Double knock to preserve glyphosate

The sequential application of two separate herbicide treatments has become the most common ‘double knock’ approach used in weed management. Unfortunately, these approaches have added cost, complexity and scheduling issues to weed management programs but have been required for two main reasons: 1. To control herbicide-resistant weed populations, that may have been selected by prolonged use of a similar mode of action chemistry; and 2. Control of weed species or stages that are unsuccessfully controlled with single herbicide applications. Paraquat has been the key active ingredient used in the second knock situation and can provide effective management of a wide range of grass and broadleaf weeds. However, it is clear we require other options to use in this management window to: 1. Avoid the more rapid selection of paraquat resistance; and 2. Provide options that may improve weed control in situations where paraquat efficacy is not adequate. Since winter 2016, NGA has been screening a range of herbicides, to identify options that have the potential for this usage pattern. The two key broadleaf weeds being targeted are common sowthistle (Sonchus oleraceus) and flaxleaf fleabane (Conyza bonariensis). DOWNLOAD FACTSHEET

Lance and Fallon Wise and Lance’s parents, Murray and Janette crop 1600 ha and run cattle on 45 ha of non-arable rocky ridges. The locality and soft black plains enable the Wises to grow a range of crops including sorghum, mungbean, chickpea, barley and wheat, along with the occasional crop of faba bean, millet and canola. In an effort to reduce their reliance on glyphosate and overcome some of the disadvantages of long-term no-till farming, the Wises have reintroduced light cultivation and are moving toward more targeted application of glyphosate and other herbicide products using an optical sprayer. “Our usual rotation is a legume followed by a cereal, such as mungbean, to sorghum and chickpea to wheat or barley,” says Lance. “We use either a short fallow or double crop to change from a winter to summer crop program and aim for at least one crop every 12 months from each paddock.” Having been no-till farming for 25 years the Wises have seen the weeds transition to those species that proliferate in the absence of cultivation; weeds like fleabane, urochloa, feather top Rhodes grass, cow vine and bladder ketmia. In an effort to reduce their reliance on glyphosate and overcome some of the disadvantages of long-term no-till farming, the Wises have reintroduced light cultivation and are moving toward more targeted application of glyphosate and other herbicide products using an optical sprayer. Nine years ago they had success using the Kelly disc chain to manage urochloa on their less well-structured red earth soils. More recently they have used this implement to target herbicide tolerant feathertop Rhodes grass and fleabane on their main black clay soils. “We are using a chemical / tillage double knock to good effect on feathertop Rhodes grass in the fallow,” says Lance. “We apply glyphosate and then follow up five days later with the Kelly chain, which does a terrific job of pulling out sick, dead and small plants at an operation speed of 11 to 12.5 km/hr.” “The same system works well on fleabane too, pulling up plants with foot-long roots from soft soil, although a higher rate of glyphosate is required. It doesn’t work well for weeds like prairie grass that have deep, fibrous root systems.” The Wises have found the chemical / tillage double knock to work well on feathertop Rhodes grass and fleabane. Used in reverse order, Lance has found the light cultivation is an effective way to stimulate germination after a poor sorghum crop to sprout volunteers and then spray out the crop. Along with the benefits of partial stubble incorporation on soil microbial activity and ease of sowing, Lance says the two short chains in the middle fill in the 3 m wheel track to even out the paddock, avoiding the need to do extra wheeltrack renovation operations. About the Kelly chain “We also use the Kelly chain to incorporate pre-emergent herbicide after sowing to a maximum depth of 2 cm. This does not disturb seed, which is all sown at least 4 cm deep with a presswheel, and the soil is moved sideways without destroying the cracks in the soil or drying out the profile,” he says. “Weed seed is not buried deeply so it doesn’t come back to haunt you years later.” Lance avoids using the Kelly chain more than once in a season on the red soils, which can get very dusty and are prone to hardsetting on the surface. At the end of harvest Lance and Murray assess the stubble load and weed pressure in each paddock. They usually spray glyphosate after a rain event and either double knock with the Kelly chain or spray paraquat through their Weedseeker optical sprayer. After using the Kelly chain, Lance follows 30–45 days later with the Weedseeker rig to clean up any survivors. “The Weedseeker is a new fallow option for us and means that we can treat weeds that we might otherwise ignore, apply higher rates, and use more expensive products to control small areas or patches of weeds,” says Lance. “The 36.6 m boom carries 96 sensors so there are not many weeds that go