Read time: 2 minutes

Herbicide resistance management is not a one-year decision

Herbicide resistant weeds can be controlled within a few years using a planned strategy, according to Dr Chris Preston, University of Adelaide (UA) Associate Professor—Weed Management.

Dr Preston advocates using a few different integrated weed management tactics to maintain pressure on weeds. He suggests that the costs associated with herbicide resistant weeds are so great that there are situations where weed control opportunities may need to take priority over other crop outcomes.

Starting with the worst weed and worst paddock, Dr Preston said it is possible to see significant results within two years, particularly with weeds like annual ryegrass that do not stay dormant in the soil for a long time. The next step is to include weed seed set prevention strategies for all weeds in all paddocks.

“A reduction in weed populations can be achieved if in-crop tactics are used in consecutive years to minimise seed set,” he said. “A single operation to reduce weed seed set with no follow-up is rarely as effective as using a number of weed control tools.”

“Weeds that have survived the growing season and set seed at harvest time are the source of a continuing weed burden on cropping land,” said Dr Preston. “Growing crops specifically for the weed control options they offer is one tactic to combat weed seed set.”

Promoting strong crop competition is an important tactic that should be factored into every crop production phase, especially for cereals. “High seed rates, narrow row spacing, early sowing when the soil is still warm and correct use of pre-emergent herbicides all create the best opportunity for the crop to out-compete weeds,” said Dr Preston.140207 chris preston2 _photo jenny barker

“Growing a brown manure crop or croptopping can be effective options in the long term,” he said. “Using a variety of tactics will remove herbicide resistant weeds whilst conserving herbicide chemistry.”

All pulses can be grown as brown manure crops and will add vital nitrogen and organic matter to the soil. Growing pulses, such as field peas or lupins, specifically for the opportunity to croptop can also be very effective if the operation is done when the weeds are most susceptible. “Croptopping is less effective if it is done as a last minute response to a weed blow-out. If it is planned and the weeds are monitored to determine the best time to apply the croptop herbicide then it will achieve a significant reduction in weed seed set.”

Croptopping is usually used to control grass weeds but it is important to note that it can be difficult to implement in lentils and faba beans and does not usually offer a high level of effective weed control in chickpeas.

In the lead-up to harvest it is worth considering the use of mechanical harvest weed seed control tactics such as producing hay or windrow burning.

“Producing oaten hay is a very valid weed control strategy to use against both broadleaf and grass weeds,” said Dr Preston. “After cutting the hay it is essential to go back with a clean-up operation using a paraquat-based herbicide, which is very effective on new regrowth.”

Mechanical harvest weed seed control tools are a good addition to a weed strategy, with windrow burning being an effective tactic against wild radish.

“The overall key is to be diligent. Monitor the effectiveness of each weed control tactic used and be ready to implement operations at the right time,” he said. “Intensive management of small problem areas or patches can greatly reduce the threat of a weed explosion.”

Related Articles

View all

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

Subscribe to the WeedSmart Newsletter