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Dealing with a feathertop Rhodes grass problem?

with Andrew Storrie, Agronomist, Agronomo

Feathertop Rhodes grass, with its distinctive seed head, is steadily travelling across the country ­– even being seen along the Eyre Highway crossing the Nullarbor Plain. Agronomo agronomist, Andrew Storrie says prolific seed production and its ability to withstand herbicides makes this weed difficult, but not impossible, to control.

“The key is to concentrate on preventing seed set,” he says. “Feathertop Rhodes grass seed is relatively short-lived in the soil, so with a dedicated approach it is possible to run down the seed bank reasonably quickly.”


“Often the first obvious sign of feathertop Rhodes grass will be a few large plants with the seed head ‘skeletons’ pointing toward the sky,” he says. “FTR grass is moving rapidly along roadsides and most invasions occur from a boundary fence, with seed blown into a paddock. Once on a property it can spread easily on machinery and along irrigation channels.”

Feathertop Rhodes grass is an exotic species that first gained a foothold in the northern grain growing region in response to a shift to low tillage production systems. It is now an emerging weed across the southern and western grains regions of Australia.

“FTR grass is an annual plant that flowers within three to four weeks of germinating and will keep tillering and producing seed while soil moisture is available,” he says. “Not all the seed germinates with the first rain event so there are usually multiple germinations over summer, however most seed in the seed bank is from a single season of seed production rather than accumulating over years.”

Although the light seed is wind dispersed, most seed will fall within five metres of the parent plant, making patch management a viable control option.

Andrew Storrie, Agronomo agronomist says growers need to look for the Archilles Heel of hard to kill weeds like sowthistle, flaxleaf fleabane and feathertop Rhodes grass.

Andrew Storrie, Agronomo agronomist says growers need to look for the Archilles Heel of hard to kill weeds like sowthistle, flaxleaf fleabane and feathertop Rhodes grass.

What are the herbicide control options for feathertop Rhodes grass?

Short answer: FTR grass is naturally very tolerant of glyphosate and there are several cases of highly resistant populations. Many growers are using Group A knockdown herbicides but this could be a short-lived option.

Longer answer: Although Group A herbicides are currently an effective control for feathertop Rhodes grass, resistance is inevitable and field observations suggest it could have already occurred in some paddocks.

Group A herbicides are registered for use in various summer and winter broadleaf crops and in fallow ahead of a mungbean crop (Permit 12941). This permit states that a fallow application must be applied to small weeds and followed with paraquat within 10 days.

Group A herbicides are very sensitive to plant stress and, like paraquat, good coverage is essential for reliable results. Given these constraints, it may be prudent to ‘save’ this herbicide for in-crop grass weed control and look for other ways to control FTR grass in summer fallows.

What patch management methods do you recommend?

Short answer: Physical removal of isolated plants, patch cultivation, burning and optical spray technology.

Longer answer: FTR grass invasion often begins with a few ‘mother’ plants. The plants have shallow root systems and are easy to remove. If this is done before the seed drops, significant costs can be avoided.

Seed buried below a few centimetres of soil will not germinate and most will be non-viable in 12 months, even if another cultivation returned then to the soil surface. Light cultivation of a patch is a very viable option to stimulate germination and allow effective herbicide control of small plants. Under the right conditions fire is also effective to kill the plants and seed on the surface.

Optical spray technology is the only option for treating plants larger than fist size. Permit 11163 provides a range of herbicide options for use with optical sprayers. There are also products from Nufarm and CropCare registered for application with an optical sprayer.

Feathertop Rhodes grass seed head.

Feathertop Rhodes grass seed head.

What can I do to control FTR grass that establishes under the crop?

Short answer: Harvest early following a pre-harvest desiccation and come back immediately with a control tactic to target small FTR grass plants.

Longer answer: Pre-harvest desiccation will allow earlier harvest, particularly in crops like canola that allow more light in toward the end of the season. This is particularly useful if there has been a blow-out in FTR grass numbers.

An application of paraquat + amitrole soon after an early harvest can provide good control and be followed with a residual herbicide, such as Balance® applied ahead of a chickpea crop, to extend the control into the summer months.

FTR grass is sensitive to crop competition. All efforts to increase crop competition through crop and variety choice, narrower rows and stubble management will suppress FTR grass that might otherwise germinate as the temperature rise above 20 degrees C in spring.

Other resources:

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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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