undetected in the fallow.” At the end of last year, the Wises started sub-soil ripping to a depth of 35 to 40 cm on 75 cm spacing to increase water capture and break up the sub-soil compaction to improve crop growth. Six weeks ahead of planting they apply the final glyphosate spray and then add fertiliser, which they incorporate with the Kelly chain, with the added benefit of removing any weeds present. Liquid fertiliser applied at seeding promotes early seedling vigour and growth, which gives the crops a competitive advantage over weeds. They plant using a Tobin planter that achieves a good even strike in stubble, starting on the red soils as soon after rain as possible, then moving onto the black soils. Pre-emergent herbicide is applied after sowing legume crops and incorporated using the Kelly chain. Herbicide is applied in crop as well as for desiccation purposes in sorghum, mungbeans and chickpea. Being a spray contractor, Lance has also invested in an air boom on his sprayer that enables him to have much greater control of droplet size to match the environmental conditions, while also covering a larger area in a day. He says the elliptical cone delivers spray in both a forward and rear motion to achieve better coverage, even at lower water volumes. The controls in the cab allow the operator to adjust the spray quality from fine to extra course without changing nozzles on the boom and there is no need to have all the different nozzles to suit different conditions and products. The Wises operate a 12 m controlled traffic system and plant all their crops on 375 mm row spacings. Lance has increased the planting rate in sorghum from plants 40 cm apart in the row to 25–30 cm apart to quickly to shade the interspace and suppress weed growth.

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) TFB01 >75 100 75 TFB02 75 75 75 TFb-Suscept 0 0 0 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). Other resources Resistance risk to knock-down herbicides on irrigated cotton farms Protecting knock-down herbicide options What are the herbicide options for the summer fallow

Orange Park is an 8200 ha corporate dryland cropping operation with eight main blocks, all within a 30 km radius of Lockhart, NSW where John introduced the use of the double break from cereals 10 years ago.   To get the best result possible from OP canola, ‘Orange Park’ manager John Stevenson (left) has their seed professionally graded and only plants seed sized over 2 mm diameter. Karl Grocke (right) has joined the team at Orange Park on their graduate program, making the most of the opportunity to learn from great operators like John. “Our rotation is driven by herbicide resistant weed management,” he says. “A double break, such as hay/canola, pulse/canola, fallow/canola, is implemented once in a 7-year rotation. The rotation is flexible but we do what we can to avoid growing three cereal crops in a row because the result is inevitably a weed blow-out.” The main weeds on Orange Park are annual ryegrass in-crop and fleabane in summer, which has been increasing since 2005, particularly in wet years. Wild oats is a lesser but persistent weed. “We have low level glyphosate resistance and varying levels of Group A resistance to contend with,” says John. “We don’t use much Group B chemistry either, except occasionally in imi-tolerant crops, partly because of decontamination issues with canola, but mainly because of poor efficacy.” The wettest September on record in 2016 saw about 10% of the crop across the Orange Park operation inundated – compromising their weed control and nitrogen management. “Essentially we lost a year of weed control with ryegrass blow outs due to poor crop competition and not being able to do timely herbicide applications,” says John. “We also saw a shift in the weed spectrum with more carryover of weeds that thrive in wet conditions, such as toad rush and lesser loosestrife.” Crop rotation and double break cropping The rotation that is working well on Orange Park since 2008 consists of two cereals followed by canola, then another two cereals and finally a double break before returning to cereals. John incorporates as much diversity within the system as possible to maximise returns and keep pressure on weed numbers. Canola, wheat, barley, pulses (including faba bean, lentils and vetch hay), oaten hay for export, and strategic fallowing to conserve moisture, all feature in the list of options. The diversity in crops enables the rotation of herbicide groups, including pre-emergent herbicides ahead of each crop. Imi-tolerant crops such as Hurricane lentils, IT-canola, possibly barley in the future add to the mix although John often grows hybrid IT-canola conventionally simply for its inherent yield advantage. “The price of lentils is attractive however logistics are difficult, as grain needs to get to Horsham, over 6 hours away,” he says. “Realistically, canola and feed grain for poultry and feedlots are our mainstay crops.” To maximise the competitiveness of canola crops John grows some hybrid crops and also grades OP canola to 2 mm diameter as a cost-effective way to improve crop establishment and early vigour that also suppresses early weed growth. John uses TT canola when weeds become a problem