Read time: 3 minutes

Avoid transporting herbicide resistance across the landscape

Herbicide resistant weeds are spreading through Queensland’s landscapes using all the available modes of transport—vehicles, animals, wind and water.

Good weed management practice is the only way to slow the dispersion of resistant weeds. The first step is addressing the practices that contribute to the herbicide resistance. The second step is to look at how resistant weeds might move around the landscape.

Finding new and varied ways to control weeds is the challenge for both farmers and the managers of our ‘linear corridors’, such as roads, railways and irrigation channels.

Some weeds are more vulnerable to herbicide resistance that others, but the repeated and frequent use of just one chemical and few, if any, non-chemical weed control tactics increases the chance of herbicide resistance in all species. This has great potential to remove currently effective herbicides from the available options for weed control.

If a high risk weed is managed using high risk tactics then herbicide resistance will occur sooner rather than later. Queensland Department of Agriculture, Fisheries and Forestry (DAFF) senior research scientist, Dr David Thornby, said this resistance can occur on grain farms and in the connecting corridors across the landscape, and transfer from one site to another can easily occur.

“The way weeds are managed in each situation is the main determinant of herbicide resistance risk,” he said.

“If a grains paddock is connected to other land use zones, such as a roadside, cotton paddock or a livestock enterprise, there is a risk that herbicide resistance can develop on these other zones, if high risk tactics are employed, and be transferred to the grains paddock, and vice versa.”

DAFF senior research scientist, Dr David Thornby, said herbicide resistance can occur in weeds on grain farms and in the connecting corridors across the landscape, and transfer from one site to another can easily occur.

DAFF senior research scientist, Dr David Thornby, said herbicide resistance can occur in weeds on grain farms and in the connecting corridors across the landscape, and transfer from one site to another can easily occur.

Dr Thornby said the more ‘connected’ a paddock was to the landscape the higher the risk of herbicide resistance potentially being transported across the landscape.

Some managers of non-cropping areas such as roadsides, irrigation channels, national parks, railway tracks and powerlines, are limited in their weed control resources and options.

For many, herbicides are the only viable management strategy and sometimes there are community or political pressures to avoid using certain herbicides. This often leads to an over-reliance on glyphosate.

“Glyphosate is the most widely used and useful herbicide available but its repeated and frequent use places strong selection pressure on a weed population, leading eventually to glyphosate resistance,” he said.

“If this herbicide is ‘lost’ to land managers, on farms or on public land, weeds will gain the upper hand and there will be few control options available.”

In practical terms, Dr Thornby recommends that weed management teams responsible for weed control on public land and the linear corridors across the landscape assess their weed management plans and practice.

“Our workshops and meetings with land managers across Queensland have identified a wide range of weed control practices,” he said. “There are some very high risk operators and some organisations that were more informed about the risks of herbicide resistance than others.”

Dr Thornby encourages all land managers to critically assess their weed control tactics and to make a concerted effort to implement as many tactics as possible, rather than following a standard spraying routine.

“If resources are limited though, the greatest effort should be directed at areas that are exposed to the highest management risk and monitoring the effectiveness of each control event,” said Dr Thornby.

“There is a very high risk of herbicide resistance of grass weeds developing after about 15 years, if high risk management practices are routinely applied.”

Weeds are capable of developing cross-resistance to multiple herbicide modes of action so non-chemical control measures need to be included in any weed management strategy. Preventing seed set and destroying weed seeds is known to be effective in combating herbicide resistance.

The WeedSmart initiative provides a 10 point plan that is designed mainly to assist broadacre crop farmers but the principles are also relevant to weed control teams in non-farming organisations when planning their weed management strategy.

Related Articles

View 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. Prevent spray-drift 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 Adjuvants 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. Other resources 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

Subscribe to the WeedSmart Newsletter