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Group G herbicides & effectiveness for herbicide resistance

with Dr Chris Preston, University of Adelaide

There has been a gap in the pre-emergent herbicide options for growers to control broadleaf weeds, particularly in pulses, but also in cereals.

Dr Chris Preston, Professor, Weed Management at The University of Adelaide says a suite of new Group G herbicides will go a long way to help growers control brassica and thistle weeds and rotate away from the imi chemistry that has been heavily used in recent years.

Dr Chris Preston, University of Adelaide says using the suite of new Group G herbicides in a tactical way to control early germinating broadleaf weeds could help keep pulses profitable in southern farming systems.

“The new Group G herbicides will be of particular value in pulse crops,” he says. “Until recently Group G products have only been used in small quantities, predominantly as a knockdown spike ahead of planting, but several of the new products have pre-emergent herbicide use patterns.”

Group G mode of action inhibits part of the process for making chlorophyll, and the leaves die as a result. Uptake of the herbicide is usually through the leaf surfaces – either through contact on emerged weeds or taken up from the soil as the weed seedlings break through the soil surface.

“The introduction of Terrain (flumioxazin) as a pre-emergent broadleaf herbicide in front of faba beans offered a new and helpful use pattern in the pulse phase,” Chris says. “Terrain is also a good fenceline weed control option on heavier soils, provided all surface vegetation is removed prior to application.”

Syngenta’s yet-to-be-released Group G product, Reflex (fomesafen) is expected to offer pulse growers pre-emergent control of broadleaf weeds that might be resistant to other MOA groups. This product can be used either pre-emergent incorporated by sowing (IBS) or post sowing, pre-emergent (PSPE), and will provide an alternative to imi herbicides for the control of weeds like sowthistle and prickly lettuce in pulse crops. Lentils are less tolerant than other pulse crops, so Reflex can only be used IBS in lentils.

With the release of these new products Chris is reminding growers and agronomists of the importance of ensuring that herbicide product choice is always based on addressing the weeds present that are likely to cause economic loss or produce large quantities of seed.

“Mixing and rotating herbicide modes of action is one of the WeedSmart Big 6 tactics, but crop competition is just as important,” he says. “Pulses are particularly sensitive to competition from weeds in the early crop stages, so using these Group G herbicides in a tactical way to control early germinating broadleaf weeds could be a very good way to keep pulses profitable in our southern farming systems.”

The sustainable use of Group G and other new herbicides coming to market will be a key topic at WeedSmart Week 2020 in Clare, SA. The companies developing these new Group G products will be present to answer questions and provide advice. Click here to register for WeedSmart Week 2020.

Should I mix and rotate these new herbicides with existing products?

In brief: Yes. Read the labels as they become available and look for opportunities to mix and rotate within and between seasons and crops.

The details: For example, Terrain offers broad spectrum weed control in tank mixes with TriflurX, Terbazine, Avadex Xtra, Kyte, Simagranz and Rifle. Terrain has a narrow weed spectrum for the rate registered in-crop for faba beans.

With some clever planning these products can help ‘bring back’ some previously lost chemistry using the mix and rotate tactic, extending the effective life of a broader range of herbicide options. When coupled with some non-herbicide tactics the grower can regain control of herbicide resistance on their farm and operate in a low-weed situation.

What new use patterns are available with the new Group G herbicides?

In brief: Pre-emergent with IBS and knockdown options.

The details: The first Group G with pre-emergent properties to hit the Australian market was Terrain (flumioxazin), from Nufarm with new use patterns registered for wheat and pulses. Other products with pre-emergent properties expected to come to market in the next year or two are Syngenta’s Reflex (fomesafen) and BASF’s Voraxor (saflufenacil + trifludomoxazin). Voraxor can be used pre-emergent in cereals and also as a knockdown spike. Terrador (tiafenacil) from Nufarm will have knockdown spike use and is safe for cereal or pulse planting just one hour after application.

More details about the new products can be found in the Group G Tech Note.

The early trials with Reflex look very promising and widespread use of this herbicide is expected in WA, where farmers have battled wild radish control in lupins for decades.

Is resistance to Group G a problem?

In brief: Not yet in Australia.

The details: There are no recorded cases of weeds resistant to Group G herbicides in Australia. Although the Group G mode of action has been commonly used for over 15 years, these products have generally been used in small quantities.

Resistance to Group G is becoming widespread in North America and we will soon have resistance problems if we do not practice the Big 6 tactics to minimise weed seed production.

