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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. “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

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

The three things that the spray operator can and must control are nozzle choice, boom height and ground speed of the spray rig. Recently, there has been plenty of attention on some of the new technologies involving weed detection and artificial intelligence (AI), and companies like Goldacres are keen to deliver these to growers as they come to market, but their sales and marketing operations manager, Stephen Richards says the technologies behind effective and reliable droplet delivery to the target remain central to their spray rig designs. Goldacres sales and marketing operations manager, Stephen Richards says the technologies behind effective and reliable droplet delivery to the target are central to their spray rig designs. “At the end of the day, if the droplets of product don’t hit the target at the required rate you might as well have left the spray rig in the shed,” he says. “The best way to ensure the correct dose is applied and avoid spray drift is to pay close attention to setting the rig up correctly and operating it well.” In the last 20 years there has been a quiet revolution in nozzle design and much of this has been driven by the need to eliminate the risk of spray drifting downwind or being caught up in temperature inversion layers. “Years ago the standard nozzle was the XRT-jet flat fan nozzle that operated at a pressure of 1 to 4 bar, which gave good coverage in ideal spray conditions, but also produced more fine droplets that easily drift,” says Stephen. “Modern nozzles have been designed with the emphasis on producing medium to coarse droplets and using higher water rates to achieve adequate coverage.” The modern nozzles also have a wider pressure range of 1 to 6, or 1 to 8 bar, making the one nozzle type suitable for a variety of applications. When considering ground speed, Stephen says the technologies behind even rate delivery through the boom have made it possible for machinery manufacturers to build sprayers that can operate at higher speeds and cover more area in a day. “The Goldacres self-propelled sprayers have had a 3-tier nozzle system for about 20 years, where the first set of small nozzles come on when the machine is operating at 5 to 10 km/hr then the second and third sets activate when the machine is operating at higher speeds,” he says. This ensures that the correct product rate is applied at the headlands and wherever the operator needs to slow down. Another option is the ‘pulse width modulation’ system to adjust the volume through the nozzles in response to changes in ground speed. “Pulsing is particularly good for turn compensation with a large boom, where nozzles near the outside tip are typically moving twice as fast as nozzles near the machine,” says Stephen. “This means product would be under-applied at the tip and over-applied near the centre. Consistent under-dosing of herbicide is a particular risk in the evolution of herbicide resistance.” As boom length increases so does the need for high tech suspension and rate compensation for variable speed and turning. The boom height is also critical in reducing drift risk associated with the air turbulence behind the spray rig. A 20 cm change in height from the recommended 50 cm above ground to 70 cm can quadruple the quantity of air-borne droplets. “With booms now as wide as 48 m the suspension system is more important than ever,” says Stephen. “Goldacres machines use a system that minimises yaw, roll and pitch of the boom to give a stable spray platform and optimise spray coverage in undulating or uneven paddocks.” Before heading out to spray Stephen recommends operators check for blocked nozzles and at the start of each season, do a jug test to check for nozzle wear. The large investment in spray technology can be undone if nozzle choice and maintenance is neglected. “The jug test needs to show that each nozzle is delivering within 10 per cent of the nominated volume per minute for the specific nozzle type and size,” he says. “The cost of a new set of nozzles pales in significance against the cost of product wastage, a spray failure or the evolution of herbicide resistance on your farm.” Before heading out to spray Stephen recommends operators check for blocked nozzles and at the start of each season, do a jug test to check for nozzle wear. The WeedSmart Big 6 tactics that form an integrated weed management program to reduce the risk of herbicide resistance in weeds are supported by companies like Goldacres, who understand the importance of effective and safe herbicide application. Goldacres is working with Bilberry to perfect the artificial intelligence systems required to bring green-on-green weed detection to Australian farmers. These systems, along with the optical spraying technology that has been used for spot-spraying in fallows for over 20 years, are expected to deliver more targeted herbicide use into the future.

