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Glyphosate has been the go-to product for keeping weeds in these areas under control for a long time but unfortunately it is often the only product used and the weeds are commonly quite large when they are sprayed. The result is that glyphosate resistance can, and does, quietly build up in these zones in a wide variety of weed species. Fencelines will always be a potential source of weed seed but there are ways to ensure that the seed from these areas is not already resistant to the herbicides when it blows into the production areas. Farmanco agronomist, Brent Pritchard, collected the suspect capeweed samples on a farm near Borden in Western Australia. The capeweed had evolved resistance to glyphosate in an un-cropped drainage area, where it had routinely been sprayed with glyphosate, and had then invaded the adjacent field. The cropped area had been managed with a diverse rotation of wheat, TT canola, pasture and fallow over a 17-year period. The capeweed samples also showed signs of resistance to metosulam (Eclipse®) and diflufenican (Brodal®), but were susceptible to a range of other herbicides including clopyralid, MCPA, bromoxynil, diuron, metribuzin, simazine, Spray.Seed® and Velocity®. Dr Yaseen Khalil, a researcher in the agronomy team at the Australian Herbicide Resistance Initiative (AHRI), conducted the resistance screening and confirmed the resistance status of the capeweed population. AHRI’s Dr Yaseen Khalil confirmed the resistance status of the capeweed samples and is urging growers to take a more diverse approach to weed management in non-cropped areas around the farm “There is no doubt that an integrated approach to weed management needs to be applied to non-production areas such as fencelines, around buildings, along tracks and roads and around irrigation infrastructure,” says Dr Khalil. “Probably the first step is to stop using glyphosate alone in these areas unless you are able to reliably apply a double knock to every application. Evolving resistance to this useful herbicide in non-productive zones is counter productive at the least.” Wherever possible, apply glyphosate in a mix with other herbicides effective on the target weeds, then follow with a second knock. The main problem on fencelines is the lack of competition to weeds. If pastures are part of the crop rotation it may be possible to establish the pasture species along the fenceline and leave them in place when the paddock returns to the cropping phase. Similarly, the crop can often be sown right up to the fence and the first round or two mown or baled for hay prior to harvest. If there are livestock in the production system they can be used to graze the perimeter in the fallow or in suitable crops. Mowing or baling the perimeter of the crop can halt the incursion of weeds into the crop area. Establishing cover using desirable perennial species and eliminating fenceline spraying could be a long-term solution to stop fencelines being a source of herbicide resistant weeds. If this is not practical, or if the non-crop area must be kept bare for other reasons, such as managing insect pests, close attention must be paid to using alternative herbicides, double knocking, mixing and rotating herbicides and eliminating survivors. Applying the WeedSmart Big 6 tactics to non-crop areas is a pre-emptive strike on ‘home-grown’ herbicide resistance. More resources: AHRI Insight – World-first: glyphosate resistant capeweed Management of herbicide resistant weeds on fencelines Don’t jeopardise glyphosate for clean fencelines

You can listen to the article being read above! We all know that old saying – ‘one year seeding, seven years weeding’ or some variant of it, and know it is true. But it is easy to overlook just how important weed seedbank management is, until herbicide resistance begins to reduce the efficacy of previously reliable tools. For a few decades herbicides really took the focus away from seedbank management because the chemical options were so effective at killing weeds that they appeared to be a complete solution to weed management. But all along, growers, agronomists and researchers have known it was too good to last. The WeedSmart Big 6 strategy has struck a chord because it is a useful check list that can be used to prompt growers to consider using a selection of the many available weed control tools. No one tool will do the job – just as herbicides alone have failed, so too will harvest weed seed control or crop competition if they are not part of a planned and multi-pronged assault on the weed seedbank. This is the underlying principle for integrated weed management. In economic simulations conducted using the RIM and WeedRisk models in 2006, agricultural economists Randall Jones and Marta Monjardino showed that although many things impact on the