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Does chaff in a chaff line suppress weeds?

with Annie Ruttledge, Weeds researcher, DAF, Queensland

In the wake of rapid adoption of chaff lining, the newest harvest weed seed control tool developed by Australian farmers, a substantial research effort has been made to validate the efficacy of this practice.

Chaff lining involves depositing weed seed-laden chaff in a narrow line behind the header. Some growers using this practice have suggested that as the chaff in the chaff line rots away, much of the weed seed also decays in the process. Researchers working across the northern grains region have now gained a deeper understanding of what happens to weed seed in a chaff line.

Dr Annie Ruttledge, DAF Qld weeds researcher has been investigating weed emergence from chaff lines.

Department of Agriculture and Fisheries, Queensland weeds researcher Dr Annie Ruttledge and several collaborating scientists have been looking into different aspects of weed seed decay and weed suppression in the chaff line.

“Non-herbicide tools like chaff lining are very important to help manage the onset and spread of herbicide resistance in weeds,” says Dr Ruttledge. “The idea with harvest weed seed control tactics is to collect any weed seed present at harvest height, usually above 15 cm. With chaff lining, these weed seeds are deposited in a narrow line of chaff behind the harvester.”

“Burial in the chaff line can suppress emergence in some weed species, but it does not guarantee that no weeds will emerge from chaff lines,” she says. “Harvest weed seed control tools like chaff lining and chaff tramlining concentrate the weed seed into confined zones where emergence can be monitored and action taken as required, without treating the whole paddock.”

Chaff lining (and chaff tramlining) deposit weed-laden chaff in a narrow line behind the harvester or directed onto the tramlines.

Does the chaff line suppress weed emergence?

Short answer: Yes, if the weed seed is buried deeply enough in the chaff. Many weeds in no-till and reduced-till farming systems prefer to germinate on the surface where there is plenty of light.

Longer answer: Our trials investigated the effect of chaff on germination rates of annual ryegrass and common sowthistle. The sowthistle seed was more readily prevented from emerging than annual ryegrass seed, probably due to the different requirements of the species for light. Maximum suppression of annual ryegrass emergence was achieved with a chaff load of 42 t/ha, which can be feasibly produced in a 3.5 t/ha cereal crop. In contrast a chaff load of just 12 t/ha of wheat chaff was sufficient to suppress emergence of common sowthistle seed.

Work done by our collaborator Dr John Broster, CSU found that chaff from cereal crops generally provided better suppression of annual ryegrass compared to canola and pulse chaff. For all chaff types the higher the rate per hectare the better the suppression.

One of the experiments involved determining the level of chaff required to effectively suppress emergence of annual ryegrass and common sowthistle.

What’s the difference in suppression in chaff lines compared to chaff tramlines?

Short answer: Chaff tramlining effectively halves the amount of chaff in each line, potentially reducing the suppressive potential of the chaff.

Longer answer: Placing a single line of chaff behind the harvester (or directing all the chaff from a chaff deck into one tramline) maximises the amount of chaff and therefore the level of weed suppression. Different crop types, sowing rates and crop yield all influence the quantity of chaff produced.

In addition to looking at suppression of emergence, we looked at weed seed decay under field conditions. In these trials there was no evidence that weed seeds rotted more rapidly in a chaff line than on the soil surface. However, we expect that environmental conditions play a large part in weed seed decay so the results could vary according to season, with more rotting likely in a wet year than in a dry year. The depth and persistence of chaff cover and the type of weed species are other factors that would influence seed persistence in chaff lines or chaff tramlines.

Harvest weed seed control tactics aim to collect weed seed at harvest and concentrate it in a small zone where weeds can be targeted at a fraction of the cost of whole-paddock treatments.

What are the options for treating the weeds in the chaff line or tramlines?

Short answer: Farmers are leading the way with practical solutions to manage weeds along chaff lines and chaff tramlines.

Longer answer: Some growers use an optical sprayer or a boom with nozzles only operating on the chaff line or tramlines to apply a mix of herbicides that may be too expensive to apply to the entire paddock. Weed seed that is collect at harvest and placed in the chaff line, may have survived in-crop herbicide applications and may be herbicide resistant. Susceptibility testing can help identify herbicides that can provide effective control.

Non-herbicide options are to wait for germination and chip or trample the weeds. In a mixed farming operation, sheep can graze the chaff lines rendering most of the weed seed unviable, and the sheep will benefit from an additional feed source.

Growers who have been using chaff lining and chaff tramlining for several years report that the high concentration of weed seed leads to a high level of competition between the weeds and this is compounded with competition from the following crop. Over time, seed set reduces and any weed seed produced will be collected and returned to the chaff line the following year. When chaff is deposited on the wheeltracks, weeds that emerge face dry, compacted conditions and are often subject to frequent passes with heavy machinery.

This chaff tramline was sprayed out using a shielded sprayer.

Does it matter if I use a draper or a stripper front?

Short answer: No, not in terms of amount of weed seed harvested, provided you set up and operate with weed seed collection in mind.

Longer answer: Our collaborators Dr John Broster, Dr Michael Walsh and Annie Rayner conducted trials with stripper and draper fronts at two trial sites. The results at one site showed that it is possible to achieve the same level of weed seed collection with the two harvester front options. At the other trial site, the stripper front was not as effective as a result of wider row spacing, higher operating height and faster operating speed compared to the draper front at the same site. The key to success with chaff-only harvest weed seed tools is getting the weed seed in the front and effectively separating the weed seed and chaff from the straw component. The WeedSmart website provides practical information about setting up different harvesters and operating them for effective harvest weed seed control.

Bear in mind that a stripper front will generate a lot less chaff than a cutter type front, and so this is likely to influence weed suppression and the rate of weed seed decay.

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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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