issue and Clearfield canola in low weed population paddocks where he can also apply atrazine on volunteer faba beans. This year John planted 650 ha trial of lentils in a block that would normally have been fallowed. “There was good residual soil moisture from last year’s wet winter and we tried a late planting system to reduce the density of the lentil crop to keep air flowing through canopy, hopefully reducing the incidence of disease,” he says. “Weed control in the Hurricanes XT lentils has been exceptional although the crop has demonstrated the need for better pH amelioration at depth.” “Including lentils will spread the workload and we can target a different weed germination cohort,” he says. “Having a different group of herbicides available will also help add diversity to our control program.” Row spacing, CTF and stubble management John manages Orange Park as eight 1000 ha management units, where a unit may consist of several blocks in close proximity. He is looking at opportunities to implement east–west sowing in suitable blocks to maximise yield and reduce weed pressure. While on a Nuffield Scholarship trip to New Zealand, John saw crops grown on 125 mm (5 inch) row spacing where farmers were able to include ryegrass as a crop in their rotation, without concern over future weed problems. “Historically, our seeding has been on a 300 mm row spacing but we are in the process of investing in a full disc seeding system, which will allow us to narrow the row spacing to 175 mm (7 inches),” says John. The disc seeder will enable John to retain stubble from their 1.7 t/ha pulses, 3.4 t/ha wheat and 4 t/ha barley crops within their 18 m system with 3 m CTF tramlines. Harvest weed seed control John and his team have been narrow windrow burning for three years. Having tried this harvest weed seed control tactic in canola, wheat and barley crops they have found the safest and best results are achieved in canola. “Realistically, narrow windrow burning is not very compatible with the council’s fire regulations so we are looking at alternatives,” says John. “Cereal stubble burns for 4 or 5 hours, and whirlwinds can easily shift burning chaff 300 m into a neighbour’s stubble.” A downside to narrow windrow burning is that more moisture is stored under the narrow windrows, which can cause uneven crop germination the following season. John has been successfully using narrow windrow burning in canola for several years. Twelve days after sowing, this Lancer wheat crop shows the variability in germination where the rows under the narrow windrow have access to more moisture. John is looking into other options for harvest weed seed control and will probably phase out narrow windrow burning over the next few years. Chaff deck systems suit CTF and although John thinks they have merit, there are limitations on the options available when using contract harvesters. Chaff lining seems easier to implement and is likely to feature in the near future. “Narrow rows help push weed seed heads to the top of the canopy,” says John. “Two plants per m2 is too many, especially if the weed is able to adopt a prostrate habit and evade collection at harvest. The big challenge is getting inexperienced header drivers to harvest low, even though we pay the contractor a premium to have this happen.” John uses spray topping in canola, and sometimes wheat and barley, along with hay production to take out late germinating weeds. Fallow management Over summer, John uses a contractor with an optical (camera) sprayer to treat survivors with paraquat following a broadacre spray of glyphosate. He says the optical sprayer is also a valuable tool to help manage cud weed, which is becoming more prevalent. There is no routine cultivation in the system however John does incorporate lime with a speed tiller before planting and a full tillage using a flexicoil is done when there is levelling work required. “When we change over to the disc seeder we will probably need to cultivate to remove the ridges left by the tined system,” he says. “The disc seeders can’t handle too much chaff on the ground either, so burning is still an option for weed control, or when slugs and mice are a problem.” Soil fertility and weeds Across the Orange Park operation John has acidic (pH 4.5) red soil ridges and some sodic vertosols to contend with. He is using variable rate technology (VRT) to apply lime to the red ridges using rates ranging from 1 to 3 t/ha to lift the pH to 5.5. “We saw an immediate response of lower ryegrass population in low pH blocks after lime application due to stronger crop competition,” he says. “Variable rate application and soil testing has been very cost effective for us and has helped reduce weed numbers and improves herbicide efficacy. Fixing soil pH gives the best return on investment through improved nutrient availability, which supports higher yield and better weed control.” John is also using soil sensing and VRT to address constraints in nitrogen and phosphorus. The red soil ridges on ‘Orange Park’ are acidic (pH 4.5) so John is using variable rate lime applications to lift the average pH in these soils to 5.5. In doing so he has observed an immediate reduction in ryegrass numbers due to the improved crop competitiveness.