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How can I be certain that herbicide residues in the soil have fully degraded at planting?
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)
Maximising the weed control value of my crop rotation
with Kevin Morthorpe, Trait & Seed Technology Stewardship Manager, Pioneer Seeds A diverse crop rotation is the twine that holds a good farming system together and underpins an effective weed management program. Kevin Morthorpe, Pioneer Seeds’ Trait & Seed Technology Stewardship Manager says herbicide tolerance traits in crop hybrids can be used to maximise competition against weeds and increase the herbicide options available to growers while optimising yield and profitability of the crop sequence in rotations. Kevin Morthorpe (left) – Pioneer Seeds’ Trait & Seed Technology Stewardship Manager, with Dr Ray Cowley – Canola Research Scientist, Corteva Agriscience and Pioneer Seeds’ Rob Wilson – Strategic Customer & Market Development Manager and Clint Rogers – Western Regional Sales Manager & Canola Product Lead at a canola research trial near Jindera in southern NSW. Plant breeders continue to introduce herbicide tolerance traits in a number of crops in Australia, including corn, canola, pulses, cereals, grain sorghum, summer forages and cotton. “For example, in canola there are several herbicide tolerance traits and they are primarily available in hybrids,” he says. “This means growers get both improved crop performance due to hybrid vigour and more flexibility in herbicide use patterns.” The increased vigour of canola hybrids also generates greater biomass production and early canopy closure that suppresses growth and seed set of weeds that germinate in-crop, complementing the use of pre-emergent herbicides. “Hybrids super-charge crop competition through a strong root system and vigorous growth,” Kevin says. “From an economic angle, hybrids optimise yield in both high input and tough environments. In fact, we see more growers selecting hybrids when producing canola in tough conditions.” Since the release of the first herbicide tolerant canola in 1991, the popularity of herbicide tolerance has seen a 98 per cent adoption of canola varieties with tolerance to imidazolinone (Clearfield), triazine (TT) or glyphosate (RR). In the last 15 years, the area sown to hybrid canola has risen to an impressive 47 per cent in Australia. With glyphosate tolerant canola hybrids entering South Australia in 2021 and new hybrid releases, the hybrid percentage will increase further over coming years. With glyphosate tolerant canola hybrids entering South Australia in 2021 and new hybrid releases, the hybrid percentage will increase further over coming years. Kevin says that Pioneer Seeds have seen increasing demand for Clearfield canola in recent years following a dip in popularity. Through strategic application of herbicide tolerant traits in diverse crop rotations it seems that farmers are overcoming the resistance problems that were prevalent with the Clearfield technology and can now re-introduce these varieties and take advantage of the weed control benefits and high yields they offer, and manage herbicide residues in the soil. “A diverse rotation of crops and pastures is one of the WeedSmart Big 6 tactics, which Pioneer Seeds endorses wholeheartedly to protect the longevity and effectiveness of herbicide tolerance traits,” he says. “Through an effective crop rotation you can tick off all the herbicide and non-herbicide tactics needed to drive down weed numbers.” How do I make the most of a hybrid crop? In brief: Employ best practice agronomy. The details: Grain hybrids are vigorous plants that produce increased biomass and grain yield. To do this, they must be supported with adequate crop nutrition. When properly fed, hybrids will provide increased crop competition and achieve greater water use efficiency compared to their conventional counterparts. Growing a hybrid crop with herbicide tolerance traits does not equate to a full weed control program. These crops must be used within the WeedSmart Big 6 framework, within a diverse crop rotation and using herbicide tactics such as double knocking alongside cultural practices such as harvest weed seed control and crop competition to reduce seed set. They also combine well with pre-emergent herbicides to achieve excellent early weed control and suppress seed set in any late germinating weeds. Can I use hybrid crops with herbicide tolerance to fix a weed blow-out? In brief: No. This technology is not suitable for salvage operations. The details: When Roundup Ready canola varieties were first