Optimising efficacy of pre-emergent chemistry

Take home messages

• There are four main causes for pre-emergent herbicides to fail to control weeds: herbicide resistance, too little persistence, too little rainfall and too much rainfall.
• Understanding the properties of pre-emergent herbicides can ensure the best product or mixture of products are employed in each situation.
• Early post-emergent use of pre-emergent herbicides is a good way of increasing annual ryegrass control, but each herbicide needs to be used in a particular manner to get the best out of it.

Background

Pre-emergent herbicides are now widely used to control annual ryegrass and some other weeds in cropping in Southern Australia. Pre-emergent herbicides are more complex to use than post-emergent herbicides, as they need to be in the right location as the weed seeds germinate. This requires an understanding of how herbicides move in soil, where the herbicides are taken up by the germinating seedling and where weed seeds are most likely to be located.

Most pre-emergent herbicides are taken up by weed seeds by the roots or the mesocotyl (the part of the shoot immediately adjacent to the seed). The main exception to this is triallate and the Group 14 herbicides. Triallate is taken up by the coleoptile and Group 14 herbicides are taken up by the shoot as it moves through the herbicide layer in the soil. For all other herbicides, it is essential for the herbicide to reach the weed seeds to be effective.

In no-till farming systems, most weed seeds fall on to the soil surface and will still be close to the soil surface at sowing time. Any disturbance of the soil surface over summer will tend to bury weed seeds. However, weed seeds will only be buried to the depth of the disturbance. Weed seeds that were set in previous years that have not germinated will have been buried by the previous sowing operations. The deeper weed seeds get buried, the further the herbicides need to move through the soil to reach them. Tillage operations to control summer weeds, for example, will tend to bury weed seeds and make it harder for the less soluble herbicides to reach them.

Weed seeds are moved with the seeding operation. Where they end up depends on the type of equipment used. Seeds that are in the soil thrown from the furrow with a knife-point seeder will get separated from the herbicide. Some seeds will remain in the furrow or end up on the shoulder and will avoid the herbicide, particularly with use of the less soluble herbicides.

Understanding herbicide behaviour in soils

There are several factors that influence how far herbicides will move through soil. These include soil type, soil organic matter, rainfall and herbicide chemistry. There are two key factors of herbicide chemistry that are important: solubility of the herbicide; and its ability to be bound in soil (Table 1). Solubility determines how likely the herbicide will be dissolved in soil water and hence, be moved by rainfall. Binding to soil components tends to slow the herbicide movement through the soil.

Table 1: Behaviour of some pre-emergent herbicides used for grass weed control.

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

Herbicides with high water solubility and low binding to organic matter, such as metazachlor, will tend to move further through the soil profile and can be lost below the root zone of weeds. Herbicides with low water solubility and high binding to soil components, such as trifluralin or aclonifen, will not move very far. The distance that herbicides with other properties will move depends a lot on the amount of rainfall and the properties of the soil.

Sandy soils tend to have larger particle sizes and hence, larger pores between the particles. This allows water to move more quickly through the soil and hence, take more herbicide with it. Sandy soils also tend to have lower organic matter contents. Organic matter tends to slow the movement of herbicides through soil, particularly for herbicides that have high binding to organic matter.

The main causes of pre-emergent herbicide failure

There are essentially four main causes for pre-emergent herbicides to fail to control weeds. Firstly, there is herbicide resistance. The latest resistance survey indicates resistance to trifluralin is present in 38% of crop fields in South Australia and 21% of crop fields in Victoria. Resistance to Boxer Gold is present in 1% of crop fields in South Australia and in 9% of crop fields in Victoria. If herbicide resistance is confirmed or suspected, alternative pre-emergent herbicides should be used.

Secondly, insufficient persistence of herbicides. This is particularly a problem for products, such as Boxer Gold, prosulfocarb and Tenet, where the efficacy of the herbicide declines rapidly after application. Later emerging weeds are able to avoid the herbicide. This is more of a problem in higher rainfall zones or in longer seasons. However, over recent years, some annual ryegrass populations have evolved increased seed dormancy in response to continuous cropping, making short persistence herbicides, such as Boxer Gold, less effective. The solution is to use longer persistence products and mixtures to obtain more control of the weeds.

