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How can I respond to emerging weed threats?

with Bhagirath Chauhan, Principal Research Fellow, Queensland Alliance for Agriculture and Food Innovation

Emerging weeds are generally weeds that have been present in an area but for some reason have not been of any great concern—until something changes with a run of wet or dry season, or a change in farming practice.

Dr Bhagirath Chauhan, Principal Research Fellow with the Queensland Alliance for Agriculture and Food Innovation is leading a project to study the characteristics of 10 weed species that are becoming increasingly important in the northern grains region of Australia.

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“Growers in the region put forward the list of three winter weed and seven summer weed species, which they thought were of greatest concern,” said Dr Chauhan. “Over the next four years we will be studying the influence of rainfall pattern and cropping history on the seed dormancy and persistence of these species.”

The study will focus on better understanding the seed bank biology of sowthistle, turnip weed and Mexican poppy in the winter and feathertop Rhodes grass, windmill grass, liverseed grass, button grass, caltrop, bladder ketmia and sweet summer grass in summer.

“We want to know if these species are more or less of a potential problem in higher rainfall areas such as Dalby or drier areas such as St George,” he said. “We will also see if crop competition and additional resources, such as fertiliser, affect dormancy and germination patterns of these weeds.”

“Seed dormancy and persistence in the soil are very important aspects of weed ecology and can be used to manage weed population with both herbicide and non-herbicide measures.”

Dr Bhagirath Chauhan, Principal Research Fellow with the Queensland Alliance for Agriculture and Food Innovation is studying the ecology of ten emerging weeds in the northern grains region to help identify the most effective control tactics.

Dr Bhagirath Chauhan, Principal Research Fellow with the Queensland Alliance for Agriculture and Food Innovation is studying the ecology of ten emerging weeds in the northern grains region to help identify the most effective control tactics.

Why is information about seed dormancy and persistence important?

Short answer: These characteristics vary considerably between plant species but once understood they help identify management practices that will work best.

Longer answer: Seed dormancy is a measure of the conditions required for the seed to germinate. For instance, one species may not germinate until a certain soil temperature is reached while another species may respond to soil moisture or light. Persistence is a measure of how long a seed can remain viable in the soil. Some species remain viable for decades while others only a few months while awaiting the right conditions for germination.

How can I use this information to manage weeds?

Short answer: Look for ways to run down the seed bank.

Longer answer: For example, sow thistle has no dormancy—as soon as it rains the seed will germinate, making it easier to time an effective herbicide application. Mexican poppy shows strong dormancy over summer then germinated in a flush when it rained in autumn. Turnip weed has been considered a winter weed however we are seeing it germinate as early as February. Knowing the triggers for germination helps plan a control program.

Understanding the persistence characteristic of these weeds will take longer but will alert growers to the potential benefit of cultivation to control weeds that do not persist for long once the seed is buried or crop competition for weed seeds that do not remain viable for long on the soil surface.

Weed seeds are grown in semi-controlled field conditions to determine the effect of rainfall patterns and other environmental influences on seed dormancy and persistence.

Weed seeds are grown in semi-controlled field conditions to determine the effect of rainfall patterns and other environmental influences on seed dormancy and persistence.

Is turnip weed likely to be as big a problem for northern region growers as its Western Australian relative, wild radish?

Short answer: Possibly, but there are differences between the species.

Longer answer: Until the last few years turnip weed has been mainly found on roadsides rather than in cropping paddocks but it is now a common weed in crops. Like wild radish it produces vast quantities of seed however the seed is less dormant and less persistent in the soil. The seed coat still needs to rupture for germination to occur but fluctuations in moisture and temperature, along with insect damage means dormancy is readily broken down and turnip weed is germinating earlier in the year than expected. There have been cases of resistance to Group B herbicides in turnip weed so it is important to manage any survivors after applying these herbicides. A planned survey of 600 turnip weed populations will illustrate the current density of turnip weed and herbicide resistance testing against current herbicides will better inform researchers and growers.

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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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