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The best ways to control herbicide resistant wild oats

with Richard Daniel, CEO Northern Cropping Alliance

Wild oats takes a little longer to evolve herbicide resistance than some other grasses, but once resistance has occurred it is a challenging weed that requires a concerted effort to bring under control.

Although herbicide resistance has been known in wild oats since the late 1990s, growers have generally had some effective in-crop herbicide options that kept numbers at a manageable level. However, in recent years it has become increasingly obvious that Group A herbicides are no longer a viable option to control wild oats in many farming systems.

Richard Daniel, Northern Grower Alliance CEO says that now growers are faced with less herbicide options and few cultural practices to control wild oats, they will need to stack control tactics that work in their farming system.

Richard Daniel, CEO of the Northern Grower Alliance (NGA) says that although wild oats is self-pollinating and is a hexaploid species (comparatively more DNA than diploid species such as annual ryegrass), attributes which have delayed resistance, there are populations in Australia that are resistant to all post-emergent herbicides registered for its control.

“Faced with less herbicide options and few cultural practices to control wild oats, growers will need to stack control tactics that work in their farming system,” he says. “Canadian research has demonstrated the value of stacking tactics such as crop rotation plus high seeding rates plus a competitive cultivar to control wild oats.”

Richard says the most valuable tool growers have in the northern grains region is the ability to summer crop. He recommends a summer cropping phase to break the cycle and drive down numbers in the wild oats seed bank.

“Two consecutive winter fallows gives the best opportunity to control wild oats,” he says. “A single winter fallow and then double cropping back to a winter crop does less than half a job, particularly if the paddock is sown to chickpea.”

“Chickpea poses a higher risk because it is generally less competitive against wild oats. This risk can be reduced if the chickpea crop is sown later, which gives the grower the opportunity to apply a winter fallow weed control treatment on the first germination of wild oats to give the crop a competitive advantage.”

What is the most effective crop rotation to combat dense populations and resistant wild oats?

Short answer: A summer cropping phase.

Longer answer: For best results, implement a summer cropping phase that allows for winter fallows on both sides of the summer crop/s to control wild oats.

For this strategy to be effective for wild oats management, it is important to avoid planting the summer crop too early where there is a risk of a late flush of wild oats emerging after planting.

When returning to a winter crop, consider sowing a competitive crop such as barley first. Position chickpea carefully within the rotation, aiming to only sow chickpea into paddocks with low wild oats numbers.

Although wild oats is a prolific seed producer, about 75 per cent of seed is depleted within 12 months of entering the seed bank. This means that consistent pressure on the seed bank is a very effective control measure and large populations can be run down in 3–5 years if seed production is prevented.

A summer crop phase between two winter fallows is an effective tool to drive down wild oats numbers, allowing more options in the next winter crop season.

How can I reduce the risk of wild oats escapes in chickpea crops?

Short answer: Residuals + delayed planting + crop competition.

Longer answer: In-crop herbicides are limited to Group A in chickpea and there is widespread resistance to this mode of action in wild oats. This means that residual (pre-emergence) herbicides can be important to provide early weed control. Delayed sowing is another useful tactic that generally does not come with a yield penalty in chickpea. By delaying sowing the grower has the opportunity to apply a fallow management tactic to the first, and largest, germination of wild oats seed. Delayed sowing may have other benefits in restricting early crop biomass production and avoiding flowering during conditions when temperatures are too cold for pod set to occur.

Any effort to improve crop competition, such as narrower row spacing, will be rewarded with reduced weed biomass and seed set.

Wild oats can out-compete chickpea and deposit copious amounts of seed in the seed bank if left unchecked.

What are the best tactics to control wild oats escapes?

Short answer: Aggressively deploy patch management tactics.

Longer answer: Wild oats tends to establish in patches, which are particularly easy to see in chickpea crops. Use as many tactics as possible to keep weed seed set to a minimum, using non-Group A herbicides, harvest weed seed capture and even a longer (2 or 3 years) summer cropping phase. Each of these tactics on their own may only provide 50 to 60 per cent control but when applied in close succession they are likely to effectively control weed numbers. Swathing is one harvest weed seed control tactic that is used to good effect in cereals and although difficult to implement, wick-wiping with glyphosate in chickpeas is another option to consider to prevent seed set late in the winter crop phase. Infrequent but intense cultivation may also be an option for growers to regain control on a paddock or patch level.