economic assessment of weed management practices, there is strong evidence that when seasonal risk is taken into account, and the economic assessment is for a period of 20 years, integrated weed management consistently out-performs herbicide-only systems, regardless of the weed in question. Herbicides provide high level control and are considered an essential component of broadacre cropping systems, however, other tactics that specifically target weeds that have escaped herbicide control are what make IWM systems more profitable in the long-run (see Table 1). For weeds like wild radish, which produce large quantities of seed that can remain viable in the soil for many years, taking a non-integrated approach of using post-emergent herbicide only has the potential to ‘crash the system’, from an economic point of view. It will always be a numbers game and IWM consistently wins, usually by a considerable margin, primarily due to lower weed seedbank numbers and conservation of the highly effective herbicide resource for tactical use over time in integrated weed management systems. TABLE 1 The economic impact ($/ha) of different crop and IWM systems on meana annualised discounted returns for wild oats, wild radish and annual ryegrass in a southern New South Wales cropping system (4-year crop phase followed by 3-year perennial pasture phase).   Economic return ($/ha)a   Wild oats Wild radish Annual ryegrass Continuous cropping       No IWM 268 (± 35) -9 (± 27) 284 (± 34) IWM 332 (± 38) 315 (± 37) 335 (± 38) Crop + pasture rotation No IWM 288 (± 29) 157 (± 25) 284 (± 28) IWM 319 (± 32) 300 (± 30) 320 (± 31) a The shown in brackets following ± are the standard deviation. Source: Jones R, Monjardino M and Asaduzzaman Md (contributors) (2019). Section 1: Economic Benefits of Integrated Weed Management, in: A.L. Preston (Ed) 2019. Integrated weed management in Australian cropping systems. Grains Research and Development Corporation. Use the WeedSmart Big 6 to prepare an IWM plan for your farm To develop an integrated weed management plan (IWM), it is useful to collate some historical information about past weed control activities, test weeds for herbicide resistance and use the WeedSmart Big 6 to match opportunities and weeds with suitable and effective control tactics, remembering that there are many weed control tools at your disposal. With your agronomist’s assistance, aim to create a plan that maps out when each tactic will be applied. Ideally, try to include three or more of the Big 6 tactics in each crop, fallow or pasture phase. Diversity is key. Some people prefer to have a set cropping sequence while others choose the crops in response to seasonal or market conditions, but either way it is important to look for ways to add as much diversity to your farming system as possible and to keep downward pressure on weed numbers at every opportunity. While preventing weed seed production completely is unrealistic in the real world, a focus on the weed seedbank will pay dividends in the long run.

You can listen to this article being read above! WeedSmart extension agronomist, Peter Newman is urging growers to think twice before mixing glyphosate with 2,4-D when treating weeds with low to medium levels of resistance to glyphosate. Antagonism between the two products has been widely known for some time and in most circumstances careful product choice and a slight rate adjustment is all that is required to compensate for the compromised performance of glyphosate. New research from the Australian Herbicide Resistance Initiative (AHRI) has shown that high label rates of glyphosate can often control grass weeds with low level glyphosate resistance; but adding 2,4-D amine or ester to the glyphosate can result in these weeds surviving the spray. Once low to medium glyphosate resistance has established in a population of awnless barnyard grass, using a tank mix of glyphosate and 2,4-D is not going to work. With investment from the Grains Research and Development Corporation and others, AHRI researcher Jingbo Li and others studied two populations of awnless barnyard grass with relatively low-level glyphosate resistance and found this phenomenon is due to the 2,4-D dramatically affecting uptake and translocation of glyphosate from the tank mix. “For awnless barnyard grass a susceptible population was 100 per cent controlled using 0.5 L/ha glyphosate 540 while 11 per cent survived when the same rate of glyphosate was mixed with 1 L/ha of 2,4-D amine 700,” says Peter. “For the low-level resistant population, a higher rate of 1 L/ha of glyphosate was required to achieve 100 per cent control but when this rate of glyphosate was mixed with the 1 L/ha of 2,4-D amine, 90 per cent of the weeds survived. A similar result was found using 2,4-D ester.” Survival of awnless barnyard grass seedlings with low level