Take home messages

• Care needs to be taken with pre-emergent herbicide applications in light soils and where crop tolerance is not high.
• Less soluble pre-emergent herbicides are safer to use for dry sowing.
• When using pre-emergent herbicides as early post-emergent applications, the products should be applied to small ryegrass seedlings – ideally at the one-leaf stage.
• New registrations for Group 14 herbicides are expanding the ways these herbicides can be used.

Using pre-emergent herbicides safely and effectively

Control of annual ryegrass has become much more dependent on pre-emergent herbicides due to widespread resistance to post-emergent herbicides. One of the problems with greater use of pre-emergent herbicides is that damage to the crop can occur. Achieving good crop safety with pre-emergent herbicides requires understanding herbicide behaviour, soil types, rainfall patterns and crop tolerance. Many pre-emergent herbicides will cause crop damage if they get into the crop seed row. Therefore, care needs to be taken to ensure this does not occur. The normal knife-point and press-wheel seeding system throws herbicide-treated soil out of the crop row, increasing the crop safety for pre-emergent herbicides.

Table 1 provides information on the behaviour of recently registered pre-emergent herbicides and compares the newer products to existing products. The key factors in understanding risk of crop damage with pre-emergent herbicides are water solubility and binding to soil. The more soluble a herbicide is, the further it will move through the soil with each rainfall event. On the other hand, higher binding to soil components will reduce herbicide movement.

Table 1: Behaviour of some pre-emergent herbicides used for annual ryegrass control.

*Boxer Gold contains both prosulfocarb and S-metolachlor, Mateno Complete contains aclonifen, pyroxasulfone and diflufenican.

For dry sowing situations, it is best to choose herbicides with lower solubility, as the herbicides will move further through the soil with the first rainfall event on dry soil than they will if there is already moisture in the soil. This will be even more important if the first rainfall event is substantial. The more water-soluble herbicides are better suited to situations where rainfall has already occurred, and the soil is moist at sowing. Using herbicides for which the crop has higher inherent tolerance is better in dry sowing situations. For example, Sakura rather than Boxer Gold for wheat and Boxer Gold rather Overwatch for barley.

In 2021, Overwatch was released and used widely on barley crops. There was a lot of crop effect on barley in particular, with whole crops being bleached. Mostly, the crops recovered and there was no obvious effect on yield. However, in some crops, the plant population was thinned, resulting in yield losses. Factors that contributed were relatively dry soil at sowing, good rainfall after sowing and extended cloudy weather with little sunshine that delayed the recovery of crops. What we learned from 2021 is that barley is more sensitive to Overwatch than originally anticipated and that, in situations that are conducive to crop effect, excessive damage may occur. Damage to barley can be reduced by sowing the crop at least 3cm deep and ensuring that herbicide-treated soil does not move back into the seed row.

New registrations and label changes have increased the number of options for early post-emergent use of pre-emergent herbicides. This provides the opportunity to obtain better control of annual ryegrass. In lower rainfall areas, Boxer Gold, Arcade or Tenet are the best choices as they need less rainfall to activate. Timing of early post-emergent herbicides on annual ryegrass is essential to get correct. Ideally, annual ryegrass should be at the 1 to 2-leaf stage at application. If choosing Mateno Complete for early post-emergent control, then the herbicide should be applied at the 2-leaf stage of the crop, regardless of weed size. All of the herbicides in Mateno Complete have low water solubility, so this product needs more rainfall to become activated and start controlling the weeds. This means the herbicide needs to go out as soon as possible, so the weeds do not get too large if there is a dry period after application.

With all early post-emergent applications, using a different pre-emergent herbicide prior to sowing is essential. If sufficient rain does not occur soon after the early post-emergent herbicide is applied, then annual ryegrass may not be effectively controlled. Early post-emergent applications should be used strategically to improve annual ryegrass control, not as the main tool for annual ryegrass management.

Group 14 herbicide uses: spikes or residuals

There have been a number of new products containing Group 14 (old Group G) herbicides released in recent years. This has increased the options available but has also introduced additional complexity. Based on our previous experience with these herbicides, there has been a tendency to think of Group 14 herbicides as having low solubility, resulting in contact herbicide behaviour and limited movement in the soil. However, there is now a wider range in solubility of Group 14 herbicides, with some more recently registered herbicides having much higher water solubility than has been traditionally associated with Group 14 herbicides (Table 2). This changes the ways that these herbicides behave in soil and plants.

Table 2: Water solubility and binding to organic matter of Group 14 herbicides.

*Voraxor contains both saflufenacil and trifludimoxazin.

Traditionally, Group 14 herbicides have been used as spikes with glyphosate or paraquat knockdown applications. However, the new chemistry available has allowed other use patterns in grain cropping. Terrain, Reflex and Voraxor now all have registrations for residual use prior to sowing to provide suppression or control of broadleaf weeds. Voraxor also provides some suppression of annual ryegrass; however, a pre-emergent ryegrass herbicide will be required in most cases. The greater water solubility of fomesafen (Reflex) and saflufenacil (in Voraxor) makes them more effective as pre-emergent herbicides. It is important to understand that all Group 14 herbicides are absorbed primarily by the shoot, with little absorption by roots. This means that weeds pick up the herbicide as the shoot moves through the herbicide band. The greater movement within the soil increases the chance that a weed seedling will encounter the herbicide. However, greater mobility in the soil does mean increased risks of crop damage, so it is important to adhere to the label advice on crop safety.

The greater water solubility of saflufenacil and tiafenacil also tends to provide Terrad’or, Sharpen and Voraxor with a broader range of activity as a spike with knockdown herbicides. This is likely because there is more movement of the herbicide within leaves. These newer products provide better control of weeds like fleabane and capeweed, but also more control of annual ryegrass than older products.

When using spikes with double knocks, there are choices about which of the knockdown herbicides to put the spike with. Increasingly, there is a focus on putting products that require a crop oil with the second paraquat application rather than the first glyphosate application, particularly where glyphosate-resistant weeds are expected. If putting the Group 14 herbicide in the second knockdown application, it is essential to ensure that the second knockdown goes out to ensure effective 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.

Useful resources

Mixing knockdown partners with Group G herbicides 

Soil behaviour of pre-emergent herbicides in Australian farming systems: a reference manual for agronomic advisers

Contact details

Chris Preston
School of Agriculture, Food and Wine, University of Adelaide
0488 404 120
christopher.preston@adelaide.edu.au