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Diversity – it’s importance in the battle with herbicide resistance

with Mike Ashworth, Research Agronomist, AHRI

Everyone would like the answers to farming’s problems to be simple, but the fact is, all over-simplistic solutions are prone to collapse. This has been repeatedly demonstrated with different weed species adapting to challenge different farming systems or practices.

Australian Herbicide Resistance Initiative research agronomist, Dr Mike Ashworth says weeds use two mechanisms to survive weed control methods – they can either become ‘immune’ or ‘evade’ the control tactic.

Dr Mike Ashworth says the best way to keep all weed control tactics alive is to be unpredictable in your choice of crops, planting times, herbicide choices and harvest weed seed control methods.

“There is extensive research in the area of herbicide resistance showing the molecular, genetic and biological mechanisms that different weeds can use to become immune to herbicide treatments,” he says. “Just as the systematic use of herbicides will lead to resistance, the routine use of non-herbicide weed control tactics will select for individual plants that can evade that tactic.

In one study, Dr Ashworth started with wild radish plants that had not been subjected to harvest weed seed control and systematically collected seed from early flowering individuals. The seeds were planted to produce the next generation for further selection.

“Within five generations of selection the wild radish population had more than halved the time to flowering – from 60 days to just 29 days,” he says. “As a result these plants will be carrying well-matured pods at harvest, and will be more likely to drop their seed, effectively evading seed capture at harvest. This is even more likely in years where there are periods of water deficit, high temperature or high wind.”

The good news is that the early flowering wild radish populations produced less biomass prior to flowering than the later flowering individuals, likely making them more susceptible to crop competition. Unfortunately, these early flowering plants also had poor structural integrity, with more pods set below the harvest cutting height.

How can I stop weeds evading weed control tactics?

Short answer: Avoid predictable and repetitive farming systems and weed control tactics, and aim to establish highly competitive crops.

Longer answer: The more diversity the better. Look for ways to add different crops to the rotation, mix and rotate herbicide modes of action, use different row spacings, alter sowing times and implement different strategies before and at harvest.

Take advantage of weed control that comes through increased crop competition. For example, the early flowering wild radish plants are smaller, potentially making them more resistant to harvest weed seed control, but more susceptible to crop competition.

All weed control strategies are most effective when weed numbers are low so concentrate on ways to drive down weed numbers, prevent seed set and remove survivors.

What about crop competition – can weeds evade that?

Short answer: Yes, weeds will respond to increased crop competition.

Longer answer: In a highly competitive crop a few individual plants are likely to gain an advantage through increased vigour or plant height. If left unchecked these individuals can set seed and the population can increase in size. This is less likely to become a problem if the farming system includes a range of crops and pastures of varying heights for example. A trait that is an advantage in one situation is often a disadvantage in another.

The combination of highly competitive cropping with harvest weed seed control is a great example. Vigorous plants often hold their seed high in the crop canopy and so are readily collected at harvest in a competitive crop environment. On the other hand, these weeds would flourish and potentially lodge in a less competitive crop, thereby evading harvest weed seed control.

This pot trial is investigating the fitness penalty that may be associated with the early flowering trait of these specially selected wild radish plants when faced with stiff crop competition from a wheat crop compared to no competition.

How are weeds evading pre-emergent herbicides?

Short answer: Through selection for later germination.

Longer answer: Pre-emergent herbicides are being increasingly relied on to provide early weed control. Individual weed seeds that remain dormant for longer and germinate later in the season can evade pre-emergent herbicides altogether or are exposed to a low dose that can lead to resistance.

By mixing up planting dates and using different pre-emergence herbicides with different modes of action, growers can disrupt the shift in dormancy.

Weeds that emerge later, will be more susceptible to crop competition from an early seeded crop. Get the jump on weeds and make sure your crop is up and away.

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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.
Ask an Expert

Does ambient temperature affect herbicide performance?

with Chris Preston, Associate Professor, Weed Management
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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