resistance to glyphosate. Left: Zero survival from 1 L/ha Glyphosate 540 application. Right: 85 per cent survival to 1 L/ha Glyphosate 540 + 1.03 L/ha 2,4-D Ester 680 mixture. In another, more resistant, population of awnless barnyard grass the same scenario played out, albeit with an even higher rate of 3 L/ha of glyphosate to achieve 100 per cent control. In this population the survival rate was 77 per cent for the tank mix. “What this means for growers is that once glyphosate resistance has established in a grass weed population, using this particular tank mix is not going to work,” says Peter. “A grower with glyphosate resistant grass weeds would be better served by applying the higher rate of glyphosate on its own, or perhaps with a different mixing partner, to achieve maximum control. It is then necessary to look at building in additional tactics to keep weed numbers low into the future with less reliance on glyphosate.” While not examined in this study, 2,4-D antagonism of glyphosate is reported on several other species including Johnson grass, wheat, barley and wild oats. 2,4-D is also reported as antagonistic of Group A herbicides on species such as wild oats and annual ryegrass. Although mixing these two herbicides can provide a valuable multi-shot control of both grass and broadleaf weeds, the pros and cons need to be carefully evaluated. “The other thing to remember is that the maximum level of control when using glyphosate is achieved when the best formulation is applied to young weeds at higher label rates,” says Peter. “These factors are generally within the grower’s control, even if they cannot control the weather conditions or plant stress levels, which also impact on glyphosate efficacy.” To keep glyphosate as a viable option into the future Peter also recommends applying a double knock tactic after each application of glyphosate. He says following glyphosate with paraquat has been an effective double knock for many years but there are other options to consider, including strategic tillage and alternative herbicides. Other resources AHRI Insight: 2,4-D antagonises glyphosate, especially in glyphosate resistant weeds

Listen to the article being read above! The common practice of planting sorghum on wide rows has made this crop notorious as a weak link crop that can allow key summer grass species to set bucket loads of seed. The take home message from four years of research at Narrabri, NSW and Hermitage, Qld, is that halving sorghum row spacing can halve weed seed production in both feathertop Rhodes grass and awnless barnyard grass. With investment from the GRDC, researchers from the University of Sydney and Queensland Department of Agriculture and Fisheries (DAF) have conducted a range of field trials to identify ways to increase the competitiveness of sorghum and summer pulses. Dr Michael Widderick, DAF principal research scientist says the findings from these field trials have shown that a change to narrower row spacing for sorghum greatly suppressed weed growth and seed production, without reducing crop yield. “This is very significant for sorghum growers who have struggled with controlling these grass weeds in wide-row configurations,” he said. “These weeds are difficult to control with herbicides and there are few chemical options available to growers to control grasses in a grass crop. Any non-chemical strategies that reduce seedbank replenishment are very valuable to growers.” Dr Widderick said sorghum is often grown on one metre row spacing with an expectation that the crop will have access to more soil moisture. A considerable downside to planting on the wider row configuration is that canopy closure does not occur, allowing weeds to proliferate in the inter-row. With soil moisture at a premium, there is nothing spare to waste on growing summer weeds. For a sorghum crop to be competitive against weeds it requires adequate stored soil moisture (or access to irrigation) to establish the crop and achieve canopy closure as quickly as possible. This is most reliably done at a row spacing of 50 cm and this trial demonstrated that row spacing did not significantly impact crop yield within a season.   