Thirdly, too little rainfall after application of the herbicide. This is normally a problem for the less soluble products, such as Sakura, propyzamide and Mateno Complete. It typically occurs where there has been good rainfall prior to application of the herbicide, that causes annual ryegrass to germinate. Without sufficient follow-up rainfall after herbicide application, the herbicides are not activated in time to control the weeds. Trifluralin, despite its low water solubility, is not usually affected by this issue, as it becomes a gas on contact with water allowing it to be absorbed by the germinating weeds. Mixtures with herbicides that have different properties can overcome this problem. Useful mixtures have been Sakura plus Avadex Xtra and Sakura plus trifluralin. An alternative is to use Boxer Gold or prosulfocarb as an early post-emergent salvage option to control the annual ryegrass that has got through.

Finally, there is too much rainfall after application of the herbicide. This can move the herbicide out of the root zone of the germinating weeds. This mostly occurs with the more soluble herbicides, such as Tenet and Luximax, and mostly on lighter soil types. However, it can be a problem for many herbicides if there is sufficient rainfall. Herbicide leaching can also occur on heavier soil types that have low organic matter. There are additional factors to consider. There have been more problems with Luximax moving out of the weed root zone than Overwatch, despite their similar behaviour. Some of this is because Luximax is less likely to be bound by organic matter and some is due to the longer persistence of Overwatch. The roots of the weeds will eventually grow into the herbicide, but if there is insufficient herbicide remaining, or the weeds are too large, they will not be controlled. Herbicides will move further with the first rainfall event in dry soil, so the more soluble herbicides should be avoided in dry sowing situations. Using herbicides with lower water solubility in higher rainfall regions, will manage this problem. However, in situations where unexpected high rainfall occurs after sowing, there is only the early post-emergent salvage option available.

Using pre-emergent herbicides in the early post-emergent timing

There are several pre-emergent herbicides that are also registered for early post-emergent use. When using pre-emergent herbicides in this way, it is important to understand that they will not control established weeds and so, a pre-emergent application should always be used first. Early post-emergent herbicide applications can increase the amount of annual ryegrass controlled, control annual ryegrass in the furrow and on the shoulder, and extend the length of control.

It is essential to have rainfall after application to get herbicides to control annual ryegrass at the early post-emergent timing. How much rainfall depends on the herbicide product. Of the cereal herbicides, Boxer Gold requires the least amount of rainfall, followed by Arcade with Mateno Complete requiring more rainfall. This means that Boxer Gold and Arcade can be used for salvage weed control where pre-emergent herbicides failed to adequately control annual ryegrass, whereas Mateno Complete will be less effective at this. Boxer Gold and Arcade are best applied when annual ryegrass is at the 1-leaf stage but can be used up to the 3-leaf stage.

The higher rainfall requirement to activate Mateno Complete means it is best applied before the next flush of annual ryegrass has germinated. Mateno Complete is best used as a strategic approach to maximise annual ryegrass control through the season. This means using a pre-emergent herbicide prior to sowing and then applying Mateno Complete at the appropriate crop stage, regardless of whether annual ryegrass is present or not. The herbicide will then be available once sufficient rainfall occurs to germinate the annual ryegrass.

Tenet has more water solubility than the other herbicides and needs less rainfall after application to activate. However, the rate of Tenet that can be used early post-emergent in canola is insufficient to give reliable control of annual ryegrass. Therefore, another herbicide active on annual ryegrass, such as clethodim, needs to be applied with it.

A note about herbicide persistence

Having longer persistence is good for annual ryegrass control but can cause problems for future crops. Be aware that sensitive crop plant backs have both a time and a rainfall component for Sakura, Mateno Complete and Overwatch. My thinking about early post-emergent Mateno Complete use was that it would provide a solution for growers in higher rainfall zones. When using this herbicide early post-emergent in lower rainfall zones, you need to be aware that you have reduced the opportunity of the herbicide to be degraded before the next crop is sown. This is particularly true in years with a dry spring, such as 2023. There is a requirement of 250 mm of rainfall prior to sowing barley, canola and pulse crops. November and December rains in 2023 mean that many growers will not now have a problem with residues of this herbicide. However, fields that did not receive sufficient rainfall in this period could still have a problem with residues in the soil. If in doubt, plant a tolerant crop.

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

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

Contact details

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