Weeds researcher Dr Peter Boutsalis says there needs to be a shift in thinking about fenceline weed management to delay the onset of herbicide resistance.
Dr Boutsalis, from the University of Adelaide and Plant Science Consulting, says spraying fenceline weeds in early autumn rather than spring is preferential.
“In early autumn, weeds are younger and easier to control,” he says. “You can also incorporate residual herbicides when spraying in early autumn.”
He says this should result in a clean fenceline during the growing season. If survivors are noticed, he suggests chipping the weeds out or spraying them with a herbicide from another mode of action group.
Herbicide resistance testing is useful to determine which herbicides can be deployed, he says.
A previous project, with GRDC investment, looked at fenceline weed control because many growers found that glyphosate was not controlling annual ryegrass along fencelines.
“It was because of repeated use of an easy-to-use herbicide and no crop competition,” he says. “It starts with just a few plants, and as weeds are repeatedly sprayed with the same herbicide, you see more plants survive along the fenceline.”
For example, annual ryegrass resistance to glyphosate can go from zero to 100 per cent in just a few years.
Pollen movement causes the spread of glyphosate resistance genes to other annual ryegrass plants in the paddock. This is in addition to machinery movement close to fencelines, he says.
“If you’re using glyphosate on the fenceline over summer or autumn before sowing, you’ll see glyphosate-resistant annual ryegrass creeping into the paddock.
“Within one trial, we sampled plants perpendicular to the fenceline and found that as you moved out from the paddock’s edge, glyphosate resistance was more frequent closer to the fenceline and less frequent further into the paddock.”
Another issue, he says, is that some growers are using whatever herbicide is left in the spray tank to manage fenceline weeds, which is generally something that is used to manage weeds in-crop.
He says spraying selective herbicides along fencelines increases the chance of weeds surviving, leading to resistance to those products when they are used in-crop.
Fenceline spraying is frequently done in spring because people tend to be less busy.
“But during spring, weeds tend to be larger, so they are harder to kill," he says. "If you spray weeds when larger, they are less likely to die because they have larger root systems.”
Dr Boutsalis says spraying weeds in early autumn before sowing is the best time to treat fencelines with residual herbicides.
“Bromacil (Uragan®WG) is one of the pre-emergence herbicides that can be used for fenceline weed management. However, if trees are nearby, this herbicide should be avoided,” he says.
“Another product is indaziflam (Alion® 500 SC Herbicide) from Bayer. The same herbicide active from Envu (Esplanade® Herbicide) is registered for fencelines and other non-cropped areas.
“Indaziflam controls weed seeds from germinating but does not control established plants. It provides long residual control without harming trees growing nearby.”
Glyphosate and paraquat
Dr Boutsalis says about 20 annual ryegrass samples sent to him for herbicide resistance testing during 2023 were tolerant to glyphosate and paraquat.
“Some weeds were from fencelines, and others were from within paddocks.
“Growers can send surviving weeds to us during the growing season or collect seeds for herbicide resistance testing at the end of the year.”
WeedSmart western extension agronomist Peter Newman encourages using multiple herbicides in fenceline brews.
“Mix and rotate fenceline brews,” Mr Newman says. “If you’re just using one herbicide such as glyphosate or paraquat, you’re doing it wrong.
“I’m a big fan of cropping to the fence then spraying out the firebreak later with a robust mix. This adds some competition.”
Avoid crop herbicides
Dr Boutsalis says fenceline weed control should not be done with herbicides used in crops. If it is and any survivors are observed, they should be killed to stop resistance from developing.
“Some growers are using clethodim plus glyphosate, which works well, and it is an effective mix for Roundupâ Ready canola,” he says.
“But if you’re using that mix on your fencelines or in your fallow, and you see resistant survivors, you may no longer be able to use it in your Roundup® Ready canola.”
If glyphosate or paraquat do not work, he says the knockdown herbicide amitrole (Amitrole T) can be used along fencelines at robust rates before sowing wheat or barley.
He says another pre-emergent active, flumioxazin (Terrain® 500 Herbicide), can be applied to fencelines before rainfall at the start of the season.
Professor Chris Preston from the University of Adelaide says that to achieve the best out of flumioxazin, it is important to apply the product at label rates to bare soil.
“It is important that the soil is not disturbed to achieve the best results.”
There are other approaches that some growers are using to manage fenceline weeds. Professor Preston says he knows of one grower using a low rate of paraquat on fenceline weeds to stop seed-set.
“This grower wants weeds growing on fencelines because of light, erodible soils but does not want the weed seeds from the fenceline moving into paddocks,” he says.
“By reducing the number of seeds produced, the weeds don’t move from the fence, but there are enough that drop near the fence that grow and stop the fragile soil from blowing away.”
Some growers use mowing or slashing to stop seed-set, he says. “It may not be practical on big farms, but you could certainly use slashing on smaller farms.”
Another option is cutting the outer lap of the crop for hay, mainly to create a firebreak, but it stops the weed seeds from moving into the paddock, so it does not matter how many weeds are on the fenceline, he says.
