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Is poor weed control due to herbicide resistance?

with Maurie Street, CEO, Grain Orana Alliance (GOA)

If a grower experiences a poor spray outcome on annual ryegrass with glyphosate, or seemingly needs to continually increase rates to achieve the same level of control, it might seem obvious that herbicide resistance is the most likely problem.

On the contrary, Maurie Street, CEO at Grain Orana Alliance says that this is not always the case, having investigated ways to regain control of problematic ryegrass populations.

Grain Orana Alliance CEO Maurie Street says growers and agronomists need to critically assess any spray job that does not achieve the expected results. Photo: GRDC

“Annual ryegrass has always been present on most farms in the Central NSW cropping region and many populations are resistant to Group A and B herbicides,” says Maurie. “So, when growers started to have trouble controlling this weed with glyphosate it seemed natural to assume that populations were becoming resistant to our most relied on knockdown herbicide.”

In an effort to finesse the available control options, GOA established trials over three years with GRDC investment on seven sites where poor control of annual ryegrass over previous years had resulted in increased weed seed banks.

“The first thing we did was collect samples from each site and have them tested using the Quick Test method for glyphosate resistance,” says Maurie, “We were surprised to discover that five of the populations were in fact susceptible, even at lower label rates of glyphosate, and the other two populations were only moderately resistant to the lower rate and 100 per cent susceptible to higher label rates.”

What this suggests is that there can be something other than resistance contributing to herbicide failures. Testing will reveal if herbicide resistance is at play and identify herbicide products and rates that can be expected to provide acceptable control. Next, critically assess the spray operation and identify factors that could have affected the efficacy of the spray job. Finally, look for ways to implement the WeedSmart Big 6 tactics in your weed control program to keep weed numbers low.



Trials that ran at 7 sites for 3 years revealed that many things can contribute to a spray failure and growers should not rush to the conclusion that herbicide resistance is always the cause.

So, if the ryegrass was susceptible to glyphosate, why was control poor?

Short answer: Most likely a combination of reasons.

Longer answer: Instances of poor weed control after a herbicide application could be associated with one or more factors such as poor spray water quality, incorrect spray timing, inappropriate sprayer set up delivering less than optimal spray droplet size and/or water rates, products with sub-optimal surfactant loadings, or environmental stress affecting the plant and or the herbicide activity. Or in some cases, the herbicide rate is too low for the job at hand.

In the event of an apparent herbicide failure, review and investigate all aspects of the application – including Stress, Timing, Application and Rate (STAR). Don’t just assume that herbicide resistance is to blame.

In the paddocks used for this trial, early testing for herbicide susceptibility would have revealed that glyphosate was still effective, although some higher application rates would be needed in some fields to achieve adequate levels of control. Attention could then be turned to other factors that might have caused the poor control of annual ryegrass in these paddocks.

Is it safe to use higher rates of glyphosate and how does it help?

Short answer: Using a rate at or near the upper end of the allowed range for glyphosate can improve efficacy in both resistant and susceptible populations. Always stay within the label rate range when applying herbicide.

Longer answer: In the case of glyphosate resistant plants, the resistance is often still rate related. Consequently, increasing product rates will effectively control plants with lower level of resistance.

In the case of both resistant and non-resistant plants, increasing glyphosate rates may contribute to more effective control by counteracting poor application, improving control of older or stressed plants, overcoming reduced efficacy due to using poor quality water and when treating plants covered by dust. Higher label rates can also improve glyphosate activity on plants exposed to the higher temperatures that can arise in early autumn or late spring (Boutsalis et al. 2015).

In these GOA trials the use of higher glyphosate rates gave acceptable control in all but one paddock. The population in question was confirmed susceptible and although the application conditions were noted as ‘very dry and the weeds somewhat stressed with warm spray conditions’, another trial site sprayed under similar conditions, using the same water source, spray set up and products achieved acceptable control.

The reason for the failure on a susceptible population was not confirmed in the trial but highlights the importance of susceptibility testing both to determine if resistance is a contributing factor and if increasing the application rate is likely to be an effective strategy.

In these GOA trials the use of higher glyphosate rates gave acceptable control in all but one of the problematic paddocks.

What’s more important, adjuvants and surfactants or product rate?

Short answer: Generally speaking, increasing the product rate gives the most consistent improvement in control.

Longer answer: In the trial, a range of glyphosate formulations, adjuvants and surfactants were tested for efficacy at the seven sites. There was no consistent difference in performance of Roundup® CT®, Roundup ULTRA® MAX (a premium, fully loaded product) and a low priced- generic brand, if robust rates were used. At the lower rates tested, Roundup ULTRA® MAX often performed better than the other two products tested.

Similarly, when surfactants or additives were applied with robust rates of glyphosate there was no consistent advantage to the addition of BS1000®, LI700®, Liase®, Wetter TX® or Activator®. At lower glyphosate rates, the addition of these surfactants sometimes improved control, but often not to the levels achieved with higher rates of glyphosate, and the response was inconsistent. Having said this, if surfactants or additives are required to improve water quality they should always be used.

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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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