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Pre-em herbicides for grass weed control in break crops

In the central western grains region of NSW, annual ryegrass is becoming increasingly difficult to control. WeedSmart champion farmer and Grain Orana Alliance (GOA) CEO, Maurie Street has been leading on-farm trial work to identify improved control measures in break crops that can add diversity to farmers’ weed control systems.

Grain Orana Alliance CEO, Maurie Street led trialwork across central western NSW to investigate the pre-emergent herbicide options for controlling weeds in broadleaf crops.

Grain Orana Alliance CEO, Maurie Street led trialwork across central western NSW to investigate the pre-emergent herbicide options for controlling weeds in broadleaf crops.

“In the last few years we have seen a large increase in the number of annual ryegrass and wild oats plants surviving the traditional herbicide treatments used in cereal crops,” he says. “Recent herbicide resistance surveys have made it clear that key herbicides used in cereal crops are losing their effectiveness. Over 90% of samples collected were resistant to Group B herbicides such as Logran and over 70% of samples were resistant to Group A herbicides such as Axial and Hoegrass.”

He says the situation has led to high levels of seed set in the cereal phase, which is putting considerable pressure on the few remaining in-crop grass selectives, that is Select or clethodim, in the following broadleaf break crops.

The 2014 weed survey revealed that resistance to clethodim is increasing rapidly with 60% of submitted samples showing resistance. Continuing to spray high density populations with clethodim will soon see the loss of this herbicide option in the region. Maurie says the industry risks losing the use of Select altogether if something is not done quickly to bring the weed population back under control.

This prompted GOA to establish GRDC-funded trials across central western NSW to investigate the effectiveness of a range of pre-emergent herbicides in chickpea, lupin and canola crops last winter.

The trial identified pre-emergent product combinations that provided significantly better control (right) than the district standard practice (left) for herbicide resistant annual ryegrass.

The trial identified pre-emergent product combinations that provided significantly better control (right) than the district standard practice (left) for herbicide resistant annual ryegrass.

Although the use of pre-emergent herbicides is not new to growers, the trials have demonstrated that there are several options available that can achieve better results than the current district standard practice.

In chickpeas, the standard approach in the region is to use simazine and Balance as a pre-emergent weed control tactic. Generally this combination provided poor control, with control in some trials being as low as 50%, which was no better than in the untreated control strip. This left populations of 200 plants per m2 that growers would have to target in-crop with clethodim.

“While we only have one year of results, it is clear that there are a number of other options that are far more effective than the district standard practice,” says Maurie. “Generally, the newer pre-emergent herbicides such as Sakura and Boxer Gold showed improved weed control over the standard.”

The most striking results over all the trial sites was the improvements gained through multiple product combinations, or tank mixes, of pre-emergent chemistries compared to single product treatments.

The simple addition of trifluralin to herbicides already used in the district resulted in much higher levels of control. For canola growers the recent registration of the propyzamide product, Rustler, has shown great potential for improved ryegrass control, particularly when combined with other products such as atrazine or trifluralin. In some trials untreated populations of 320 plants per m2 were reduced down to an impressive 10 plants per m2.

The new registrations of Sakura and Boxer Gold in pulses, and Rustler in canola, have broadened the options available to growers. Observations from the trials suggest that it would be unwise for growers to simply change from one pre-emergent herbicide to another because generally product combinations were more effective than any single product.

“It is also important that we introduce as many different tactics as possible into the cropping rotation, all aimed at keeping weed density low,” he says. “Any reliance on a small range of products or tactics will inevitably bring us back to the same situation that we are finding ourselves in now.”

When considering a multi-product treatment, care is required to ensure the products are compatible and that the use is registered for the crop. It is essential to apply each product at its full label rate.

“The use of these strategies must be considered a short-term option to bring the weed population down while taking steps to reduce the reliance on herbicides that has landed many growers in this situation in the first place,” says Maurie. “Other tactics that will support and protect both pre-emergent and knock-down herbicides include fallow management, harvest weed seed control and hay making.”

These general principles also apply to other grass weeds such as wild oats and barley grass that have also been increasing in number and often display multiple-MOA resistance.

When planning an integrated weed management strategy, Maurie suggests growers consider what other weeds are present and the effectiveness of the alternatives treatments on these species, the comparative costs, crop tolerances, plant-back or herbicide residue constraints and the resistance status of the weeds present.

For more information about the GOA trials visit their website.

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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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