Another option is cultivation, but Dr Catherine Borger, weeds discipline leader at the Western Australian Department of Primary Industries and Regional Development (DPIRD), says not all growers would have a plough narrow enough to fit between their fences and crops. The disadvantage of ploughing is that light soils could suffer from wind erosion.
“A good option is removing your fencelines altogether if you do not have sheep or cattle,” Dr Borger says. “Sometimes cropping right up to the fenceline and then slashing it out at the end of the year might be easier than using herbicides for weed control.”
Pasture along fencelines
A novel solution to managing fenceline weeds was explored in a WeedSmart podcast. This involved planting lucerne along fencelines to outcompete the weeds.
WeedSmart northern extension agronomist Paul McIntosh says he has seen lucerne outcompete feathertop Rhodes grass when it was established along a fenceline.
“Research at Wagga Wagga, New South Wales, showed lucerne seedlings were also able to outcompete annual ryegrass seedlings,” he says.
“If you can establish 50 to 70 lucerne plants per square metre into weed-free soil with a pHCa of more than 5.0, the pasture should outcompete any future weed growth.”
Other growers have reported planting perennial phalaris and fescue along paddock boundaries and say no fenceline spraying is needed.
Electric weed control
An alternate option for fenceline weed management is under investigation by DPIRD researchers.
In an Australian first, the team is exploring electric weed control, with collaboration from AGXTEND (a brand of CNH Industrial) and investment from GRDC, Wine Australia and the Cotton Research and Development Corporation.
DPIRD research scientist Miranda Slaven says DPIRD is investigating the AGXTEND XPower machine, powered by Zasso™, which uses the tractor’s mechanical power to produce a high-voltage current to burst weeds’ cells.
“It kills the weeds or suppresses their growth,” Ms Slaven says. “The current is applied directly to weeds through metal electrodes mounted behind or in front of the tractor.”
In 2023, DPIRD found that electric weed control is effective on glyphosate-resistant annual ryegrass.
When glyphosate (1.5 litres per hectare of RoundUpâ Ultra Max) was applied, Ms Slaven says the annual ryegrass density averaged 64 plants per square metre.
“A double knock of glyphosate followed by paraquat with diquat (1.5L/ha Spray.Seed® 250) was also applied and reduced the annual ryegrass density to 53 plants/m2,” she says.
“However, electric weed control was more effective, reducing the density to 22 plants/m2 when the machine was driven at two kilometres/hour and 4km/h.”
When the technology becomes available in Australia, Ms Slaven says, electric weed control could be used as an alternate weed management method along fencelines.
Dr Borger says electric weed control is available now in Europe with a two to 3m application width. “They’re not yet certified in Australia,” she says. “The manufacturer expects them to be available for sale in the second quarter of 2024.”
Although electric weed control cannot be used in rainy weather, she says it can be used in windy conditions or when the weeds are stressed.
Ms Slaven says that although fire risk is an issue, there is a low risk if applications are undertaken during spring and winter, after rainfall, or when the topsoil and any residues are moist.
Spray application specialist Bill Campbell from Bill Campbell Consulting says setting up for spraying fencelines is about:
- using a coarse to medium droplet;
- using robust water rates (60 to 100L/ha); and
- managing drift to prevent crop loss.
He says careful set-up can also help avoid crop herbicide residue issues. This is particularly important when growing malting barley.
“Then we need to achieve effective coverage, which is a component of water volume and droplet size,” he says.
“We can transpose the minimum standards of paddock spraying to fenceline spraying where we try and aim for good to excellent coverage with 40 drops per square centimetre and 12 to 15 per cent of area covered.”
He says the tricky part of working with fenceline sprayers is that many are homemade and do not have automatic rate controllers.
“Setting them up and calibrating fenceline sprayers is quite simple when you know how to do it, but people find those tasks quite tricky,” he says.
“It’s about calculating the nozzle flow rate and your speed so that if you spray your fencelines with 80L/ha of water, for example, the sprayer applies 80L/ha of water.”
The first step, he says, is to determine the flow rate required for a nozzle. This is done by using the formula:
Flow rate per nozzle (litres per minute) = water rate (litres per hectare) x speed (km/h) x nozzle spacing (width) ÷ 600.
The next step is checking the standard nozzle chart to determine which size nozzle to use and the operating pressure required.
“If you set the pressure correctly with the relief valve and drive at the right speed, you can ensure you are applying herbicide at the right rate,” he says.
According to Mr Campbell, weeds are selected for resistance over many years when fencelines are sprayed with multiple low-dose applications.
“Most new boomspray units now have dedicated plumbing systems for the fenceline nozzle on both wings to prevent underdosing.”
More information: Peter Boutsalis, [email protected]; Chris Preston, [email protected]; Catherine Borger, [email protected]; Paul McIntosh, [email protected]; Miranda Slaven, [email protected]; Bill Campbell, [email protected]
Samples of seeds or plants can be tested for herbicide resistance by:
Plant Science Consulting
22 Linley Avenue
Prospect South Australia 5082
School of Agricultural and Wine Sciences
Charles Sturt University
Locked Bag 588
Wagga Wagga New South Wales 2678