released there was an expectation that these traits could be used to reverse a weed infestation. This proved not to be the case. Hybrid crops are best used in low weed density situations where they can effectively drive down the weed seedbank. They should be grown in rotations that include an effective double-break, brown manure crop or a pasture phase. Having hybrid crop options for both summer and winter growing seasons increases the opportunities to tackle weeds throughout the year or to use different fallow herbicides while maintaining the ability to safely grow crops in the following season. New glyphosate tolerance traits (Truflex® and Optimum GLY®) and the stacking of herbicide tolerance traits of triazine tolerant and Clearfield® (TT+CL) have expanded the safe window for herbicide application in canola. Are residues in grain a concern when using stacked trait herbicide tolerant hybrids? In brief: Not if the stewardship program is followed. The details: New glyphosate tolerance traits (Truflex® and Optimum GLY®) and the stacking of herbicide tolerance traits of triazine tolerant and Clearfield® (TT+CL) have expanded the safe window for herbicide application in canola. This gives more options, more flexibility and more crop safety through the rotation. The stewardship program for the herbicide tolerant trait hybrids describe herbicide use patterns that growers must follow to confidently avoid the accumulation of herbicide residue in the grain and ensure that Australian maximum residue limits (MRLs) will not be exceeded. [Note that MRLs in other countries may be different to the Australian MRL. Find out more at Grain Trade Australia] To avoid problems with crop safety within the rotation it is important to maintain accurate paddock records to avoid applying herbicide to the wrong crop variety and ensure susceptible crops are not sown into paddocks with herbicide residues in the soil. On the flip-side, herbicide tolerance in crops increases the options for crop selection within the rotation. Also, keep in mind the importance of controlling any volunteers from a herbicide tolerant crop in the summer fallow or following crop.
Harvest weed seed control in a nutshell
*Note: In Australia we call the whole machine a harvester, not just the cutting front. At harvest time many weeds that have grown in the crop still have seed held in the seed head. These seeds enter the harvester along with the grain and most exit the harvester and are spread across the paddock in the chaff and straw. Collecting these weed seeds at harvest and either destroying them or depositing them in a known location where they can be monitored and controlled later, is an excellent way to stop weeds in their tracks. Brome grass is the most costly weed for Mallee farmers to manage, even though herbicide resistance in brome grass is currently low in the region. If you are considering adding harvest weed seed control (HWSC) to your weed control program there are excellent resources on the WeedSmart website to help guide you through the initial decisions and the implementation of this important weed control tool. Key messages: Decide on which system fits your farm best. Get maximum weed seed into the header. Know how to manage the collected weed seed. Which system is best? HWSC is being rapidly adopted in Australia and other countries around the world. There are six systems currently being used on Australian farms and they have all been developed by farmers. Research has demonstrated that all are very effective weed control tactics, achieving over 80 per cent control and for some nearly 100 per cent. There are six systems currently used to collect and manage weed seed at harvest: chaff carts chaff lining chaff decks (chaff tramlining) impact mills Bale Direct narrow windrow burning While they are all effective, they vary considerably in capital and ownership cost, nutrient removal costs, operational costs and labour costs. Some HWSC tactics involve the purchase of substantial machinery – such as an impact mill, chaff cart or chaff deck – but the operational and labour costs might be lower than methods such as narrow windrow burning, which involves low set-up costs but higher nutrient losses and labour costs associated with burning. Invariably narrow windrow burning is the most expensive option in the long-run due to the high nutrient removal cost. To calculate the cost of each method for your farm you can use a calculator developed by AHRI’s Peter Newman. https://www.weedsmart.org.au/calculating-the-cost-of-hwsc-for-your-farm/ The HWSC tools all involve some modification to the harvester. The simplest modification