We’re here to help make winning the battle against crop weeds simple.
There are 6 core points for you to follow and some additional WeedSmart Wisdom points for you to consider. Discover practical tip and tricks to implement these strategies on-farm and further information on the research to back up these steps. This plan is all about the farmer calling the shots, not the weeds.
WeedSmart WisdomView all
Never cut the herbicide application rate
Scientific studies have demonstrated that resistance can rapidly evolve in weeds subjected to low doses of herbicide. Some weeds can develop resistance within a few generations.
Full rates when mixing herbicides too!
When mixing herbicides it is important that each product is still applied at the full label rate to ensure high mortality.
Applying different chemicals in one mix can provide an additive advantage. It is important to understand the mode of action of each herbicide on the plant when preparing a herbicide mix. This is just as important for pre-emergent grass weed mixes as it is for post-emergent mixes aimed at broadleaf weed control. ALWAYS READ THE LABEL.
Surrounding weed seeds with a combination of pre-emergent herbicides with different modes of action can give a high level of control and help extend the useful life of all the chemicals used. The high level of control must be supported with additional control measures for all survivors. All products with different modes of action must be applied at full label rates for this to be an effective strategy.
Mixing two chemicals with the same mode of action can achieve some additional efficacy, however, the mix should deliver the combined full rate to ensure a lethal dose. The amount of stubble present and crop safety are all important considerations when mixing chemicals. For example, when using a tank mix of Avadex® and trifluralin to control ryegrass in wheat, the rates used will vary depending on the sowing system and level of stubble retention. Be sure to get good advice.
Many herbicides on the market are a combination of two or more modes of action within the one product. These products must be applied at the full label rate to be effective. Having dual action does not negate the need to change herbicide products and rotate modes of action. Repeated use of any single strategy will reduce the effectiveness of that strategy over time.
Spray well – correct nozzles, adjuvants and water rates
Spray application is a technical field and growers need to make sure their equipment and application techniques are spot-on. The GRDC Spray Application GrowNote provides detailed information and about 80 videos to demonstrate key skills.
The focus of spraying herbicide needs to be on doing the job right so the weeds receive the correct dose and die, and this includes reducing the air borne fraction to a bare minimum.
Bill Gordon’s 10 Tips for Reducing Spray Drift
Choose all products in the tank mix carefully.
Understand the product mode of action and coverage requirements.
Select (and check) the coarsest spray quality that will provide effective control.
Expect that surface temperature inversions will form as sunset approaches and will likely persist overnight and even beyond sunrise on many occasions. DO NOT SPRAY.
Use weather forecasts to inform your spray decisions.
Only start spraying when the sun is about 20 degrees above the horizon and when the wind speed has been above 4–5 km/hr for more than 20–30 minutes, and clearly blowing away from any adjacent sensitive crops or areas.
Set the boom height to achieve a double overlap of the spray patterns.
Avoid higher spraying speeds.
Leave buffers unsprayed if necessary and come back.
Continue to monitor conditions, particularly wind speed, at the site during the spray operation
High water rates don’t have to slow you down
Some growers are concerned that increasing the water rate when applying herbicide will slow down their spray operation and cost them money. However, the biggest financial loss during spraying usually comes from a failed spray job.
To keep your spray operation as time efficient as possible when using more effective and reliable application volumes, you can:
Use nurse tanks around the farm to reduce the time spent travelling back to a central re-fill point.
Use a larger pump, e.g. 2.5 inch, to make re-filling quicker.
Pre-mix the batch while the sprayer is operating. Many mixes can be held in the mixing tank for up to 6 hours. However, wettable granules and suspension concentrates will need agitation to keep them in solution.
For pre-emergent herbicides in high stubble situations, carrier volume has a large effect on the level of control achieved. Across four trial sites Dr Borger’s research demonstrated that ryegrass control with trifluralin or Sakura® increased from 53% control when the carrier volume was 30 L/ha to 78% control when the carrier volume was increased to 150 L water/ha in high
Water quality and mixing order
Water quality is often overlooked as a possible contributor to herbicide failure and can lead to confusion over the herbicide resistance status of weeds on a property.
Water should be considered as one of the chemicals in any mix, given that water quality varies markedly depending on its source. Getting the mixing order right is essential for effective spray results.
Don’t start mixing until the water quality is right
Podcast – Mixing herbicides
Sometimes adding an adjuvant is beneficial and sometimes it is detrimental; and there is an art to knowing how to best deploy these additives.
When weeds are susceptible to the applied herbicides, the effectiveness of adjuvants generally goes un-noticed. Correctly applied adjuvants can reduce the impact of low level herbicide resistance by helping to maximise the amount of herbicide taken up by the plant.
Clean borders – avoid evolving resistance on the fence line
About one-quarter of glyphosate-resistant populations within broadacre cropping situations across Australia come from fencelines and other non-cropping areas of the farm.
Along paddock borders, where there is no crop competition, weeds can flourish and, if not controlled, set lots of seed. The traditional approach has been to treat these weeds with glyphosate to keep borders clean but after 20-odd years this option is now failing and paddock borders are becoming a significant source of glyphosate-resistant weed seed.
