Tips for managing weeds in fencelines

Author: | Date: 28 Mar 2017

A fenceline where glyphosate has been used in the foreground and Terrain in the background

A fenceline where glyphosate has been used in the foreground, and Terrain in the background four months after application.

When it comes to dealing with weeds in fencelines researcher Chris Preston understands growers want to battle the issue when ‘they’re not busy’ with the pressing task of farming.

But speaking at the Grains Research Development Corporation (GRDC) Update in Goondiwindi recently the researcher, warned that relying solely on knockdown herbicides, like glyphosate, to control weeds in fencelines and crop margins, risked creating resistance issues for the whole farm.

Dr Preston, who chairs the Glyphosate Sustainability Working Group, said growers had commonly used the broad-spectrum herbicide along fencelines, because it controlled both seedlings and larger plants, did not leach through the soil, and was easy and cheap to use.

“But glyphosate is also the most important herbicide we have for fallow management, pre-crop seeding and inter-row weed management and its use in all of these places has resulted in the evolution of glyphosate resistant weeds,” he explained.

“Once these weeds occur in one location on the farm they can easily move to other locations. Fencelines and crop margins are particularly problematic, because due to the lack of competition in these locations glyphosate resistance weed survivors can set a lot of seed.”

Weed management options for fencelines and crop margins

However, he said there were several management options for growers to use for weed control in these areas and changing tactics would reduce the looming glyphosate resistance issue.

“The area around crops can be cultivated, slashed, rolled for hay, or grazed by stock to control the weeds,” Dr Preston said.

“But I understand all these options can result in other problems: cultivation can leave the area prone to erosion and reduce trafficability; slashing and hay making takes extra time and grazing can be a challenge unless you already have livestock on farm.

“Increasing competition will also limit the impact of glyphosate resistant weeds, but does mean limiting the area left bare.”

He said other effective strategies included planting crops closer to fences, or removing fences and cropping over the area, as well as ensuring crop margins changed from year to year.

Chris Preston

Researcher Chris Preston has looked at best practice methods for growers to manage weeds in fencelines.

Post emergent chemical control options

Yet the University of Adelaide-based expert said he also recognised that for many on-farm situations, herbicides would remain the preferred option for weed control in fencelines.

“But clearly, relying on glyphosate alone will be a risky option for resistance and for the Group B options available resistance has already occurred,” he said.

As part of a GRDC-funded research project he has investigated a range of chemical alternatives for weed control along fencelines in Queensland and New South Wales farming operations. Among the chemicals trialled was bromacil (Uragan®). 

“Uragan is a Group C herbicide with grass and broadleaf activity, is not overly mobile and importantly belongs to a sub-group not being used in the cropping phase and to which we don’t have resistance. This means if resistance does occur in the fenceline, it is likely to have less impact in crop,” Dr Preston said.

“We conducted a series of trials looking at control of glyphosate resistant annual ryegrass in fencelines in the southern region. 

“Post emergent control in August/September suits growers of winter crops, because it is a time when little else needs to be done. However, it does mean application of herbicides to larger well established plants.”

As part of this research a variety of treatments were applied at two trial sites in South Australia. Mixtures of other herbicides with glyphosate proved ineffective in fully controlling annual ryegrass in the fencelines at either site. However, Spray.Seed® at high rates, Alliance® and Basta® plus Amitrole T were more effective, as was Uragan mixtures with Spray.Seed or Basta. A double knock application of two treatments of Spray.Seed 14 days apart also worked well.

“What we learned from these trials is that simply mixing another herbicide with glyphosate was not going to be effective, except where populations were small and without high levels of resistance,” Dr Preston said.

“Even then, this approach just places the selection pressure on the mixing partner. Use of a residual herbicide with appropriate activity on the weeds and registration with an alternative knock down herbicide was the best approach. A strategy of two applications of paraquat-based products was effective and could be used in situations where Uragan wasn’t suited.”

Pre-emergent control options

An alternative approach for growers was to apply a residual herbicide to bare ground along fencelines or in crop margins as a means of stopping weeds from establishing. 

“Uragan can be used for this, however, a few other herbicides are also available for residual, broad spectrum weed control in fencelines. These typically need to be applied to bare soil for best effect. These herbicides include simazine, fluometuron and imazapyr, but all are herbicides where resistance has occurred,” Dr Preston said.

The researcher said it was anticipated that flumioxazin (Terrain®) will be registered for this purpose soon. 

He said significantly higher rates of Terrain than used as a spike with glyphosate will be needed, but at these rates the herbicide would provide long residual control of weeds, without risk of movement through the soil and damage to trees.

“To get the best out of Terrain it will be important to achieve a surface seal on the soil to stop germinating weeds. This means using the high rates and applying them to relatively bare soil. It is important that the soil is then not disturbed to achieve the best results.”

An integrated approach

In summary when it comes to fencelines growers need to be using something with residual activity, that they aren’t using elsewhere on the farm, Dr Preston explained.

“In trials in south east Queensland Terrain worked effectively and was a good fit for Queensland, but the key message remains use chemical groups along fence lines and in crop margins that you aren’t using elsewhere in the paddock.”

In Summary:

  • Persistent use of glyphosate in fencelines and crop margins will lead to glyphosate resistant weeds that can then invade the cropped area of the paddock. 
  • A range of non-chemical options are available, that may suit some growers.
  • A combination of an alternative knock down herbicides with bromacil has proved effective in managing glyphosate resistant annual ryegrass in fencelines.
  • Applying residual herbicides to bare ground is an alternative herbicide option to using glyphosate.

More Information

Chris Preston, University of Adelaide
08 8313 7237

Useful Resources>

GRDC Project Code UA00124

Region North