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Early intervention: patch management of herbicide resistance

Local knowledge and early intervention can slow the development of herbicide resistance in weeds. This is the view of DAFF Queensland senior research scientist, Dr David Thornby, who has been using computer modelling to investigate the way herbicide resistant populations become established on farms.

“Several management strategies have been developed that are effective in reducing the weed seed bank even after herbicide resistant weeds have become widespread on a farm, though they are most effective if they are implemented before resistance is widespread,” he said. “Our modelling shows that, under the right conditions, local eradication of resistant biotypes is possible, provided action is taken early and the weed species is not very mobile.”

The Spatial Herbicide Resistance Analyser (SHeRA, developed with funding assistance from the Cotton Research and Development Corporation) is used to predict the effect of management tactics on the fates of individual weeds and the movement of seeds and pollen. “Our main interest is in glyphosate resistance but the principles are the same for other herbicides,” he said.

“When patches of resistant weeds are small, up to 100 m2, growers can readily consider intensive management options that would be considered too expensive or risky to be done over the whole paddock or the whole farm.”

Dr Thornby is suggesting that growers become more conscious of the signs of herbicide resistance. “Glyphosate is such a widely used product that a large proportion of growers are likely to have been selecting for glyphosate resistance over many years,” he said. “Other than in very high value and high input systems, glyphosate is relied on as a relatively inexpensive method of controlling a wide range of weeds, particularly in no-till systems.”

“In dryland cropping systems glyphosate-resistant awnless barnyard grass is a significant problem and sow thistle and sweet summer grass are emerging as being at risk of developing resistance.”

Weeds with high rates of reproduction are the most likely to develop resistance. In addition to using strategies to reduce the risk of herbicide resistance, such as rotating chemical modes of action and including non-chemical weed control methods, Dr Thornby is urging growers to assess the efficacy of each herbicide application.

“Herbicide resistance can start with one resistant plant that survives the chemical application,” he said. “If that single plant is noticed and removed before setting seed then a large problem can theoretically be averted. The difficulty is in finding the surviving plants and having strategies in place to remove them immediately.”


SHeRA is used to predict the likely expansion of a herbicide resistant patch through seed dispersal and pollen flow to surrounding susceptible weed populations. The management practices of the grower can restrict or assist the expansion of the patch. For instance, continuing to use glyphosate alone on resistant weed populations promotes rapid expansion while intense intervention within and around the patch, such as double knocking, chipping or cultivating can contain, and potentially eradicate, the resistant population.

“Because of gene flow in pollen it is important to actively manage the patch and the area surrounding it,” said Dr Thornby. “The patch itself can be considered the ‘eradication zone’, the area surrounding it might be the ‘containment zone’ and the rest of the paddock might be managed using best practice spray techniques and agronomy. This way the risk of widespread herbicide resistance is contained while not adding significantly to the production costs.”

“This approach relies on finding patches while they are a manageable size and taking action immediately.”

Herbicide resistant patches frequently occur in non-cropped areas such as along fencelines, irrigation channels and around farm infrastructure so these ‘hot spots’ need close monitoring so they do not become a source of resistant seed that could be carried into cropping areas.

If patches of resistant weeds are evident inside a paddock, chances are that it is the in-crop or fallow weed management practices that are the main cause. Dr Thornby suggested that cultural practices need to be given more consideration as weed control methods to avoid over-reliance on herbicides.

“Minimum tillage has many benefits but there is a place for cultivation to control weeds,” he said. “But cultivation alone is not the answer either because some weeds prosper with soil disturbance. The routine use of harvest weed seed control methods, growing competitive crops and using crop rotations to manage weeds are also very important and require an understanding of weed ecology and the use of local experience.”

“When choosing which crop type and variety to grow in rotation, weed management and herbicide resistance need to be part of the decision,” he said. “Non-competitive crops provide resistant weeds with an opportunity to spread, which then limits your crop choices and increases production costs in the future.”

Strategic management of patches is unlikely to be effective against highly mobile weeds. Some weeds, such as fleabane are mobile because their seeds are widely transported on the wind. Other weeds, such as ryegrass have high rates of gene flow in pollen and are likewise considered highly mobile.

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Never cut the herbicide application rate

Scientific studies have demonstrated that resistance can rapidly evolve in weeds subjected to low doses of herbicide. Some weeds can develop resistance within a few generations. Full rates when mixing herbicides too! When mixing herbicides it is important that each product is still applied at the full label rate to ensure high mortality. Applying different chemicals in one mix can provide an additive advantage. It is important to understand the mode of action of each herbicide on the plant when preparing a herbicide mix. This is just as important for pre-emergent grass weed mixes as it is for post-emergent mixes aimed at broadleaf weed control. ALWAYS READ THE LABEL. Surrounding weed seeds with a combination of pre-emergent herbicides with different modes of action can give a high level of control and help extend the useful life of all the chemicals used. The high level of control must be supported with additional control measures for all survivors. All products with different modes of action must be applied at full label rates for this to be an effective strategy.   Mixing two chemicals with the same mode of action can achieve some additional efficacy, however, the mix should deliver the combined full rate to ensure a lethal dose. The amount of stubble present and crop safety are all important considerations when mixing chemicals. For example, when using a tank mix of Avadex® and trifluralin to control ryegrass in wheat, the rates used will vary depending on the sowing system and level of stubble retention. Be sure to get good advice. Many herbicides on the market are a combination of two or more modes of action within the one product. These products must be applied at the full label rate to be effective. Having dual action does not negate the need to change herbicide products and rotate modes of action. Repeated use of any single strategy will reduce the effectiveness of that strategy over time.  

