New pre-emergence herbicides – opportunities and challenges

Take home messages

• There are many old and new pre-emergent herbicides available, but most are designed for annual ryegrass control.
• Choice of herbicide should consider soil type, seeding system, dry seeding, soil organic matter and likely rainfall after application.
• Dry conditions around sowing make pre-emergent herbicides performance variable, mixing products with different behaviour can improve performance.

Choosing between new and old pre-emergent grass herbicides for grass control

Pre-emergent herbicides have become the main tool for annual ryegrass control in cereals and there are now many products available. This presents a range of opportunities as well as challenges. Choosing the correct product for the circumstance requires understanding the main factors that influence pre-emergent herbicide behaviour.

Soil type

Soil type and organic matter content both influence pre-emergent herbicide behaviour. Herbicides move with soil water, so will move further in lighter soils due to the larger size of spaces between the soil particles. Lighter soils also tend to have lower organic matter. Herbicide binding to organic matter is variable. Some herbicides bind tightly, which slows their movement through the soil. Other herbicides bind weakly and so will move further, but also move further in soils of low organic matter.

Non-wetting sands cause specific problems for pre-emergent herbicides. As most herbicides move through the soil in water, their distribution in non-wetting sands tends to be uneven. Where water penetrates, it will carry the herbicide with it. Where water does not penetrate, there will be less herbicide present.

Herbicide chemistry

The key elements of herbicide chemistry to be concerned about are water solubility, binding to organic carbon and persistence. More water-soluble herbicides will tend to move further into soil with rainfall, whereas less soluble herbicides will move less. Herbicides that bind tightly to organic carbon will tend to be held up by organic matter in the soil, whereas those that have low binding will move through the soil with every rainfall event.

When considering herbicide persistence, what we are interested in is the effective persistence of the herbicide. That is how long the herbicide will continue to control weeds. There is not really a value that captures this, other than experience. Persistence is influenced by the herbicide rate used, the chemistry of the herbicide, soil organic matter, soil moisture and temperature. The aim is to have herbicides persist long enough to control weeds, but not to harm subsequent crops.

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

*Boxer Gold contains two ingredients, prosulfocarb and s-metolachlor.

Rainfall

Herbicides with low solubility require more rainfall to activate them than more soluble herbicides. The exception to this is trifluralin, which becomes a gas on contact with water. Too much rainfall can move more soluble herbicides into the crop root zone and result in crop damage. However, it is not just the amount of rainfall, but also when it falls that matters. Herbicides applied to dry soil will move further with rainfall than those applied to wet soil. This makes some more water-soluble herbicides risky for use with dry seeding. Particular care needs to be taken choosing herbicides for dry sowing conditions. Dry conditions after herbicide application can make performance of herbicides more variable, depending on whether there is already moisture in the soil.

Seeding setup

Much of the crop tolerance to pre-emergent herbicides is due to positional selectivity. That is the herbicide is applied in a way that limits its interaction with the crop seed. Typically, this is done with a knifepoint and press-wheel seeding system where herbicide treated soil is thrown out of the crop row. For several the pre-emergent herbicides, this is the only safe way to use them. Where crop tolerance is higher, such as wheat tolerance to Sakura®, other seeding setups are also safe, such as knife points and harrows, where some herbicide treated is moved into the crop row. Only a few pre-emergent herbicides are safe for use in disc seeding systems. On the other hand, for some herbicides, like Luximax®, there is a need to ensure crop seeding depth is adequate to achieve safety.

Efficacy on weeds other than annual ryegrass

The pre-emergent herbicides we have for grass weed control in Australia is largely dictated by whether they will control annual ryegrass. Control of other grass weeds is variable. Figure 1 shows the efficacy of several pre-emergent herbicides used at the field rate on control of barley grass in pots under ideal conditions. TriflurX®, Butisan® and Ultro® provided good control in these experiments, but the other herbicides were variable. This means that choosing herbicides where multiple grass weeds are present is more difficult.

Figure 1. Emergence of barley grass in well-watered pots after application of various pre-emergent herbicides at the field rate.

The situation becomes more complex when conditions are not ideal for herbicide activity. Figure 2 shows the same experimental design, except there was no watering for 7 days after herbicide application, when watering was resumed. In this trial, all the herbicides performed less well than if there was watering immediately after herbicide application. Some of the variation in performance of pre-emergent herbicides is due to dry periods after application. This may make herbicides perform less well on weeds like barley grass that are able to bury themselves in the soil than for annual ryegrass.

Figure 2. Emergence of barley grass in pots kept dry for 7 days after application of various pre-emergent herbicides at the field rate.

One way of achieving better performance under dry conditions is to mix pre-emergent herbicides that have different properties. Due to its lack of movement through the soil profile and ability to be activated by low amounts of rainfall, trifluralin is often a good choice for dry seeding conditions. It is also less affected by dry conditions after seeding than some other pre-emergent herbicide choices. However, generally trifluralin provides limited control of weeds like barley grass and brome grass. In our trials on barley grass, mixtures of trifluralin and Sakura have generally performed well.

Pre-emergent herbicides often only provide control for a limited period after sowing. It is essential that other tactics, such as enhancing crop competition, are used to reduce the seed set of weeds that escape the herbicide.

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