This has been the key message of weed management experts in Australia ever since 1996, when Australia’s worst weed, annual ryegrass, was found to be resistant to our most useful herbicide, glyphosate.   A few years later, the Australian Glyphosate Sustainability Working Group (AGSWG) was set up under the CRC for Australian Weed Management (Weeds CRC) to bring together commercial and research expertise from around the country with a determination to ‘keep glyphosate resistance rare’. With investment from the Grains Research and Development Corporation, AGSWG established a database of confirmed cases of glyphosate resistance in Australia and developed information products for all users of this important weed control tool. After guiding growers and agronomists through a critical 15 years of managing glyphosate resistance in Australia the AGSWG has been disbanded, however the work of advising farmers and other weed managers will continue. Keep weed numbers low and do everything you can to prevent resistant weeds from setting seed. Australian Herbicide Resistance Initiative (AHRI) director, Professor Hugh Beckie, says glyphosate means so much more than weed control to Australian farmers, particularly for dryland cropping. “This herbicide has been the means of achieving incredible productivity increases in dryland crop production, initially providing an alternative to tillage for fallow weed control and thus conserving soil moisture over summer,” he said. “It is also now used as a broad spectrum knockdown pre-seeding and post-harvest in many crops and in RoundUp Ready cotton and canola.” “As predicted, the incidence of glyphosate resistance is ramping up, having been heavily relied on for weed control since its introduction to Australia in 1976,” said Prof Beckie. “It is important to understand that glyphosate is not only used extensively on farms but also along roadways, fence lines, railway lines, in public parks and in home gardens. This means that resistance can, and does, evolve in many different settings and can move across the landscape in weed seeds and pollen.” According to the International Herbicide-Resistant Weed Database there are currently 20 species and thousands of populations known to have evolved resistance to glyphosate in Australia.    While this is a serious situation, and glyphosate resistance can no longer be considered ‘rare’, it is still possible to regain control of weed populations that have evolved resistance. One of the useful products that AGSWG published was a series of factsheets outlining the practices that should be followed and those that should be avoided. These factsheets have recently been updated and published on the WeedSmart website. There is a factsheet for each of the main glyphosate user groups – grain producers, cotton growers, horticulturalists, orchardists and vinegrowers, irrigators and managers of public lands and utilities. While the principles remain the same for all industries, there are some practical variations in implementation. Using a diverse weed control program and taking care to apply glyphosate in the optimal way can tip the scales in the grower’s favour and keep this valuable product as an option well into the future. Download glyphosate factsheets Northern grains and cotton factsheet Winter grains and irrigation factsheet Orchards and vineyards factsheet Roadside and railways factsheet Vegetable production factsheet

GRDC Grower Relations Manager – North, Richard Holzknecht, says while it is important that growers control fallow weeds early to maximise efficacy, it’s equally important spraying is only undertaken when weather conditions are right. Spray equipment also needs to be set up and operated appropriately. “While spraying at night and in the morning is not restricted, product labels state that chemicals should not be applied when hazardous inversions are present,” Mr Holzknecht said. He warned off-target damage could occur from physical drift and inversion drift, which posed a significant risk during summer spraying as day/night fluctuations in temperature often result in inversions forming overnight and or early in the morning. “Wind speed, in particular, should be monitored at least every 15 to 20 minutes and if the wind drops, spraying should stop,” he said. The main factors influencing drift potential were weather conditions at the time of spraying and how spray machinery was operated in terms of spray quality, speed and boom height. Photo GRDC “So, planning and being proactive is extra important. Growers need to talk with their neighbours to determine the location of any sensitive crops, such as cotton, and ensure they understand label recommendations and permit regulations, particularly those governing the use of 2,4-D.” Mr Holzknecht said the main factors influencing drift potential were weather conditions at the time of spraying and how spray machinery was operated in terms of spray quality, speed and boom height. In an inversion, chemical droplets can remain suspended in concentrated form and be carried significant distances. “It is important growers understand the weather conditions that indicate an inversion is present and avoid spraying during these times. “Surface temperature inversions are often associated with calm, low wind conditions, dust remaining suspended, fog or mist forming in low areas and sounds travelling long distances. All these signs indicate the risk of inversion drift is significantly high.” Mr Holzknecht advised growers and spray contractors to closely monitor weather conditions. The Grains Research and Development Corporation (GRDC) is actively investing in spray application research and training to assist industry in implementing best-practice spray systems, and it recently released a new video explaining the key factors affecting spray drift. Source article: Spray safely to reduce drift risk this summer Spray drift in-depth resources GRDC Spray drift hub Stop the drift webinar Are you going spraying, or killing weeds? How do you manage summer weeds without spraying at night? Spray wisely and well webinar Effect of formulation and environment on dicamba volatility webinar Spray well – correct nozzles, adjuvants and water rates

