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How can I get the most bang from crop-topping canola?

with Greg Condon, Grassroots Agronomy and AHRI

In an environment of increasing herbicide resistance, getting back in the driver’s seat with weed control relies on stacking tactics and not leaving all the heavy lifting to just one or two strategies.

Greg Condon, Grassroots Agronomy and Australian Herbicide Resistance Initiative (AHRI) southern extension agronomist, works with growers to develop and implement combinations of control measures that complement each other to drive down weed seed set.

Greg Condon, Grassroots Agronomy and Australian Herbicide Resistance Initiative (AHRI) southern extension agronomist, advocates crop-topping plus harvest weed seed control in canola as an effective way to drive down annual ryegrass and wild radish numbers.

“One of the combinations we are advocating is crop-topping plus harvest weed seed control,” he says. “Canola is a good candidate for this package, particularly to drive down annual ryegrass numbers.”

“Being able to apply the registered glyphosate products early in the crop senescence in canola gives growers a better opportunity to interrupt weed seed set. Following the herbicide with a non-herbicide tactic like harvest weed seed control helps protect the herbicide chemistry by taking another swipe at weed seed that may have evaded the crop-topping tactic.”

Greg says the over-the-top option is generally a more practical option than applying the registered glyphosate products under the cutterbar if the crop is windrowed. 

“Knowing the glyphosate resistance status of weeds in a paddock is really important,” says Greg. “If the weeds present have resistance to glyphosate then crop-topping is ineffective and should not be used.”  

On some farms canola may be grown as often as every second year, putting heavy pressure on the crop-topping tactic. Under these circumstances, adding harvest weed seed control and maximising crop competition is absolutely essential.  

What herbicides are registered for late season weed control in canola?

Short answer: Diquat and two specific glyphosate formulations*.

Longer answer: Products registered for application prior to direct heading or before/under the windrow* are weedmaster DST and Roundup Ultra Max. Weedmaster DST has a withholding period of five days. Diquat is also registered for use in canola (WHP 4 days).

The glyphosate products registered for canola can be applied to mature standing crops with a minimum of 20 per cent seed colour change to dark brown or black. Higher end label rates are recommended when crops or weeds are dense. Glyphosate must not be applied to crops intended for seed. Refer to the label prior to use and follow the instructions.

Glyphosate must not be applied to standing crops and again at windrowing.   

*Pintobi Attack is also registered for this use pattern but is not a commercial product at this time.

What other benefits do I get from crop-topping in canola?

Short answer: A head start on summer weed control.

Longer answer: Crop-topping with high water rates gives good coverage and can penetrate dense crops to kill small, germinating weeds (such as sowthistle) as well as reducing weed seed set in mature weeds (such as annual ryegrass and wild radish). This essentially provides the first summer weed control application and should reduce the use of glyphosate over summer.

Crop-topping has the added benefit of providing early control of summer growing weeds like sowthistle while they are still small.

What if I suspect glyphosate resistance is present?

Short answer: Reduce glyphosate use across the cropping program and look for ways to protect this chemistry by driving down weed numbers.

Longer answer: Collect weed seed samples and have them tested for herbicide resistance. Knowing what still works is just as important as confirming your suspicions. If immature weeds are present they can be collected and sent for a QuickTest that will give you an opportunity to take action within the season.

Research by Dr Peter Boutsalis and others has confirmed that late season applications of glyphosate on glyphosate-resistant annual ryegrass provides no control and is essentially a waste of money. This research showed that 80 per cent of the seed on glyphosate resistant annual ryegrass remains viable after crop-topping.

Two take home messages from this research are: 1. treating younger plants at lower temperatures can improve glyphosate efficacy on resistant biotypes and 2. crop-topping with glyphosate is not effective on glyphosate resistant ryegrass.

Before crop-topping canola it pays to understand the herbicide resistance profile of the weeds present. Crop-topping glyphosate resistant annual ryegrass is not effective as 80 per cent of the weed seed remains viable after crop-topping.

Which harvest weed seed control method is the best crop-topping partner?

Short answer: All harvest weed seed control (HWSC) methods provide very similar results – the key is the do everything you can to get the weed seeds into the front of the header.

Longer answer: Harvesting low and setting up your harvester to maximise weed seed capture are very important. Canola is a good crop for implementing HWSC because harvesting low does not add significantly to the amount of material that must be processed through the header. Many growers are also using canola chaff captured through HWSC tactics such as chaff carts, chaff lining or chaff decks as a high quality feed source for sheep when grazed.

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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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