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Stewardship of synthetic auxin herbicides

with Roberto Busi, Research Fellow, AHRI

Globally, resistance to the world’s oldest herbicide is relatively rare. Unfortunately, one of the economically significant examples of auxin (2,4-D, dicamba and MCPA) resistance is in wild radish populations in Western Australia.

Roberto Busi, research fellow with the Australian Herbicide Resistance Initiative (AHRI), says the recent and future release of new crops with resistance to synthetic auxins (USA only) will heighten the risk unless stewardship measures are followed.

Dr Roberto Busi, AHRI research fellow says researchers around the world are working to better understand the way auxin herbicides work and the mechanisms plants use to evolve resistance.

“The synthetic auxin herbicides have been in wide-spread use since their discovery in the 1940s. According to a 2014 Dow AgroSciences report, herbicides such as 2,4-D and dicamba are used on about 200 million ha world-wide. They mimic the activity of natural plant hormones and seem to have multiple sites of action along with both physiological and biochemical effects that lead to impaired plant growth and death,” he says. “The rate of resistance evolution has been lower than other herbicide modes of action.”

“2,4-D resistant wild radish was first documented in Western Australia in 1999,” says Dr Busi. “Since then surveys have revealed some populations with high-level 2,4-D resistance and a total of 60 per cent of randomly surveyed samples in 2003 that has levelled out at around 74 per cent in both 2010 and 2015.”

Multiple resistance to at least two herbicides is quite common in wild radish populations in Western Australia, generally to chlorsulfuron and 2,4-D, however the last survey also showed alarming levels of resistance to diflufenican. However, resistance to atrazine, glyphosate or Group H herbicides remains rare or is not reported.

“Western Australian farmers have been able to find other ways to reduce the overall number of wild radish plants in their crops by running down the soil seed-bank and this is the correct way to overcome resistance,” says Dr Busi. “If auxin herbicide use were to increase or be used more frequently in Australian cropping rotations growers would need to implement non-herbicidal strategies that reduce the risk of herbicide resistance.”

Group I resistance has also been recorded in wild radish in South Australia, Victoria and New South Wales and in other species including Indian hedge mustard, sowthistle and capeweed.

Why did wild radish in WA evolve 2,4-D resistance when it is so rare globally?

Short answer: Wild radish in the northern WA wheatbelt infested paddocks at high density and the repeated use of Group B and 2,4-D rapidly selected for high levels of resistance.

Longer answer: Growers and agronomists are now well aware that simply swapping from one herbicide to another does not help manage herbicide resistance. This lesson was learnt the hard way in WA where 2,4-D initially provided excellent control of Group B-resistant wild radish. Failure ensued because 2,4-D was applied frequently to dense populations of wild radish, a species that is a prolific seeder and the seed can persist in the soil for many years. In two study populations we found that 2,4-D resistance was clearly inherited as a single, nuclear (pollen transmitted) dominant or near-dominant gene trait. This contributed to the speed of resistance evolution to 2,4-D in wild radish. In other species the traits conferring 2,4-D resistance may be less frequent or be less genetically dominant, and that slows the evolution of resistance.

Are 2,4-D resistant plants less fit than susceptible plants?

Short answer: Following a 2,4-D application, surviving weeds are less competitive.

Longer answer: True fitness penalty has not been established for 2,4-D resistant weeds. A ‘fitness penalty’ is when a resistant plant, growing in the absence of the herbicide is less fit than a susceptible plant.

AHRI research has demonstrated that when 2,4-D resistant plants are sprayed with 2,4-D they are suppressed, show strong symptoms of herbicide damage, but do not die. However, if these plants are sprayed when they are small and grow in competition with a crop, the resistant weeds may die or may produce far less seed than it would in a non-competitive situation.

Crop competition can therefore be used as a weapon against 2,4-D resistant weeds while continuing to use 2,4-D or MCPA as part of a tank mix.

The release of new auxin herbicide formulations in Australia means farmers have more options for broadleaf weed control but researchers emphasise the heightened need for stewardship to reduce the risk of resistance and the importance of crop competition in managing resistance.

How can tank mixes extend the use of herbicides that weeds are resistant to?

Short answer: Multiple resistance to auxin herbicides and other MOA is rare in individual plants. Focus on low seed-banks, use herbicide mixes and mix it up with non-herbicide strategies to keep ahead of resistance.

Longer answer: The most important objective is to aim for low weed numbers. In this situation, while a population of wild radish in a paddock may be resistant to a number of herbicide modes of action the chance of an individual plant being resistant to more than one MOA is very low. In plants such as wild radish that are cross-pollinated there is a lot of mixing of the genetic material so stacking of resistant traits is initially uncommon, particularly for resistance traits that are recessive. Consequently, if a tank mix of three compatible herbicides with different modes of action is applied, with each herbicide at robust label rates, an individual plant is highly likely to be susceptible to at least one of the herbicides applied and so overall plant numbers are reduced through the use of the tank mix.

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Ask an Expert

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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