Spray well – correct nozzles, adjuvants and water rates

Spray application is a technical field and growers need to make sure their equipment and application techniques are spot-on. The GRDC Spray Application GrowNote provides detailed information and about 80 videos to demonstrate key skills. Prevent spray-drift The focus of spraying herbicide needs to be on doing the job right so the weeds receive the correct dose and die, and this includes reducing the air borne fraction to a bare minimum. Bill Gordon’s 10 Tips for Reducing Spray Drift Choose all products in the tank mix carefully. Understand the product mode of action and coverage requirements. Select (and check) the coarsest spray quality that will provide effective control. Expect that surface temperature inversions will form as sunset approaches and will likely persist overnight and even beyond sunrise on many occasions. DO NOT SPRAY. Use weather forecasts to inform your spray decisions. Only start spraying when the sun is about 20 degrees above the horizon and when the wind speed has been above 4–5 km/hr for more than 20–30 minutes, and clearly blowing away from any adjacent sensitive crops or areas. Set the boom height to achieve a double overlap of the spray patterns. Avoid higher spraying speeds. Leave buffers unsprayed if necessary and come back. Continue to monitor conditions, particularly wind speed, at the site during the spray operation High water rates don’t have to slow you down Some growers are concerned that increasing the water rate when applying herbicide will slow down their spray operation and cost them money. However, the biggest financial loss during spraying usually comes from a failed spray job. To keep your spray operation as time efficient as possible when using more effective and reliable application volumes, you can: Use nurse tanks around the farm to reduce the time spent travelling back to a central re-fill point. Use a larger pump, e.g. 2.5 inch, to make re-filling quicker. Pre-mix the batch while the sprayer is operating. Many mixes can be held in the mixing tank for up to 6 hours. However, wettable granules and suspension concentrates will need agitation to keep them in solution. For pre-emergent herbicides in high stubble situations, carrier volume has a large effect on the level of control achieved. Across four trial sites Dr Borger’s research demonstrated that ryegrass control with trifluralin or Sakura® increased from 53% control when the carrier volume was 30 L/ha to 78% control when the carrier volume was increased to 150 L water/ha in high Water quality and mixing order Water quality is often overlooked as a possible contributor to herbicide failure and can lead to confusion over the herbicide resistance status of weeds on a property. Water should be considered as one of the chemicals in any mix, given that water quality varies markedly depending on its source. Getting the mixing order right is essential for effective spray results. Don’t start mixing until the water quality is right Podcast – Mixing herbicides Adjuvants Sometimes adding an adjuvant is beneficial and sometimes it is detrimental; and there is an art to knowing how to best deploy these additives. When weeds are susceptible to the applied herbicides, the effectiveness of adjuvants generally goes un-noticed. Correctly applied adjuvants can reduce the impact of low level herbicide resistance by helping to maximise the amount of herbicide taken up by the plant.

Clean borders – avoid evolving resistance on the fence line

About one-quarter of glyphosate-resistant populations within broadacre cropping situations across Australia come from fencelines and other non-cropping areas of the farm. Along paddock borders, where there is no crop competition, weeds can flourish and, if not controlled, set lots of seed. The traditional approach has been to treat these weeds with glyphosate to keep borders clean but after 20-odd years this option is now failing and paddock borders are becoming a significant source of glyphosate-resistant weed seed. Weed researcher Eric Koetz said the limited options for managing weeds along irrigation infrastructure and other non-crop areas is a problem and is putting additional pressure on knock-down herbicides in irrigated systems. In some situations, cultivation can be used to kill the weeds and provide a firebreak, but on light soils this may pose an erosion risk and mowing or slashing may be safer options. Another possible tactic is to continue using herbicides but to ensure that a clean-up operation is carried out before any survivors can set seed. Some growers are choosing to increase the heat on weeds along the borders by planting the crop right to the fence and then baling the outside lap and spraying with a knockdown herbicide to kill any weeds and provide a firebreak. Another good option in some situations is to maintain a healthy border of vegetation using non-invasive grasses. In Queensland, buffel grass is a good example of a grass that can outcompete other weeds while not invading crop lands. If only herbicides are used on fencelines, resistance is inevitable. Surviving weeds on fencelines have no competition and access to plenty of soil moisture, so they set a lot of seed and resistance can easily flow into neighbouring paddocks. Other resources It’s time for a glyphosate intervention Farm hygiene cottons on – Cleave Rogan, St George What’s new in management of herbicide resistant weeds on fencelines? Keeping the farm clean – Graham Clapham, Norwin Don’t jeopardise glyphosate for clean fencelines Keeping fencelines clean Resistance risk to knock-down herbicides on irrigated cotton farms

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