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How can studying resistance genetics in weeds help farmers?

with Dr Qin Yu, Research Associate Professor, Australian Herbicide Resistance Initiative.

The old saying ‘know thy enemy’ certainly applies to managing herbicide resistance. Dr Qin Yu’s research into the biochemical and molecular processes plants use to evade destruction is essential for the development of effective strategies to manage resistance.

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Working with 33 populations of annual ryegrass that had survived treatment with Hoegrass® (diclofop), Dr Yu and her team identified the resistance mechanisms being used. Their findings were both alarming and illuminating—91% of the resistant populations studied possessed target site mutations, 80% possessed metabolic mutations, meaning 70% were using both types of mechanisms to survive herbicide treatment.

“Target site mechanisms are relatively easy to identify and study,” says Dr Yu. “We now have a good understanding of these mechanisms, which can lead to high levels of resistance. The resistant plants can usually be removed using alternative herbicides of the same mode of action.”

“Non-target site mechanisms are more difficult to study and harder to manage,” she says. “This type of resistance is related to the metabolism of the herbicide within the plant, making it more complex and unpredictable. This is harder to manage in the field.”

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How can target site and non-target site mechanisms occur in the same individual plant?

Short answer: Through gene sharing.

Longer answer: Some weed species have a very high capacity to respond and adapt to environmental conditions, including herbicide applications. Cross-pollinator weeds such as annual ryegrass readily exchange genetic material, with each new generation having the capacity to accumulate resistance genes from different individuals.

What is the value of theoretic science such as resistance genetic studies?

Short answer: Better information for better decisions.

Longer answer: Advances in diagnostic technology to determine what mechanisms are present in a resistant weed population can identify effective management strategies. Finding ways to capitalise on the ‘fitness penalty’ that may come with herbicide resistance, such as increasing crop competition or using other non-chemical control tools, is an important outcome of this research.

What can resistance genetic studies tell us about using selective herbicides?

Short answer: Resistance can occur within just a few years if low rates are used or survivors are ignored.

Longer answer: Selective herbicides work by taking advantage of a crop’s greater ability to metabolise a herbicide compared to the target weed’s lesser ability to metabolise the herbicide. Studies of annual ryegrass and wild oats have proven that both weeds possess the genetic capacity to breakdown selective herbicides if these herbicides are applied at sub-lethal rates. Resistant populations can be resistant to other selective herbicides, even ones that are not yet on the market. Applying full label rates and removing any survivors is essential to preserving these chemicals for use in-crop.

What are some ways to improve herbicide efficacy in resistant populations?

Short answer: Ambient temperature makes a difference to the efficacy of some herbicides and the resistance level.

Longer answer: Some resistance mechanisms are temperature-dependent. Understanding these mechanisms can help researchers optimise the way growers use herbicides that the weeds are resistant to. For example, glyphosate should be applied during relatively cool (but not warm) temperatures for better control of resistant winter-grown ryegrass, and applied at warm (but not hot) temperatures to improve control of resistant summer-grown barnyard grass.

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Ask your questions about about genetic research that is helping to manage herbicide resistant weeds using Facebook, Twitter@WeedSmartAU or leave a comment below.

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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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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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