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Is it possible to apply additional competition to inter-row weeds?

with Hanwen Wu, Principal Research Scientist, NSW DPI

The inter-row space provides an ideal environment for weeds to grow, particularly if pre-emergent herbicides are not applied or are less effective than expected.

NSW Department of Primary Industries principal research scientist, Dr Hanwen Wu says filling the inter-row space with a productive species might be another way to suppress weed growth and reduce seed production of herbicide resistant weeds.

Dr Hanwen Wu, NSW DPI is investigating new ways to increase crop competition, particularly in the crop inter-row space.

“There is very strong evidence that narrower rows are an excellent way to increase crop competitiveness but there are some practical limitations,” he says. “We have looked at a combination approach, of planting most of the seed in rows and the rest broadcast to ‘fill in the gaps’. We have called this the ‘compound sowing technique’.”

In 2016 Hanwen set up two field trials at different locations near Wagga Wagga, NSW to assess the effect of this system on weeds and crop yield in both narrow (22.5 cm) and wider (45 cm) row spacings, with and without IBS trifluralin. Three broadcast species were evaluated – wheat, gland clover and French serradella. The two sites were assessed to have an initial annual ryegrass density of 48 and 25 plants/m2.

“We sprayed out the broadcast legumes in early September to prevent them competing with the crop for moisture,” says Hanwen. “Further trials are needed to test a range of parameters such as suitable legume species, optimal seeding rates, proportion of seed broadcast, row spacing for the conventional seeding and different pre-emergent herbicide options.”

Although the 2017 season did not allow Hanwen to replicate this trial he is keen to do more trials in 2018.

“We think this technique has merit and our initial trial suggested that weed suppression can be achieved without any yield penalty,” he says. “There even seems to be situations where a yield increase can be achieved in response to reduced weed pressure.”

Crop competition is rightfully attracting more attention from farmers and researchers in the war on herbicide resistant weeds. It is a numbers game and crop competition can play an important role in vastly reducing weed seed set.

Which was the most competitive broadcast species?
Short answer: Wheat.

Longer answer: At the weediest site the broadcast wheat treatment, without IBS trifluralin reduced annual ryegrass biomass by 71–77 per cent at both the narrow and wider row spacings. In the presence of less weeds the broadcast wheat still reduced weed biomass by 50 per cent in the narrow rows and 27 per cent in the wider row configuration.

IBS trifluralin further increased weed suppression at both sites and both row spacings. At the weedier site, annual ryegrass biomass was suppressed by 88–90 per cent. Where there were less weeds present the addition of IBS trifluralin increased biomass suppression from 27 to 70 per cent at the wider row spacing.


Compound sowing technique (conventional + broadcast sowing) dramatically increases crop competition in the inter-row compared to conventional sowing. Of the three broadcast species tested, wheat provided the strongest suppression on weed biomass.

What was the effect on yield?

Short answer: The wheat yield increased by 15–22 per cent at the weediest site when wheat was used as the broadcast species.

Longer answer: In the favourable season of 2016, only broadcast wheat generated a yield increase, and only in the presence of higher weed pressure. None of the broadcast treatments caused a yield reduction at either site. Further trials are required to evaluate the impact of site and seasonal climatic conditions on the weed control and crop yield associated with the compound sowing technique.

The broadcast legumes may provide additional soil fertility and moisture retention benefits while maintaining crop yields. More work is needed to identify more competitive legume species to have a greater impact on weed biomass and to identify the optimal timing to kill broadcast legumes to maximise weed suppression and minimise yield loss.


Using a broadcast legume that is sprayed out in September could have additional soil health and moisture retention benefits and warrants further investigation.

Have any farmers tried this idea?

Short answer: Yes.

Longer answer: Leigh Bryan at Swan Hill has tested this idea on his farm – he calls it zero-row spacing. Also in 2016, a strip-trial in barley resulted in the zero row spacing strip yielding 4.994 t/ha compared to 4.889 t/ha in the conventionally sown crop at 37.5 cm spacing. This was achieved with no pre-emergent or in-crop herbicide applied.

Leigh has noticed that the random placement of stubble is easier to sow through the next year and it still provides trellising for pulse crops and shades the soil to conserve moisture and reduce soil surface temperatures.

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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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