Left: weeds growing uninhibited in the inter-row space of sorghum sown at 1 m row spacing. Right: Fewer weeds can establish when the canopy closes in sorghum sown at 50 cm row spacing. Two of the most difficult to control summer weeds, feathertop Rhodes grass (FTR) and awnless barnyard grass (ABG) can produce 40,000 and 42,000, seeds per plant respectively. Other studies have found these numbers could be even higher, so every effort to reduce seed production is worthwhile. Both these species have populations confirmed as resistant to glyphosate, and recently a population of FTR was confirmed to be resistant to haloxyfop (Group A). Including a poorly competitive sorghum crop in the crop rotation provides a weak link in any strategy to reduce the weed seedbank for these weeds, and potentially allows a blow-out in herbicide resistant biotypes, making future control in other crops or summer fallows very difficult. Dr Widderick said sorghum competitiveness across all seasons and both sites was increased with narrow row spacing (50 cm) and a plant density of 10 to 15 plants/m2. In the 2017/18 season at Hermitage, the researchers demonstrated that planting sorghum at a density of 10 to 15 plants/m2 reduced seed production of both weed species reduced by over 50 per cent compared to the seed production at the low crop density of 5 plants/m2. In the same season, cultivar choice, sorghum density (5, 10, 15 plants/m2) and row spacing (50, 75 and 100 cm) had no statistically significant effect on crop yield. Armed with this information, the 2018/19 sorghum trial at Hermitage was sown at a crop density of 10 plants/m2, and the effect of row spacing (50 cm and 100 cm) on weed production was measured. Biomass and seed production of ABG was reduced by 55 per cent and 65 per cent, respectively when the sorghum was sown at the narrower spacing. Similarly for FTR, the 50 cm row spacing reduced biomass and seed production by 48 per cent and 56 per cent, respectively. Graphs: Awnless barnyard grass (ABG) (left) and feathertop Rhodes grass (right) biomass and seed production as affected by sorghum row spacing at Hermitage, Qld 2018/19. Within each graph, different letters indicate significant (P<0.05) difference after pairwise comparison. Crop competition is a ‘free kick’ non-herbicide tactic in the WeedSmart Big 6 strategy to manage herbicide resistance in weeds. There is now solid evidence that growers can maintain crop yield and reduce summer grass seed production by planting sorghum crops at a density of 10 plants/m2 and a row spacing of 50 cm. There are some residual herbicide options for the control of these summer grass weeds in sorghum. However, their efficacy can differ greatly depending on the season and will rarely provide full control of FTR and BYG. A combination of residual herbicides and a competitive crop is likely to have an additive effect and reduce seed production on surviving weeds. This research project also demonstrated that mungbeans are more competitive on 50 cm row spacing, making any changes to seeding equipment worthwhile as it would suit the whole summer crop program in the northern region. Central Queensland sorghum grower experience Organic grain producers Paul and Cherry Murphy have always relied on crop competition as an integral component of their weed management program in all crops, including sorghum, at ‘Kevricia’, near Capella in Central Queensland. With many years of experience growing sorghum on 50 cm row spacing Paul says the suppressive ability of the closed canopy certainly reduces weed growth and seed set in-crop. Paul Murphy, Central Queensland, has been sowing sorghum on 50 cm row spacing for many years to suppress weeds in their organic farming system. “We have been working off a plant density of around six plants per metre square as a rule of thumb that seems to work in most seasons on our farm,” he says. “In seasons where soil moisture might be limiting we have seen higher density crops fall over, and so have leant towards the lower planting rate. But the 10 plants per metre square would certainly increase the competitiveness of the crop in seasons where there is sufficient moisture.” Paul is pleased to see researchers doing more work on row spacing and plant density, which is difficult to really tease out in commercial settings where there are too many potential variables. “In sorghum there is a complexity associated with plant density, tillering and row spacing that needs scientific trials like this to really determine the optimal combination for maximum yield and weed control in a variety of seasonal scenarios,” he says. This season Paul will be breaking with tradition and planting sorghum on wider row spacing as he now has a Garford camera-guided inter-row cultivator. He hopes the wider spacing will only be required for this season while he makes the adjustments required to have the machine suit their controlled traffic configuration. The Murphy’s inter-row cultivator is capable of working in crops planted on 50 cm row spacing once it has been adjusted to suit their CTF configuration. “Once we are ready to plant the winter crop I hope to be able to plant on 50 cm spacing again and still use the inter-row cultivator,” he says. “The cameras on the cultivator guide the alignment of the tynes to follow the plant row with a 1 cm accuracy, and can be used when the crop is 10 to 40 cm high.” As organic growers the Murphys don’t use any herbicides and so early weed control can be difficult, but this inter-row cultivator will help remove any weeds that emerge with the crop and then crop competition can suppress any later germinations. Other resources GRDC Update paper: Growing competitive sorghum and mungbean crops to suppress summer weeds  Creating stiff competition against summer weeds  Managing barnyard grass in summer crops and fallow 

