Managing weeds in fencelines

Take home messages

• 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.

The problem with fencelines and crop margins

Fencelines and other uncropped areas of the farm are where weeds can invade and become established. From there the weeds can move into the cropped area, particularly with harvest operations. Increasingly farmers have been keeping these areas bare of vegetation. However, this causes issues with weed invasion due to the lack of competition in the area.

A common approach to managing fencelines and crop margins is to use glyphosate for weed control. Glyphosate is ideal in this situation, because it is a broad-spectrum herbicide that controls both seedlings and larger plants, does not leach through the soil, is easy to use and is cheap. However, glyphosate is also the most important herbicide used for fallow management, prior to crop seeding and for inter-row weed management. It use in all of these places has resulted in the evolution of glyphosate resistant weeds. Once these weeds occur in one location on the farm they can easily move to other locations. Fencelines and crop margins are particularly problematic areas due to the lack of competition present. This means that any glyphosate resistant individual that survives can set a lot of seed.

Weed management options for fencelines and crop margins

There are a number of management options for weeds in fencelines and crop margins. Glyphosate resistance is occurring in this area as a result of the intensive use of glyphosate, limited use of any other controls and the lack of competition. Changing management practices will reduce the problem.  The area adjacent to the crop can be cultivated, slashed, rolled for hay, grazed by stock or treated with herbicide.  The aim is to reduce the ability for problem weeds to set seed that can be moved to crop areas of the farm.

All of the options for management can result in other problems. Cultivation can leave the area prone to erosion and reduce trafficability. Slashing and cutting for hay takes extra time and may be a problem for large farms. Grazing can be problematic in sourcing stock, if not already on the farm, and stopping them from straying into the crop.

Increasing competition over the area will help limit the impact of glyphosate resistant weeds. This means limiting the area left bare to the minimum. This could be achieved through growing the crop closer to fences followed by slashing to create a fire break, removing fences and cropping over the area or not putting crop margins in the same place every year. One idea that has been proposed is to grow native grasses in these areas. While they could be very useful in providing competition against weeds, unfortunately many native species are not tolerant of herbicides used in the cropped area and eventually the grasses would weaken and gaps would appear allowing weeds to invade.   

Post emergent chemical control options

We recognise that for many situations, herbicides will remain the preferred option for weed control in fencelines. Clearly, relying on glyphosate alone will be a risky option for resistance and for the Group B options available resistance has already occurred. Therefore we explored a range of other options. Among them 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.

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.

A variety of treatments were applied at two trial sites in South Australia in August 2011. The trial was assessed by counting the number of seed heads in December 2011. The annual ryegrass was more resistant to glyphosate and weed populations were larger at Hilltown compared with Ungarra (Table 1). Mixtures of other herbicides with Roundup PowerMax® were not effective 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. Spray.Seed was more effective where the annual ryegrass population was lower at Ungarra than at Hilltown.  The double knock application of two treatments of Spray.Seed 14 days apart was also effective.

Table 1. Control of glyphosate-resistant annual ryegrass with alternative herbicides at Hilltown and Ungarra in 2011.

* treatments in each column followed by different letters are significantly different (P=0.05).

# Uragan label rate for non-crop areas is 3.5-6.5 kg/ha, with the 2.0kg rate on the label for ‘retreatment’. Basta label rate is 1.0-5.0L. Amitrole T label rate is 1.1L/100L water

A second trial was conducted in 2013 at Clare and Kapunda in South Australia. Again, treatments were applied in August and assessments made in December. This trial looked at the rates of Uragan required for mixtures to be effective. In this trial, Roundup PowerMax at 2 L ha^-1 was relatively ineffective at both sites (Table 2). At both sites, Uragan at 2 kg ha^-1 was effective in mixtures with either Spray.Seed or Basta; however in these high annual ryegrass populations, the higher rate of Uragan was sometimes better.

Table 2. Effect of herbicides on annual ryegrass seed head production in the fence line trials at Clare and Kapunda in 2013

* treatments in each column followed by different letters are significantly different (P=0.05).

# Uragan label rate for non-crop areas is 3.5-6.5 kg/ha, with the 2.0kg rate on the label for ‘retreatment’.

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. Even then, this 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 cannot be used.

Pre-emergent control options

An alternative approach is to apply a residual herbicide to bare ground 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.

It is anticipated that flumioxazin (Terrain®) will be registered for this purpose soon. Much higher rates of Terrain than used as a spike with glyphosate will be needed. Terrain at these rates will provide long residual control of weeds, without risk of movement through the soil and damage to trees, that can occur with some other herbicides. 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 bare soil. It is important that the soil is then not disturbed to achieve the best results.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support.

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