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How does renovating non-wetting sands help weed control?

with Peter Newman, Western Extension Agronomist, AHRI

Some weed control strategies work well in a wide range of situations and some are more targeted. This is one of the targeted strategies, but it does have potential application on some 5 million hectares, or almost one-fifth, of Australia’s cropping land.

Peter Newman, Australian Herbicide Resistance Initiative (AHRI) western extension agronomist, is an enthusiastic proponent of renovating non-wetting sands to ‘grow more crop and less weeds’.

Peter Newman, Australian Herbicide Resistance Initiative (AHRI) western extension agronomist, says weed seed burial works best if the topsoil is fully inverted using a mouldboard plough and then left undisturbed indefinitely. It relies on the grower taking the opportunity to implement a robust integrated weed management program, such as the WeedSmart Big 6.

“The package that works best is controlled traffic farming plus soil inversion plus lime,” he says. “There is very solid proof that increasing the clay content in the topsoil, through mouldboard ploughing, spading or Plozza ploughing, improving pH and burying weed seed will have an enormous effect on farm profitability.”

“Part of the overall benefit is ‘re-setting’ the weed seed bank. Seed burial works best if the topsoil is fully inverted using a mouldboard plough and then left undisturbed indefinitely. It relies on the grower taking the opportunity to implement a robust integrated weed management program, such as the WeedSmart Big 6.”

Peter says the research he and others have done over a period of more than a decade shows that for weed control, the mouldboard plough does the best job. Other claying options can be very effective and less time consuming if the weed burden is not excessive.  

“The results have consistently shown that mouldboard ploughing will bury 99 per cent of the weed seed and crops will respond with a large yield increase,” says Peter. “Some weed species remain viable at depth for only a few years while some other seeds can remain viable for five or more years – but even with these species such as wild radish, only a low percentage of the seeds actually survive that length of time.”  

Most growers who have embarked on a renovation program on non-wetting sands return to a controlled traffic, zero-tillage farming system and expect to not ever have to repeat the operation. This leaves the herbicide resistant weeds safely buried and out of harm’s way. Having re-set the weed seed bank they are then able to concentrate their efforts on keeping weed numbers low – the very best way to avoid herbicide resistance.

    

Is the full restoration process economically worthwhile on non-wetting sands?

Short answer: Yes.

Longer answer: The change in water use efficiency is what drives the turn-around in profitability on this soil type. Growers can expect WUE to increase by about 40 per cent, so if the non-wetting sand is achieving a WUE of 12 kg/mm then crops grown after the renovation program will achieve WUE of around 16 kg/mm. This jump in productivity makes a massive difference to profitability.

In round figures, the mouldboarding/spading operation costs $70/ha, deep ripping to 60 cm depth costs $70 and 4 t/ha of lime sand (spread) costs $100 (depending on location). Mouldboard ploughing (or spading) plus lime has been shown to achieve a return on investment of over 500 per cent over 10 years. An even higher return on investment can result if compacted soils are deep ripped.

This return on investment justifies further investment in gear for controlled traffic farming (to avoid the future need to ameliorate compaction) and to implement a weed control program that relies less on herbicides. 

Amelioration of non-wetting sands makes an enormous difference to the profitability of these sandplain soils and can also re-set the seed bank of herbicide resistant weeds to next to zero.

Does soil inversion affect herbicide use and efficacy?

Short answer: It can. Growers need to be particularly careful with pre-emergent herbicide use for a few years.

Longer answer: Following a renovation operation, additional care is needed to ensure crop safety in relation to pre-emergent herbicides. The reason for this is likely to be the increased wet-ability and the low organic matter in the ‘new’ topsoil, resulting in less binding of herbicide compared to the pre-renovation soil behaviour. Some growers avoid using pre-emergent herbicides for a year or two and re-introduce their use carefully. Trifluralin has been particularly damaging in some situations. Other growers have been able to continue using pre-emergent herbicides while paying particular attention to soil throw out of the furrow.

On the flip side, weed control can improve following soil amelioration because the herbicides are now incorporated into wettable soil and weed seeds no longer get trapped in pockets of dry soil.

Is the weed seed burial a weak link in this system?

Short answer: Only if the sub-soil is disturbed again within five or six years.

Longer answer: Non-wetting sands are prone to compaction and the alleviation of a hardpan contributes to the yield response that results from the full soil amelioration operation. In some instances, future deep ripping can bring buried weed seeds back to the soil surface. This is hard to avoid as the crop often responds favourably to deep ripping, so growers just need to be aware of the potential risk and plan their weed control accordingly.

Once the weed seed bank is ‘re-set’, how do I stop weed blow-outs after ameliorating the soil?

Short answer: Implement as many of the WeedSmart Big 6 tactics as you can.

Longer answer: We know that herbicide-only weed control is no longer an option so it is essential to use a combination of herbicide and non-herbicide tools to stop seed set and keep weed numbers low. No matter what the soil type, you need to be doing some form of harvest weed seed control right now. There are a heap of options and some are very easy and relatively cheap to adopt. Following that you need to look at ways to increase crop competition and maximise the effectiveness of the herbicide options that are still available. Herbicide resistance can be beaten!

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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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