With investment from GRDC, researchers led by Professor Bhagirath Chauhan at the University of Queensland, have shown that both windmill grass and feathertop Rhodes grass can greatly reduce yield in mungbean, yet both weed species retain a large portion of their seed when the mungbean crop is ready for harvest. This gives growers the opportunity to use several tactics to reduce the seedbank of these two species while growing mungbean. Professor Chauhan says that even at the most competitive row spacing of 50 cm, mungbean yield was halved when there were around 40 windmill grass plants/m2 or just 11 feathertop Rhodes grass plants/m2 growing in the crop. Feathertop Rhodes grass competes strongly and produces masses of seed if it gains a foothold in a mungbean crop. “The good news is that both species have a high level of seed retention at harvest because mungbean is such a quick growing crop,” he said. “This gives growers the chance to vastly reduce the amount of new seed entering the seedbank.” “Even though these weeds have high seed retention at harvest they also produce a huge quantity of seed,” he says. “At peak weed density in our field trials feathertop Rhodes grass produced over a quarter of a million seeds per metre square and windmill grass produced around 100,000 seeds per metre square. So, even if a small portion of this seed enters the seedbank it can still equate to a large number of seeds to potentially germinate the following spring.” Feathertop Rhodes grass is known to begin germinating in late winter and early spring, well before a mungbean crop is planted so every effort should be made to eliminate all flushes of this weed prior to planting mungbean. Haloxyfop is currently registered for fallow control of feathertop Rhodes grass ahead of mungbean production and can be used to reduce the weed burden prior to planting mungbeans in the most competitive configuration of 50 cm row spacing. To reduce the risk of Group A resistance, use a double knock in this pre-plant situation to control any Group A herbicide survivors of these difficult grass weeds. Paraquat is the usual chemical double-knock partner in these situations and should be applied to small, unstressed weeds within 7 to 10 days after the application of haloxyfop. Both these weed species can germinate close to the same time as the mungbean crop, so early weed control is essential to maximise yield and minimise early weed competition. Although these two grass species are susceptible to several pre-emergent herbicides, only flumioxazin (Valor) is registered for use in mungbean. This Group G herbicide can be applied at least two months pre-sowing to provide enhanced knockdown and residual control of feathertop Rhodes grass in mungbeans, taking care to follow the ‘critical comments’ to avoid crop injury. Extra emphasis should be put on ensuring the paddock is as clean as possible prior to planting mungbeans. Inter-row cultivation may be an option provided the young plants are not injured, as wounds can allow entry of diseases such as tan spot or halo blight. Clethodim applied before the mungbeans begin to flower will provide effective in-crop control of small, late germinating grass weeds. Mungbean crops are commonly desiccated prior to harvest using either Reglone or glyphosate. Both of these Chloris weed species are generally unaffected by these herbicides as mature plants, so the desiccation of the crop is unlikely to stop weed seed set. Mechanical options such as swathing are currently under investigation and may provide a more reliable way to stop seed set on these weeds prior to harvest. Professor Bhagirath Chauhan, University of Queensland, says windmill grass and feathertop Rhodes grass both retain a large portion of their seed at the time of mungbean harvest, making harvest weed seed control an practical option to help reduce the weed seedbank. “Mungbean is a good candidate for harvest weed seed control, using chaff lining, impact mills and the like, because the crop is harvested at ground level so any weed seed held on the plants should enter the harvester front,” says Professor Chauhan. The WeedSmart Big 6 approach to help manage resistant and hard to control weeds combines the power of multiple tactics throughout the year and across a full crop sequence to reduce weed seed set. Although feathertop Rhodes grass and windmill grass both produce vast quantities of seed, the seed is very short-lived. If left on the soil surface the seed remains viable for only one to two years. All efforts to prevent seed set will be rewarded with a rapid decline in the weed seedbank for these two difficult grasses. GRDC has recently updated the ‘Integrated weed management of feathertop Rhodes grass’ manual, which provides detailed information on the ecology of this important weed, along with the tactics and strategies that can be used throughout a cropping sequence to manage the seedbank. Other resources Giving summer legumes the competitive edge FTR grass demands attention to stop seed set  Creating stiff competition for summer weeds GRDC manual: Integrated weed control for feathertop Rhodes grass 2020 update