Desiccation and crop topping with pre-harvest herbicide application is a useful way to reduce seed set in late germinating weeds and is an effective harvest aid for cereal grain, pulse and oilseed crops.  Given the scrutiny that glyphosate is currently receiving Paul McIntosh, WeedSmart’s northern extension agronomist, says it may be a good time to start looking for alternative means of reducing weed seed set prior to harvest and avoiding any potential issues with market access. WeedSmart’s northern extension agronomist, Paul McIntosh has been investigating alternatives to glyphosate as a desiccant in mungbeans. “Currently, there are five herbicides registered for late season use in a variety of crops,” he says. “Glyphosate and diquat (or Reglone) are registered for use in wheat and barley in some states, canola, chickpea, lentil, faba bean, field pea, mungbean and soybean. For some of these crops, growers are also able to use paraquat, metsulfuron methyl or saflufenacil (Sharpen).” Although there are many benefits to the practice from a weed control perspective, there are also market forces at play that could curtail the future use of pre-harvest herbicides.    “It might be a good time for growers to re-visit some of the non-herbicide options for reducing seed set,” says Paul. “One possibility is to trial swathing in pulse crops like chickpeas, faba beans and mungbeans. Early commercial scale trials suggest that it could be very effective and could also have the additional benefit of hastening crop maturity, bringing harvest forward.” “In combination with harvest weed seed control, swathing is a valuable WeedSmart Big 6 tactic to manage the weed seed bank,” he says. “Swathing adds another non-herbicide tool to a diverse program, particularly for pulse crops that are often not very competitive, and for weeds that typically shed seed before the crop is ready to direct harvest.” Are there other herbicide options for crop desiccation if the current products are banned? In brief: Not really. In most instances, glyphosate is the most effective crop desiccant product. The details: Glyphosate is already a key component of cropping systems, particularly in no-till systems. In crops like mungbeans that have semi-indeterminant maturity traits that make them want to keep on growing, glyphosate applied at the label rate can give mixed results. The Australian Mungbean Association recently commissioned weeds researcher Dr Bhagirath Chauhan, QAFFI to investigate the efficacy of a range of possible alternatives to glyphosate as a desiccant, but there were no stand-out herbicide candidates. This small plot trial also included the non-herbicide option of swathing, and the results were very promising.   Has anyone trialed swathing commercial mungbean crops? In brief: Yes. A grower on the Darling Downs trialed swathing two mungbean crops in March and April 2020, the first being 0.4 ha within a larger paddock that was desiccated with herbicide, and the second was an 8 ha block. The details: These two trial paddocks were very successful and the grower was encouraged by the yield and grain quality of the swathed areas. This has generated significant interest from other growers and agronomists in the northern grains region. The crops were swathed at the standard 90 per cent physiological maturity, the same timing used for chemical desiccation in mungbeans. Harvest was delayed in the 0.4 ha block due to two falls of rain, 12 mm and then 18 mm, which meant the windrows remained in the paddock for 14 days. The crop produced 1.6 t/ha of reasonable quality grain with no evidence of dust. Picking up the mungbean windrow after a two week delay due to wet weather. The crop in the 8 ha block was shorter and sparser than the small trial block. Four days after this block was windrowed it was harvested with a Smale pea front at the correct moisture, suggesting that low yielding crops with reduced dry matter could be harvested earlier. The crop yielded just below 1 t/ha of excellent quality grain, with very few pods being left on the ground. What are the potential benefits and costs of swathing? In brief: The costs will be very similar to chemical desiccation and there could be extra benefits as the practice is fine-tuned. Swathing and windrowing costs around $35 to $40 per ha, similar to chemical desiccation, but the operation may take more time. The details: The first benefit is the avoidance of pre-harvest chemical application, removing the potential for desiccant chemical residues in the grain. The second big benefit is that it may be possible to bring harvest forward. Even if swathing is done when the crop is 90 per cent physiologically mature, the same as for chemical desiccation, the crop can be harvested within a few days and could be off the paddock nearly two weeks earlier than a desiccated crop. The clincher is the possibility of swathing before the crop reaches 90 per cent maturity. If this can be done without compromising grain size and quality, it could have very significant benefits for weed control. Many weeds in the northern cropping region set seed before traditional desiccation and harvest time and so if the crop can be cut earlier there is a chance that less weed seed will mature. Weed seed heads present in the mungbean windrow. It is early days for the revival of swathing in the northern cropping region and there are many things to be tried and tested. Early successes have also been seen in sorghum, faba beans and chickpeas.

