption, translocation and metabolic degradation of herbicides applied to plants. Herbicides applied under the wrong conditions can appear to fail, however the reason may not be herbicide resistance. Dr Chris Preston, Associate Professor, Weed Management at The University of Adelaide says most herbicides have a temperature range at which they are most effective in controlling target weeds. “Applying herbicides outside the optimal temperature range is likely to contribute to a spray failure, even in susceptible populations,” he says. “Alternatively, applying herbicides within the correct temperature range can improve the control in populations known to have a level of resistance to that herbicide.” Dr Chris Preston suggests testing whole plants rather than seed for responses to a range of post-emergent herbicides. The Quick-Test is conducted in the same growing season as herbicide will be applied so the testing will occur under similar conditions to field conditions. Dr Preston says the effect of frost on the efficacy of clethodim is a striking example. Spraying clethodim in non-frosty conditions achieves vastly better results than spraying after three days of frost, even on populations that are resistant to this chemical mode of action. “Combining the optimal temperature with optimal weed size will give the best results possible,” he says. “The current common practice of applying clethodim to tillered ryegrass in the coldest months is not making the best use of this herbicide.” As a general rule of thumb, Group A (fops), paraquat (Group L) and glyphosate (Group M) are more effective at lower temperatures while Group A (dims), atrazine (Group C) and glufosinate (Group N) are more effective at higher temperatures. However, weeds that are resistant to paraquat become less resistant in warmer temperatures. “The other implication of this research is the effect of ambient temperature on herbicide test results,” says Dr Preston. “Seed collected in winter and grown out in the glasshouse in summer will be tested for resistance in conditions that are not representative of field conditions when growers are next treating that weed species. The Quick-Test using whole plants overcomes this problem and improves the reliability of herbicide susceptibility testing.” How can I get the best performance out of clethodim? Short answer: Avoid applying clethodim during frosty periods. Longer answer: Twice as much clethodim is required to kill susceptible annual ryegrass if the product is applied after three days of frost. Even higher rates are required if the plants have resistance to clethodim. Planning to apply clethodim for grass control outside the coldest months of June and July, and avoiding night spraying in winter, will see better results in both resistant and susceptible populations, particularly in tillered plants. Clethodim is most active when temperatures are over 20 degrees C. Weed seed that is tested during summer may return false negative results, which could translate into spray failure in the field the next season. Twice as much clethodim is required to kill susceptible annual ryegrass if the product is applied after three days of frost. Even higher rates are required if the plants have resistance to clethodim. When it is it too hot for glyphosate? Short answer: Efficacy is much better at 20 degrees C than at 30 degrees C. Longer answer: Spraying glyphosate resistant barnyard grass at lower temperatures is more effective than under hotter conditions. If barnyard grass is tested for herbicide resistance during the cooler parts of the year it may appear susceptible to the field rate of glyphosate but then when this rate is applied to the population in summer there may be many survivors. When glyphosate is taken up rapidly it tends to limit its own translocation, which can mean that although symptoms may appear more rapidly in warmer temperatures, plant kill is less reliable. Which herbicide resistance test should I use? Short answer: The weed resistance Quick-Test for post-emergent herbicides. Longer answer: The Quick-Test involves testing whole plants rather than seed for responses to a range of herbicides and rates. The Quick-Test is conducted in the same growing season as herbicide will be applied so the testing will occur under similar conditions to field conditions. The results of the Quick-Test are available within the same season, potentially giving growers an opportunity to apply an effective weed control tactic before the end of the season. The Quick-Test is not available for many pre-emergent herbicides. The Quick-Test is available through Plant Science Consulting and results are normally available after four weeks. Relevant links Maximising clethodim performance and the impact of frost fact sheet Keeping clethodim working in broafleaf crops Plant Science Consulting herbicide resistance testing – Quick-Test GRDC Update Paper – New developments and understanding in resistance mechanisms and management