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Why look for summer weeds in spring?

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

Sowthistle, windmill grass and feathertop Rhodes grass are problematic weeds for grain growers across Australia. All three are known to evolve resistance to glyphosate, the mainstay herbicide in no-till farming.

These three weeds have several key biological traits in common that make them difficult to control, particularly once the winter crops have been harvested and these weeds have uninhibited access to whatever soil moisture and nutrients are available over summer.

Dr Chris Preston, Chair of the Glyphosate Sustainability Working Group says these three weeds have some natural tolerance of glyphosate, particularly once they have some size, and some populations have been identified that are resistant to robust rates, even when applied to small, actively growing plants.

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“All three species respond to warming temperatures in spring and can germinate late in-crop where they are difficult to control with herbicides,” says Chris. “Windmill grass is a short-lived perennial that can regrow from the crown, giving it a distinct advantage when soil moisture is limited.”

“Another trait that these species share is the short period of seed dormancy. This means that seed will germinate almost immediately after rainfall events in spring once the soil has started to warm and, once established, these plants are also very tolerant of heat and moisture stress.”

“Sowthistle can also germinate in autumn but these populations are easier to control as part of the seeding operation,” he says.

Dr Chris Preston says sowthistle, feathertop Rhodes grass and windmill grass are of concern in all Australian grain growing regions, not just the Northern region.

Dr Chris Preston says sowthistle, feathertop Rhodes grass and windmill grass are of concern in all Australian grain growing regions, not just the Northern region.

Why is sowthistle more of a problem in some years than others?

Short answer: Sowthistle seed does not persist in the soil for very long. More rainfall over spring and summer leads to larger populations.

Longer answer: Burying seed can extend the life of the seed so cultivation is generally not recommended. Sowthistle incursions often begin in non-crop areas where there is little competition. Sowthistle resistance to Group B herbicides is widespread, resistance to Group M (glyphosate) has been confirmed in the northern region and resistance to Group I herbicides has been confirmed in the southern region. This means that one or more of the common summer fallow herbicides do not work on many populations of sowthistle.

If these summer weeds are so hard to control with herbicides, what options do growers have?

Short answer: Herbicides can still play a part but applications need to be well-timed and survivors removed.

Longer answer: The key to control lies in preventing emergence to run down the seed bank. These weeds are not great candidates for harvest weed seed collection even though they are often present at harvest, because the seed is light and easily spread on the wind. They generally don’t perform well in competitive situations so they often become prolific in non-cropped areas like roadsides and fencelines. Avoid spraying these areas with glyphosate alone whenever possible and use non-herbicide options like slashing, pasture and hay making to prevent seed set.

Large sowthistle plants produce huge quantities of air-borne seed but the seed does not persist for long, giving growers the opportunity to intensively manage incursions and non-crop areas to keep weed numbers low.

Large sowthistle plants produce huge quantities of air-borne seed but the seed does not persist for long, giving growers the opportunity to intensively manage incursions and non-crop areas to keep weed numbers low.

Are feathertop Rhodes and windmill grasses really a problem outside the northern region?

Short answer: Feathertop Rhodes and windmill grass are closely related C4 species, which means they grow rapidly in warm, sunny conditions, so they are prevalent in the northern region. However, they are found on roadsides all over Australia – representing a considerable risk in all regions.

Longer answer: These two grasses produce seed that does not persist in the soil for very long, in fact all the viable seed shed germinates the following spring and summer. Like sowthistle, these grasses prefer to have no competition. Feathertop Rhodes incursions often begin in a patch around a mature plant and experience in the northern region suggests that intensive patch management is effective and worthwhile. The most effective control is a double knock strategy of glyphosate plus a Group A herbicide, followed by paraquat. As Group A herbicides are prone to resistance, use of Group A herbicide must be followed with a robust second knock.

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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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