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Does delayed sowing help manage weed populations?

with Dr Gurjeet Gill, Associate Professor – Weed and Crop Ecology, University of Adelaide

The answer to this question is a very simple ‘no’. Waiting for weed seeds with longer dormancy to germinate before sowing costs yield and weeds often set more seed in late sown crops.  

Dr Gurjeet Gill, Associate Professor of Weed and Crop Ecology at The University of Adelaide says sowing a competitive crop ‘on time’ has better outcomes for both crop yield and suppressing weed seed production.

Dr Gurjeet Gill, Associate Professor of Weed and Crop Ecology at The University of Adelaide is one of the four experts presenting the new WeedSmart Crop Competition 101 online course.

“The lack of effective in-crop herbicides leaves growers with few chemical options when it comes to controlling weeds like annual ryegrass and brome grass that are emerging later in the crop,” says Gurjeet. “Our field trials in South Australia demonstrated that sowing ‘on time’ is the best way to maximise crop yield and suppress weeds that germinate in-crop, both with and without the use of pre-emergent herbicides.”

The time of sowing x seeding rate x herbicide field trials were conducted at several sites in South Australia in 2018 and 2019 with GRDC investment.

“The other aspect of these trials was investigating the effect of seeding rate on weed density and seedhead production,” says Gurjeet, “Higher seeding rate increased the yield in wheat at Minnipa at both times of sowing, and did not increase screenings.”

Early sown crops consistently produce more crop and less weeds.

Dr Gill is one of the presenters in WeedSmart’s new Diversity Era ‘Crop Competition 101’ course, which can be completed online in less than 10 hours, giving you a solid grounding in ways to effectively implement tactics that give crops a competitive advantage over weeds.

This free online course can be found at

Why are weeds in cropping systems becoming more dormant?

Short answer: Increased cropping intensity and routine use of pre-emergent herbicides selects for the longer dormancy trait in annual ryegrass and brome grass.

Longer answer: Weed populations have a mix of individuals with different levels of seed dormancy. In self-regenerating pastures, there is likely to be a penalty for high seed dormancy and germinating later than the neighbouring plants. Therefore, high dormancy late germinating plants remain a minority in the population.

However, the situation changes when growers switch to intensive cropping where knockdown herbicides routinely kill the very early germinating plants. In such systems, weeds that emerge with or soon after the crop have a greater survival because they escape the effects of the knockdown herbicides. After several years of cropping, weed populations change from being early germinating to later germinating. These later germinating weed populations are less responsive to delayed sowing and are now common in southern farming systems.

See the Short answer: Grow the most competitive crop possible – cultivar, seeding rate, row spacing, row orientation and time of sowing all have an impact.

Longer answer: In these trials, time of sowing was by far the major contributor to weed suppression and crop yield – even when no pre-emergent herbicide was applied.

Delaying sowing to wait for weeds to germinate after breaking rain is usually counter-productive, unless the delay results in better soil moisture conditions for pre-emergent herbicides. But even when there is a weed control benefit from the later sowing date there is likely to be a penalty on crop yield of at least 20 per cent.

This was demonstrated at the Minnipa site in 2018, where a delay in sowing of wheat reduced in-crop ryegrass density and its seed production, but there was a yield penalty of 25 to 43 per cent.

Also in 2018, the delayed sowing treatment at Marrabel, saw a large reduction in brome grass plant density in barley — however, weed seed production on these fewer plants was high. Delayed seeding also reduced barley grain yield by almost 30 per cent.

In barley, the additional in-crop use of Intervix completely prevented weed seed set at both the on-time and delayed time of sowing. As resistance to Intervix is still quite rare in brome grass, use of Clearfield® crops can be a highly effective part of the management program.

Field trials at various locations across South Australia clearly demonstrated that sowing a competitive crop ‘on time’ has better outcomes for both crop yield and suppressing seed production on annual ryegrass and brome grass. Photo: University of Adelaide.

What is the effect of seeding rate?

Short answer: Higher crop seeding rate can greatly reduce weed seed production.

Longer answer: In these trials, doubling the crop seeding rate from 100 to 200 plants per m2 usually reduced weed seed production by 30 to 40 per cent. In the barley trial, the performance of the late planted crop was improved when the higher seeding rate was used.

In many other trials, very high seeding rates (such as 400 plants per m2) have been shown to vastly reduce annual ryegrass numbers. Using variable rate seeding, growers can consider sowing known weedy patches at very high seed rates (e.g. 250 to 300 plants per m2) simply to outcompete weeds.

Using a sowing rate at the upper end of the recommended range for the chosen cultivar is good practice to help support the efficacy of pre-emergent herbicides early in the season and provide strong competition for weeds that emerge later in the season.

Seed dormancy explainer

Seed dormancy is usually associated with a ‘dark’ requirement, where seeds can remain dormant on the surface, where they are exposed to light, but when they are ‘planted’ the dark requirement is filled and germination follows. These weeds are the target of pre-emergent herbicides in no-till farming systems.

What growers have observed, and researchers have tested, is that some weeds remain dormant even after the dark requirement has been fulfilled, suggesting that some other trigger may be at play. In brome grass, for example, it has been demonstrated that some seeds do not germinate until a ‘cold requirement’ has been fulfilled. These weed seeds remain dormant in the soil until the night temperature reaches 4 degrees C, often well after any pre-emergent herbicide applied at seeding has degraded.

Once the weeds with the cold dormancy trait have established and set seed they can become the dominant in-crop weed pressure to impact crop yield, and future applications of pre-emergent herbicides will have a limited effect on the population.

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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.