More resources:

Group G Tech Sheet

Nufarm Terrain – Group G pre-emergent and knockdown

  • 500 g/kg flumioxazin
  • New registrations for wheat and pulses in 2019.
  • Pre-emergent weed control in lucerne, wheat, faba bean, chickpea, field pea and along fencelines. Broadleaf weed control only in pulses.
  • Control emerged weeds with knockdown before applying Terrain in wheat and pulses or along fencelines.
  • New chemistry for IBS use pattern.
  • Improves brown-out and weed control of knockdown herbicides. Pre-plant knockdown spike ahead of barley, chickpea, cotton, faba beans, field peas, lentil, lupin, maize, mungbean, oats, sorghum, soybean, sunflower, wheat. Add to glyphosate and paraquat/diquat products. Requires adjuvant.
  • Not suitable for lighter soils.
  • Residual activity on annual ryegrass, sow thistles and wild radish.
  • Broad spectrum weed control in tank mixes with – TriflurX, Terbazine, Avadex Xtra, Kyte, Simagranz, Rifle.

Syngenta Reflex – Group G pre-emergent

  • 240 g/L Fomesafen SL
  • Registration expected early 2021.
  • Suitable for a range of pulses
  • Wide range of broadleaf weeds (including sowthistle and prickly lettuce) – check label when available.
  • IBS or PSPE uses. IBS knife points and presswheels only in lentils.

BASF Voraxor – Group G knockdown and pre-emergent herbicide

  • 250 g/L saflufenacil plus 125 g/L trifludomoxazin
  • Both group G actives
  • Used in wheat, durum and barley against a wide range of grass and broadleaf weeds
  • IBS knife points and presswheels only
  • Registration expected in 2020 and launch in 2021
  • Knockdown and residual control of broadleaf weeds plus suppression of ryegrass.
  • Broadleaf residual control for 8 to 12 weeks (at higher application rates)
  • Partner with glyphosate as a knockdown spike or mix with paraquat for double knock applications.

Nufarm Terrador – Group G knockdown spike

  • 700 g/kg tiafenacil WG
  • Registration and commercial launch expected in 2021.
  • Likely use pattern, partner with glyphosate as a knockdown spike or mix with paraquat for double knock applications. Expect suppression of ryegrass as a knockdown partner with glyphosate or paraquat. 1 hr plant back for cereals and pulses, probably 7–14 days for canola, depending on rate.

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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)
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Managing summer weeds without spraying at night?

Concerns are being raised about the practical implications of this for summer weed control programs. Mary O’Brien, a private consultant with extensive experience in managing spray drift, is keen to see growers fully adopt spray application practices that maximise herbicide efficacy and minimise off-target drift.   Mary O’Brien says the ‘community drift’ that can occur when a number of applicators are each putting a small amount of product in the air at the same time can have very damaging effects on off-target sites. “The bottom line is that allowing spray to drift is like burning money,” she says. “Any product that doesn’t hit the target is wasted and the efficacy of the spray job is reduced, mildly resistant biotypes may survive as a result of low dose application and there is potential damage to sensitive crops and the environment.” “The difficulty is that many growers want to spray at night to cover more ground when conditions are cooler and potentially weeds are less stressed. Having a restriction on night spraying does restrict the time available to cover the areas required.” Having heard these concerns from growers across the country Mary keeps coming back to the fact that if there was a limitation to capacity at planting or at harvest, growers would scale up to get the job done in a timely manner. “Buying another spray rig or employing a contractor is an additional cost, especially after a couple of tough seasons, but I really think this is insignificant against the cost of losing key products and the resultant escalation in herbicide resistance to the remaining herbicides,” says Mary. “This problem is not confined to 2,4-D or even to herbicides. I recently spoke to a stone fruit grower who was forced to dump his whole crop after a positive MRL return for a fungicide he had never even heard of, let alone used.” What about just slowing down and lowering the boom during night spraying? Short answer: This, coupled with a good nozzle, will reduce drift but it will never eliminate it. Longer answer: The correct ground speed and boom height will have a large effect on the amount of product that remains in the air. The problem is that it only takes 1 per cent of the product remaining in the air to cause off-target damage. Once there are a few operators putting just 1 per cent of their product in the air at the same time, the amount of product quickly accumulates and can potentially be very damaging. Mary calls this ‘community drift’. Isn’t it better to spray weeds at night when it’s cooler? Short answer: Not really. Longer answer: Research by Bill Gordon showed that even if you keep everything else the same, night spraying can put at least three times more product in the air than daytime application, even if weather conditions are similar and there is no temperature inversion in place. The main difference between day and night is how the wind is moving across the landscape, rather than the wind speed. Under inversion conditions, the air moves parallel to the ground surface and this means that the product can move significant distances away from the target before coming to the ground. To achieve the best results through daytime spraying, applicators should focus on treating small, actively growing weeds. When there is good soil moisture, weeds are unlikely to be stressed even when the temperature is quite high. Temperature inversion conditions are more common at night and in the early morning. These conditions generate a laminar flow of air across the landscape allowing small droplets to travel many kilometres away from the target site before coming to ground. Can I use other products at night and just avoid using 2,4-D? Short answer: The current changes to 2,4-D labels has drawn a lot of attention but the problem is the same for all crop protection sprays – herbicides, fungicides and insecticides. Longer answer: Different products have different properties and some may work better at night but the problem is the sensitivity of some crops to certain products, such as 2,4-D. All products are tested for their efficacy and the label provides detailed information about the required spray quality and spray application conditions. Many products have explicit label instructions regarding wind speed, temperature inversions (or laminar flow) and night spraying. Given the high risk of drift at night, applicators need to be very confident that there is no inversion present, and weather conditions should be measured at least every 15 minutes to ensure wind speed remains above 11 kilometres per hour. An on-board weather station is the best way to monitor conditions. A visual demonstration using smoke to simulate the the lateral movement of small spray droplets when a temperature inversion is in place. What can I do to improve spray efficacy and avoid spray drift? Short answer: If you do just one thing – change your nozzle. Longer answer: All the factors that increase drift also reduce efficacy. To improve efficacy and reduce drift, use a better nozzle (larger spray quality) and appropriate water rates (matched to spray quality and stubble load), slow down and keep the boom low. Wind is required to push product downward and onto the target, and remember that the 3–15 km/h wind speed is for day time conditions only, this does not apply at night.
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Testing for herbicide resistance

