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The safest way to manage pre-em herbicides at seeding?

with Sam Kleemann, Research Associate, University of Adelaide

Achieving effective pre-emergent weed control while protecting crop safety at seeding is no mean feat, with plenty of variables to manage.

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Dr Sam Kleemann, research associate at the University of Adelaide, has investigated seeding systems and pre-emergent herbicide combinations that give the best results under different circumstances.

“Effective early weed control is more important than ever, and pre-emergent herbicides have a big role to play, often delivering multi-generation weed control while the crop gets established,” he says. “The difficulty is that these herbicide products vary significantly in their solubility and their tendency to bind to organic matter, and this can lead to considerable variability in weed control efficacy and crop safety.”

Dr Kleemann says that understanding the way different products behave will influence the decisions growers and advisors make when selecting products and setting up their equipment for seeding.

Dr Sam Kleemann says seeder set up and product choice are pivotal decisions when using pre-emergent herbicides to control weeds early in the crop cycle. Photo: Alistair Lawson

Dr Sam Kleemann says seeder set up and product choice are pivotal decisions when using pre-emergent herbicides to control weeds early in the crop cycle. Photo: Alistair Lawson

“There are a few rules of thumb that can help at seeding,” he says. “The first is to minimise soil disturbance so that weed seeds remain on the soil surface as much as possible.”

“The second is to remember that pre-emergent herbicides can cause crop damage. It is essential to separate the product from crop seed.”

“The third rule is to choose the right herbicide for the job and follow label recommendations closely. There are many differences in the properties of pre-emergent products such as their volatility and their rate of degradation in sunlight.”

Tined seeding systems fitted with knife-points generally provide for better crop safety when using pre-emergent herbicide than low disturbance discs. Triple discs are safer than single disc seeders provided adequate soil throw is achieved.

Why is minimising soil disturbance at seeding so important?

Short answer: Weed seeds on or very near the soil surface have the most exposure to the concentrated band of herbicide.

Longer answer: If weed seeds are buried and mixed through the soil they have a greater chance of avoiding the herbicide than when they are concentrated on or near the surface. If a weed seed germinates and establishes a root system before intercepting the pre-emergent herbicide, the weed is often able to grow through the herbicide layer. While it may be suppressed, it probably will not die.

What is the best way to keep herbicide away from the crop seed?

Short answer: Follow the label instructions carefully. Pre-emergent herbicides are mostly non-selective and will cause crop damage if they are thrown or washed back into the furrow.

Longer answer: Seeding systems that throw soil away from the furrow provide the greatest protection for the crop. Knife-point seeders generally provide sufficient herbicide incorporation while maintaining separation from the crop seed. Avoid excessive soil throw from one furrow to the next. Pre-emergent herbicides are not recommended for use with low soil disturbance disc seeders due to the unreliability of herbicide separation from crop seed and increased risk of crop damage.

Why do the label instructions for pre-emergent herbicide products vary so much?

Short answer: Each product has different properties that impact on efficacy and crop safety.

Longer answer: Properties such as solubility, need for incorporation, binding on organic matter, rate of breakdown in the environment and so on make a big difference to how a herbicide is used. Use this information to make product choice decisions that take into account the stubble load, short-range rainfall forecast, sowing date and seeding equipment and set-up.

How to ask a WeedSmart question

Ask your questions about protecting emerging crops for pre-emergent herbicides at seeding 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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