Weed researcher Eric Koetz said the limited options for managing weeds along irrigation infrastructure and other non-crop areas is a problem and is putting additional pressure on knock-down herbicides in irrigated systems.
In some situations, cultivation can be used to kill the weeds and provide a firebreak, but on light soils this may pose an erosion risk and mowing or slashing may be safer options. Another possible tactic is to continue using herbicides but to ensure that a clean-up operation is carried out before any survivors can set seed.
Some growers are choosing to increase the heat on weeds along the borders by planting the crop right to the fence and then baling the outside lap and spraying with a knockdown herbicide to kill any weeds and provide a firebreak.
Another good option in some situations is to maintain a healthy border of vegetation using non-invasive grasses. In Queensland, buffel grass is a good example of a grass that can outcompete other weeds while not invading crop lands.
If only herbicides are used on fencelines, resistance is inevitable. Surviving weeds on fencelines have no competition and access to plenty of soil moisture, so they set a lot of seed and resistance can easily flow into neighbouring paddocks.
It’s time for a glyphosate intervention
Farm hygiene cottons on – Cleave Rogan, St George
What’s new in management of herbicide resistant weeds on fencelines?
Keeping the farm clean – Graham Clapham, Norwin
Don’t jeopardise glyphosate for clean fencelines
Keeping fencelines clean
Resistance risk to knock-down herbicides on irrigated cotton farms
Clean seed – don’t seed resistant weeds
This can potentially reduce crop yield and almost certainly means that the weeds will set abundant seed and most likely shed that seed before the crop is harvested, increasing the weed pressure in future years.
The best way to ensure clean crop seed is to buy certified weed-free seed each year. But many growers prefer to retain some grain on-farm for planting the next year. For best results growers usually harvest seed from their cleanest paddocks and conduct some form of seed cleaning either on or off-farm.
However, research shows that there is a tendency to underestimate weed seed contamination in seed retained for planting.
An AHRI study on 74 farms across the Western Australian grainbelt showed 73% of cleaned crop seed samples had some level of weed seed contamination. The up-side is that 25% of cleaned samples were weed-free, so it can be done!
This means that many unknowingly introduce significant levels of weed and volunteer crop seeds into the farming system at seeding time, even when crop seed has been cleaned. More alarmingly, many of these weed seed populations are resistant to a range of commonly used post-emergent herbicides.
Uncleaned crop seed samples can have almost 25 times more contamination than cleaned crop seed. It is important to remember that resistance will evolve faster from introducing resistant weed seeds into a paddock, compared to resistance evolving independently in that paddock.
The cleaning method used strongly influences contamination levels – a ‘gravity table’ is the most effective, followed by other methods such as rotary screens and sieves.
Contamination levels of each cleaning method for all contaminants.
Crop type also has a significant effect on the amount of contamination, with wheat containing much higher annual ryegrass seed numbers than barley, possibly because barley was more likely to out-compete weeds during the growing season.
Another advantage of having seed professionally cleaned and graded is that larger crop seeds can be retained, promoting stronger seedling vigour and higher germination rates.
Systems promoting farm hygiene such as meticulous seed cleaning and sanitising tillage, sowing and harvesting equipment between paddocks will help prevent the introduction of new weed species, noxious weeds and herbicide resistance.
Testing for herbicide susceptibility and resistance
There are several reasons why weeds might survive a herbicide treatment but it is increasingly common for herbicide resistance to be the culprit.
Testing weeds for herbicide susceptibility and resistance can save growers thousands of dollars, making the investment of a few hundred in testing very worthwhile.
There are two main tests – The Quick Test and The Seed Test
The Quick Test is done using weed plant samples collected in-crop and provides the results within a few weeks.
The Seed Test is done on weed seed samples, usually collected around harvest time and the results take a few months.
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.
The value of the Quick Test is that you can find out what herbicides still work on the weeds collected and this gives you the option to use a different herbicide to treat the weed patch within the same season and before the plants set seed.
Gathering samples for the Quick Test
collect from the middle of the patch of weeds that are suspected to be resistant
if the weeds are large, collect 20 plants
if the weeds are small, collect 50 plants
shake off the loose dirt and place the sampled weeds in a zip-lock plastic bag
do not add water to the bag
keep the sample cool
if possible, collect and send samples on a Monday or Tuesday
sample from different patches in the paddock, note the location/s and keep samples from different patches separate
send by express post to Plant Science Consulting Check the website for details about the services offered, costs and specific instructions before submitting samples.
Gathering samples for the Seed Test
Collecting weed 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.
Keep samples from different locations separate and details noted on the bag. Only use paper bags (double layer) to collect and send seed samples. Ensure bags are sealed so that the samples don’t mix during transit.
There are three weed seed testing services in Australia:
Plant Science Consulting
CSU Herbicide Resistance Testing
UWA Herbicide Resistance Testing
Visit the website/s for details about the services offered, costs and specific instructions before submitting samples.
How to collect samples for the Quick Test
How the Seed Test works
In the podcast below, Dr Peter Boutsalis provides practical tips on collecting weed seeds for herbicide resistance testing. You can also see visual examples of weed seeds ready for collection on the podcast page here.
Testing for herbicide resistance with Dr Peter Boutsalis
Testing for herbicide susceptibility pays off
What herbicides still work
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.