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Management of herbicide resistant weeds on fencelines

with Dr Chris Preston, Chair of the Australian Glyphosate Sustainability Working Group

Weeds growing along fences and other crop border areas are quietly gaining resistance to important knockdown herbicides like glyphosate, setting plenty of seed and are ready to move into neighbouring crops.

Dr Chris Preston, Chair of the Glyphosate Sustainability Working Group, says glyphosate products have been so effective for the past 15 to 20 years that many growers have relied solely on these products for weed control in their non-crop areas.

“The problem that has arisen is that any escapes – weeds that survive the glyphosate treatment – are able to flourish in an environment where there is little competition for moisture, nutrients or light,” he says. “This means they can set large quantities of seed and it doesn’t take much for this seed to move into the crop area, already highly resistant to a key herbicide mode of action.”

The other major problem that occurs is due to all weeds growing along the fenceline being treated with the same product. This means that weed species that already have a high tolerance to glyphosate, and are not listed on the product label, are able to gain a stronger foothold.

“This can be clearly seen along roadsides around the country where hard to kill grasses like feathertop Rhodes grass have become a dominant species,” says Dr Preston. “In situations where other weed control options are limited, it is difficult to contain the spread of these weeds.”

Fencelines and crop borders represent a significant threat to crop production due to the often-unnoticed evolution of glyphosate resistance in weeds.

What other options do I have for managing fenceline weeds?

Short answer: Using diverse herbicide and non-herbicide tactics including double-knock, chipping survivors, slashing, burning, livestock, cultivation and different herbicide modes of action, including residuals.

Longer answer: Consider the situation, that could realistically arise, where glyphosate is not effective on any weeds present on the fencelines or crop border areas. What would you do? Making a change to how you manage the borders now could extend the useful life of glyphosate in your farming system. Herbicide resistance is best addressed by combining a variety of control tactics to drive down weed numbers and prevent seed set.

What other herbicides are a good option for fencelines?

Short answer: Herbicides that are not used in crop production could be valuable in maintaining clean borders.

Longer answer: One example is bromacil, a Group C herbicide that controls a range of both grass and broadleaf weeds. This herbicide is effective on small weeds post-emergence and also offers residual activity to suppress germinations. It would be best applied during the winter cropping phase when winter-growing weeds are small, rather than leaving treatment until August or September when the target weeds are large. In the northern region, spring is a good time to be targeting summer-growing weeds along fencelines. However, if you do have to use bromacil on larger weeds, it must be applied with an effective knockdown herbicide partner. Complete control of glyphosate resistant annual ryegrass has been achieved in trials using paraquat/diquat + bromacil and glufosinate + bromacil, applied post-emergent.

Bromacil can also be applied to bare soil to stop weeds establishing. Other residual herbicides currently registered, including simazine, fluometuron and imazapyr, all have recorded instances of herbicide resistance and so are not robust options for fenceline weed control. Flumioxazin is soon to be registered for application to bare ground and will offer growers another residual herbicide option without the risk of damaging trees.

A new registration for flumioxazin to be applied to bare soil in non-crop areas will increase the residual herbicide options available to growers. This photo shows the efficacy of flumioxazin compared to glyphosate (front), four months after application.

What happens if a fenceline gets incorporated into a cropped area when I realign paddocks?

Short answer: Crop competition becomes an important control tactic.

Longer answer: If fencelines are no longer required, removing them will make it easier to manage crop borders around the farm. Where crops are planted across old fencelines there is often additional weed pressure for the first few years. If possible, thoroughly clear away weeds during the paddock re-development and plan to grow very competitive crops in the first few seasons at least. The ‘fenceline effect’ is often observed for several years so it pays to keep the competitive pressure on.

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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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