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Do canola biotypes have allelopathic effects on weeds?

with Jim Pratley, Research Professor in Agriculture, Charles Sturt University

Allelopathic traits were never important in plant breeding in the past because there was a suite of herbicides available to control weeds. Now, with high levels of herbicide resistance, around the world there is a renewed focus on allelopathic effects in several crops, including wheat, rice and canola.

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Canola plants have varying abilities to interfere with the germination and growth of other nearby plants, including weeds. This interference is usually a combination of allelopathy and competition. Determining which canola varieties carry the genetic capability to produce allelopathic substances that suppress important weeds is of great interest to Charles Sturt University (CSU) research professor, Jim Pratley and his team.

“Modern plant breeding programs assess the productivity and other traits of a new hybrid in a weed-free environment,” says Professor Pratley. “This means that genotypes that have allelopathic capability do not have an opportunity to demonstrate their value to a real-world farming system where weed pressure is an inevitable part of plant production.”

Research at CSU has focused on screening canola genotypes for their effectiveness in controlling ryegrass and found that some varieties in the current genotype collection provide good non-herbicide control over the target weed, annual ryegrass, and a number of other species such as shepherd’s purse, Paterson’s curse and brassica weeds like wild turnip.

“More non-herbicide tools are required to help manage herbicide resistant weeds,” he says. “Crops that can be sown into weedy paddocks and that can suppress weed seed germination using allelopathy or reduce the number of weeds setting seed through crop competition offer a useful tool for growers.”

Professor Jim Pratley (right) and PhD student Md Asaduzzaman are very keen to see the research work on allelopathy in canola continue as the importance of non-herbicide weed control measures become increasingly important in farming systems.

Professor Jim Pratley (right) and PhD student Md Asaduzzaman are very keen to see the research work on allelopathy in canola continue as the importance of non-herbicide weed control measures become increasingly important in farming systems.

How do you identify the difference between allelopathy and competitiveness in a hybrid?

Short answer: It can’t be done in the field. This is for the lab only.

Longer answer: In the laboratory, the canola plants are grown in agar, in a weed-free environment for one week. Then the target weed seed is introduced and the development of the weed root system is monitored. The plants do not need to compete for light, nutrients or water so any suppression of root development can be ascribed to the herbicidal effect of the substances that the canola plants have exuded into the agar.

Can a variety be both competitive and allelopathic to weeds?

Short answer: Yes, but not always.

Longer answer: Once the allelopathic capability of a variety is established, researchers grow the variety in the field with no herbicides applied to control weeds. Any additional weed control that is achieved in the field can be ascribed to the ability of the hybrid to out-compete the weeds present for light, nutrients and water resources.

The team at CSU have screened half of the available canola genotypes to rank them according to their ability to contribute the genes required to produce the allelopathic substances to suppress annual ryegrass and other key weed species.

The team at CSU have screened half of the available canola genotypes to rank them according to their ability to contribute the genes required to produce the allelopathic substances to suppress annual ryegrass and other key weed species.

Is canola the only crop that has allelopathic potential?

Short answer: No. Researchers around the world are also finding wheat and rice cultivars vary in their allelopathic capability. In other countries, this information is being actively pursued in the respective plant breeding programs. Research in this field in Australia is sadly lagging behind.

Longer answer: Work in the area of allelopathy is not new but it has not featured as a high priority trait in plant breeding programs in modern agriculture, particularly since herbicides have been widely used in cropping systems. Many older varieties and cultivars have much stronger allelopathic ability than modern ones. This trait can be re-introduced into high performance varieties and hybrids using either traditional plant breeding methods or gene transfer. Keep in mind that weeds also have allelopathic capability and are not afraid to use it.

How to ask a WeedSmart question

Ask your questions about using crops with allelopathic capability to manage herbicide resistant weeds on  Facebook or Twitter @WeedSmartAU or leave a comment below.

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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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Testing for herbicide resistance

“Testing takes the guesswork out of the equation and gives farmers baseline information that they can use to monitor changes in the weeds on their farms,” he said. “If low level resistance is identified early there are many more management options available compared to situations where full blown resistance has taken hold.” Dr Boutsalis said the over use and over reliance on particular herbicides will unavoidably lead to herbicide resistance developing. “We often hear of farmers applying herbicide even though they are not sure if it will work,” he said. The $300 to $400 cost of testing is insignificant compared to the cost of wasted herbicide, lost production and the costs of driving down a large seed bank of resistant weeds. What herbicide resistance tests are available to farmers in Australia? Short answer: The ‘quick’ test using the whole plant and the ‘seed’ test. Longer answer: The ‘quick’ test uses plant samples collected on farm and sent to the laboratory. The plants are revived and planted into pots then tested against the required herbicides. The ‘seed’ test requires the collection of ripe seed, which is planted out at the laboratory. After dormancy has been broken and the seedlings have started to grow they are tested for their response to herbicides. Both tests are equally accurate. The ‘quick’ test can not test for resistance to some pre-emergent herbicides, such as trifluralin. Which is the most common test that farmers use? Short answer: The seed test. Longer answer: Collecting seed before or at harvest is the most common method used. The collected seed must be mature, from green to when the seed changes colour. Before harvest collect 30 to 40 ryegrass seedheads or several handfuls of wild oats seed. After harvest it is common to find seedheads still in the paddock or samples of contaminated grain can be sent for analysis. Where is the best place to collect samples? Short answer: From suspicious or high risk areas. Longer answer: Herbicide resistance can develop in high risk areas like fencelines or at random through a paddock. Visual observations and changes on the yield monitor in the header can indicate good places to collect seed. If collecting plant samples, look for weeds at the early tillering stage that appear to have ‘escaped’ previous herbicide treatment. Collect 50 to 100 small plants or fewer larger plants. Shake off the soil from the roots, place in a plastic bag and send to the laboratory. What’s involved in sending samples? Short answer: Pick, pack, register and ship. Longer answer: Each sample needs to arrive at the laboratory with suitable identification and instructions. Register the samples online to get a unique sample number and to provide the information required, such as which herbicides you want to test against. Plant Science Consulting and Charles Sturt University both offer commercial herbicide resistance seed testing. Find the details under Point 4 of the 10 Point Plan on the WeedSmart website.   How to ask a WeedSmart question Ask your questions about the spread of herbicide resistance, or any herbicide resistance management strategy, using this blog or using Twitter @WeedSmartAU.

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