“Testing takes the guesswork out of the equation and gives farmers baseline information that they can use to monitor changes in the weeds on their farms,” he said. “If low level resistance is identified early there are many more management options available compared to situations where full blown resistance has taken hold.” Dr Boutsalis said the over use and over reliance on particular herbicides will unavoidably lead to herbicide resistance developing. “We often hear of farmers applying herbicide even though they are not sure if it will work,” he said. The $300 to $400 cost of testing is insignificant compared to the cost of wasted herbicide, lost production and the costs of driving down a large seed bank of resistant weeds. What herbicide resistance tests are available to farmers in Australia? Short answer: The ‘quick’ test using the whole plant and the ‘seed’ test. Longer answer: The ‘quick’ test uses plant samples collected on farm and sent to the laboratory. The plants are revived and planted into pots then tested against the required herbicides. The ‘seed’ test requires the collection of ripe seed, which is planted out at the laboratory. After dormancy has been broken and the seedlings have started to grow they are tested for their response to herbicides. Both tests are equally accurate. The ‘quick’ test can not test for resistance to some pre-emergent herbicides, such as trifluralin. Which is the most common test that farmers use? Short answer: The seed test. Longer answer: Collecting seed before or at harvest is the most common method used. The collected seed must be mature, from green to when the seed changes colour. Before harvest collect 30 to 40 ryegrass seedheads or several handfuls of wild oats seed. After harvest it is common to find seedheads still in the paddock or samples of contaminated grain can be sent for analysis. Where is the best place to collect samples? Short answer: From suspicious or high risk areas. Longer answer: Herbicide resistance can develop in high risk areas like fencelines or at random through a paddock. Visual observations and changes on the yield monitor in the header can indicate good places to collect seed. If collecting plant samples, look for weeds at the early tillering stage that appear to have ‘escaped’ previous herbicide treatment. Collect 50 to 100 small plants or fewer larger plants. Shake off the soil from the roots, place in a plastic bag and send to the laboratory. What’s involved in sending samples? Short answer: Pick, pack, register and ship. Longer answer: Each sample needs to arrive at the laboratory with suitable identification and instructions. Register the samples online to get a unique sample number and to provide the information required, such as which herbicides you want to test against. Plant Science Consulting and Charles Sturt University both offer commercial herbicide resistance seed testing. Find the details under Point 4 of the 10 Point Plan on the WeedSmart website.   How to ask a WeedSmart question Ask your questions about the spread of herbicide resistance, or any herbicide resistance management strategy, using this blog or using Twitter @WeedSmartAU.

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