You can listen to the article being read above! Whether they are resistant to herbicide or not, weeds generally do not compete well with vigourous crops, but in the fallow they can rapidly take advantage of the lack of competition for resources. Department of Agriculture and Fisheries researcher, Dr Annie Ruttledge has been running experiments at Kingaroy to investigate the benefits of bringing crop competition into the fallow phase of cropping systems in southern Queensland. Dr Annie Ruttledge, Department of Agriculture and Fisheries, Queensland weeds researcher is conducting trials to identify cover crop species suited to southern Queensland that have weed-suppressive traits. With investment from GRDC the cover crop project, led by Charles Sturt University, is investigating the weed-suppressive power of various cover crop species suited to either summer or winter fallows at three locations in the northern grain growing region – Kingaroy, Narrabri and Wagga Wagga. At the Kingaroy site, both winter and summer-growing cover crops were shown to suppress weeds by over 85 per cent and up to 95 per cent, compared to an untreated fallow where the sown weeds were not inhibited by a cover crop. While this level of control is worthwhile on its own, it is also backed up with either a chemical or non-chemical tool to terminate the cover crop and kill any survivor weeds. “In winter in Kingaroy, the best cover crops for weed control were grazing oats and tillage radish,” says Annie. “These species provided early season ground cover and suppressed our mimic annual grass weed, Italian ryegrass, by up to 94 per cent relative to the weeds-only fallow. None of the cover crop species we tried were able to suppress the quick-growing mimic broadleaf weed, Oriental mustard.” Winter-growing cover crop monocultures and mixtures. In summer, Annie says the best cover crop options for Kingaroy were white French millet, Japanese millet, forage sorghum and buckwheat. Again, early-season biomass and ground cover was the key to suppression of both grass and broadleaf weed mimics by up to 95 per cent when compared to the weeds-only fallow. Summer-growing cover crop monocultures and mixtures. So far in this trial, there has been no measurable weed suppression benefit in sowing mixed species cover crops rather than monocultures. However, a mixed species cover crop may be preferred if a grower is wanting to achieve multiple outcomes. For example, grazing oats may be selected as a fast growing and highly competitive species and teamed with a less competitive legume to boost soil nitrogen stores. “Obviously, the species selected will depend on the growing region and soil type,” she says. “Cover crops also provide many other services to the farming system and so the grower could select a cover crop species, or mix of species, that would also provide a break from disease or insect pressure, increase moisture infiltration, build up organic matter or break down compaction.” Source: Charles Sturt University Cover crops are an extension of the WeedSmart Big 6 tactic of providing crop competition to suppress weed growth and reduce the weed seed bank in an integrated weed management program. Annie says that light interception is a critical driving force in the effectiveness of cover cropping for weed control. In selecting cover crops for weed suppression, choose species that grow well in your locality and that restrict light penetration to the soil through strong early growth and the development of a dense canopy. For greatest benefit, terminate cover crops at maximum biomass, which should coincide with the beginning of flowering; however, earlier termination may be required if soil moisture is limiting. There is a large body of research work now underway to investigate other aspects of incorporating cover cropping into farming systems in various regions. While this work focuses on weed suppression, other researchers are looking into soil water and nutrient use efficiency under different conditions and in various cropping systems. Other resources Summer cover crops video DAF Day family case study Cotton cover crops Cover crops research update video presentation GRDC Update paper – Cover crops to provide groundcover in dry seasons