is for chaff lining and narrow windrow burning, where a simple chute is attached to the rear of the harvester to direct the residue into a band on the ground, running the same direction as the harvester has travelled. These chutes are often constructed and fitted on-farm. All the other systems are commercial modifications that are fitted to the harvester – chaff decks and impact mills – or trail behind the harvester – chaff cart and Bale Direct. WeedSmart resources: Videos from the HWSC course outline the science and practice of HWSC https://www.weedsmart.org.au/resources/hwsc/ Calculating the cost of HWSC https://www.weedsmart.org.au/calculating-the-cost-of-hwsc-for-your-farm/ Stepping into chaff lining https://www.weedsmart.org.au/stepping-into-chaff-lining/ Using your harvester to destroy weed seeds https://www.weedsmart.org.au/using-your-harvester-to-destroy-weed-seeds/ Get the weed seeds into the header Harvest weed seed control only works on weed seed that enters the header. Getting the weed seed into the header relies on the seed being held in the seed head at the time of harvest. The seed head must also be at harvestable height. Consider the weed spectrum and the likelihood of seed capture. Even if some seed has shed, chances are there will be other seed heads that have not yet shed and even this will assist with reducing the amount of seed entering the seed bank. There are four chaff-only systems and two all-residue systems. The chaff-only systems – chaff carts, chaff lining, chaff decks and impact mills – require the harvester to be set up to separate chaff and straw, and to keep the weed seed in the chaff stream. This may require modifications to the harvester rotor and sieves and the installation of a baffle to keep the weed seed in the chaff stream. If you choose the Bale Direct system or narrow windrow burning, all the straw and chaff ends up in the same place, so no other modification to the harvester is needed. WeedSmart resources: Harvester setup for HWSC https://www.weedsmart.org.au/webinars/harvester-set-up-for-harvest-weed-seed-control-hwsc-for-all-header-colours/ Getting weed seed into the chaff stream https://www.weedsmart.org.au/setting-up-harvesters-to-capture-weed-seed-in-the-chaff/ Using HWSC in different weed spectrums https://www.weedsmart.org.au/is-harvest-weed-seed-control-a-real-option-for-managing-northern-region-weeds/ Manage the weed seed after harvest If you choose an impact mill as your HWSC tool then the tactic is completed in one pass at harvest, with nothing extra to do. All the residue is spread in the field and the weed seeds are rendered unviable. All the other HWSC tools involve some action after harvest to remove or destroy the weed seed collected at harvest. Chaff decks deposit the weed seed-laden chaff in one or both harvester tramlines or wheeltracks. Some growers find that the chaff rots and the weed seeds die, but in other environments growers find that it is necessary to control weeds that germinate in the tramlines using herbicide or non-herbicide tactics applied just to the tramlines. Chaff carts can be emptied as they fill in the paddock or emptied at a central point. Many growers use chaff piles as a high nutrient value stockfeed, others burn the piles and others leave them unburned in the paddock and sow through them the following season. Chaff lines are usually left unmanaged with the expectation that the following crop will provide adequate competition to the weeds to minimise weed growth and seed production. The Bale Direct system results in large bales of crop residue that can be sold into suitable markets. Distance to market is usually an important factor in the success of this system for HWSC. Narrow windrow burning uses fire to destroy the weed seed in the Autumn following harvest. There are significant labour costs and safety risks to consider along with the loss nutrients and ground cover. Key resources to learn more: Diversity Era online course – Harvest weed seed control 101 https://www.diversityera.com/courses/harvest-weed-seed-control-101 Kondinin Group Residue Management at Harvest – Weed Seed Options research report https://www.weedsmart.org.au/app/uploads/2018/06/RR_1802_weedsmart.pdf Kondinin Group Harvest Weed Seed Warriors research report https://www.weedsmart.org.au/app/uploads/2020/05/RR_February_2020_Weedsmart.WS_.2020.pdf Grower experiences: Chaff decks and chaff lining in a high rainfall zone https://www.weedsmart.org.au/case-studies/esperance-growers-using-chaff-decks-and-chaff-lining/ Keeping pressure on brome grass with HWSC https://www.weedsmart.org.au/case-studies/bruce-family-alford-sa/