Windrowing barley is an effective strategy to prevent seed set in wild oats.

Resources:

https://www.weedsmart.org.au/windrowing-collects-early-shedding-weed-seed/#more-5254

https://www.weedsmart.org.au/podcasts/cropping-resistance-forum-overview/

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How do you manage 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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Does ambient temperature affect herbicide performance?

with Chris Preston, Associate Professor, Weed Management
 at 
The University of Adelaide Temperature affects the absorption, translocation and metabolic degradation of herbicides applied to plants. Herbicides applied under the wrong conditions can appear to fail, however the reason may not be herbicide resistance. Dr Chris Preston, Associate Professor, Weed Management
 at The University of Adelaide says most herbicides have a temperature range at which they are most effective in controlling target weeds. “Applying herbicides outside the optimal temperature range is likely to contribute to a spray failure, even in susceptible populations,” he says. “Alternatively, applying herbicides within the correct temperature range can improve the control in populations known to have a level of resistance to that herbicide.” Dr Chris Preston suggests testing whole plants rather than seed for responses to a range of post-emergent herbicides. The Quick-Test is conducted in the same growing season as herbicide will be applied so the testing will occur under similar conditions to field conditions. Dr Preston says the effect of frost on the efficacy of clethodim is a striking example. Spraying clethodim in non-frosty conditions achieves vastly better results than spraying after three days of frost, even on populations that are resistant to this chemical mode of action. “Combining the optimal temperature with optimal weed size will give the best results possible,” he says. “The current common practice of applying clethodim to tillered ryegrass in the coldest months is not making the best use of this herbicide.” As a general rule of thumb, Group A (fops), paraquat (Group L) and glyphosate (Group M) are more effective at lower temperatures while Group A (dims), atrazine (Group C) and glufosinate (Group N) are more effective at higher temperatures. However, weeds that are resistant to paraquat become less resistant in warmer temperatures. “The other implication of this research is the effect of ambient temperature on herbicide test results,” says Dr Preston. “Seed collected in winter and grown out in the glasshouse in summer will be tested for resistance in conditions that are not representative of field conditions when growers are next treating that weed species. The Quick-Test using whole plants overcomes this problem and improves the reliability of herbicide susceptibility testing.” How can I get the best performance out of clethodim? Short answer: Avoid applying clethodim during frosty periods. Longer answer: Twice as much clethodim is required to kill susceptible annual ryegrass if the product is applied after three days of frost. Even higher rates are required if the plants have resistance to clethodim. Planning to apply clethodim for grass control outside the coldest months of June and July, and avoiding night spraying in winter, will see better results in both resistant and susceptible populations, particularly in tillered plants. Clethodim is most active when temperatures are over 20 degrees C. Weed seed that is tested during summer may return false negative results, which could translate into spray failure in the field the next season. Twice as much clethodim is required to kill susceptible annual ryegrass if the product is applied after three days of frost. Even higher rates are required if the plants have resistance to clethodim. When it is it too hot for glyphosate? Short answer: Efficacy is much better at 20 degrees C than at 30 degrees C. Longer answer: Spraying glyphosate resistant barnyard grass at lower temperatures is more effective than under hotter conditions. If barnyard grass is tested for herbicide resistance during the cooler parts of the year it may appear susceptible to the field rate of glyphosate but then when this rate is applied to the population in summer there may be many survivors. When glyphosate is taken up rapidly it tends to limit its own translocation, which can mean that although symptoms may appear more rapidly in warmer temperatures, plant kill is less reliable. Which herbicide resistance test should I use? Short answer: The weed resistance Quick-Test for post-emergent herbicides. Longer answer: The Quick-Test involves testing whole plants rather than seed for responses to a range of herbicides and rates. The Quick-Test is conducted in the same growing season as herbicide will be applied so the testing will occur under similar conditions to field conditions. The results of the Quick-Test are available within the same season, potentially giving growers an opportunity to apply an effective weed control tactic before the end of the season. The Quick-Test is not available for many pre-emergent herbicides. The Quick-Test is available through Plant Science Consulting and results are normally available after four weeks. Relevant links Maximising clethodim performance and the impact of frost fact sheet Keeping clethodim working in broafleaf crops Plant Science Consulting herbicide resistance testing – Quick-Test GRDC Update Paper – New developments and understanding in resistance mechanisms and management

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