In the course of a chemical fallow there are often several applications of herbicide and some residues may still be present on or near the soil surface when it is time to plant the next crop. In particularly dry years, residues may even carryover from the crop prior to the fallow. NSW Department of Primary Industries soil scientist, Dr Mick Rose, says there has been concern in recent years about the effect these residues may have on soil microbial activity and on the establishment and growth of crops following the fallow, even after the plant back period. Dr Mick Rose, DPI NSW soils researcher, is developing tests and predictive models to support growers in their decisions about crop choice after using residual herbicides. (Photo: GRDC). “Glyphosate has been the most commonly used knockdown herbicide in northern fallows for several decades and more recently growers have been looking to use more diverse programs that include chemicals with residual activity on weeds,” he says. “The increased use of imazapic and diuron have been of most concern to growers when choosing the next crop, particularly after a low rainfall fallow period.” With investment from GRDC, Mick has been working on a project led by Dr Michael Widderick from the Department of Agriculture and Fisheries, Queensland to develop a soil test for imazapic and diuron residues that will indicate damaging residue levels and help growers to decide which crops would be safe to plant in a paddock. “We are determining the threshold levels of residues of these two herbicides at which crop damage is likely for six crops, both winter and summer growing, in a range of soil types,” he says. In earlier work he also looked at the level of glyphosate residue in soils around the country at planting time and the impact these residues have on soil biological processes. “We found that residues of glyphosate were commonly detected in the soil at planting but there was no indication that the herbicide was adversely affecting soil biological activity,” says Mick. “This suggests that the label recommendations are suitable and the proper application of glyphosate in Australia is not posing a threat to soil health.” “For growers to be able to keep using glyphosate they need to implement the WeedSmart Big 6 strategy, including using diverse chemistry in fallows,” he says. “Residual herbicides are a useful tool for growers but there are some gaps in our knowledge about how these herbicides break down in different soils and under different seasonal conditions.” Why not just follow the plant back recommendations on the label? In brief: The label provides the minimum plant back period provided certain environmental conditions are met. There is a possibility of crop injury even though plant back periods are observed. The details: Many factors affect the bioavailability of a herbicide in the soil. For example, even though a clay soil and a sandy soil might have similar residue levels, more herbicide will be available for uptake in the sandy soil. More rain will increase the rate of breakdown, but it is not known exactly how much rain will ensure the specific soil is ‘safe’ to plant into. Another important factor is that many things can contribute to a germination failure. In some situations, residual herbicide may be suspected as the culprit, but can be difficult to either rule it in or out with certainty when diagnosing the reason for a problem at planting. If herbicide residues in the plant tissue can be shown to be phytotoxic, then another, less susceptible crop could be sown into the paddock. Dr Annie Ruttledge, DAF Qld weeds researcher, inspecting chickpea plants growing in soils containing different levels of imazapic and diuron herbicide residue. What effects can herbicide residues have on emerging crops? In brief: The herbicide itself can inhibit germination and growth, or it can exacerbate other factors, such as root disease. The details: At different levels of bioavailability, herbicide residues will have different effects on crop plants. If the herbicide is readily available to the plant, then susceptible crops will take it up from the soil and it can have phytotoxic effects ranging from suppressed vigour to yellowing and potentially plant death. Testing the plant tissue of a struggling crop can show if the leaves contain sufficient herbicide to have caused the observed symptoms. Some herbicide residues in soil can also ‘prune’ plant roots, particularly the fine roots that help access moisture and nutrients. Obviously, if the young plants are struggling to access resources then they will be less vigorous and possibly die. Damaged roots are also more susceptible to water stress, disease and poor nodulation in legumes, making it difficult to determine the initial cause of the problem in the field. If herbicide residues are shown to be the problem then a more tolerant crop can be sown, speeding up the breakdown of the residue and there will be more rainfall events before the next cropping season comes around. Seedling emergence and establishment is being measured for six crops (winter and summer) in the presence of different levels of herbicide. What pre-planting soil tests are being developed to give growers confidence to plant? In brief: The current project is establishing phytotoxicity thresholds for six summer and winter crops in a range of soil types, for two herbicides – imazapic and diuron. The details: By mid-2021 the aim is to have established the thresholds so that soil could be tested pre-plant to determine what crops would be safe to plant. This will give growers confidence to use these herbicides in a diverse strategy to manage weeds like feathertop Rhodes grass in the fallow, while avoiding germination or establishment failures in the following crop. Spray records play an important role in the management of these herbicides and mistakes can easily be made if the spray history for the past several years is not taken into account. In time, growers and their agronomists will gain a better understanding of how these herbicide residues behave in the soils on a particular property and will be able to make herbicide application and crop rotation decisions with more confidence. In another project with the Soil CRC, Mick is developing a predictive model for herbicide breakdown for a wider range of herbicides used in southern and western cropping systems. Until these tests and models become available, the use of an in-field or pot bioassay with a susceptible crop can be helpful in determining potential plant back issues.   Related resources Herbicide residues in soil – the scale and significance (GRDC Update paper) Herbicide residues in soil (GRDC Podcast)