War on weeds - Boort region (latest tips)

Take home messages

• The main herbicide resistant grass weeds in the Wimmera and Mallee are annual ryegrass and brome grass.
• Understanding the behaviour of pre-emergent herbicides in soil is crucial to obtaining efficacy.
• Uragan® or Terrain® are useful alternatives to glyphosate for controlling weeds on fence lines, there are fewer options on irrigation channels.

Resistance to herbicides in the Mallee and Wimmera

The Wimmera and Mallee were last surveyed for herbicide resistance in 2015. For annual ryegrass, resistance to trifluralin, Intervix® and glyphosate had all increased, but there was little resistance to the new pre-emergent herbicides Boxer Gold® and Sakura® (Table 1). Glyphosate resistance was low in both areas but is now common enough to be picked up in our random weed surveys.

Table 1. Extent of herbicide resistance in annual ryegrass collected in random surveys in Wimmera and Mallee in 2015. Populations are considered resistant if there is more than 20% survival.

For brome grass resistance was found to the Group A and Group B herbicides (Table 2). All of the resistant populations were from the Mallee. While resistance to the Group B sulfonylurea herbicides was present, no resistance to Intervix® was found.

Table 2. Extent of herbicide resistance in brome grass collected in random surveys in Wimmera and Mallee in 2015. Populations are considered resistant if there is more than 20% survival.

Getting pre-emergent herbicides to work

The key to understanding the behaviour of pre-emergent herbicides is to understand what they are doing in the soil. Solubility of the herbicide, how much it binds to soil components (mostly organic carbon), soil type, rainfall patterns, and where the weed seeds are located are important in understanding the behaviour of herbicides.

Solubility in water and binding ability to organic carbon (KOC) of several pre-emergent herbicides are listed in Table 3. The way to consider the information in Table 3 is that the greater the solubility of an herbicide the more potential it has to move in the soil in response to rainfall. The lower the KOC, the less likely the herbicide is to become bound to soil organic matter. Therefore, Butisan® will be the most mobile herbicide and trifluralin the least mobile (n.b. Butisan® is more mobile than S-metolachlor despite having slightly lower water solubility because it has much lower binding ability to organic matter). Atrazine will move further than diuron, particularly on lower organic matter soils, even though solubility is similar, because diuron is more likely to interact with soil organic matter.

Table 3. Solubility in water and binding to organic carbon (KOC) of common pre-emergent herbicides.

Note: *Boxer Gold® is a mix of prosulfocarb and S-metolachlor

The next part of ‘getting pre-emergent herbicides to work’ is the location of the weed seeds in the soil. Most of these herbicides are taken up at or below the weed seed. If the seed is on the soil surface, the low solubility herbicides will work well, as they will have a small distance to move before encountering the weed seed. However, if the seed is buried, herbicides like Sakura® will perform less well than the more soluble herbicides like Boxer Gold®.

Finally, rainfall patterns are important. If the season opens with small, patchy falls of rain, the more water-soluble the herbicide the better it will perform, as they take less moisture to activate. The exception to this is trifluralin, which is taken up as a gas and turns into a gas as it encounters water. This means trifluralin is often the best herbicide for patchy starts to the season. On the other hand, if the crop is dry sown and a heavy rainfall event occurs, herbicides can easily move into the crop root zone and cause crop damage. The more soluble herbicides are most likely to do this, although in previously dry soils all herbicides move faster than if the soil is wet.

Management of glyphosate resistant weeds on fence lines and irrigation channels

Glyphosate resistant weeds occurring on fence lines and crop margins can cause problems by moving into the cropped areas. Fence lines suffer from having no competition, so any surviving weed has full access to resources and can set a lot of seed. There are several species with glyphosate-resistance that can become problems in this area including annual ryegrass, windmill grass, brome grass and fleabane. The problem is even worse for irrigation channels, as the seeds of glyphosate resistant weeds can shed into the water and be carried downstream affecting other crops. Glyphosate resistant annual ryegrass and fleabane have been reported from irrigation channels.

Some years ago, research was conducted on the management of glyphosate resistant annual ryegrass on fence lines that led to the registration of Uragan® for this purpose. There are some other options that can be used during spring, but they tend not to work as well (Table 4). One of the problems with controlling all vegetation on fence lines is that the soil along the fence lines can become open to wind erosion on light soils if they are continually kept bare. Sowing as close as possible to the fence can reduce the amount of soil exposed.

Table 4. Control of glyphosate-resistant annual ryegrass on fence lines with alternative herbicides at Hilltown and Ungarra in 2011.

Note: fb = followed by; Letters denote significant differences p<0.05.

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

An alternative to Uragan® that has been registered since our work was conducted on annual ryegrass is Terrain®. Terrain® needs to be applied to bare ground and needs 15mm of rain within a few weeks to activate it, consequently autumn or winter applications will be most effective. Terrain® is a good choice where young trees are close to the fence line.

Control of glyphosate resistant weeds on irrigation channel banks is more complicated as there are fewer herbicide options. The options that are available include: glyphosate (some formulations only), amitrole, dichlobenil, diquat, imazapyr (some products only) and pendimethalin. There are numerous restrictions on the use of these herbicides on irrigation channels, so the label needs to be consulted before choosing a product to use.

Two experiments were conducted for the control of glyphosate-resistant fleabane on channel banks in Victoria (Table 5). The population at Oaklands was very resistant to glyphosate and that at Goolgowi much less resistant. None of the treatments provided excellent control and there was considerable variability at Goolgowi, probably due to a mixture of resistant and susceptible individuals in the population. These experiments demonstrate the difficulty of controlling glyphosate resistant weeds on channel banks with other herbicides.

Table 5. Control of glyphosate-resistant fleabane on channel banks with alternative herbicides at Oaklands in 2015 and Goolgowi in 2017.

Note: fb = followed by; Letters denote significant differences p<0.05.

Useful resources

New options for southern fenceline weed control

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.

Contact details

Christopher Preston
University of Adelaide
0488 404 120
christopher.preston@adelaide.edu.au