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How to regain control of herbicide resistant sowthistle?

with Dr Chris Preston, University of Adelaide

Having once been a fairly innocuous weed on farms around Australia, sowthistle is now a major weed in many farming systems and has become very difficult to control.

Dr Chris Preston, Associate Professor, Weed Management
 at The University of Adelaide says that changes in emergence patterns and resistance to a wide array of herbicide groups has seen sowthistle adapt well to modern farming systems.   

Dr Chris Preston, University of Adelaide says that in some paddocks, tight lentil wheat rotations has caused a complete failure in sowthistle control every second year, adding massive amounts of seed to the seed bank.

“We have seen two different scenarios unfold in the southern and northern grain growing regions of Australia,” he says. “In southern regions, sowthistle is a major problem in lentil crops where it is able to take advantage of unused soil moisture in spring to establish in gaps in the crop. Having long been resistant to the ‘SU’ herbicides in Group B, the frequent use of imi-tolerant lentil varieties has led to the rapid evolution of resistance to the ‘imi’ herbicides in Group B, leaving no in-crop options for control.”

This means the only form of herbicide control available late in the season is the use of desiccants. Desiccants for use in pulses, such as Sharpen, often do not kill the sowthistle plants and so it is able to recover and go on to set more seed.    

“In the northern cropping region the problem lies in summer fallow management,” he says. “Farmers are observing rate creep with both glyphosate and 2,4-D, where higher rates are now required to achieve the same level of control. While glyphosate resistance has been confirmed in the north, there are no confirmed cases of 2,4-D resistance in the north, but I have no doubt that this will be confirmed in the field sooner rather than later due to the amount of phenoxy herbicides being applied in fallows.”

With limited herbicide options available, finding ways to disrupt the expansion of sowthistle populations will need to be front and centre when planning a weed management program.

How can crop rotation help manage sowthistle?

Short answer: Crop competition and herbicide rotation.

Longer answer: Lentils and several other pulses have been very profitable and this has supported tight lentil wheat rotations in some areas. In some paddocks this has caused a complete failure in sowthistle control every second year, adding massive amounts of seed to the seed bank. Spreading out the less-competitive crops such as lentils and choosing highly competitive crops to add to the rotation will help drive down weed numbers. Sowthistle is well-known as a non-competitive weed when small.   

What other summer fallow methods can be used?

Short answer: Be careful with tank mix partners, look at other herbicide options and try changing the winter / summer crop mix.

Longer answer: In the northern region, the use of glyphosate + 2,4-D followed by paraquat as the double knock is unlikely to be effective long-term. When mixed together, glyphosate and 2,4-D are antagonistic on sowthistle, reducing the efficacy of both tank mix partners. Sowthistle also has the potential to evolve resistance to both glyphosate and 2,4-D, which would make the first knock ineffective. With few other herbicide options other than perhaps the use of residual chemistry, growers will need to implement other control measures.

To drive down numbers, control in spring is critical. Changing the combination of summer and winter cropping could enable better control in spring.

Glyphosate resistance in sowthistle has been confirmed in the north, and while there are no confirmed cases of 2,4-D resistance in the region, Dr Preston has no doubt that this will be confirmed in the field sooner rather than later due to the amount of phenoxy herbicides being applied in fallows.

Does size matter?

Short answer: Yes, even resistant sowthistle plants are more susceptible when they are very small.

Longer answer: The label on glyphosate products typically specifies two different rates according to plant size classes and following these instructions is very important.

If a lower rate is applied to larger weeds they can take a long time to die (if at all) and may well set seed before the plant dies, making the application a waste of money.

One of the difficulties with managing sowthistle is the fact that it can germinate at any time of the year if soil moisture is available. In favourable conditions, this weed can set seed within months and has the capacity to produce about 6000 seeds per plant.

Is mechanical weed control an option for sowthistle?

Short answer: Both harvest weed seed control and strategic cultivation could play a part in reducing seed set.

Longer answer: HWSC is not very effective in capturing sowthistle seeds, which easily disperse on the wind. The removal and destruction of immature seed heads following desiccation will still have a positive effect on weed seed numbers.

Infrequent cultivation that buries seed to a depth of 2 cm or more can help to reduce emergence however repeated use of cultivation can bring seed back to the soil surface where germination can occur. Sowthistle emergence occurs primarily from seeds close to the soil surface with up to 30% of viable seeds emerging over 5 months. Seed can emerge from a depth of up to 2 cm with approximately 4% emergence after 6 months. Seed buried below 5 cm is unable to emerge and can persist at depth.

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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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