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Is one-time tillage a weed control option in a no-till farming system?

with Yash Dang, Senior Research Fellow, University of Queensland

Many would say that the widespread adoption of no-till and minimum till farming underpinned the expansion of cropping in the northern region and saved many farming families from economic hardship.

While costs and erosion damage were reduced, the heavy reliance on herbicides has resulted in a significant shift toward weeds that can thrive in this farming system. Weeds that were previously not considered a problem are now making farming unprofitable or impossible on some no-till paddocks.

Dr Yash Dang, University of Queensland research fellow, says ‘no-till’ does not have to mean ‘never till’. Occasional cultivation can be used as a weed control tactic without having a detrimental effect on the soil resource.

University of Queensland senior research fellow, Dr Yash Dang says the removal of cultivation has also led to an accumulation of certain immobile nutrients such as phosphorus, zinc and potassium in the dry surface layer of the soil where plant roots can not access them.

“Cultivation has a role in distributing nutrients, managing soil and stubble borne diseases and controlling certain weeds,” he says. “The complete removal of tillage for 15 to 20 years or more on some farms has led some farmers to the conclusion that they can not continue as no-till farmers.”

Yash undertook a 4-year project, starting in 2012, to investigate the effect of tillage on a range of soil properties in no-till paddocks throughout the northern cropping region – from Emerald, Qld to Dubbo, NSW.

“We applied tillage using disc and tyne implements and also tested the timing and frequency of tillage operations,” says Yash. “On well-structured soils there was no detrimental effect as a result of the cultivation. Even on more difficult soils, such as those with sodic subsoil or with hardsetting tendencies, one-off tillage operation at the correct soil moisture content caused only limited damage to the soil and repair was evident within two or three years.”

The research also demonstrated that cultivation is a viable weed control tactic within an otherwise no-till system, to prevent seed set of weeds such as fleabane and feathertop Rhodes grass, which flourish in a chemical-dominant control program.

“The positive effect on weed numbers is usually short-lived and has the potential to have negative effects in the years after cultivation,” says Yash. “Growers considering the re-introduction of occasional cultivation must consider all the pros and cons. Cultivation is just another tool in an integrated weed management system – not a stand-alone solution.”

How often should I use cultivation in my no-till system?

Short answer: As a last resort.

Longer answer: A move to no-till farming has provided significant benefits to the soil resource and to farmer’s profitability. This research does not suggest a return to full cultivation, however, the trials showed that one-off, occasional tillage does not have significant detrimental effects on the soil. Care is required in terms of timing, type and frequency of tillage on sodic soils and soils with hardsetting characteristics.

There is a soil moisture loss associated with cultivation so this should be taken into account when making the decision to cultivate.

The type of implement used for a one-off cultivation had little impact on the soil properties. Non-inversion tine or disc implements provided effective weed control when used before weeds flowered and set seed.

When should I use tillage?

Short answer: Before the weeds flower and set seed.

Longer answer: Cultivation should be a last resort measure to treat patches or paddocks where the weed pressure has reached unacceptable levels and where the species present do not have seed that remains viable for many years once buried.

Weeds such as fleabane and feathertop Rhodes grass are good candidates for occasional tillage as a means of preventing seed set. Numbers of these species can be driven down quickly through a dedication to preventing seed set for just a few consecutive years.

In most situations a single pass cultivation is sufficient to achieve the desired effect of reducing the target weed population. If the problem is severe then this single pass operation may be required for a few consecutive years. Timing is critical to achieve good weed control while not sacrificing a planting opportunity.

What other benefits can I expect from occasional tillage?

Short answer: A possible yield increase.

Longer answer: Cultivation will speed up mineralisation in the soil and the breakdown of organic matter. The distribution of immobile nutrients, such as phosphorus, potassium and zinc, deeper into the plant root zone. In a no-till system these nutrients tend to accumulate in the top few centimetres of soil, which is often too dry for plant roots to access. The release of nutrients may support short-term yield response.

Cultivation will also disrupt insect and disease cycles, potentially improving yield and reducing control costs. Pathogens causing diseases such as crown rot of wheat, yellow spot of wheat, ascochyta blight of chickpea and stalk rot of sorghum, can build up in the stubble and soil in a no-till farming system. Likewise soil insects such as Helicoverpa, armyworms and black field earwigs can proliferate in the surface soil.

The type of implement used for a one-off cultivation had little impact on the soil properties. Non-inversion tine or disc implements provided effective weed control when used before weeds flowered and set seed. Photo: L Wise

What are the risks associated with occasional tillage?

Short answer: Potential nutrient and soil moisture loss, and exposure to erosion.

Longer answer: Tillage is accompanied with a loss of soil moisture, however, at most of the trial sites stored moisture improved in subsequent years due to improved infiltration. If possible, cultivation should be done when there is a good chance of sufficient rainfall before the next planting opportunity. Mineralised nutrients and the soil itself is more exposed to runoff following cultivation.

In situations where weed seed has been buried previously, cultivation may bring these seeds back to the surface and initiate fresh germinations. Paddock history is an important consideration in the decision to cultivate.

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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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